Silicon Availability from Chemically Diverse Fertilizers and Secondary

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Sustainability Engineering and Green Chemistry

Silicon availability from chemically diverse fertilizers and secondary raw materials Olivier Duboc, Anja Robbe, Jakob Santner, Giulia Folegnani, Perrine Gallais, Camille Lecanuet, Franz Zehetner, Peter Nagl, and Walter W. Wenzel Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06597 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Environmental Science & Technology

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Silicon availability from chemically diverse fertilizers and

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secondary raw materials

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Olivier Duboc1, Anja Robbe1, Jakob Santner2*, Giulia Folegnani1, Perrine Gallais1, Camille

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Lecanuet1, Franz Zehetner3, Peter Nagl4, Walter W. Wenzel1

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1

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Lorenz-Strasse 24, 3430 Tulln, Austria. 2

10 11

14

University of Natural Resources and Life Sciences, Division of Agronomy, KonradLorenz-Strasse 24, 3430 Tulln, Austria.

³ University of Natural Resources and Life Sciences, Vienna, Institute of Soil Research,

12 13

University of Natural Resources and Life Sciences, Institute of Soil Research, Konrad-

Peter Jordan Straße 82, 1190 Vienna, Austria. 4

Department of Lithospheric Research, University of Vienna, Althanstraße 14 (UZA II), 1090 Vienna, Austria.

15 16

* Corresponding author: [email protected]

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Number of Tables: 1

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Number of Figures: 4

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Keywords: silicon; fertilizer; solubility; ferrihydrite, phytolith, biochar, LD slag.

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Abstract Crops may require Si fertilization to sustain yields. Potential Si fertilizers include

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industrial by-products (e.g. steel slags), mined minerals (CaSiO3), fused Ca-Mg-

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phosphates, biochar, ash, diatomaceous earth and municipal sewage sludge. To date, no

26

extraction method was shown to accurately predict plant availability of Si from such

27

chemically diverse Si fertilizers. We tested a wide range of products in greenhouse

28

experiments and related the plant Si content to Si extracted by several common Si fertilizer

29

tests: 5-day extraction in Na2CO3 / NH4NO3, 0.5 mol L-1 HCl, and Resin extraction. In

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addition, we tested a novel sink extraction approach for Si(OH)40, which utilizes a dialysis

31

membrane filled with ferrihydrite (‘Iron Bag’). Wheat straw biochars and ash exhibited

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equivalent or marginally higher Si solubility / availability compared to wheat straw. Thermo-

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chemically treated municipal sewage sludge, as well as diatomaceous earth, did not

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release substantial amounts of Si. The Resin and the Iron Bag extraction methods gave

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the best results to predict plant availability of Si. These methods better reproduce the

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conditions of fertilizer dissolution in soil and around the root by (1) buffering the pH close

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to neutral, and (2) extracting the dissolved Si(OH)40 with ferrihydrite (Iron Bag method) for

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maximal quantitative extraction.

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40 41 42


Graphical abstract

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1. Introduction Although not considered an essential plant nutrient, Si has positive effects on plant

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health, in particular resistance to biotic and abiotic stress 1. Silicon deficiency is of concern

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in tropical regions where Si accumulators such as rice or sugarcane are grown, and soil

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plant-available Si (PAS) may be low in highly weathered soils 2. In temperate regions too,

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some of the major crops are Si accumulators (mainly cereals and grasses, but also

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soybean, sugar beet and tomato 1) and there are concerns about negative impacts of

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intensive agricultural practices on PAS 3,4 Recently, we identified a substantial proportion

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of Austrian agricultural topsoils to be very low in PAS, indicating that also part of the

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temperate zone soils may require Si management and fertilization (own unpublished

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results). Growing interest in the role of Si in agriculture requires improved PAS thresholds

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for Si application 5 and characterization of PAS in potential Si fertilizers 6–8.

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Steel slags from blast oven furnaces (BOF slag), or from Linz-Donawitz converters (LD-

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slag) are major potential sources for Si fertilizer production and therefore have received

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attention in fertilizer testing 6,7,9,10. Phytoliths, i.e. plant-borne amorphous SiO2, are an

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important Si pool in the biogeochemical Si cycle, and their importance for PAS increases

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with soil weathering stage 2, so that recycling of Si-rich crop residues such as straw can be

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crucial to sustain soil fertility 1,11. Plant biomass can be directly recycled to the field with

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crop residues or after straw and stubble incineration, which remains a common practice in

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several parts of the world, for example in Asia 12. However, biomass often undergoes a

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cascade of agro-industrial uses in the form of manure, biogas slurry, compost, ash, etc.

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before returning to the field. Studies have shown that straw incineration at low temperature

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(up to 400 – 500°C) could increase straw Si solubility 13, but that at higher temperatures

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(600 – 900°C) Si solubility strongly decreases due to crystallization 13–15. 4 ACS Paragon Plus Environment

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Pyrolysis is an alternative to incineration and is increasingly seen as a value adding

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process. Biochar from (phytolith-rich) Miscanthus was found to be a good source of plant

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available Si 16,17. Diatomaceous earth (DE), which contains diatom frustules (cell walls)

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consisting of amorphous SiO2, is another emerging fertilizer source (e.g. AgriPower

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Australia Ltd). Furthermore, municipal sewage sludge (MSS) may also contain significant

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amounts of Si, and has been suggested as a potential source for recycling biological Si 3.

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New thermo-chemical treatments of MSS such as the Ash-Dec process 18 aim at

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increasing P bioavailability but may also mobilize Si.

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Common extraction techniques for Si fertilizers are based on quasi-equilibrium in water,

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neutral, acidic or alkaline aqueous solutions, or on induced solubilization with organic

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ligands such as citrate. A 5-day extraction with Na2CO3 and NH4NO3 and a pH-buffered

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H2O extraction protocol 9 were among the best predictors (R2 between 0.6 and 0.7) of Si

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availability to plants, being clearly superior to e.g. 0.5 mol L-1 HCl, 0.5 mol L-1 acetate, and

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5% citric acid 6,19. The 5-day Na2CO3 + NH4NO3 extraction was validated in a single-

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laboratory study for testing solid Si fertilizers with Si concentrations up to 84 mg g-1 10.

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However, after testing different industrial by-products (mainly steel slags), Haynes et al. 7

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concluded that no single fertilizer extraction technique could predict Si availability from Si

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fertilizers. Moreover, Zellner et al. 8 extracted a wide range of products (including slags

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and biochars) with the 5-day Na2CO3 + NH4NO3 method and obtained only a poor

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correlation with plant uptake (R² = 0.46). Hence, none of the available methods

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satisfactorily predicts Si bioavailability from the increasing range of potential Si sources,

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i.e. apart from Ca- and Mg-silicates also products based on plant Si (phytoliths), or DE as

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an emerging Si fertilizer.

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Duboc et al. 20 showed that an ion-sink technique based on a dialysis membrane tube filled with ferrihydrite (referred to as ‘Iron Bag’; IB), was very well suited to assess the P 5 ACS Paragon Plus Environment


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availability from very contrasting recycling fertilizers. In brief, the method is based on a

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zero-sink created through adsorption of phosphate on the ferrihydrite. After a given

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extraction time - a few days to weeks - the ferrihydrite is retrieved, dissolved with

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concentrated H2SO4 and analysed for P. Silicic acid, Si(OH)4, is the soluble, plant-

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available form of Si which, similarly to phosphate, is also adsorbed on ferrihydrite.

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Maximum Si adsorption is expected around the pKa of silicic acid (pH 9.5) 21, but

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adsorption also occurs at pH between 3 and 6 22,23. Therefore, this method may have

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potential to extract Si through sink-driven fertilizer dissolution.

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The objectives of this study were to (1) assess the sink method (IB) as a new approach

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to characterize Si solubility in chemically diverse Si fertilizers, and in particular to test

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whether it can provide a better estimate of Si availability to plants compared to existing

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methods, and (2) to provide an overview of Si plant availability in various potential Si

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sources for agriculture, including commercial fertilizers and secondary raw materials /

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recycling products such as slags, biochars, ashes, or sewage sludge, differing largely in

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their physical and chemical properties.

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2. Materials and Methods For all analyses (fertilizer / soil / plant extractions or digestions), we used laboratory

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water type I (18 MΩ cm-1) and plastic vessels that were either new or washed successively

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in a 5 % HNO3, and in a 0.1 mol L-1 NaOH bath for ≥ 5 h each and rinsed with type I water

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after each bath.

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2.1. Characteristics of fertilizers and soil

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2.1.1. Sample description

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Tested products are listed in detail in Supporting Information (Table S1). They included

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the following : (1) municipal sewage sludge (MSS) and -ash (MSSA) and their respective

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conversion products (AshDec-Na 18, pyrolysis, and hydrothermal treatments), (2)

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diatomaceous earths (DE), (3) one untreated LD slag and four LD slags amended with

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MSSA in their slag stage as phosphorus source 24, (4) wheat straw with its respective

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biochars and ash (Str, Str-BC and Str-Ash), (5) a commercial, fused Ca-Mg-P fertilizer

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(ThermoPhos), and (6) several reference materials (sodium metasilicate (Na2SiO3), SiO2,

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fumed SiO2, and calcium metasilicate (CaSiO3)). Additionally, mixed samples were

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produced from these individual substances to obtain fertilizer materials with intermediate

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Si solubility, so as to fill the gap in the regression obtained with plant uptake in the first pot

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experiment, which consisted of two clusters (see EXP 1 with S. cereale in Figure 4).

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Mixing ratio were chosen based on known solubilities of the different components with the

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Iron Bag extraction method (see method description below), and therefore primarily

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designed to fill the gap in the Iron Bag regression.

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Total Si in the materials (except straw) was determined by X-ray fluorescence 7 ACS Paragon Plus Environment


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spectrometry (XRFS), which was carried out on fused beads (precalcined sample powder,

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1:10 with LiTB+LiMB(66:34) flux) using a sequential XRF spectrometer PANalytical

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PW2404. Sample preparation by fusion with a flux enables the analysis of wide range

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element concentrations and improved mathematical corrections of the sample matrices'

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influences, thus it was the preferred procedure in this case. See Supporting Information for

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more details. Total Si in straw was determined using a molybdate blue colorimetric assay

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after oven-induced digestion (both methods described below).

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2.1.2. Sample preparation

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All fertilizers were milled to < 200 µm in a vibratory ball mill (Retsch ® MM 200) using

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stainless steel grinding equipment. Individual milling steps were limited to a maximum of

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3 min. After each milling step the sample was sieved and the fraction retained in the sieve

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was milled again. In order not to discard any fraction, this process was repeated until all

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material passed through the 200-µm sieve.

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2.1.3. Soil selection

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Given raising interest in plant Si nutrition in temperate regions we used a soil from

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Sigmundsherberg, Lower Austria, a carbonate-free soil with a pH in H2O (1 : 2.5, w/v;

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< 2 mm fraction) of 6.05 ± 0.17, and pH of a saturated soil paste of 6.96 ± 0.41 (< 4 mm

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fraction as used in pot experiments). CaCl2-extractable Si 25 was 13.1 ± 0.8 mg Si kg-1.

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The soil was selected based on a screening of 270 agricultural topsoils in Austria, as it’s

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CaCl2-extractable Si concentration falls within the lower 10% of the concentration range

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covered, and is also clearly below the band of upper limits of Si deficiency (20 - 45

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mg Si kg-1) reported in the literature 25–28.


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2.2. Pot experiments Two pot experiments were conducted in the greenhouse to estimate plant availability of

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Si from the fertilizer materials. Experiment 1 (EXP 1) was conducted with Secale cereale

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L. (var. “Amilo”) with four replicates per treatment, and experiment 2 (EXP 2) with Lolium

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perenne (var. “Grasslands NUI”) in triplicate. Secale cereale, a common crop in temperate,

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regions was chosen for EXP 1 because it is a common arable crop belonging to the

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Poaceae family with known accumulation potential for Si 29. Lolium perenne, a widespread

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grassland species and animal feed was chosen for EXP 2 due to its even larger Si

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concentration in shoots according to a dose response trial that was run in parallel to EXP 1

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(see below and Figure S1). Using two different crop species is deemed to increase the

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validity and relevance of our conclusions. One kilogram soil was added to each pot, the

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finely ground fertilizer was thoroughly mixed at a rate of 75 and 150 mg Si (kg dry soil)-1 in

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EXP 1 and EXP 2, respectively. The first rate was chosen to cover the range of fertilization

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of macronutrient, which show similar shoot concentrations as Si in graminaceous species.

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We increased the Si addition rate in EXP 2 based on a dose-response trial run in parallel

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to EXP 1, showing a linear increase of Si uptake by both species between 0 to 160 mg Si

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(kg dry soil)-1(see Figure S1 in Supporting Information). Straw was not used as a Si source

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in the pot experiments due to its impracticably low Si concentration (8 mg g-1).

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Fertilizer powder was mixed with the dry soil, transferred into the pots, and deionized

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water was added to reach 80% of WHC. The pots with moistened soil were then

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equilibrated in the greenhouse for one week. Afterwards, 40 seeds of S. cereale were

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planted per pot and thinned to 25 plants seven days after planting (DAP) in EXP 1. For

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EXP 2, 750 mg of L. perenne seeds were placed on the soil surface and covered with 50 g

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of fine soil. Plants were watered daily with type I water, maintaining the pots as closely as

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possible to 80% of WHC by weighing. The pots were placed randomly on the table and 9 ACS Paragon Plus Environment


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moved regularly.

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From 7 DAP onwards, 50 mL of a nutrient solution (modified from Middelton and

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Toxopeus 30) was added weekly, containing: 4 g L-1 NH4NO3, 1 g L-1 NaH2PO4 ∙ 2H2O,

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0.4 g L-1 Ca(H2PO4)2 ∙ H2O, 1.47 g L-1 K2SO4, 444 mg L-1 MgSO4 ∙ 7H2O, 360 mg L-1

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CaCO3, 7.2 mL L-1 of 1 mol L- 1 HCl (to dissolve the CaCO3), 600 µg L-1 H3BO3, 158 µg L-1

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CuCl2, 5.5 mg L-1 MnCl2 ∙ 4H2O, 80 µg L-1 (NH4)Mo7O24 ∙ 4H2O, 300 µg L-1 ZnCl2 and

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2.4 mg L-1 Fe EDDHA. This resulted in a total addition of 350 mg N, 74 mg P, 165 mg K,

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82 mg S, 11 mg Mg, 52 mg Ca, 26 µg B, 19 µg Cu, 382 µg Mn, 12 µg Mo, 36 µg Zn and

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34 µg Fe over the course of the experiment. Controlled growth conditions included

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humidity of 60 %, day / night cycle of 16 / 8 h, with day / night temperature of 25 / 15 °C

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(EXP 1, conducted in winter) and 30 / 20 °C (EXP 2, conducted in spring). Artificial lighting

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with sodium vapor lamps complemented daylight and guaranteed minimum

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photosynthetically active radiation (PAR) of 300 to 400 μmol m−2 s−1.

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Shoots were harvested at 42 DAP by cutting evenly with scissors at the level of the

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pot’s edge (ca. 2 cm above soil surface level), washed with deionized water and dried for

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48 h at 65 °C. After weighing the dried biomass, the sample was milled twice for 20 s in a

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Retsch ® GM 200 mill at 8500 RPM, and digested with the oven-induced digestion (OID)

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method 31 with minor modifications (see Supporting Information). In brief, 100 mg sample

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was heated to 95°C in 50 mL centrifuge vials for 30 min with 80 µL octyl alcohol and 2 mL

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30% H2O2. Then, 4 mL 500 g L -1 NaOH was added and the samples were heated to 95°C

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again for 5 h. Finally, 1 mL 5 mmol L-1 NH4F was added and the solution was brought to

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50 mL with type I water.

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2.3. Si solubility in the fertilizer materials

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For all extraction procedures, samples were weighted directly into the extraction vial on 10 ACS Paragon Plus Environment

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an analytical scale with a precision of ± 0.1 mg. After extraction, all samples (except Iron

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Bag method) were filtered through Munktell 14/N folded filter paper and analysed for their

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Si concentration using a molybdate blue colorimetric assay (see below).

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Fertilizer extractions were replicated 3 - 9 times to better assess their reproducibility and

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precision. Some replications were done in different batches, with several days to months

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of interval and different operators.

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2.3.1. Quasi-equilibrium Si extraction methods from fertilizers

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We performed five different fertilizer extractions. The benchmarks were H2O with

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Amberlite GC50 (termed ‘Resin’) 9, and the 5-day extraction with Na2CO3 + NH4NO3 (‘5-

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Day’) 10 since both were found to better predict Si plant availability than other methods in

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recent studies. We also extracted with 0.5 mol L-1 HCl (‘HCl’) as described in Haynes et al.

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7

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initially developed for soil amorphous Si 32. We also performed an extraction in pH-

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buffered H2O (buffer: 30 mmol L-1 3-morpholinopropane-1-sulfonic acid) (‘MOPS’) at pH 7.

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The latter was performed to compare with the Iron Bag extraction method (conducted in

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the same aqueous MOPS matrix; see below) but without the sink effect of ferrihydrite.

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Details of the methods are given in Table 1.

using a solid : liquid ratio of 1 : 100, and with 0.2 mol L-1 NaOH (‘NaOH’), an extraction

Table 1: Characteristics of the quasi-equilibrium fertilizer extractions

Extraction name

Solution(s)

Sample weight

Extraction volume and [vial size]

Extraction time & speed

NaOH

0.2 mol L -1 NaOH

75 mg

30 mL [50 mL]

HCl

0.5 mol L-1 HCl

100 mg

15 mL [20 mL]

5-day

0.094 mol L -1 Na2CO3 + 0.2 mol L -1 NH4NO3 (1:1)

70 mg

35 mL + 35 mL [100 mL]

120h, 5 RPM overhead 1h, 20 RPM overhead 1h 20 RPM overhead + 5 day resting

Reference Georgiadis et al. 32 Haynes et al. 7 Sebastian et al. 10



Resin MOPS

222 223

a

H2O + 100 mg Amberlite CG50 a 30 mmol L-1 3morpholinopropane1-sulfonic acid, pH 7

40 mg

80 mL [100 mL]

40 mg

80 mL [100 mL]

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96 h, 7 RPM overhead 96 h, 7 RPM overhead

Kato and Owa 9

same CAS number (9002-29-3) as Amberlite IRC-50 which was used by Kato and Owa 9

2.3.2. Depletion-induced Si solubilization from fertilizers (Iron Bag method)

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We used a sink method that employs iron oxide slurry-filled dialysis membranes for

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measuring water-soluble fertilizer Si quantitatively. It was initially developed by Freese et

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al. 33 for soil P extraction, and adopted successfully by Duboc et al. 20 for quantitatively

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extracting P from a range of P fertilizers. Briefly, an Iron Bag consists of a ca. 28 cm long

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visking dialysis tubing (12.000 - 14.000 Dalton MWCO; ⌀ 15,9 mm / width when flat: 25

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mm) filled with 20 mL of a ferrihydrite suspension (ca. 6.9 g Fe L-1). The Iron Bags are

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placed in a fertilizer suspension and are gently agitated during the extraction period. The

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working principle of IB extraction is that Si is bound on the ferrihydrite contained in the

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membrane bag, thereby lowering the solution Si concentration, which in turn promotes

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desorption/dissolution of fertilizer Si. Further details about Iron Bag preparation are

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described in Duboc et al. 20.

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We evaluated the performance of the Iron Bag technique using different approaches,

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including Si sorption experiments form solutions containing varied concentrations of Si in

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the absence and presence of P, and by comparison of a 7-days one-step IB extraction with

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the cumulative amount of Si obtained in three subsequent steps after 42 days. The

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methods and results of the evaluation experiments are shown in the Supplement,

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indicating that the method is not limited by the capacity of the Iron Bags.

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The amount of fertilizer weighed into 250 mL containers varied - according to their Si

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content and expected solubility - between 20 mg (e.g. SiO2 Fumed) to 120 mg (low Si

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content - straw) but was 30 mg in most cases. Then, 150 mL of a 30 mmol L-1 MOPS 12 ACS Paragon Plus Environment

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buffer, adjusted to the pH of a saturated paste of our test soil (pH = 7), were added. One

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Iron Bag was added per container and the containers were placed on an overhead shaker

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and were shaken at 5 RPM.

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The extraction proceeded for six weeks, during which the Iron Bags were removed and

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replaced once (after 15 days) or twice (after 7 and 21 days) before final sampling 42 days

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after the start of the extraction. A single replacement after 15 days was sufficient in most

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cases to reach maximal Si extraction, except for fumed SiO2. Particles adhering to the

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surface of the membrane were washed back into the bottle with type I water, and care was

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taken to avoid contamination of the ferrihydrite sample by cleaning the membrane with

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tissue paper before opening it. After opening, the ferrihydrite slurry was transferred into

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100-mL vials, and the membrane was further washed with type I water to collect all the

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ferrihydrite. One milliliter 96 % H2SO4 was added to dissolve the ferrihydrite, afterwards

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the solution was diluted to 0.25 mol L-1 H2SO4. Samples were analysed for their Si

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concentration using a molybdate blue colorimetric assay (see below).

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2.4. Si availability in fertilized soil

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We expected to obtain the closest relation of plant Si uptake and fertilizer Si solubility by

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incubating the fertilizer material with the soil used in the plant uptake experiment, as it is

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the fertilizer-soil interaction that determines Si availability. Although this assay is

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impossible to do in fertilizer testing procedures, as the soil to be fertilized is unknown, we

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included it as a benchmark for our bioavailability study.

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One-hundred grams of 4 mm-sieved, air-dried soil was weighed into 250 mL plastic

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vessels (one for each fertilizer). Fertilizer was added at 150 mg Si (kg dry soil)-1 (as in EXP

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2) and mixed homogeneously with the soil. Type I water was added to reach 70 % of WHC

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and shaken again for homogenization. Then, the vessels were covered with parafilm and 13 ACS Paragon Plus Environment


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incubated at 25°C for 8 days. After the incubation, the soil was oven-dried for 48 h at 50°C

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and sieved to < 2 mm. Si availability in the soil samples was determined in 0.01 mol L- 1

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CaCl2 extracts 25: 3 g soil was mixed with 30 mL solution and placed on an overhead

271

shaker at 5 RPM for 18 h (instead of 16 h in the published method). After filtering, the

272

extracts were analysed for their Si concentration using a molybdate blue colorimetric

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assay (see below).

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2.5. Colorimetric measurement of Si (molybdate blue)

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The staining procedure was performed with reagent solutions (acidified molybdate,

276

tartaric acid and reducing agent) prepared according to Morrison and Wilson 34 and

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Webber and Wilson 35. Timing of the reactions was according to Morrison and Wilson 34.

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More details about dilutions and volumes of reactants are given in Supporting Information

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(Table S2).

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2.6. Data evaluation

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Data evaluation and figures were done with the software ‘R’, version 3.5.1 36 following

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data entry and preparation on spreadsheet. Results of fertilizer extraction are given as

283

mean ± u, where u represents the propagated uncertainty which includes (1) an analytical

284

uncertainty of 3 % and (2) the repeatability / reproducibility assessed through replicated

285

extractions. Results of plant experiments are expressed as mean ± standard deviation

286

(SD).

287

When comparing multiple groups, analysis of variance was performed from summary

288

data, i.e. mean, standard deviation & sample size with aovSufficient (package HH 37).

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Tukey HSD post-hoc test at α=0.05 was performed with TukeyHSD while letters for

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significant groups were produced with multcompLetters2 (package multcompView 38). 14 ACS Paragon Plus Environment

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Linear fitting for plant shoot Si content vs. Si extracted from fertilizer was done with lm.

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The control receiving no Si was considered as a sample in all scatterplots and linear fits

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and considered equivalent to applying a fertilizer without any extractable Si. Root mean

294

squared deviation (RMSD) was calculated as: 𝑛

295

296

𝑅𝑀𝑆𝐷 =

∑𝑖 = 1(𝑦 ― 𝑦̂ )² 𝑛

where y and ŷ are the observed and fitted shoot Si content, respectively.

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3. Results and discussion

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3.1. Plant shoot biomass and Si concentration

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We observed considerable, significant differences in plant Si concentration between

300

fertilizers (Figure 1), whereas plant shoot biomass (7.04 and 5.56 g pot-1 in EXP 1 and

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EXP 2, respectively) was not significantly affected by Si fertilization treatment. Overall,

302

plant Si concentration in treatments fertilized with products DE-1, MSSA, SiO2 and fumed

303

SiO2 as well as some mixtures thereof (Mix-1B, Mix-3B) did not differ from the control. In

304

contrast, treatments LD-1 to LD-5, Na2SiO3, Str-Ash, Str-BC 700, ThermoP and CaSiO3

305

resulted in the highest plant Si contents. Lack of fertilizer effect on plant biomass either

306

means (1) that no significant stress occurred which could have been alleviated by

307

improved Si nutrition, or (2) that Si supply from the soil was adequate without fertilization

308

despite the relatively low PAS content (13.1 mg Si kg-1 in CaCl2). Tissue Si concentration

309

ranged from 1.97 to 3.52 mg Si g-1 in S. cereale and from 3.51 to 6.12 mg Si g-1 in L.

310

perenne. This is roughly comparable with published values for those species 29.

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312 Figure 1: Biomass and Si concentration (mean + SD) in shoots of S. cereale (EXP 1, n = 4) and L. perenne (EXP 2, n = 3). Different letters above bars indicate significantly different groups (Tukey HSD, α = 0.05).

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3.2. Comparison of Si extractions

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3.2.1. Fertilizer Si extraction by quasi-equilibrium and sink methods

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Overall, the IB method extracted more Si from most samples compared to other

316

extractions, often approaching 100% of total Si (Figure 2). Although extraction time was

317

longer for IB than for the other extraction methods, Figure S3 shows that on average,

318

close to 90% of the cumulated IB-extractable Si was already extracted after seven days,

319

indicating a stronger sink than provided by the 5-day, Resin and MOPS methods,

320

respectively. This result is similar to the results for P extracted by IBs in Duboc et al. 20, i.e.

321

that the major proportion of “water-insoluble” compounds readily solubilized when the 16 ACS Paragon Plus Environment

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322

dissolved species were continuously removed from the solution by the strong sink. Among

323

others, the case of SiO2 Fumed (13.9 vs. 82% of total Si extracted in MOPS and IB

324

respectively) is exemplary of the specific effect of the ferrihydrite sink - pH being equal in

325

both extractions.

326

As expected, the NaOH and HCl extractions showed a clearly opposing extraction

327

behaviour (Figure 2): most Ca-Mg silicates (LD samples, and to a lesser extent CaSiO3

328

and ThermoP) were soluble in HCl, but not in NaOH, while the contrary was observed for

329

samples containing amorphous SiO2 such as Str-BC 400 and 700, and the SiO2 samples.

330

The DE samples were expected to be highly soluble in the NaOH extract, but this was not

331

confirmed. The Na2SiO3 sample was 100% soluble in all extracts except HCl (ca. 50 %).

332

Results of Resin and MOPS extractions were very similar, except for SiO2-Fumed, Str

333

and Str-derived samples. These extractions had identical solid : liquid ratio and extraction

334

periods, while the pH was slightly higher in MOPS than in Resin (7 vs. 6 to 6.5 depending

335

on the fertilizer). This suggests that the pH of the extraction solution is the major

336

parameter controlling the Si dissolution in these extracts. Therefore, Amberlite mainly

337

controls Si extraction by buffering the pH, while cation complexation and associated co-

338

dissolution of Si does not contribute strongly to dissolution.

339

The HCl extraction was the least reproducible extraction (see large error bars in Figure

340

2); this could be due to the short extraction time (1 h), which may not allow for equilibrium

341

to be reached.



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18


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343 Figure 2: Silicon extracted from fertilizer by the five extraction methods, in % of total Si. Data plotted as mean + u (n = 3-9 measurements except Iron Bag (IB) for AshDec-Na and SiO2: n = 1 and 2 respectively). Values above bars give the percentage of total Si extracted. Light grey bars (AshDec-Na, DE-2 and Str) are products that were not used in the pot experiments. To ease reading of the multipanel figure, the first two characters of fertilizer names are printed below each panel.

344 345

3.2.2. Silicon extraction from fertilized soil Silicon extracted from soil by the CaCl2 extraction ranged from 14.6 to 57.5 mg Si kg-1

346

(Figure 3). Given that 20 - 45 mg Si kg-1 has been reported as upper limit of Si deficiency,

347

albeit with most studies having been conducted on rice and sugar cane 25–28 the majority of

348

fertilizers (exceptions: AshDec-Na, DE-1 and -2, MSSA, SiO2 and SiO2-Fumed) raised the

349

PAS level to > 20 mg Si kg soil-1 (Figure 3). The rate of Si addition for the CaCl2 extraction

350

experiment was 150 mg Si kg soil-1, as used in EXP 2.

351



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Figure 3: Silicon extracted by 0.01 mol L-1 CaCl2 from soil incubated for eight days with 150 mg Si (kg dry soil)-1 added in form of different fertilizers. Data plotted as mean + SD (n = 3 measurements). Different letters above bars indicate significantly different groups (Tukey HSD, α < 0.05). Printed numbers on top correspond to bar height in mg Si kg-1. Light grey bars (AshDec-Na, DE-2 and Str) are products that were not used in the pot experiments.

352

3.3. Suitability of extractions to predict fertilizer PAS

353

When plotting plant Si content against the proportion of fertilizer Si extracted by the

354

different methods (Figure 4) the IB, Resin and MOPS extraction methods stand out as

355

prediction tools for fertilizer PAS: with RMSD between 1.4 and 2.3 and R² between 0.73

356

and 0.90, these methods provide better predictions of fertilizer Si availability than the other

357

methods (RMSD between 3.1 and 4.5, and R² between 0.04 and 0.52). As expected, the

358

best fit was obtained by the CaCl2 extraction of fertilized soil (Figure 4, grey panel) as it

359

samples Si according to its dissolution behavior in the experimental soil without modifying

360

pH, and has been established as a reliable indicator for soil PAS for rice and sugarcane in

361

studies involving various fertilizers 7 and soils 27. However, the improvement provided by

362

the soil incubation assay compared to the extractions was small (RMSD; R2: 2.1-2.3; 0.76-

363

0.79 in IB, Resin, MOPS vs. 1.7; 0.86 in incubation assay).

364

In soils, plant roots and soil minerals act as ion sinks thus driving the dissolution of

365

nutrients from fertilizers. The outperformance of other extractions by the IB method

366

indicates that the latter indeed mimics this process well by providing a sink for Si(OH)40.

367

This is in line with previous findings of P extraction by IB 20. A similar mechanism has

368

recently been highlighted by Riotte et al. 39, who concluded that plant uptake and

369

adsorption of dissolved Si onto Fe-oxihydroxides controlled the rapid decrease of

370

dissolved Si concentration in irrigation water added to a paddy field. Moreover, the method 20 ACS Paragon Plus Environment

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371

extracts more Si than Resin and MOPS, making it an interesting tool for fertilizer PAS

372

quantification.

373

pH strongly affects fertilizer dissolution (see HCl vs. NaOH extractions in Figure 2), and

374

it is expected that maintaining pH of the extraction solution close to that of a soil (roughly

375

between 5.5 and 8 in most agricultural soils) would result in a better estimate of fertilizer Si

376

plant availability. With the Resin method, Kato and Owa 9 emphasized pH buffering as a

377

major reason for the better correlation of this extraction method with plant Si uptake,

378

compared to unbuffered (H2O) or strongly acidic (HCl) extractions. However, they also

379

mentioned that cation adsorption by Amberlite might enhance Si solubilisation through

380

cation depletion in the extraction solution; indeed, in most soils similar processes would

381

occur, with cations tending to be adsorbed onto exchange sites. As mentioned above, our

382

results for Resin were similar to those of MOPS (Figure 2) in which only a pH buffer was

383

added, suggesting that cation binding is not an important process in the Resin extraction.

384

385 Figure 4: Correlation between plant shoot Si content and Si extracted from fertilizer by 0.2 mol L-1 NaOH, 0.5 mol L-1 HCl, the 5-day extraction with NH4NO3 and Na2CO3, H2O + resin, 30 mmol L-1 MOPS, and the Iron Bag extraction. Soil extraction (grey panel) was performed with 0.01 mol L-1 CaCl2 on soil incubated separately for 8 days (not directly on the soil from EXP 2) with fertilizer added at



Page 22 of 30

the same rate as in EXP 2. Fertilizer addition was 75 and 150 mg Si (kg dry soil)-1 in EXP 1 and 2, respectively. p value for slope is given with two significant digits, unless < 0.0001.

386

3.4. Plant available Si in major product categories

387

3.4.1. Mineral Si fertilizers and LD slags

388

By-products of the steel industry are often investigated for PAS 6,7,9,19 as they have long

389

been known as potential Si-fertilizers. Our data confirm the high Si solubility (soil

390

incubation in Figure 3, and IB extraction in Figure 2) as well as plant availability (Figure 1)

391

from LD slags which is close to 100% according to IB extraction. Similarly high IB

392

extractability (and plant availability) was obtained for the commercial Si fertilizer ThermoP

393

and the reference CaSiO3. Results for these two materials also demonstrate the poor

394

performance of the 5-day and HCl extractions, according to which these products would be

395

classified as low PAS fertilizers (Figure 2).

396

3.4.2. Straw, biochar and ash

397

According to the IB extraction (Figure 2), the soil incubation (Figure 3) and the plant

398

experiment (Figure 1), the proportion of PAS in straw biochars and straw ash was

399

equivalent. Iron Bag extractability of Si (Figure 2) was 80 – 90 % of total Si in biochars and

400

ash (Str-BC 400, Str-BC 700, Str-Ash), and 70% in Str.

401

Xiao et al. 14 reported a marked increase in Si solubility (sum of 28 successive CaCl2

402

extractions) in rice straw biochars produced at intermediate temperatures (350 and 500°C)

403

as compared to rice straw, while at higher temperatures (pyrolysis at 700°C, and ash from

404

combustion at 900°C) Si solubilities decreased markedly. However, their results were

405

presented as soluble Si concentration (mg Si g-1) without correcting for the increasing total 22 ACS Paragon Plus Environment

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406

Si concentration in the biochar/combustion products. As shown in Table S3 (Supporting

407

Information), the soluble Si fraction either remained unchanged or steadily decreased with

408

increasing treatment temperature. The soluble Si fraction from biochars (28 – 57% of total

409

Si) was also lower than with IB in our study.

410

The high Si solubility from Str-Ash in our study is in contradiction with previous studies

411

where high combustion temperatures (600 – 900°C) strongly reduced Si solubility 13–15.

412

The main difference is that we combusted the straw for only 15 min (instead of 3-6 h in the

413

other studies), which was likely too short for Si crystallization to occur. We chose a shorter

414

combustion time in order to obtain results that are more relevant to what would happen

415

during field incineration of crop residues 40, where temperature is expected to drop soon

416

after the combustion is finished. Our data suggest that incineration in the field may not

417

cause a major decrease in straw Si solubility.

418

3.4.3. Diatomaceous earth

419

Diatomaceous earth has not been studied extensively as a Si fertilizer and the available

420

data shows rather contrasting results. Diatomaceous earth was found to increase soil

421

solution Si on the short term (only at 30 days but not at other sampling times) in an

422

alkaline and an acidic soil 41 and to increase plant Si uptake and yield 42,43, although the

423

effect was often dependent on soil pH and texture. In another study, Seyfferth and

424

Ferndorf 44 did not measure an increase of Si after addition of DE, neither in soil pore

425

solution nor in the crop biomass. Moreover, Sandhya et al. 43 could not establish the

426

presence of an amorphous Si fraction (which would originate from diatom frustules) in the

427

commercial DE used in their study, and they attributed the better growth and Si uptake in

428

DE-amended treatments to increased water retention. In an isotope dilution study, Riotte

429

et al. 39 found only a marginal contribution of DE fertilizer to Si uptake of rice. 23 ACS Paragon Plus Environment


430

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In the present study we did not include a commercial DE fertilizer. DE-1 and DE-2 were

431

commercialized as hygiene product for animal husbandry and household (e.g. pest

432

control). But considering the inconsistent results with commercial fertilizer products in

433

literature, the very low solubility of DE in all but the strongly alkaline NaOH extraction

434

(Figure 2), and the absence of an effect on soil PAS (Figure 3) is not surprising in our

435

study. Solubility in the NaOH extraction solution indicates the presence of an amorphous

436

fraction, but the products may mainly consist of other silicates as in the case of Sandhya et

437

al. 43. Given the sparse data, DE fertilizers should be further studied in the future to better

438

understand the reasons for presence - or absence - of effects on soil PAS and / or plant

439

yield.

440

3.4.4. Municipal sewage sludge

441

All MSS-based samples exhibited only marginal release of Si (Figure 1). Given that

442

most biological Si (phytoliths) is concentrated in non-edible plant parts, biogenic Si

443

fractions in municipal wastewater can be expected to be low. Moreover, as they are quite

444

soluble, phytoliths would tend to dissolve in wastewater.

445

Part of the dissolved Si (of geogenic and biogenic origin) should get adsorbed and

446

immobilized on Fe-oxihydroxides during P removal in the wastewater treatment plant. The

447

fact that NaOH extraction of MSSA solubilized 20% of total Si (Figure 2) may indicate that

448

some Si is adsorbed on Fe-oxides in this sample. The AshDec process (thermo chemical

449

treatment of MSS or MSSA under reducing conditions with addition of Na2SO4 18) did not

450

mobilize Si as it does for P (Figure 2). Similarly, only minor amounts of Si (0.26 to 1.07 mg

451

Si g-1) were extracted by IB from a MSS and its pyrolysis and hydrothermal conversion

452

products (samples MSS, MSS- BC 500, MSS-BC 700, MSS-HC; see Table S4 in

453

Supporting Information). Although total Si was not determined for these products, IB 24 ACS Paragon Plus Environment

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454

extractable Si is so low that no fertilizer value can be expected. Our results also answer

455

the question raised by Vandevenne et al. 3 as to whether MSS could represent a

456

substantial or relevant pool of biological Si for recycling.

457

3.5. Implications for Si fertilizer testing, fertilization and agro-municipal Si

458 459

cycling In terms of testing fertilizer PAS, we conclude that where a wide range of Si sources is

460

being compared, pH-buffered aqueous extraction methods (Resin, MOPS) are better

461

suited than other established quasi-equilibrium extractions (5-day, HCl). While these

462

methods are well suited for routine analysis, the more laborious sink extraction method

463

(Iron Bag) is another effective tool not only to predict fertilizer PAS but also for maximum

464

quantitative extraction of the PAS fraction. In particular, we have highlighted that products

465

with a high content of PAS such as Str-Ash, ThermoP and CaSiO3 would have been

466

considered medium to low PAS fertilizers according to the MOPS, Resin and 5-day

467

extractions. The implications are highly relevant, and we believe that the Iron Bag

468

extraction can improve Si fertilizer characterization - in particular in the context of research

469

and development. These methods should be compared in further experiments to

470

understand which one performs best under various conditions (i.e. different types of soils,

471

crops, and fertilizers).

472

Regarding agro-municipal cycling of Si, neither straw pyrolysis nor combustion is likely

473

to have a negative impact on Si cycling, besides enhancing solubilization due to

474

destruction of plant fiber and cell walls. Silicon recycling through MSS - and its conversion

475

products - does not seem to be an option given their very low extractable Si content. We

476

found no evidence that DE provides PAS, and some previous studies with soil incubation

477

of commercial DE fertilizers also failed to do so, so that the relationship between 25 ACS Paragon Plus Environment


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478

composition and PAS release from DE fertilizers should be better assessed in the future.

479

Amendment of (P-enriched) LD slag can be beneficial, although long-term effects of Cr

480

and V accumulation needs to be considered in determining application rates 45.

481


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482


4. Acknowledgements

483

We are grateful to the following people: Gerhard Soja, Christoph Pfeifer and Lisa

484

Sekulic for their support with straw pyrolysis and combustion and for providing MSS

485

sample and conversion products. Pete Nelis (NelisFert Ltd.; Tirau, NZ) for providing

486

ThermoP, Martin Rex (FEhS – Institut für Baustoff-Forschung e.V.; Duisburg, DE) for

487

providing LD-1, and Christian Vogel as well as Hannes Herzel (Bundesamt für

488

Materialforschung, Berlin, DE) for providing the AshDec-Na sample. Philipp Schuster (Fritz

489

Mauthner Handelsges.m.b.H. & Co. KG, Vienna, Austria) for providing a sample of S.

490

cereale seeds.

491

Funding: We acknowledge the financial support by the Austrian Research Promotion

492

Agency (FFG) through the Research Studio Austria FERTI-MINE [Project number:

493

844744].

494 495 496

5. Supporting Information Methods: Fertilizer sample description (Table S1), X-ray fluorescence spectrometry

497

method details; Si dose response trials for S. cereale and L perenne (Figure S1); Iron Bag

498

method assessment for Si absorption (Figures S2 – S4); Modifications to the oven-induced

499

digestion (OID) for plant tissue Si; Si colorimetric method details (Table S2).

500 501

Results and Discussion: Biochar (Table S3 showing results of Xiao et al. 14); Municipal Sewage Sludge and conversion products (Iron Bag extractable Si, Table S4)

502



503

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Silicon Availability from Chemically Diverse Fertilizers and Secondary

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