Improving the Water Holding Capacity of Soils of Northeast Brazil by

Chapter 16

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Improving the Water Holding Capacity of Soils of Northeast Brazil by Biochar Augmentation A. S. Mangrich,*,1,2,5 E. M. C. Cardoso,1 M. E. Doumer,1 L. P. C. Romão,2 M. Vidal,3 A. Rigol,3 and E. H. Novotny4 1Department

of Chemistry, Federal University of Paraná, Coronel Francisco Heráclito dos Santos s/n, Curitiba, PR, Brazil 81531-980 2Department of Chemistry, Federal University of Sergipe, Rodovia Marechal Rondon s/n, São Cristovão, SE, Brazil 49100-000 3Department of Chemistry, University of Barcelona, 1-11 Martí i Franquès, Barcelona, BCN, Spain 08028 4Embrapa Soils, Jardim Botânico 1024, Rio de Janeiro, RJ, Brazil 22460-000 5National Institute of Science and Technology - Energy and Environment (INCT E&A), Campus Universitário de Ondina, Salvador, BA, Brazil 40170-115 *E-mail: [email protected].

The Northeast of Brazil, a semi-arid region, frequently experiences severe drought. Despite rainfall during two or three months of the year, the presence of soils with low water retention capacity, together with intense insolation, results in infiltration of water to deeper soil layers, rapid evaporation, and deficiency of water during the rest of the year. In this work, we propose the use of soil conditioners derived from agricultural and industrial wastes to improve soil water supply. Five biochars, prepared by slow pyrolysis, were produced from green coconut shells, orange peel, palm oil bunch, sugarcane bagasse, and water hyacinth plants. Charcoal fines obtained from the metallurgical industry were also used. The soils investigated were two Quartzarenic Neosols, QN1 and QN2, from Sergipe State in Northeast Brazil. After mixing with 5% (m/m) of biochar, both soils showed increased water retention capacity, compared to the original samples. The best water retention was achieved using the biochars from palm oil bunch and sugar cane bagasse (absolute increases of 5.5 and 6.5%, © 2015 American Chemical Society In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

respectively) for soil QN1, and the biochars from sugar cane bagasse and water hyacinth plants (absolute increases of 7 and 8%, respectively) for soil QN2. These results could be explained by the polarity of the biochars, as shown by their hydrophilicity, measured by 13C NMR spectroscopy, as well as by the increased presence of micropores, revealed by SEM analyses, that could physically retain water.


Introduction The Northeast of Brazil is a semi-arid region with a dry and warm climate and rainfall that is concentrated between March and May. Annual precipitation typically varies between 500 and 800 mm, although it can be as low as 400 mm in some areas (1, 2). The uneven distribution of rainfall over time, combined with intense insolation, results in low soil water retention capacity, water infiltration to deeper soil levels, rapid evaporation, and deficiency of water during most of the year. Under these conditions, agricultural activity is of low intensity and sporadic, with poor soil productivity due to hydric deficiency (3, 4). The shallow sandy soils are unable to effectively retain water and require irrigation to provide sufficient water for crops during the growth stage (2). The expansion of agroindustry in Brazil, despite its many benefits, has resulted in concerns regarding the disposal of the large quantities of waste biomass generated. The environmental impacts caused by these materials are associated with their high contents of organic material, as well as the high costs of transport and application to soil. Therefore, they often accumulate close to processing facilities, sometimes exceeding the capacity for natural decomposition and leading to the production of substances that may be toxic, cumulative, and difficult to degrade. One possible option is to use this biomass in more environmentally friendly agricultural practices, with the return of the waste biomass to the soil, benefiting from the presence of organic residues, although one difficulty is the rapid decomposition of these materials spread over the soil due to climatic conditions that favor microbial activity (5, 6). In the State of Sergipe (Northeast Brazil), around 1.7 Gg of agroindustrial wastes are generated annually from the processing of coconuts, oranges, and sugarcane, amongst other crops, and more than 19 Gg of wastes are generated from forestry activities. Green coconut shells can pose a substantial risk to public health. Coconut shell halves are concave and retain rain water, enabling the development of Aedes aegypti, the mosquito that transmits dengue fever, chikungunya, and yellow fever, among other viruses, mainly in countries with tropical climates such as Brazil (7). At the same time, low levels of soil organic matter result from unsatisfactory management practices and the lack of incorporation of crop residues into the soil (8). The use of more persistent organic residues, such as biochar, therefore offers the potential to improve the physical, chemical, and biological properties of the soil, including its water retention capacity. 340 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Biochars, the charcoals produced for agricultural purposes, are formed during the thermal decomposition of organic materials in the absence, or low supply, of oxygen (7, 9, 10). Used as a soil amendment, biochar can have beneficial effects on various soil properties and processes (11), such as nutrient availability (12), microbial activity, soil organic matter content, water retention, and carbon sequestration (13, 14). It can reduce the need for fertilizers, decrease the emissions of greenhouse gases (15, 16), and alleviate erosion and leaching of nutrients (17). The use of biochar in agricultural soils has been suggested as an effective tool for long-term mitigation of the adverse effects of drought in Northeastern Brazil (7), improving both the physical properties of the soil and crop productivity (5, 18). The water holding capacity (WHC) is one indicator of soil quality and productivity (19). Biochars can improve soil properties, depending on the physical and chemical nature of the biochar, as well as the rate of application to the soil (20, 21). Since the addition of biochar increases the availability of soil organic matter, the water holding capacity, and the bioavailability of nutrients, it can significantly enhance microbial activity and consequently soil aggregate formation and stability (22). Furthermore, increase in the WHC after the addition of biochar (14, 23) is related to the porosity of the material and its high specific surface area, and the magnitude of the effect depends on the type of raw material and the conditions used for its pyrolysis (24). Glaser (5) described an increase in WHC of 18% for Anthrosols rich in vegetal charcoal, and a similar increase was reported for sandy soils by Lehmann (25). Tryon (26) showed that the available humidity in a sandy soil increased linearly with the rate of application of wood charcoal. Several other recent studies have also reported the potential of wood biochar to increase the WHC of sandy soils (13, 27, 28). Abel (27) investigated the effects of the addition of biochar on water retention in sandy soils, and concluded that in addition to acting as a soil amendment, the material showed potential to improve water retention. Therefore, the use of biochar can play an important role in water management, while helping to reduce inputs of agricultural fertilizers and maximize crop productivity. The aim of the present work was to evaluate the WHC of soil from the semiarid Northeastern region of Brazil after the application of biochars prepared at low temperature. Improved water retention would enable a reduction of the amount of water required for irrigation in this region, where water resources are scarce.

Materials and Methods Preparation of the Biochars The biomasses used were green coconut shells - Cocos nucifera - (CS), orange peel - Citrus sinensis - (OP), oil palm bunch - Elaeis guineensis - (PO), sugarcane bagasse - Saccharum officinarum - (SB), and water hyacinth plants - Eichhornia crassipes - (WH), as well as charcoal fines (CR). The biomasses were dried in an oven at 105 °C for 24 h. The biochars were then prepared by pyrolysis of the materials at a heating rate of 5 °C min-1 up to 350 °C under a controlled atmosphere. The samples were placed in a tunnel oven (FT-HI/40, EDG Equipamentos) and the system was completely sealed, except for an exit for gases that were bubbled 341 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

through distilled water. The only oxygen that participated in the thermochemical reaction was that contained in the pyrolysis tube. After preparation, the biochars were ground to a particle size of 2 mm using a knife mill (Willye Star FT 50).


Characteristics of the Soil Samples of two different soils were collected in the pediplain of Sergipe State, Northeast Brazil. Both soils were Quartzarenic Neosols (denoted QN1 and QN2). The contents of sand, silt, and clay were 820, 60, and 120 g kg-1, respectively, for soil QN1, and 910, 10, and 80 g kg-1, respectively, for soil QN2. The soils showed low contents of total carbon: 3.0 and 1.9 g kg-1 for soils QN1 and QN2, respectively. XRD analysis identified the minerals quartz, hematite, and gibbsite in both soils. Aging Experiments and Water Holding Capacity (WHC) The WHC was determined using a methodology adapted from Case (14). The experiment was performed in duplicate, and evaluation was continued for a period of 140 days. A factorial experimental design was used, with two treatment factors (two soils and six biochars plus control) and two replications. In each experimental unit, the WHC was determined every two weeks, totaling ten repeated measurements. The data were evaluated using repeated measures analysis of variance (rANOVA), after homogeneity of variances and normality of residuals evaluations. The soils were sieved (2 mm), and 95 g portions were placed in PVC tubes (Ø = 50 mm; H = 75 mm) that were closed at the base with a fine mesh cloth. The biochars were mixed with the soils in a proportion of 5% m/m (5 g) of the total dry weight of the soil/biochar mixture. A control without biochar was also used in the aging experiment. The soil/biochar mixture was equivalent to an application rate in the field of 120 ton ha-1, calculated assuming 12 cm depth and bulk density of 1.2 g cm-3. After thorough mixing, the samples were saturated with water for 1 h. The PVC tubes were then supported on top of beakers in humidified plastic buckets, which were closed in order to avoid evaporation. After 3 h, the samples were removed and weighed, heated at ~60 °C for 4 days, and then re-weighed. The samples were placed in open PVC tubes at field capacity (by addition of 10 mL of water) and kept at ambient temperature for approximately 7 days. Ten wetting/drying cycles were used. The maximum WHC obtained under laboratory conditions was then calculated using Equation 1:

13C

NMR and SEM Analyses and pH Determination

Solid-state 13C NMR spectra were obtained to characterize some of the properties of the biochar samples, including hydrophilicity/hydrophobicity and their interaction with water. Solid-state 13C NMR spectra were acquired using 342 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


a Varian Inova (11.74 T) spectrometer at 13C and 1H frequencies of 125.7 and 500.0 MHz, respectively. Samples were packed into 5 mm diameter cylindrical zirconia rotors with Kel-F rotor end caps. The pulse sequence employed was variable-amplitude cross-polarization. Measurements were carried out using magic-angle spinning (MAS) of 15 kHz, a cross-polarization time of 1 ms, an acquisition time of 15 ms, a recycle delay of 500 ms, and higher-power two-pulse phase modulation (TPPM) proton decoupling of 70 kHz. The cross-polarization time was chosen after experiments with different contact times, and the recycle delays were chosen to be five times longer than the longest 1H spin–lattice relaxation time (T1H), as determined by inversion-recovery experiments. The relative contents of the various C-groups were determined by integration of the signal intensity in the following chemical shift regions: alkyl (0-51 ppm); N-alkyl and methoxyl (51-61 ppm); O-alkyl (61-95 ppm); di-O-alkyl (95-110 ppm); aryl (110-140 ppm); O-aryl (140-160 ppm); carboxyl (160-185 ppm); and carbonyl (185-215 ppm). Afterwards, the hydrophilicity index values, used to characterize the interactions of the chars with water molecules, were calculated according to the ratio between polar (N-alkyl and methoxyl; O-alkyl; di-O-alkyl; O-aryl; carboxyl; and carbonyl) and non-polar (alkyl and aryl) organic groups. The micromorphology of the biochar, such as the pore system and roughness, is an important determinant of water-biochar interactions and can be evaluated by scanning electron microscopy (SEM). SEM analysis was performed after mounting the biochar samples on carbon substrates, using a JEOL JSM 6360 microscope operated at 15 kV. The preparation of the samples was performed using a Balzers Union SDC 030 sputtering system. The images were recorded using the proprietary SEM interface software. The pHs of the biochars and the biochar/soil mixtures were determined as described by Carrier (29). The biochar samples (0.5 g) were agitated using a shaker (TE-421, Tecnal) for 30 min with 10 mL of distilled water or an aqueous solution of 1 mol L-1 KCl (99% purity, Merck Chemicals Ltd.). The suspension was allowed to stand for 10 min before measuring the pH with glass and reference (Ag/AgCl) electrodes, using a potentiometer (MPA 210, MS Tecnopon). The ΔpH value was obtained as follows: ΔpH = pH (biochar/KCl solution) - pH (biochar/water) (30).

Results and Discussion 13C

NMR, SEM Analyses and pH Determination

The 13C NMR analyses of all the samples, with the exception of CR, presented typical signals of the biochar precursors (Figure 1). The parent material signals included those for cellulose (O-alkyl and di-O-alkyl), lignin (O-aryl, aryl, and methoxyl), and alkyl groups that probably derived from decarboxylated fatty acids, in accordance with the mild pyrolysis conditions employed for these samples (31–33). Due to the high and variable content of residual cellulose, the hydrophilicity index values also varied among the samples (the correlation between these two parameters was R = 0.94), with the PO sample showing the highest hydrophilicity. 343 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The 13C NMR spectrum for the CR sample (Figure 1) was dominated by the nearly symmetrical and featureless aryl signal centered at 128 ppm, characteristic of charcoal obtained under intermediate carbonization conditions, unlike the narrower upfield shifted aryl signal typical of highly polycondensed char (34–36). This predominance of aryl structure and lack of O-substituted functional groups resulted in the CR sample having the lowest hydrophilicity index value.

Figure 1. 13C NMR spectra of the biochar samples. The numbers below the sample identifiers are the corresponding hydrophilicity scores.

SEM micrographs of the samples (Figure 2) showed that all the biochars were highly porous, but with pores of different sizes. It has been reported previously that biochars possess an extremely complex network of pores, channels, and fibrous surfaces (37). The pore sizes of the CR and CS samples were around 10 µm, while those of PO and SB were about 6 µm, still large enough to hold microorganisms such as bacteria and mycorrhizal hyphae. The WH sample presented the roughest surface, probably due to pore collapse and filling of the porous system with ash. In all cases, the pores could be classified as micropores (

Improving the Water Holding Capacity of Soils of Northeast Brazil by

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