Characterization of Leached Phosphorus from Soil, Manure, and

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Characterization of Leached Phosphorus from Soil, Manure, and Manure-Amended Soil by Physical and Chemical Fractionation and Diffusive Gradients in Thin Films (DGT) Nadia Glæsner,*,†,‡,§,⊥ Erica Donner,† Jakob Magid,‡ Gitte H. Rubæk,§ Hao Zhang,∥ and Enzo Lombi† †

Centre for Environmental Risk Assessment and Remediation, Mawson Lakes Campus, University of South Australia, Mawson Lakes, South Australia 5095, Australia ‡ Faculty of Science, Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark § Department of Agroecology, Aarhus University, Blichers Allé 20, DK-8830 Tjele, Denmark ∥ LEC Lancaster University, Lancaster LA1 4YQ, U.K. ABSTRACT: We are challenged to date to fully understand mechanisms controlling phosphorus (P) mobilization in soil. In this study we evaluated physical properties, chemical reactivity, and potential bioavailability of P mobilized in soil during a leaching event and examined how the amounts and properties of leached P were influenced by surface application of cattle manure. Leaching experiments on manure itself, and on intact soil columns (14.1 cm inner dia., 25 cm height) before and after manure application, were carried out at an irrigation rate of 1 mm h−1 for 48 h. High concentrations of dissolved reactive P (DRP) were found in manure leachates (up to 32 mg L−1), whereas concentrations of P in soil leachates were low both before and after manure application (around 0.04 mg L−1 before application and up to 0.4 mg L−1 afterward). This result indicates that the soil retained most of the P added with manure. Manure particles themselves were also largely retained by the soil. Combined physical (centrifugation) and chemical (molybdate reactiveness) fractionation of leached P showed that leachates in the manure treated soils were dominated by dissolved unreactive P (DUP), mainly originating from manure. However, centrifugation only removed a small fraction of total particles from the leachates, indicating that the so-called dissolved fraction may be associated with low density particulate matter. Deployment of Diffusive Gradients in Thin films (DGT) devices in the leachates proved to be a good approach for measuring reactive P in soil leachates. The results indicated that total reactive P (TRP) gave a better estimate of potentially bioavailable P than both total P (TP) and DRP in these experiments.

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INTRODUCTION Phosphorus (P) is a major contributor to eutrophication of surface waters. Accumulation of P in agricultural fields due to periodic land application of animal manure in areas supporting intensive livestock production is thus of concern,1 as a portion of this soil P is susceptible to leaching. For a long time, P was not expected to be transported through the soil profile due to its strong binding to soil surfaces. However, numerous studies have reported substantial amounts of P leached through soil, both in dissolved and particulate forms.2−4 While this tendency for leaching is now well established, mechanisms controlling P mobilization and transport through soil profiles are still not fully understood. Several approaches have been used to characterize forms of P (i.e., organic/inorganic, dissolved/particulate) leached through soil. The most common approach is based on physical fractionation of dissolved and particulate P in soil leachates by filtration3,5,6 or centrifugation.7−9 However, there are problems associated with both methods. Both 0.2 and 0.45 μm filters have been used to separate particulate fractions. However, some particles may be smaller than this in two © 2012 American Chemical Society

dimensions and thus inadvertently included in the dissolved fraction.10,11 Also, P originating from colloids, oxides, and humic substances may be solubilized during subsequent chemical analysis, thus giving results that do not reflect the desired fractionation of P in soil solution.12 Furthermore, pore size of filters may alter during the filtration process due to clogging of pores.13,14 In the case of centrifugation, the fact that natural colloidal particles are often mineral particles associated with organic material15 is an issue, as centrifugations are typically based on sedimentation of mineral particles with a density much greater than organomineral particles.16 To date, researchers investigating P have mainly used filtration to fractionate the particulate and dissolved fraction, whereas centrifugation has mainly been used by researchers working on colloids. In the work reported here we have combined centrifugal fractionation of dissolved and particulate P7,9 with Received: Revised: Accepted: Published: 10564

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Table 1. Soil Characteristics mg kg−1

particle sizea a

depth (cm)

0−16 16-31 a

a

pH 1:5 ratio (mS/ cm) 5.4 6.0

a

EC (mS/ cm) 0.152 0.064

clay 0.05). In all leachates significant amounts of particles remained in solution after centrifugation, and the amounts were similar in both untreated and manure-treated columns (P > 0.05) (Figure 2d), suggesting that these particles were of soil origin. Lower density particles seem to be more readily retained than higher density particles during transport through the soil. In fact, the large majority (95%) of manure particles not removed by centrifugation were retained in the soil (calculated from cumulative number of particles reported in Figure 2d). Size fractionation of leached particles revealed that particles as large as 1600 nm (average 670 nm) were transported through the unamended soil columns (Figure 4a, Table 2). Centrifugation removed the largest particles, leaving an average size of 320 nm. After manure application, a first flush of large particles (around 1000 nm) was leached through the columns, and these were not removed by centrifugation (Figure 4b). After the first flush of particulate matter, particles present in the leachate were slightly smaller than those leached before manure application (470 nm), whereas particles in the centrifuged leachate were of a similar size before and after manure application (Table 2). Assuming centrifugal sedimentation of spherical mineral particles with a density of 2.63 g cm−1, the specified centrifugation conditions should theoretically remove particles >100 nm. We calculated the density of the observed particle sizes in centrifuged leachates to be 1.17 g cm−1 and 1.18 g cm−1 before and after manure application, respectively (Table 2) (not including the first flush (Figure 4b)). This calculation of particle density must, however, be considered with some caution as differences in refractive indexes and absorption of light may favor measurement of minerals over organic matter. These findings indicate that numerous particles, also including some that are larger than 100 nm, were still present after

Figure 2. A−C. Number of particles in leachates (black symbols/bars) and centrifuged leachates (gray symbols/bars). Note different scale on Y-axis. Each of the plotted lines represents the data for one replicate column (i.e., three replicate columns per treatment). The replicates are distinguished by different symbols. Single measurements were carried out for each sample. D. Cumulative number of particles leached. Error bars represent variations among replicate columns.

manure application (P < 0.05) (Figure 2b), reflecting the high number of particles leached directly from the manure (Figure 2c). Leaching of TOC also increased after manure application, reflecting the high leaching of TOC directly from the manure (Figure 3). Based on a cumulative number of particles reported in Figure 2d, it is possible to calculate that manure application to the soil surface caused an increase in particle leaching equivalent to 9.0% of particles leached directly from the manure. Hence, the majority of manure particles (91%) were retained in the 25 cm soil columns. This was also illustrated by low concentration of TOC leaching from soil columns after manure application compared with TOC leached directly from the manure. Hence, the majority of manure-derived organic matter was retained in the soil columns. Centrifugation generally decreased the amount of particles remaining in the leachates, but the decrease was not statistically 10567

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Figure 4. Particle size of leachates (black symbols) and centrifuged leachates (gray symbols). All plots represent replicate columns and are distinguished by different symbols. Error bars represent maximum and minimum values measured within triplicate sample measurements.

Table 2. Average Size and Density of Leached Particles particle size (nm)

untreated soil manure treated soila

leachate

centrifuged leachate

particle densityb after centrifugation (g cm−3)

670.0 (78.9) 470.3 (20.3)

328.5 (48.3) 298.5 (8.0)

1.17 (0.04) 1.18 (0.01)

Calculated without the initial flush (Figure 4b). bCalculated after Slater and Cohen (1962). Standard errors are given in brackets.

a

centrifugation and hence present in the fraction defined as the dissolved fraction. Furthermore, low densities both before and after manure application suggest that these particles are likely to be in the form of organomineral complexes.30 This has important implications for interpretation of results using a commonly employed physicochemical P fractionation scheme7−9 as this ‘dissolved’ fraction also includes particulate material. Phosphorus Leaching and Characterization of Phosphorus Forms. Concentrations of P in leachates from the unamended soil columns were very low; around 0.03 mg L−1 (Figure 5a). Only one column expressed an initial high flush (0.62 mg L−1) in the early leachates. Soil P content, Olsen P, and Colwell P (Table 1) showed values common in Australian soils with a high PBI value,31 hence large soil sorption capacity was expected. Following application of manure and onset of irrigation, P leaching increased after around 30 mm of irrigation water had been applied (Figure 5b). The leaching test conducted on the manure alone revealed that P was rapidly leached from this material, with TP concentrations in the leachate stabilizing at around 10 mg L−1 over the course of the experiment (Figure 5c). This indicates that manure P was well retained in this soil, as we did not observe an initial high leaching of P following application of manure onto the soil surfaces. Only 0.71% of TP leached directly from the manure was transported through the columns after application of manure onto the soil surface (Table 3). Rapid transport of surface applied P sources via preferential flow paths has been observed in several studies of fine-textured soils.9,32−36 As mentioned above, preferential flow was also observed in this study but was probably less pronounced as the soil was relatively coarse textured. Phosphorus released from the manure was mainly in form of DRP, but this P form was clearly retained by the soil, as there

Figure 5. Leaching of total phosphorus (TP). Note different scaling on Y-axes. All plots represent replicate columns and are distinguished by different symbols. Error bars represent maximum and minimum values measured within duplicate sample analyses, but the symbols are larger than the errors.

was no additional DRP in the manure treated leachate. Manure DUP was less well retained, as 6.12% of the DUP in the manure leachate was leached through the soil columns (Table 3). Particulate P present in the manure leachate did not form a substantial fraction of the P leached through the soil. In fact, while 9% of manure particles were leached through the columns (Figure 2), only 3.3% of manure PP (PRP + PUP) (Table 3) was transported. Dissolved unreactive P (DUP) is often assumed equivalent to dissolved organic P (DOP). The dominance of DOP leaching following manure application is consistent with some studies9,37−39 but in contrast to others.4,8 However, it should be kept in mind here that the results of the centrifugation procedure (Figures 2 and 3) show that a 10568

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Table 3. Cumulative Content (mg) of Phosphorus (P) Forms in Leachatesa untreated soil (mg) manure treated soil (mg) manure (mg) relative manure P leachingb (%)

TP

PRP

PUP

DRP

DUP

0.045 (0.016) 0.123 (0.013) 11.05 (0.794) 0.71

0.005 (0.004) 0.020 (0.005) 0.673 (0.405) 2.12

0.009 (0.001) 0.030 (0.007) 1.740 (0.434) 1.16

0.018 (0.012) 0.008 (0.001) 7.759 (0.968) -

0.012 (0.002) 0.065 (0.008) 0.878 (0.075) 6.12

a

Total P (TP), particulate reactive P (PRP), particulate unreactive P (PUP), dissolved reactive P (DRP), and dissolved unreactive P (DUP). Standard errors are given in brackets. bCalculated by dividing the cumulative P leached from the manure treated soil (minus the P leached through the untreated soil) by the cumulative P leached from the manure.

substantial number of particles were not removed by this technique. Clearly, separation between particulate and dissolved P is operationally defined with the two fractions substantially overlapping in particle sizes, primarily due to the range of particle densities present. To overcome this operationally defined limitation we tested use of DGT to characterize P, not in terms of particulate/dissolved and organic/inorganic, but rather in terms of its diffusivity/exchangeability, which may be more relevant in terms of bioavailability. In fact, diffusion of metals into DGT devices has previously been shown to mimic the continuous uptake of metals by plant roots and is therefore used as a measure of bioavailability.40 Studies using DGT in soil samples have also shown promise as a measure of plant available P,41 but the technique has, to our knowledge, never been tested on soil leachates even though this technique was initially developed for water samples.42 Examples of DGT depletion of pooled leachates are given in Figure 6; Figure 6A shows the pooled 6 + 15 h leachate sample from the column leaching high P concentrations in untreated soil; the other samples had very low P concentrations. Figure 6B represents the pooled 41 + 48 h leachate sample from one replicate column in manure treated soil, and Figure 6C represent the diluted pooled 37 + 48 h leachate sample from the three manure replicates. Pattern of depletion in the other pooled samples for the manure treated soil and manure leachates showed a similar trend. The calculated depletion curve represents theoretical uptake of P by the device over time assuming that all P is present in a freely exchangeable form capable of being measured by DGT. DGT directly measures all forms of P which can readily diffuse through the gel layer and bind to the ferrihydrite. This is likely to include some small organic forms of P but exclude particulate/colloidal forms.43 Initially, TP decreased rapidly as diffusive P was removed from solution. After all diffusive P had been removed from solution the remaining TP concentrations leveled off indicating that the remaining P was strongly bound within mineral particles or organic matter. Overall, cumulative diffusive DGT-P and TRP showed similar results (Figure 6, Table 4). Slightly more P diffused into the DGT devices than was measured as TRP in leachates of the manure. However, this was not observed after manure application onto the soil surface, thus indicating that diffusive P (i.e., bioavailable P) was retained in the soil matrix. Nevertheless, similar values of DGT-P and TRP indicated that TRP measurements gave a good indication of the bioavailable P fraction in soil leachates. Measurement of TRP directly in soil leachates is simpler than measurement of DRP and does not rely on physical separation by centrifugation; a process which largely overestimated the dissolved P fraction in leachates. It would be interesting to test DGT-P and TRP in leachates of soils where PP is the dominant form of P in leachates and to test whether measurement of TRP (release of PO43‑ from

Figure 6. Examples of DGT depletion over 120 h. A) Pooled sample of 6 + 15 h leachates from the column leaching high P concentrations in untreated soil. B) Pooled sample of 41 + 48 h leachates from one replicate column in manure treated soil. C) Diluted pooled sample of 37 + 48 h leachates of the three manure replicates. Black lines represent the theoretical depletion curve calculated with eq 1 based on the assumption that all P initially present in the leachate is DGT deployable within 120 h; white symbols represent measured total P (TP) concentrations in leachates during DGT depletion, and gray symbols represent initial total reactive P (TRP) concentrations.

Table 4. Cumulative P Diffused into DGT (DGT P) and Initial Total Reactive P (TRP = PRP + DRP) of Pooled Leachates b

manure treated soil (mg) manure (mg) relative manure P leachingc (%)

DGT P

TRP of DGT samplesa

0.031 (0.008) 8.70 0.36

0.034 (0.004) 7.17 0.47

a

These values differ slightly even though they should be equal to PRP + DRP in Table 3. This is due to the pooling of samples for DGT measurement, which might have slightly altered the concentrations. b DGT-P and TRP of DGT samples from the leachates of the manure treated soil were not statistically different (P > 0.05). cCalculated by dividing the cumulative P leached from manure treated soil by the cumulative P leached from the manure.

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particles by acidification)10 on a wider set of soils could estimate the bioavailable P fraction determined by the DGT method in soil leachates. Overall, these results indicate that the large majority of P leached from surface applied manure was retained by the soil. Of different P species analyzed by physicochemical P fractionation, reactive P forms were retained more than unreactive P forms. Our results indicate that physicochemical fractionation techniques may overestimate the presence of TDP due to persistence of low density colloids which are not removed by leachate centrifugation. However, TRP correlates well with DGT-P and as such may provide a simple proxy for P bioavailability in soil leachates. Further testing of this hypothesis should be carried out.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0045 26147726. E-mail: [email protected]. Present Address ⊥

Leibniz-Centre for Agricultural Landscape Research (ZALF), Institute of Soil Landscape Research, Müncheberg, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding support from the Australian Research Council through Discovery Project DP110103174 is acknowledged. ABBREVIATIONS: DGT diffusive gradients in thin films DIP dissolved inorganic phosphorus DOP dissolved organic phosphorus DRP dissolved reactive phosphorus DUP dissolved unreactive phosphorus P phosphorus PIP particulate inorganic phosphorus POP particulate organic phosphorus PP particulate phosphorus PRP particulate reactive phosphorus PUP particulate unreactive phosphorus TDP total dissolved phosphorus TIP total inorganic phosphorus TP total phosphorus TRP total reactive phosphorus

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