Organic amendment P compositions and availability
The greatest total P content in the present study was for SS (21 g P kg
−1 dry mass), comparable to 27 ± 9 (to a maximum 55) g P kg
−1 for sludges from 48 Swedish wastewater works (Eriksson
2001). Sludge compositions (Table
1) were similar in terms of dry matter, OM, C/N, total N, and P to those (also processed by anaerobic digestion) documented by Roig et al. (
2012). Peng et al. (
2010) reported solution
31P NMR data for sludges from 13 Chinese urban wastewater works and suggested that spectroscopic P-speciation studies complimented enzyme P studies in looking at potential bioavailability of
P
o and
P
i
. These authors showed sludge P species compositions comprised primarily inorganic ortho-phosphates with appreciable orthophosphate monoesters, similar to the current study (Table
3). Additional these authors observed minor contributions of the more labile
P
o forms pyrophosphates and diesters in several samples. The current evidence of a dominance of
P
i
by solution NMR and MRP labile solution P in water and NaOH–EDTA extracts suggests high ‘potentially available’ P in SS that could be realized according to the extent of interactions with receiving soils.
Characterization is needed to inform appropriate utilization of the growing quantities of the by-product of biogas generation (anaerobic digestate; AD). Möller and Müller (
2012) in a comprehensive review of AD in Germany reported large concentration ranges for dry matter (DM, 1.5–13 %), total N (3–14 % of DM), total C (36–45 % of DM), and total P (0.6–1.7 % of DM), reflecting a wide range in AD feedstocks. The AD in the present study from farm slurry and abattoir feedstock (Table
1) had nutrient contents at the low end of these ranges, but a large C:N ratio. The AD P species were dominated by inorganic orthophosphate P with minor contributions of monoesters and diesters (6 and 2 % of total P). The extracted NaOH–EDTA P was dominated (84 %) by MRP, but water solubilised P was dominated (80 %) by dissolved molybdate unreactive P (MUP), possibly influenced by soluble diester P. Möller and Müller (
2012) summarized the transformations of nutrients during the AD process as a reduction in DM, OM, and C:N ratio, and found no change in total N but an increasing dominance of ammonium. For P, a loss in total P was attributed to increased pH, mineralization, and precipitation reactions involving Ca- and Mg- phosphates (e.g., struvite).Mehta and Batstone (
2013) reported <10 % of total P in AD to be soluble, yet completely extracted in citrate, and they concluded P released by digestion was bound to inorganic compounds or sorbed onto solid surfaces. Although precipitation reactions limit
P
i
solubility, no crystalline P minerals were observed in the current dried sample (Fig.
S2). In this study, AD liquor was air-dried and then re-extracted into water. However, bulky organic amendments can be considered not economically viable to transport in their raw state. Drying of AD now occurs in 1 % of German biogas plants (Möller and Müller
2012).
Total P contents of the composts agree with those reported by Frossard et al. (
2002) for 15 alkaline composts (urban, garden, and farm manure substrates) of mean 3.4 (range 2.1–7.2) g P kg
−1 dry matter. In the present study, FWC has greater P (and N) content but lower C than GC (Table
1). Resulting low C:P and C:N suggest greater N and P mineralization and potential mobilization when FWC is added to soils. Phosphorus species were similar between the composts with slightly greater monoester
P
o forms in FWC (11 %) than GC (8 %; Table
3). Although NaOH–EDTA P compositions had an appreciable MUP content (30 and 33 % for GC and FWC, respectively), the water soluble P (Table
4; determined for GC only) was exclusively MRP. Frossard et al. (
2002) concluded that 30–50 % of
P
i
in composts was rapidly plant available, with another 30–50 % of
P
i
formed of Ca-precipitates, although they urged caution that air-drying of materials could have increased P mineralization and hence
P
i
quantification.
CM had the third largest P (and N) content (after SS and AD; Table
1), but the greatest contribution of monoester
P
o (Table
3) and a corresponding large fraction of MUP in NaOH–EDTA (Table
2) and water extracts (Table
4a). Many studies have used
31P NMR analysis to examine the P forms in manures (Peirce et al.
2013 and references therein). Often the specific form of inositol hexaphosphate dominates monoesters as it is not broken down in the gut of non-ruminants. Peirce et al. (
2013) found that monoester P contributed about half of the P total in fresh manures, similar to the current study. These authors studied how monoester to inorganic orthophosphate ratios of chicken manures subsequently declined following the on-farm practice of manure stockpiling over increasing times, similar to observed mineralization of monoesters in soils.
There has been considerable interest in biochar as a soil conditioner and nutrient source. Although compositions vary greatly, it has been reported that 10–60 % of P is leachable from wood-derived biochar (Muckherjee and Zimmerman
2013). However, the pine-derived commercial biochar used in the present study had very small N and P contents (Table
1), with P speciation not resolvable by NMR. Large concentrations of Ca and Fe in the NaOH–EDTA extracts suggest potential for a high degree of interaction of P with bridging cations with the implication of P being more strongly bound. On the other hand, high C contents may lead to high DOC solubility and competitive sorption to maintain solubility of any small amount of P leached.
Natural materials such as seaweed have been used historically as soil amendments but lack current scientific evaluation of their P contents and speciation. Haslam and Hopkins (
1996) focused on soil conditioning properties, reporting that kelp-amended soils had greater aggregate stability and enhanced microbial respiration and N mineralization rates at moderate applications. The current study showed monoesters contributed a quarter of total P with overall P contents similar to composts. Seaweed has potential as a P alternative given the abundant potential for supply globally, either directly or following use as an AD feedstock. Table
2 shows high K and Mg content which will add benefits when applied to the soil. However, investigations should be made of the consequences of associated marine-derived metal contaminant loading to soils (such as As).
A short pulse delay time (2 s) was used in NMR analysis. This parameter is important for attaining quantitative spectra, for example, McDowell et al. (
2006) showed for soils that short delay times bias the results in favor of nuclei such as orthophosphate that relaxes quickly, and hence apparent diesters’ concentrations decrease. The time of relaxation is enhanced in the presence of Fe and Mn. Ratios of P/(Fe + Mn) in Table
2 show that under-representation of diesters may have been an issue for SS and CM.
Potential leaching behavior of P from use of amendments
Solubility is a factor in crop availability of P species, but P leaching represents a concern on application or accumulation of P-rich organic materials in soils. In agronomic applications, such alternative fertilizer materials would be applied to soils at standard P loading rates. Hence, an improved future approach to leaching behavior may utilize fixed P mass: extract volumes (as opposed to the fixed total mass: volume approach used here) as preparation for soil leaching experiments. However, the WEP concentration order differed greatly from that of source material P contents. The WEP for CM and AD showed high P solubility but dominated by MUP (Table
4), which may be related to the organic P forms evident by NMR of monoester P for CM and diester P for AD, with the latter generally highly soluble (Table
3). Conversely, SS and GC (with NMR showing P dominated by inorganic orthophosphate; Table
3) had relatively insoluble P, dominated by MRP and exhibited strong sorption of these dilute P extracts onto the test soil (Table
4). Sorption interactions between phosphate anions and Fe, Al surface complexes (that dominate sorption in such acid Spodosols) are affected by solution pH and sorption competition with dissolved organic matter. The pH of the water extracts occupied a narrow range (pH 6.4–6.9), being relatively neutral compared with the soil pH of 4.7 (in CaCl
2). So pH differences between amendment extracts would not explain sorption behavior. It was not possible to measure the DOC concentration due to analytical issues with excessive dilutions but UV absorbance (UV
abs) was large for CM indicative of soluble aromatic humic substances. Competitive sorption between organic matter and P anions (Kang et al.
2011) would be expected to explain limited sorption of 28 % of extracted manure P onto the soil. However, the most limited (4 %) sorption of extracted AD P occurred at much smaller UV
abs and suggested that the P speciation limited interaction of AD WEP with Fe and Al surfaces in such soils.
Precipitation reactions may also impact on P solubility. Frossard et al. (
2002) found the presence of sparingly soluble, but poorly crystallized Ca–P forms (apatites, octocalcium phosphates) in composts, which they proposed were related to Ca content but also to the presence of organic substances inhibiting crystallization. The solubility of inorganic P forms is governed by associations with organic matter and other cations forming complexes or mineral phases. The molar Ca:P ratios (derived from Table
2) were close to unity but no crystalline phosphate minerals were observed by XRD in SS or AD, only calcite. Previously Frossard et al. (
2002) observed a complex mixture of octocalcium phosphates, apatites, with additionally monetite in anaerobically digested sludge.
Column systems were chosen rather than batch systems to simulate field P leaching risk since columns provide a representation of breakthrough gained from multiple samples in time and integrating physico-chemical aspects of leaching behavior (Stutter et al.
2007). Water extracts had different P concentrations and using columns experienced different loadings of P. The breakthrough curves (Fig.
S3) show
C/
C
0 = 1 attained for AD and CM. Extrapolating these simulations to field conditions would approximate to P breakthrough for AD and CM in ~2000 mm rain (1–2 years amount) for 6 cm soil depth. Attainment of equilibria for input and output concentrations suggests saturation of the surfaces but this occurred at very different total P loadings of 11 and 82 mg P kg
−1 soil for AD and CM extracts, respectively (Table
4). At the end of the column loading,
C/
C
0 concentrations of 0.1 and 0.4 had been attained for SS and GC, respectively. Hence, P sorption for these extracts was capable of continuing beyond the final loadings of 37 and 54 mg P kg
−1 soil. Due to the complex set-up and large number of samples, generated column experiments are seldom replicated, yet the results here showed close agreement between the column duplicates in terms of the breakthrough plots (Fig.
S3),
C/
C
0, and total P mass loadings (<10 % relative standard deviations; Table
4b).
The batch P desorption of soil recovered from the P-loaded columns evaluated reversibility of P fixation. One column of each pair was destructively sampled, once similarities in loadings and breakthrough of the duplicate columns were confirmed. The only appreciable desorption with water occurred for AD (5 % of sorbed P re-released), with limited additional release using 50 µM citrate (Table
4). Taken together these column sorption and desorption, experiments suggest that SS and GC would likely leach relatively small concentrations of MRP that is strongly and irreversibly sorbed by strongly reactive soils (such as Spodosols with large reactive Fe, Al concentrations). CM represents a greater leaching risk for soluble P (principally MUP) and has weaker interactions with soils due in part to high DOC concentrations. AD gave the greatest P leaching risk with a moderately high solubility and very limited soil sorption. Hence, these four alternative P amendments have very different P leaching risks and especially for AD would need strategies in place to minimize leaching, such as limiting applications to periods of crop growth. There are caveats to these findings such as the pre-treatment of drying and resolubilizing materials such as the AD. Such drying and rewetting may be expected to increase pools of soluble P, for example, associated with lysed microbial cells (Blackwell et al.
2009). However, drying pre-treatments may be part of necessary handling for bulky organic amendments.
Negative consequences of leaching have been long recognized for manures. Kang et al. (
2011) examined P leaching from columns of sandy soils under various manure fertilizer and inorganic P additions. Recovery of MRP relative to added water extractable P was 60 % for inorganic P, but >100 % for manures, and these authors explained this in terms of
P
o mineralization and competitive sorption of DOC. There is a need to extend leaching behavior studies to new waste streams such as AD. Möller and Müller (
2012) argued that although German biogas plants produce 66 million m
3 of digestate (74 Gg P), ‘no studies about the effects of AD on P losses via leaching and runoff after field application could be found.’ Walsh et al. (
2012) examined additions of chemical fertilizer, undigested slurry, and AD liquor (from a slurry feedstock similar to the material in the present study) at matched N additions onto grassland and measured soil solutions for 10 weeks post application. They found that AD and slurry gave significantly lower NO
3 and NH
4 leaching, but did not measure P leaching.