Long-term dynamics of phosphorus forms and retention in manure-amended soils

Long-term dairy manure amendment promotes legacy phosphorus buildup and mobility in calcareous soils

Continuous application of dairy manure to soils can lead to excessive phosphorus (P) accumulation (legacy P), which requires understanding for managing nutrient availability and leaching. This study was conducted in Kimberly, ID, where dairy manure or conventional fertilizer was applied to calcareous soil plots under continuous crop rotations for 8 years (2013-2020), followed by 2 years with no amendment. To understand legacy P behavior in the soils, total P, organic/inorganic P, and plant-available Olsen bicarbonate P and Truog extraction measurements were made from surface and subsurface samples. Additionally, P in soluble and less soluble calcium phosphate (Ca-P) minerals was estimated using selective extractions, and P desorption was measured in a flow-through reactor. Manure amendments resulted in increased total soil P and plant-available P, particularly in the initial 5 years. In the 0- to 30-cm depth, 54%-65% of the soil P added from manure amendments was readily soluble by the Truog P test. Phosphorus released from the 2022 manure-amended soil in the desorption experiments was about five times greater than the fertilizer-amended soil, suggesting high leaching potential. After 8 years of manure amendment, subsurface Olsen-P levels exceeded the 40 mg kg-1 management threshold, suggesting P adsorption potential of the surface had become saturated, allowing for P leaching. In the manure-amended surface soils, calcium phosphate minerals increased compared to the controls. Even after 2 years without manure amendment, soluble Ca-P mineral phases persisted in the soils, which can be a long-term source of P leaching.

Long-term phosphorus removal by Ca and Fe-rich drainage filter materials under variable flow and inlet concentrations

Phosphorus (P) losses from tile-drained agricultural fields may degrade surface water quality by accelerating eutrophication. Among the different edge-of-field technologies, compact filter systems using different filter materials have been identified as potentially effective solutions for removing P from drainage water before discharge downstream. This study investigated the long-term (>696 days) P removal efficiency of 5 different filter materials in a column setup, using artificial drainage water (pH 6). Filter materials included two iron-based granulates (calcinated diatomaceous earth (CDE), ferric hydroxide granules (CFH)), and three calcium-based granulates (seashells, limestone, calcinated silicate/calcium oxide (Filtralite-P)). Experiments were performed under variable flow rates (0.037 and 1.52 L h-1; hydraulic retention time of 26-43 min and 18-30 h) and inlet P concentrations (0.14 and 0.7 mg L-1). An overall analysis revealed that the Fe-based materials achieved higher P retention than Ca-based materials. In particular, CFH was capable of retaining 99 and 98 % of the high and low inlet P concentrations, respectively. Conversely, limestone retained only 25 % of the high P load. CDE performed moderately well, independently of the inlet P concentration. Filtralite-P and Seashells performed well at high inlet P concentration but relatively poorly at low P concentration. The sensitivity of filter material P removal efficiency to variations in P loading was generally lowest for CFH and highest for limestone.

Effect of biosolid-derived dissolved organic matter on orthophosphate sorption to soils depends on clay mineralogy and solution composition

Dissolved organic matter (DOM) from biosolids can alter the sorption of orthophosphate (inorganic phosphorus (IP)) to soils and, therefore, affect the bioavailability of IP. It is not clear how clay mineralogy and solution composition interfere with DOM effects on IP sorption by soils. Hence, we studied the effect of DOM on IP sorption to five semi-arid soils dominated by either illite/smectite (I/S) or kaolinite clays. IP sorption isotherms were constructed in either NaCl or CaCl2 background solution, with and without the addition of DOM. The IP sorption capacity maxima (SMAX, Langmuir model) of the I/S soils were 33-102% higher in the presence of CaCl2, as compared to NaCl. Although DOM had no effect on the IP-SMAX in the presence of CaCl2, it increased the IP-SMAX by 35-59% in the presence of the NaCl solution. Surprisingly, DOM sorption to the I/S soils was 30-90% greater in the presence of a Na+-dominated solution, as compared to a Ca2+-dominated solution. In contrast to the I/S soils, the SMAX of the kaolinitic soil was not affected by the background electrolyte (Na+, Ca2+) or the addition of DOM. Furthermore, the addition of IP reduced the sorption of DOM to the kaolinitic soil (by up to 50%) in both background electrolyte solutions. These results highlight the contrasting roles of divalent and monovalent cations in conjunction with DOM in IP sorption to semi-arid I/S soils. We propose a new approach based on two conceptual mechanisms to explain the DOM's enhancement of IP sorption to I/S soils. (1) Under dispersion conditions in the Na+-dominated solutions, Ca2+-mediated DOM-IP complexes bind to the clay's negative planar surfaces. (2) Under flocculation conditions in the Ca+-dominated solutions, the distance between adjacent platelets decreases, reducing both the electronegative charge spillover and Ca2+ bridge-mediated DOM sorption. In contrast, the addition of DOM to kaolinite, a multi-platelet clay with a low isomorphic negative charge, reduces IP sorption due to competitive sorption on the clay's broken edges.

Analysis of reactive phosphorus treatment by filter materials at the edge of tile-drained agricultural catchments: A global view of the current status and challenges

Phosphorus losses from agriculture have long generated concern due to the ecological impact on surface waters. Here tile-drained agricultural catchments are a critical source for concentrating and transporting phosphorus bioavailable forms or dissolved reactive phosphorus (DRP). Hence, edge-of-field technologies have been introduced to reduce DRP loads. Filter systems have received special attention due to their targeted approach using a permeable filter material (FM) rich in DRP sorbents. This review explores the performance and applicability of FMs in the aforementioned context because of the growing number of studies. An overall analysis revealed that sorption is preferable to precipitation for DRP retention at the edge-of-field, and that FM pH and particle size affect sorption properties and subsequently DRP retention and lifetime. Thus, FMs with predominant amounts of iron and/or aluminium can be recommended. Such materials generally have an appreciable availability of DRP binding sites, strong bonds with DRP and short reaction times, as well as low desorption, which lead to good operation. On the other hand, FMs with predominant amounts of calcium and/or magnesium are restricted to catchments with favourable conditions unless they have optimal reactivity for DRP. The review also found that hydraulic retention time plays a key role in the performance and applicability of FMs, especially in those dependent on precipitation reactions. Therefore, it is crucial that FMs are designed, constructed and managed according to the catchment conditions-including normally varying flow rates and DRP concentrations-in order to ensure successful operation. This reflects in long-term, high and steady net DRP retention along with low costs, thus improving the FM cost-effectiveness, besides discharging non-harmful effluents to aquatic ecosystems.

Phosphorus characteristics of Canada-wide animal manures and implications for sustainable manure management with a cleaner environment

Animal manure can be a serious environmental concern if improperly managed, particularly with regard to phosphorus (P). A good understanding on manure P chemistry is required for improving manure management in an environmentally sustainable manner. In the study, 102 representative manure samples collected from farms of major intensive livestock areas of Canada were sequentially fractionated with H₂O, 0.5 M NaHCO₃, 0.1 M NaOH, and 1.0 M HCl, respectively, for measuring inorganic (Pi) and organic P (Po). Across the manures, total P (TP) ranged from 3.71 to 17.3 g kg^-1, with total Pi and available P (i.e., the sum of H₂O-Pi and NaHCO₃-Pi) accounting for 67.0-92.4 % and 35.6-67.3 % of TP, respectively. Composting reduced the percentages of available P and Po in TP, and meanwhile increased the percentages of moderately stable HCl-Pi. Compared to other P fractions, manure available P increased more rapidly with increases in TP; once manure TP reached 7.8-9.6 g kg^-1, further TP increase enhanced transformation to more recalcitrant P (i.e., NaOH-Pi and HCl-Pi). Under Canadian conditions, manure application to meet the demand of crops for N would lead to P buildup in the soil at rates of 6.1-41.6 kg P ha^-1 yr^-1, increasing runoff P loss risk. Manure compost and poultry manure tend to cause rapid P buildup in the soil after land application and become a long-term P source for runoff losses. The results help to develop scientifically-sound manure-specific P management technologies which would enable farmers to achieve sustainable crop production with improved environment.

Accumulation and release of organic phosphorus (P) from legacy P-affected soils to adjacent drainage water

Legacy effects of P in agricultural soils have been highlighted in recent literature. However, co-accumulation and release of organic P (Po) have often been ignored in current agro-environmental assessments. The mineralizable Po fraction has a potential to increase the activity of phosphate in pore water, increasing fertility or degrading water quality. In this study, the effects of agricultural management practices (fertilizer applied corn-soybean rotation cropland and dairy manure applied pasture) on the Po/phosphate ratio were investigated in P-rich (290-1232 mg kg^-1) agricultural soils and adjacent ditchwater using experimental soil-water chemistry. The effect of agricultural management was significant on both Po and the Po/phosphate ratio in soil and adjacent ditchwater. The P_o content, dominated by orthophosphate monoesters, in the manure-amended pasture (average ~ 245 mg kg^-1) was significantly greater than that in the fertilizer-applied cropland (average 103 mg kg^-1). The Po/phosphate ratio was also significantly greater in the manure-amended pasture (0.54) than in the fertilizer-applied cropland (0.42). Similarly, water quality data also showed that ditchwater near the pasture had a significantly greater flux of dissolved non-reactive P and a greater Po/phosphate ratio compared to the water near the fertilizer-applied sites. Furthermore, a greater Po/phosphate ratio in ditchwater was often observed during wet periods, and the ratio was positively correlated to the discharge (r = 0.42, p = 0.003). The study showed the agricultural management-specific Po accumulation and release and - Po/phosphate ratio that might affect the fate of P in agroecosystems.

Phosphorus status and adsorption characteristics of perennial vegetable-cultivated soils in South China

Phosphorus (P) is an essential element for crop production and a key source of nonpoint pollution in agroecosystems. In this study, we sought to analyze P levels and the factors affecting soil P availability, via P adsorption, in a typical field system that is characterized by the year-round cultivation of vegetables. A total of 190 sites were sampled from vegetable fields in Guangdong Province, South China. Within the research area, average concentrations of 124.49 mg P kg-1 and 1.55 g P kg-1 were recorded for available P (AP) and total P (TP), respectively, which are 8.53- and 1.78-fold higher, respectively, than the corresponding values recorded in 1980. The determined P adsorption maximum (Qm) averaged at 488.38 mg kg-1, which represents a reduction of 16% compared to the values obtained four decades ago. Accumulations of both TP and AP were found to be negatively correlated with the soil’s maximum adsorption buffering capacity (MBC), although no significant correlations with the soil binding energies (k) and Qm, were seen. However, soil pH was found to be significantly correlated with k and Qm. Furthermore, both free Mn oxides (Mnd) and silt concentrations in the soil were found to contribute to explaining the variations in Qm. Collectively, the findings of this study provide evidence to indicate that there has been an excessive accumulation of P in the perennial vegetable fields of Guangdong Province over the past four decades, which may have had negative effects on the P supply potential of the soil by reducing the maximum adsorption buffering capacity.

Biosolids increase phosphate adsorption of semi-arid Mediterranean soils

The impact of biosolid compost on the adsorption of orthophosphate (IP) to Mediterranean-type soils was studied. Eight soils were amended with a stable biosolid compost (ADSC) at 9:1 and 97:3 ratios (w/w). Four soils were amended with the dissolved organic matter (DOM) fraction of the ADSC at the amount added at the 9:1 mixture (810 mg C kg^-1). Soils and their 9:1 soil‒ADSC mixtures were incubated for seven years. The maximum ADSC IP-adsorption capacity (S_MAX, Langmuir model) at native pH (≈7.5) was 850 mg P kg^-1. Mixing the ADSC with the soils increased their S_MAX values by ca. 150 and 190 mg P kg^-1 in the 9:1 and 97:3 mixtures, which exceeded additivity by 50% and 575%. The addition of DOM similarly increased the S_MAX of three out of the four soils. Following the incubation, the soils' organic-C decreased by 34% and the ADSC-derived OC decreased by 60%. Still, the corresponding soil's and mixtures' average levels of labile IP either increased (by 60%) or remained steady (at ̴30% of total-P). Incubation increased the S_MAX of three soils and five soil‒ADSC mixtures and decreased their binding affinity (k), trends which were also reflected in the quantity/intensity parameters. This study showed that amending semi-arid Mediterranean soils with stable biosolids, and their long-term oxidative co-stabilization is conducive to increase their IP binding capacity and bioavailability. Finally, the often similar effects of the compost and its DOM on IP adsorption merits further research regarding the role of cation (Ca⁺²) bridging in IP‒DOM‒solid phase interactions.

Microscale Heterogeneous Distribution and Speciation of Phosphorus in Soils Amended with Mineral Fertilizer and Cattle Manure Compost

Global concerns for the sustainability of agriculture have emphasized the need to reduce the use of mineral fertilizer. Although phosphorus (P) is accumulated in farmland soils due to the long-term application of fertilizer, most soil P is not readily available to plants. The chemical speciation of P in soils, which comprise heterogeneous microenvironments, cannot be evaluated with a high degree of specificity using only macroscopic analyses. In this study, we investigated the distribution and speciation of P accumulated in soils by using both macro- and microscopic techniques including chemical extraction, solution and solid-state 31P NMR, bulk- and micro- P K-edge X-ray absorption near edge structure (XANES), and electron probe microanalysis (EPMA). Soil samples were collected from a field in which cabbage was cultivated under three amendment treatments: i) mineral fertilizer (NPK), ii) mineral fertilizer and compost (NPK + compost), and iii) mineral fertilizer plus compost but without nitrogen fertilizer (PK + compost). Macro-scale analyses suggested that accumulated P was predominantly inorganic P and associated with Al-bearing minerals. The repeated application of compost to the soils increased the proportion of P associated with Ca which accounted for 17% in the NPK + compost plot and 40% in the PK + compost plot. At the microscale, hot spots of P were heterogeneously distributed, and P was associated with Fe and Ca in hot spots of the NPK + compost (pH 6) and PK + compost (pH 7) treated samples, respectively. Our results indicate that application of compost contributed to creating diverse microenvironments hosting P in these soils.

Use of iron oxide nanoparticles for immobilizing phosphorus in-situ: Increase in soil reactive surface area and effect on soluble phosphorus

Phosphorus (P) immobilization has potential for reducing diffuse P losses from legacy P soils to surface waters and for regenerating low-nutrient ecosystems with a high plant species richness. Here, P immobilization with iron oxide sludge application was investigated in a field trial on a noncalcareous sandy soil. The sludge applied is a water treatment residual produced from raw groundwater by Fe(II) oxidation. Siliceous ferrihydrite (Fh) is the major Fe oxide type in the sludge. The reactive surface area assessed with an adapted probe ion method is 211-304 m² g^-1 for the Fe oxides in the sludge, equivalent to a spherical particle diameter of ~6-8 nm. This size is much larger than the primary Fh particle size (~2 nm) observed with transmission electron microscopy. This can be attributed to aggregation initiated by silicate adsorption. The surface area of the indigenous metal oxide particles in the field trial soils is much higher (~1100 m² g^-1), pointing to the presence of ultra-small oxide particles (2.3 ± 0.4 nm). The initial soil surface area was 5.4 m² g^-1 and increased linearly with sludge application up to a maximum of 12.9 m² g^-1 when 27 g Fe oxides per kg soil was added. In case of a lower addition (~10-15 g Fe oxides per kg soil), a 10-fold reduction in the phosphate (P-PO₄) concentration in 0.01 M CaCl₂ soil extracts to 0.3 µM was possible. The adapted probe ion method is a valuable tool for quantifying changes in the soil surface area when amending soil with Fe oxide-containing materials. This information is important for mechanistically predicting the reduction in the P-PO₄ solubility when such materials are used for immobilizing P in legacy P soils with a low P-PO₄ adsorption capacity but with a high surface loading.

Quantifying Uncertainties in Sequential Chemical Extraction of Soil Phosphorus Using XANES Spectroscopy

Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO₃ and then 0.1 M NaOH), creating new Ca-P via formation of Ca phosphates between NaOH-desorbed phosphate and exchangeable Ca²⁺ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca²⁺ regardless of the presence or absence of CaCO₃, whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of Ca-P simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure.

Soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry drives phosphorus lability in paddy soil under long-term fertilization: A fractionation and path analysis study

Soil C:N:P stoichiometry plays a vital role in nutrient cycling in ecosystems, but its importance to P transformation in paddy soil remains unclear. We investigated the effect of soil C:N:P stoichiometry on P mobility and uptake under long-term fertilization. Three treatments, CK (no fertilization), NPK (inorganic nitrogen, phosphorus and potassium fertilization) and NPKM (combined inorganic NPK fertilizer and manure application), were selected from two long-term experiments of paddy soil that were initiated in 1991 and 1982 in Chongqing and Suining, respectively. The results showed that in comparison the control treatment, under long-term fertilization, soil pH decreased. In comparison with the NPK and CK treatments, the NPKM treatment significantly increased soil nutrient contents, P uptake and phosphatase activities. In comparison to the CK treatment, the NPK and NPKM treatments significantly decreased soil C:N, C:P and N:P ratios. In comparison to NPK and CK treatments, the NPKM treatment decreased residual-P at both sites. Compared with CK treatment, the NPKM treatments increased labile-P and moderately labile-P by 987% and 144%, respectively, and NPK treatment increased these factors by 823% and 125%, respectively, at the Chongqing site. At the Suining site, with NPKM treatment, increases in labile-P and moderately labile-P were 706% and 73%, respectively, and with NPK treatment, the increases were 529% and 47%, respectively. In contrast, non-labile-P was significantly decreased with NPKM treatment in comparison to that with NPK and CK treatments. Moreover, increases in soil C:N and C:P ratios decreased the labile-P pools and increased non-labile-P pools. A path analysis indicated that soil C:N:P stoichiometry indirectly controlled P uptake by directly affecting P transformation from non-labile to labile-P pools. Moreover, the non-labile-P in soil with high SOM and P content directly affected P uptake, indicating that soil P transformation is mainly driven by soil C and P in paddy soil. In conclusion, understanding mechanism of P mobility influenced by soil C:N:P stoichiometry could be helpful to manage soil P fertility under long-term fertilization in paddy soils of these regions.

Increased risk of phosphorus and metal leaching from paddy soils after excessive manure application: Insights from a mesocosm study

Livestock manure has gradually become an alternative fertilizer for maintaining soil fertility, whereas excessive application of manure leads to the release of phosphorus (P) and toxic metals that may cause complex environmental risks. To investigate the accumulation and migration of P within soil profiles, a mesocosm experiment was conducted to analyze the content and leaching of soil P, metals, and dissolved organic carbon after different fertilization treatments, including control (no fertilizer, CK), chemical fertilizer (CF), chemical fertilizer combined low (CF + LPM) and high (CF + HPM) rate of manure application. Results showed that a high rate of manure application significantly enhanced the accumulation of total soil P (by ~14%) and P availability (easily-available P, by ~24%; Olsen-P, by ~20%) in topsoil, and also increased the content of easily-available organic P (EA-Po) in both topsoil and subsoil compared to the CK treatment. The migration of dissolved inorganic and organic P (DIP and DOP) in leachate within soil profiles was strengthened by manure application. Moreover, significant positive correlations between P, metals, and dissolved organic carbon (DOC) in leachate indicated that downward co-migration occurred within the soil profiles, and also suggested that excessive manure application can intensify the risk of P loss by increasing the migration of manure-derived DOC. Overall, our findings provide insights into P accumulation and migration within soil profiles after excessive manure application, which is useful for predicting the potential risk of P and metal leaching from paddy soils.

Kaolinite Enhances the Stability of the Dissolvable and Undissolvable Fractions of Biochar via Different Mechanisms

Input of biomass-derived biochar into soil is recognized as a promising method of carbon sequestration. The long-term sequestration effect of biochar depends on the stability of both its dissolvable and undissolvable fractions in soil, which could be affected by their interactions with soil minerals. Here, walnut shell-derived biochar was divided into dissolvable and undissolvable fractions and then interacted with kaolinite. Stability of kaolinite-biochar associations was evaluated by chemical oxidation and biological degradation. At low dissolvable biochar concentrations, the association was mainly attributed to "Ca²⁺ bridging" and "ligand exchange", whereas "van der Waals attraction" was dominant at high concentrations. For the undissolvable biochar, kaolinite raised the activation energy of its surface by 22.1%, causing a reduction in biochar reactivity. By chemical oxidation, kaolinite reduced the C loss of total biochar by 42.5%, 33.1% resulting from undissolvable biochar and 9.4% from dissolvable biochar. Because of the presence of kaolinite, the loss of biodegradable C in total biochar was reduced by 49.4% and 48.2% from undissolvable fraction and 1.2% from dissolvable fraction. This study indicates that kaolinite can increase the stability of both dissolvable and undissolvable biochar, suggesting that kaolinite-rich soils could be a beneficial environment for biochar for long-term carbon sequestration.

Sources and Pathways of Formation of Recalcitrant and Residual Phosphorus in an Agricultural Soil

Phosphorus (P) is an essential nutrient for sustaining life and agricultural production. Transformation of readily available P into forms that are unavailable to plants adds costs to P replenishment, which eventually translates into lower agronomic benefits and potential loss of soil P into runoff may degrade water quality. Therefore, understanding the sources and pathways of the formation of residual P pools in soils is useful information needed for the development of any technological or management efforts to minimize or inhibit the formation of such P pool and thus maximize availability to plants. In this research, we paired phosphate oxygen isotope ratios (δ18OP) with solid-state 31P NMR and quantitative XRD techniques along with general soil chemistry methods to identify the precipitation pathways of acid-extracted inorganic P (Pi) pools in an agricultural soil. Based on the comparison of isotope values of 0.5 mol L−1 NaOH-Pi, 1 mol L−1 HCl-Pi, and 10 mol L−1 HNO3-Pi pools and correlations of associated elements (Ca, Fe, and Al) in these pools, the HNO3-Pi pool appears most likely to be transformed from the NaOH-Pi pool. A narrow range of isotope values of acid-Pi pools in shallow (tilling depth) and below (where physical mixing is absent) is intriguing but likely suggests leaching of particle-bound P in deeper soils. Overall, these findings provide an improved understanding of the sources, transport, and transformation of acid-Pi pools in agricultural soils and further insights into the buildup of legacy P in soils.

Distribution of phosphorous pools in western river sediments of the Urmia Lake basin, Iran

Impact of anthropogenic loading of phosphorous (P) to an aquatic ecosystem can be qualitatively assessed by measuring the buildup and distribution of P in sediments and by differentiating bioavailable and recalcitrant P pools. Distribution of P pools in sediments is affected by the physico-chemical properties including specific elements, particle size distribution, pH, electrical conductivity (EC), and carbonate content. We applied X-ray fluorescence and scanning electron microscopy (SEM) methods to characterize sediments from western rivers in the Urmia Lake basin in Iran with a particular focus on properties that are relevant to P speciation. Phosphorous pools were sequentially extracted into operationally defined exchangeable (EXCH-P), iron and aluminum oxide-bound (Fe/Al-P), calcium-bound (Ca-P), and residual (RES-P) P pools. In river sediments, the size of P pool was found to be in the order of Ca-P > RES-P > Fe/Al-P > EXCH-P indicating small fraction of bioavailable P pool and Ca-P minerals being the most dominant P sink. Carbonate-related properties had an inverse relationship with bioavailable P pools in the river sediments studied. The principal component analysis (PCA) of the sequential extraction data with sediment properties revealed that four principal components described 82.7% of total variation. Similarly, particle size-related properties were found to have the highest eigenvalues in the first PC. Electron diffraction spectra (EDS) and X-ray fluorescence (XRF) analyses showed a largely uniform distribution of P in the upstream sediment. However, limited evidence of local enrichment of P with Fe, Al, and Ca contents was observed in the downstream river sediments. Correlation of Fe/Al-P pool size with Al₂O₃ and SiO₂ contents indicated that P was associated with Al oxide and clay minerals in the sediment matrix. Overall, the results from this study provide insights into the variability of upstream and downstream river processes and their relationship with P pools with regard to their bioavailability. These results are expected to be useful in assessing the potential impact of P loading on the aquatic ecosystem in the Urmia Lake basin.

Plant available phosphorus in soil as predictor for the leaching potential: Insights from long-term lysimeter studies

This study aimed to demonstrate the impact of phosphorus (P) mineral fertilization on topsoil P content and P leaching. We evaluated 83 datasets from 25 years from lysimeter experiments involving different cropping systems (winter crop, summer crop and autumn tillage, harvested grass) or unfertilized fallow, four types of soil texture, and three levels of applied mineral P fertilizer. A positive monotonic and significant correlation was indicated between P in the topsoil determined by the double lactate method (P _DL) and the yearly flow-weight total (TP) concentrations in leachates with Spearman rank correlations r _s (r _s > 0.183) and probability (p) < 0.05. The present German recommended rates of P mineral fertilization are proposed insufficient to protect fresh and marine waters from undesired P pollution and eutrophication. A long-term reduction of excess soil P is urgent along with other measures to mitigate high P inputs to surface and ground waters.

Excessive application of pig manure increases the risk of P loss in calcic cinnamon soil in China

Soil phosphorus (P) is a critical factor affecting crop yields and water environmental quality. To investigate the degree of loss risk and forms of soil P in calcic cinnamon soil, the P fraction activities in soils were analysed using chemical methods, combined with an in situ field experiment. Seven treatments were set in this study, including control (unfertilized), no P fertilizer (No-P), mineral P fertilizer (Min-P), low (L-Man) and high (H-Man) quantities of pig manure, Min-P+L-Man, and Min-P+H-Man. The results showed that manure fertilizer could not only significantly increase maize yield but could also enhance the accumulation of soil P in organic and inorganic forms. After 23years of repeated fertilization, the soil Olsen-P contents respectively showed 64.7-, 43.7- and 31.9-fold increases in the Min-P+H-Man, Min-P+L-Man and H-Man treatments, while the soil Olsen-P in Min-P treatment only increased 23.7-fold. The soil Olsen-P thresholds ranged from 22.59 to 32.48mgkg^-1 in calcic cinnamon soil to maintain a higher maize yield as well as a lower risk of P loss. Therefore, long-term excessive manure application could obviously raise the content of soil Olsen-P and increase the risk of P loss in calcic cinnamon soil.

Diffusion-Ordered Nuclear Magnetic Resonance Spectroscopy (DOSY-NMR): A Novel Tool for Identification of Phosphorus Compounds in Soil Extracts

Liquid-state, one-dimension ³¹P nuclear magnetic resonance spectroscopy (NMR) has greatly advanced our understanding of the composition of organic phosphorus in the environment. However, the correct assignment of signals is complicated by overlapping and shifting signals in different types of soils. We applied therefore for the first time diffusion-ordered spectroscopy (DOSY) to soil extracts, allowing us to separate phosphorus components in the second domain based on their translational diffusion coefficients. After successful application to a mixture of 14 model compounds, diffusion rates correlated closely with the molecular weight of the individual compound in aqueous solution (R² = 0.97). The method was then applied to NaOH/EDTA extracts of a grassland soil, of which paramagnetic contaminations were removed with sodium sulfide following high-velocity centrifugation (21 500g, 45 min) at 4 °C. Diffusion rates in soil extracts were again closely related to molecular weight (R² = 0.98), varying from 163.9 to 923.8 Da. However, our DOSY application failed for a forest soil with low organic phosphorus content. Overall, DOSY did help to clearly identify specific NMR signals like myo- and scyllo-inositol hexakisphosphate. It thus provides a more confident signal assignment than 1D ³¹P NMR, although currently the ubiquitous use of this novel methodology is still limited to soils with high organic phosphorus content.

Opportunities for mobilizing recalcitrant phosphorus from agricultural soils: a review

BACKGROUND

Phosphorus (P) fertilizer is usually applied in excess of plant requirement and accumulates in soils due to its strong adsorption, rapid precipitation and immobilisation into unavailable forms including organic moieties. As soils are complex and diverse chemical, biochemical and biological systems, strategies to access recalcitrant soil P are often inefficient, case specific and inconsistently applicable in different soils. Finding a near-universal or at least widely applicable solution to the inefficiency in agricultural P use by plants is an important unsolved problem that has been under investigation for more than half a century.

SCOPE

In this paper we critically review the strategies proposed for the remobilization of recalcitrant soil phosphorus for crops and pastures worldwide. We have additionally performed a meta-analysis of available soil 31P-NMR data to establish the potential agronomic value of different stored P forms in agricultural soils.

CONCLUSIONS

Soil inorganic P stocks accounted on average for 1006 ± 115 kg ha-1 (57 ± 7%), while the monoester P pool accounted for 587 ± 32 kg ha-1 (33 ± 2%), indicating the huge potential for the future agronomic use of the soil legacy P. New impact driven research is needed in order to create solutions for the sustainable management of soil P stocks.

Comprehensive study of excess phosphate response reveals ethylene mediated signaling that negatively regulates plant growth and development

Excess Phosphorus (P) in agriculture is causing serious environmental problems like eutrophication of lakes and rivers. Unlike the enormous information available for phosphate starvation response (P₀), very few information is available for the effect of excess phosphate P_i on plants. Characterization of Excess Phosphate Response (EP_iR) is essential for designing strategies to increase phosphate accumulation and tolerance. We show a significant modulation in the root developmental plasticity under the increasing supply of excess P_i. An excess supply of 20 mM P_i (P₂₀) produces a shallow root system architecture (RSA), reduces primary root growth, root apical meristem size, and meristematic activity in Arabidopsis. The inhibition of primary root growth and development is indeterminate in nature and caused by the decrease in number of meristematic cortical cells due to EP_iR. Significant changes occurred in metal nutrients level due to excess P_i supply. A comparative microarray investigation of the EP_iR response reveals a modulation in ethylene biosynthesis and signaling, metal ions deficiency response, and root development related genes. We used ethylene-insensitive or sensitive mutants to provide more evidence for ethylene-mediated signaling. A new role of EP_iR in regulating the developmental responses of plants mediated by ethylene has been demonstrated.

Manure and nitrogen application enhances soil phosphorus mobility in calcareous soil in greenhouses

Over many years, high phosphorus (P) loading for intensive vegetable cropping in greenhouses of North China has contributed to excessive P accumulation, resulting in environmental risk. In this study, the influences of manure and nitrogen (N) application on the transformation and transport of soil P were investigated after nine years in a greenhouse tomato double cropping system (winter-spring and autumn-winter seasons). High loading of manure significantly increased the soil inorganic P (Pi), inositol hexakisphosphate (IHP), mobile P and P saturation ratio (PSR, >0.7 in 0-30 cm depth soil; PSR was estimated from P/(Fe + Al) in an oxalate extract of the soil). The high rate of N fertilizer application to the studied calcareous soil with heavy loading of manure increased the following: (i) mobile organic P (Po) and Pi fractions, as evidenced by the decrease in the ratio of monoesters to diesters and the proportion of stable Pi (i.e., HCl-Pi) in total P (Pt) in 0-30 cm depth soil; (ii) relative distribution of Po in the subsoil layer; and (iii) P leaching to soil depths below 90 cm and the proportion of Po in Pt in the leachate. More acidic soil due to excessive N application increased P mobility and leaching. The increase in Ox-Al (oxalate-extractable Al) and the proportion of microbe-associated Po related to N application at soil depths of 0-30 cm suggested decrease in the net Po mineralization, which may contribute to downward transport of Po in the soil profile.

Biochar efficiency in pesticides sorption as a function of production variables--a review

Biochar is a stabilized, carbon-rich by-product derived from pyrolysis of biomass. Recently, biochar has received extensive attentions because of its multi-functionality for agricultural and environmental applications. Biochar can contribute to sequestration of atmosphere carbon, improvement of soils quality, and mitigation of environmental contaminations. The capability of biochar for specific application is determined by its properties which are predominantly controlled by source material and pyrolysis route variables. The biochar sorption potential is a function of its surface area, pores volume, ash contents, and functional groups. The impacts of each production factors on these characteristics of biochar need to be well-understood to design efficient biochars for pesticides removal. The effects of biomass type on biochar sorptive properties are determined by relative amounts of its lingo-cellulosic compounds, minerals content, particles size, and structure. The highest treatment temperature is the most effective pyrolysis factor in the determination of biochar sorption behavior. The expansion of micro-porosity and surface area and also increase of biochar organic carbon content and hydrophobicity mostly happen by pyrolysis peak temperature rise. These changes make biochar suitable for immobilization of organic contaminants. Heating rate, gas pressure, and reaction retention time after the pyrolysis temperatures are sequentially important pyrolysis variables effective on biochar sorptive properties. This review compiles the available knowledge about the impacts of production variables on biochars sorptive properties and discusses the aging process as the main factor in post-pyrolysis alterations of biochars sorption capacity. The drawbacks of biochar application in the environment are summarized as well in the last section.

Effect of aging on phosphorus speciation in surface deposit of a vertical flow constructed wetland

This study was conducted to determine phosphorus (P) species captured in a vertical-flow constructed wetland (VFCW) system combining a trickling filter followed by FeCl3 injection for phosphate coagulation. Suspended solids (SS) thus formed accumulated over time at the VFCW surface and transformed into a sludge deposit layer, which was shown to concentrate most of the P captured in the system. In order to investigate the effect of aging on P species, representative SS and sludge samples were taken from a wastewater treatment plant that had been in operation for 8 years and analyzed using P fractionation, solution (31)P NMR spectroscopy, and P and Fe K-edge XANES spectroscopy. A partial mineralization of organic matter was shown by comparing organic carbon contents of SS and sludge materials. Chemical fractionations combined with P and Fe K-edge XANES spectroscopy showed that P was predominantly bound to iron within both samples in the form of ferric phosphate, rather than adsorbed onto ferric oxyhydroxide. Calcium-bound P was more significantly observed in sludge than in SS, suggesting that aging induced the recombination of part of the organic and iron-bound P species into calcium-bound forms, as a possible consequence of the partial mineralization of organic matter.

Forms of phosphorus transfer in runoff under no-tillage in a soil treated with successive swine effluents applications

Successive swine effluent applications can substantially increase the transfer of phosphorus (P) forms in runoff. The aim of this study was to evaluate P accumulation in the soil and transfer of P forms in surface runoff from a Hapludalf soil under no-tillage subjected to successive swine effluent applications. This research was carried out in the Agricultural Engineering Department of the Federal University of Santa Maria, Brazil, from 2004 to 2007, on a Typic Hapludalf soil. Swine effluent rates of 0, 20, 40, and 80 m3 ha(-1) were broadcast over the soil surface prior to sowing of different species in a crop rotation. Soil samples were collected in stratified layers, and the levels of available P were determined. Samples of water runoff from the soil surface were collected throughout the period, and the available, soluble, particulate, and total P were measured. Successive swine effluent applications led to increases in P availability, especially in the soil surface, and P migration through the soil profile. Transfer of P forms was closely associated with runoff, which is directly related to rainfall volume. Swine effluent applications also reduced surface runoff. These results show that in areas with successive swine effluent applications, practices that promote higher water infiltration into the soil are required, e.g., crop rotation and no-tillage system.

The composition, leaching, and sorption behavior of some alternative sources of phosphorus for soils

Concerns about the sustainability of inorganic fertilizers necessitate the characterization of alternative P source materials for agronomic P-efficiencies and P losses via leaching. Firstly, this study examined nutrient compositions including P speciation of seven soil amendments: sewage sludge (SS), anaerobic digestate (AD), green compost (GC), food waste compost (FWC), chicken manure (CM), biochar, and seaweed. Secondly, soil P leaching and availability was studied on a subset of four materials (SS, AD, GC, and CM). Sorption of extracts onto columns of a test soil showed strong P retention for SS and compost, but weak P sorption for CM and especially AD, suggesting short-term leaching risks for soil applied AD. Limited P desorption with water or citrate indicated sorbed P was strongly fixed, potentially limiting crop availability. These data indicate that variation in P forms and environmental behavior should be understood to maximize P usage, but minimize leaching and soil P accumulation. Hence, different alternative P source materials need differing recommendations for their agronomic management.

Long-term manure application effects on phosphorus speciation, kinetics and distribution in highly weathered agricultural soils

Phosphorus (P) K-edge XANES and Fe K-edge EXAFS spectroscopies along with sequential P chemical fractionation and desorption kinetics experiments, were employed to provide micro- and macro-scale information on the long-term fate of manure application on the solid-state speciation, kinetics and distribution of P in highly weathered agricultural soils of southern Brazil. Soil test P values ranged from 7.3 up to 16.5 times as much higher than the reference soil. A sharp increase in amorphous Fe and Al amounts were observed as an effect of the consecutive application of manures. Whereas our results showed that the P sorption capacity of some manured soils was not significantly affected, P risk assessment indices indicated that P losses should be expected, likely due to the excessive manure rates applied to the soils. The much higher contents of amorphous Fe and Al (hydr)oxides (55% and 80% increase with respect to the reference soil, respectively) in manured soils seem to have counterbalanced the inhibiting effect of soil organic matter on P sorption by creating additional P sorption sites. Accordingly, the newly created P sorbing surfaces were important to prevent an even larger P loss potential. Phosphorus K-edge XANES lent complimentary hints on the loss of crystallinity and transformation of originally present Fe-P minerals into poorly crystalline ones as an effect of manuring, whereas Fe K-edge EXAFS provided insights into the structural changes underwent in the soils upon manure application and soil management.

Effects of exogenous rare earth elements on phosphorus adsorption and desorption in different types of soils

Phosphorus (P) is an important biogeochemical element and the environmental fate of P receives increasing attention. Through batch equilibration experiments, the adsorption and desorption of P in the absence and presence of exogeneous rare earth elements (REEs) were investigated in five types of agricultural soil samples collected from China. The results showed that the addition of different doses of REEs had influences on P adsorption processes in the soils, and there were differences in different soil types and different P concentrations of the P solutions. The amount of P adsorption tended to decline when the five types of soils were amended with low concentrations of REEs. The characteristics of P adsorption were more complicated when high concentrations of REEs were added to the different soils. Affected by the high concentrations of REEs, when the P concentration of the P solution added to soils was less than 20 mg L(-1), the rate of P adsorption tended to increase in all the five types of soils. However, when the P concentration of the P solution added to soil was greater than 30 mg L(-1), the rate of P adsorption tended to decrease. The Langmuir equation fitted P adsorption in all the five types of soils well. Compared with the control, when soil samples were amended with REEs, the P desorption rates of the five types of soils increased.

An estimation of annual nitrous oxide emissions and soil quality following the amendment of high temperature walnut shell biochar and compost to a small scale vegetable crop rotation

Agricultural soils are responsible for emitting large quantities of nitrous oxide (N2O). The controlled incomplete thermal decomposition of agricultural wastes to produce biochar, once amended to soils, have been hypothesized to increase crop yield, improve soil quality and reduce N2O emissions. To estimate crop yields, soil quality parameters and N2O emissions following the incorporation of a high temperature (900 °C) walnut shell (HTWS) biochar into soil, a one year field campaign with four treatments (control (CONT), biochar (B), compost (COM), and biochar+compost (B+C)) was conducted in a small scale vegetable rotation system in Northern California. Crop yields from five crops (lettuce, winter cover crop, lettuce, bell pepper and Swiss chard) were determined; there were no significant differences in yield between treatments. Biochar amended soils had significant increases in % total carbon (C) and the retention of potassium (K) and calcium (Ca). Annual cumulative N2O fluxes were not significantly different between the four treatments with emissions ranging from 0.91 to 1.12 kg N2O-N ha(-1) yr(-1). Distinct peaks of N2O occurred upon the application of N fertilizers and the greatest mean emissions, ranging from 67.04 to 151.41 g N2O-N ha(-1) day(-1), were observed following the incorporation of the winter cover crop. In conclusion, HTWS biochar application to soils had a pronounced effect on the retention of exchangeable cations such as K and Ca compared to un-amended soils and composted soils, which in turn could reduce leaching of these plant available cations and could thus improve soils with poor nutrient retention. However, HTWS biochar additions to soil had neither a positive or negative effect on crop yield nor cumulative annual emissions of N2O.

Characterization of leached phosphorus from soil, manure, and manure-amended soil by physical and chemical fractionation and Diffusive Gradients in Thin films (DGT)

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.

Pyrolysis for biochar purposes: a review to establish current knowledge gaps and research needs

According to the International Biochar Initiative (IBI), biochar is a charcoal which can be applied to soil for both agricultural and environmental gains. Biochar technology seems to have a very promising future. Nevertheless, the further development of this technology requires continuing research. The present paper provides an updated review on two subjects: the available alternatives to produce biochar from a biomass feedstock and the effect of biochar addition to agricultural soils on soil properties and fertility. A high number of previous studies have highlighted the benefit of using biochar in terms of mitigating global warning (through carbon sequestration) and as a strategy to manage soil processes and functions. Nevertheless, the relationship between biochar properties (mainly physical properties and chemical functionalities on surface) and its applicability as a soil amendment is still unclear and does not allow the establishment of the appropriate process conditions to produce a biochar with desired characteristics. For this reason, the need of enhancing the collaboration among researchers working in different fields of study is highlighted: production and characterization of biochar on one hand, and on the other measurement of both environmental and agronomical benefits linked to the addition of biochar to agricultural soils. In this sense, when experimental results concerning the effect of the addition of biochar to a given soil on crop yields and/or soil properties are published, details regarding the properties of the used biochar should be well reported. The inclusion of this valuable information seems to be essential in order to establish the appropriate process conditions to produce a biochar with more suitable characteristics.

Interactions between soil texture and placement of dairy slurry application: II. Leaching of phosphorus forms

Managing phosphorus (P) losses in soil leachate folllowing land application of manure is key to curbing eutrophication in many regions. We compared P leaching from columns of variably textured, intact soils (20 cm diam., 20 cm high) subjected to surface application or injection of dairy cattle (Bos taurus L.) manure slurry. Surface application of slurry increased P leaching losses relative to baseline losses, but losses declined with increasing active flow volume. After elution of one pore volume, leaching averaged 0.54 kg P ha(-1) from the loam, 0.38 kg P ha(-1) from the sandy loam, and 0.22 kg P ha(-1) from the loamy sand following surface application. Injection decreased leaching of all P forms compared with surface application by an average of 0.26 kg P ha(-1) in loam and 0.23 kg P ha(-1) in sandy loam, but only by 0.03 kg P ha(-1) in loamy sand. Lower leaching losses were attributed to physical retention of particulate P and dissolved organic P, caused by placing slurry away from active flow paths in the fine-textured soil columns, as well as to chemical retention of dissolved inorganic P, caused by better contact between slurry P and soil adsorption sites. Dissolved organic P was less retained in soil after slurry application than other P forms. On these soils with low to intermediate P status, slurry injection lowered P leaching losses from clay-rich soil, but not from the sandy soils, highlighting the importance of soil texture in manageing P losses following slurry application.

Improvement of (31)P NMR spectral resolution by 8-hydroxyquinoline precipitation of paramagnetic Fe and Mn in environmental samples

Solution (31)P nuclear magnetic resonance (NMR) spectroscopy is currently the main method for the characterization of phosphorus (P) forms in environment samples. However, identification and quantification of P compounds may be hampered by poor resolution of spectra caused by paramagnetic Fe and Mn. In this study, a novel technique was developed to improve spectral resolution by removing paramagnetic Fe and Mn from alkaline extracts via 8-hydroxyquinoline (8-HOQ) precipitation. Batch experiments showed that both Fe and Mn were effectively removed by the precipitation at pH 9.0, with the removal efficiencies of 83-91% for Fe and 67-78% for Mn from the extracts of five different environmental samples, while little effect was found on concentration of total P. The (31)P NMR analysis of a model P solution showed that addition of 8-HOQ and its precipitation with metal ions did not alter P forms. Further analyses of the five extracts with (31)P NMR spectroscopy demonstrated that the 8-HOQ precipitation was an ideal method compared with the present postextraction techniques, such as bicarbonate dithionate (BD), EDTA and Chelex-100 treatments, by improving spectral resolution to a large extent with no detrimental effects on P forms.

Fate and transport of antibiotic residues and antibiotic resistance genes following land application of manure waste

Antibiotics are used in animal livestock production for therapeutic treatment of disease and at subtherapeutic levels for growth promotion and improvement of feed efficiency. It is estimated that approximately 75% of antibiotics are not absorbed by animals and are excreted in waste. Antibiotic resistance selection occurs among gastrointestinal bacteria, which are also excreted in manure and stored in waste holding systems. Land application of animal waste is a common disposal method used in the United States and is a means for environmental entry of both antibiotics and genetic resistance determinants. Concerns for bacterial resistance gene selection and dissemination of resistance genes have prompted interest about the concentrations and biological activity of drug residues and break-down metabolites, and their fate and transport. Fecal bacteria can survive for weeks to months in the environment, depending on species and temperature, however, genetic elements can persist regardless of cell viability. Phylogenetic analyses indicate antibiotic resistance genes have evolved, although some genes have been maintained in bacteria before the modern antibiotic era. Quantitative measurements of drug residues and levels of resistance genes are needed, in addition to understanding the environmental mechanisms of genetic selection, gene acquisition, and the spatiotemporal dynamics of these resistance genes and their bacterial hosts. This review article discusses an accumulation of findings that address aspects of the fate, transport, and persistence of antibiotics and antibiotic resistance genes in natural environments, with emphasis on mechanisms pertaining to soil environments following land application of animal waste effluent.

Phosphorus and nitrogen sorption to soils in the presence of poultry litter-derived dissolved organic matter

Two environmental aspects associated with land application of poultry litter that have not been comprehensively evaluated are (i) the competition of dissolved organic matter (DOM) and P for soil sorption sites, and (ii) the sorption of dissolved organic nitrogen (DON) relative to inorganic nitrogen species (e.g., NO(3)(-) and NH(4)(+)) and dissolved organic carbon (DOC). The competition between DOM and P for sorption sites has often been assumed to increase the amount of P available for plant growth; however, elevating DOM concentrations may also increase P available for transport to water resources. Batch sorption experiments were conducted to (i) evaluate soil properties governing P sorption to benchmark soils of Southwestern Missouri, (ii) elucidate the impact of poultry litter-derived DOM on P sorption, and (iii) investigate DON retention relative to inorganic N species and DOC. Soils were reacted for 24 h with inorganic P (0-60 mg L(-1)) in the presence and absence of DOM (145 mg C L(-1)) using a background electrolyte solution comparable to DOM extracts (I = 10.8 mmol L(-1); pH 7.7). Soil P sorption was positively correlated with metal oxide (r(2) = 0.70) and clay content (r(2) = 0.79) and negatively correlated with Bray-1 extractable P (r(2) = 0.79). Poultry litter-derived DOM had no significant negative impact on P sorption. Dissolved organic nitrogen was preferentially removed from solution relative to (NO(3)(-)-N + NO(2)(-)-N), NH(4)(+)-N, and DOC. This research indicates that poultry litter-derived DOM is not likely to enhance inorganic P transport which contradicts the assumption that DOM released from organic wastes increases plant-available P when organic amendments and fertilizer P are co-applied. Additionally, this work demonstrates the need to further evaluate the fate and transport of DON in agroecosystem soils receiving poultry litter applications.

Inhibition of calcium phosphate precipitation under environmentally-relevant conditions

Precipitation of Ca phosphates plays an important role in controlling P activity and availability in environmental systems. The purpose of this study was to determine inhibitory effects on Ca phosphate precipitation by Mg(2+), SO(4)(2-), CO(3)(2-), humic acid, oxalic acid, biogenic Si, and Si-rich soil clay commonly found in soils, sediments, and waste streams. Precipitation rates were determined by measuring decrease of P concentration in solutions during the first 60 min; and precipitated solid phases identified using X-ray diffraction and electron microscopy. Poorly-crystalline hydroxyapatite (HAP: Ca(5)(PO(4))(3)OH) formed in control solutions over the experiment period of 24 h, following a second-order dependence on P concentration. Humic acid and Mg(2+) significantly inhibited formation of HAP, allowing formation of a more soluble amorphous Ca phosphate phase (ACP), and thus reducing the precipitation rate constants by 94-96%. Inhibition caused by Mg(2+) results from its incorporation into Ca phosphate precipitates, preventing formation of a well-crystalline phase. Humic acid likely suppressed Ca phosphate precipitation by adsorbing onto the newly-formed nuclei. Presence of oxalic acid resulted in almost complete inhibition of HAP precipitation due to preemptive Ca-oxalate formation. Carbonate substituted for phosphate, decreasing the crystallinity of HAP and thus reducing precipitation rate constant by 44%. Sulfate and Si-rich solids had less impact on formation of HAP; while they reduced precipitation in the early stage, they did not differ from the control after 24 h. Results indicate that components (e.g., Mg(2+), humic acid) producing relatively soluble ACP are more likely to reduce P stability and precipitation rate of Ca phosphate in soils and sediments than are components (e.g., SO(4)(2-), Si) that have less effect on the crystallinity.

Phosphorus movement and speciation in a sandy soil profile after long-term animal manure applications

Long-term application of phosphorus (P) with animal manure in amounts exceeding removal with crops leads to buildup of P in soil and to increasing risk of P loss to surface water and eutrophication. In most manures, the majority of P is held within inorganic forms, but in soil leachates organic P forms often dominate. We investigated the mobility of both inorganic and organic P in profile samples from a noncalcareous sandy soil treated for 11 yr with excessive amounts of pig slurry, poultry manure, or poultry manure mixed with litter. Solution 31P nuclear magnetic resonance spectroscopy was used to characterize NaOH-EDTA-extractable forms of P, corresponding to 64 to 93% of the total P concentration in soil. Orthophosphate and orthophosphate monoesters were the main P forms detected in the NaOH-EDTA extracts. A strong accumulation of orthophosphate monoesters was found in the upper layers of the manure-treated soils. For orthophosphate, however, increased concentrations were found down to the 40- to 50-cm soil layers, indicating a strong downward movement of this P form. This was ascribed to the strong retention of orthophosphate monoesters by the solid phase of the soil, preventing orthophosphate sorption and facilitating downward movement of orthophosphate. Alternatively, mineralization of organic P in the upper layers of the manure-treated soils may have generated orthophosphate, which could have contributed to the downward movement of the latter. Leaching of inorganic P should thus be considered for the assessment and the future management of the long-term risk of P loss from soils receiving large amounts of manure.

Phosphorus leaching from cow manure patches on soil columns

The loss of P in overland flow or leachate from manure patches can impair surface water quality. We studied leaching of P from 10-cm-high lysimeters filled with intact grassland soil or with acid-washed sand. A manure patch was created on two grassland and two sand-filled lysimeters, and an additional two grass lysimeters served as blanks. Lysimeters were leached in the laboratory during 234 d with a diluted salt solution, and column effluent was passed through a 0.45-microm filter, analyzed for pH, dissolved reactive P (DRP), and total dissolved P (TDP). At the end of the experiment lysimeter soil was sampled and analyzed for pH, available P, and oxalate-extractable P, Fe, and Al. The concentration of TDP in the effluent from the sand column increased to 25 mg L-1 during the first weeks and remained above 10 mg L-1 during the rest of the percolation. In effluent from grass + patch lysimeters TDP gradually increased to 4 mg L-1. Both in the manure and in the effluent of the sand lysimeter P was found mainly in the form of DRP, but in the effluent from the grass lysimeters was found mainly as dissolved unreactive P (DUP=TDP-DRP). Earthworm activity was responsible for decomposition of the manure patch on the grass lysimeters. Manure patches and their remains were found to be a long-term source of high concentrations of P in leachates. Spreading of patches after a grazing period could reduce their possible negative impacts on the environment.

Phosphorus retention in calcareous soils and the effect of organic matter on its mobility

A survey of the interactions between phosphorus (P) species and the components of calcareous soils shows that both surface reactions and precipitation take place, especially in the presence of calcite and limestone. The principal products of these reactions are dicalcium phosphate and octacalcium phosphate, which may interconvert after formation. The role of calcium carbonate in P retention by calcareous soils is, however, significant only at relatively high P concentrations - non-carbonate clays play a more important part at lower concentrations. In the presence of iron oxide particles, occlusion of P frequently occurs in these bodies, especially with forms of the element that are pedogenic in origin. Progressive mineralization and immobilization, often biological in nature, are generally observed when P is added as a fertilizer. Manure serves both as a source of subsurface P and an effective mobilizing agent. Blockage of P sorption sites by organic acids, as well as complexation of exchangeable Al and Fe in the soil, are potential causes of this mobilization. Swine and chicken manure are especially rich P sources, largely due the practice of adding the element to the feed of nonruminants. Humic materials, both native and added, appear to increase recovery of Olsen P. In the presence of metal cations, strong complexes between inorganic P and humates are formed. The influence of humic soil amendments on P mobility warrants further investigation.

Phosphorus speciation in manure and manure-amended soils using XANES spectroscopy

Previous studies suggested an increase in the proportion of calcium phosphates (CaP) of the total phosphorus (P) pool in soils with a long-term poultry manure application history versus those with no or limited application histories. To understand and predict long-term P accumulation and release dynamics in these highly amended soils, it is important to understand what specific P species are being formed. We assessed forms of CaP formed in poultry manure and originally acidic soil in response to different lengths of mostly poultry manure applications using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Phosphorus K-edge XANES spectra of poultry manure showed no evidences of crystalline P minerals but dominance of soluble CaP species and free and weakly bound phosphates (aqueous phosphate and phosphate adsorbed on soil minerals). Phosphate in an unamended neighboring forest soil (pH 4.3) was mainly associated with iron (Fe) compounds such as strengite and Fe-oxides. Soils with a short-term manure history contained both Fe-associated phosphates and soluble CaP species such as dibasic calcium phosphate (DCP) and amorphous calcium phosphate (ACP). Long-term manure application resulted in a dominance of CaP forms confirming our earlier results obtained with sequential extractions, and a transformation from soluble to more stable CaP species such as beta-tricalcium calcium phosphate (TCP). Even after long-term manure application (> 25 yr and total P in soil up to 13,307 mg kg(-1)), however, none of the manure-amended soils showed the presence of crystalline CaP. With a reduction or elimination of poultry manure application to naturally acidic soils, the pH of the soil is likely to decrease, thereby increasing the solubility of Ca-bonded inorganic P minerals. Maintaining a high pH is therefore an important strategy to minimize P leaching in these soils.