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

The primary role of phosphate-metal- natural organic matter (humic) ternary complexes as drivers of long-term phosphorus plant nutrition in acidic soils

Several studies have shown that inorganic phosphate (Pi) can form stable complexes in soil and soil solution, with some fractions of natural organic matter (NOM), mainly humic substances, through metal bridges. However, the role of these types of complexes in plant phosphorus (P) nutrition remains unclear. On the other hand, Pi in acidic soils is rapidly absorbed and fixed, becoming unavailable for plants. In this framework, this study aims to unveil the role of Pi-metal-NOM (humic) complexes in plant P nutrition in acidic soils. To this end, we have selected nine acidic soils from Spain with different soil properties and nutrient availability. On these soils, we cultivated wheat (Triticum aestirum L., cv Fiel) for successive harvests until no new sprouts appeared. Pi fractions were characterized using two complementary Pi sequencing analyses adapted to acidic soils. The results showed that Pi-metal-NOM (humic) complexes are mainly extracted by oxalate-pH3 after extracting the more labile forms of Pi. Likewise, correlation and principal component analysis studies showed that the Pi extracted with water plus resin is linked to short-term wheat P-nutrition. On the contrary, the Pi extracted with oxalate (pH 3) (after extracting the more labile fraction of Pi) that contains the main fraction of Pi-metal-NOM (humic) complexes is mainly linked to long-term wheat P-nutrition. This fact indicates that the addition of humified organic matter to acidic soils can significantly improve the fraction of potentially available P for crops.

Colloid phosphorus transport in agricultural soils: a critical review of content characteristics, migration mechanisms, and environmental drivers

Soil phosphorus (P) loss through runoff is a significant contributor to waterbody eutrophication, particularly in regions of intensive agricultural production. One critical pathway for P loss is the migration of soil colloid-facilitated P (Pcoll), which warrants attention. However, the mechanisms underlying the generation, release, and migration of soil Pcoll remain poorly understood. This paper comprehensively reviews recent advancements in the technical methods for the extraction, separation, and physicochemical characterization of soil Pcoll. Utilizing data from published literature, we compile Pcoll contents across various soil types, with a focus on comparing Pcoll levels in agricultural soils characterized by different soil textures, fertilization treatments, and land use practices. Additionally, we analyze the relationships between soil main physicochemical parameters including pH, total organic carbon (TOC), total phosphorus (total P), and available phosphorus (available P), colloidal organic carbon (OC), calcium (Ca), magnesium (Mg), iron (Fe), and aluminum (Al) and Pcoll content. This review also summarizes the pathways of soil Pcoll loss, including surface runoff and subsurface runoff, along with the major influencing factors such as colloid properties, soil aggregates, pore structure, soil solution chemistry, crop planting structure, and fertilization management. Finally, we discuss future research directions concerning soil Pcoll, which include the establishment of standardized methods for Pcoll collection, a deeper understanding of Pcoll composition and distribution at the molecular level, investigations into the coupling effects of soil OC turnover and Pcoll cycling, and elucidating Pcoll loss mechanisms across multiple scales. Overall, this review aims to address existing gaps and shortcomings in soil Pcoll research and provide theoretical guidance for future studies.

Enhancing concurrent production of volatile fatty acids and phosphorus minerals from waste activated sludge via magnesium ferrate pre-oxidation

This study investigated the strategy of magnesium ferrate (MF) pre-oxidation to enhance acidogenic fermentation of waste activated sludge (WAS), targeting the simultaneous production of volatile fatty acids (VFAs) and phosphorus-minerals (struvite and vivianite). Results showed that such fermentation produced a high-value liquid within four days, achieving a peak VFA content of 241 ± 4 mg COD/ g VSfeed, ammonia nitrogen levels below 350 mg/L of and PO43-P under 3 mg/L. Further investigation revealed that the MF pre-oxidation raised the pH, enhanced key hydrolases activity, enriched acidogens and iron-reducing bacteria for driving the concurrent production of VFAs and phosphorus-minerals. The MF pre-oxidation promoted VFAs and phosphorus-minerals formation by enhancing the cooperation among the hydrolyzing bacteria of Acinetobacter and Proteocatella, acidogens of Fusibacter and Tissierella_Soehngenia, and iron-reducing bacteria of Dechloromonas and Thauera. This study provided an effective strategy for realizing concurrent production of high-purity VFAs along with struvite and vivianite from WAS.

Intertwined role of mechanism identification by DFT-XAFS and engineering considerations in the evolution of P adsorbents

Adsorption method exhibits promising potential in effectively removal of phosphate from wastewater, yet it faces tremendous challenges in practical application. Limited comprehension of adsorption mechanisms and the lack of evaluation method for scaling up application are the two main obstacles. To fully realize the practical application of P adsorbents, we reviewed advanced tools, including density functional theory (DFT) and/or X-ray absorption fine structure (XAFS) to elucidate mechanisms, underscored the significance of thermodynamics and kinetics in engineering design, and proposed strategies for regenerating and reusing P adsorbents. Specifically, we delved into the utilization of DFT and XAFS to gain insights into adsorption mechanisms, focusing on active site verification and molecular interaction configurations. Additionally, we explored precise calculation methods for adsorption thermodynamics and adsorption kinetics, encompassing thermodynamic equilibrium constants, reactor selection, and the regeneration, recovery, and disposal of P adsorbents. Our comprehensive review aims to serve as a guiding light in advancing the development of highly efficient P adsorbents for engineering applications.

Saltwater intrusion increases phosphorus abundance and alters availability in coastal soils with implications for future sea level rise

Sea level rise (SLR) promotes saltwater intrusion (SWI) into coastal soils globally at an increasing rate, impacting phosphorus (P) dynamics and adjacent water quality. However, how SWI influences P molecular speciation and availability in coastal soils remains poorly understood. By using a space-for-time substitution strategy, we evaluated the SWI impacts on P transformation along a SWI gradient at the Rehoboth Inland Bay, which consists of five sampling locations along a transect representing different SWI degrees. Soils were analyzed at the macro- and micro-scale using X-ray absorption near edge spectroscopy (XANES) and the modified Hedley fractionation. With increasing distance from the Bay, soil salinity (29.3-0.07 mmhos cm-1), the proportion of Fe3+ to total Fe, and P concentrations decreased. The fractionation showed that recalcitrant P was dominant (86.9-89.5% of total P). With increasing SWI, labile P increased gradually, reached a plateau, and then decreased sharply. Bulk XANES spectroscopy showed that soil P was likely dominated by iron and aluminum-associated P (Fe/Al-P), regardless of the SWI degree. Hence, with increasing SWI, P increasingly accumulated in a recalcitrant pool, mainly as Fe/Al-P. μ-XANES spectroscopy revealed that calcium-associated P (Ca-P) existed in P-rich spots of the greatest SWI soil while Al-P occurred in P-rich spots of the low SWI soil, consistent with the greater HCl-P (presumably Ca-P) in the former soil. Overall, results demonstrate that SWI impacts P availability and environmental risk in coastal soils depending on the degree of SWI. These findings have important implications for understanding soil P cycling and availability in SLR-impacted coastal areas.

Enhancing Phosphorus Release from Sewage Sludge in Anaerobic Digestion via Thermal Hydrolysis Pretreatment: Insights from Phosphorus Speciation and Molecular Biological Pathways

This study explores the mechanisms enhancing phosphorus (P) release from sludge in anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) using sequential chemical extraction, X-ray absorption near-edge structure spectroscopy (XANES), 31P NMR, and multiomics. THP-treated sludge notably increased liquid-phase P by 53.8% over 3 days compared to sewage sludge (SS), identifying solid-phase Fe-P as the primary P source. The THP+AD also provided a higher abundance of bacteria that contributed to P release through multiple pathways (MPRPB), whereas SS+AD enriched some microbial species with single P release pathway. Moreover, species co-occurrence network analysis underlined the pivotal role of P-releasing bacteria in THP+AD, with 8 out of 16 keystones being P-releasers. Among the 63 screened genes that were related to P transformations and release, the poly beta-hydroxybutyrate (PHB) synthesis genes associated with polyphosphate bacteria-mediated P release were more abundant in THP+AD than in SS+AD. Furthermore, the upregulation of genes involved in methyl phosphonate metabolism in the THP-treated sludge enhanced the methane production potential of the AD process. These findings suggested that MPRPB were indeed the main contributors to P release, and enrichment in the THP+AD process enhanced their capability for P liberation.

Missing phosphorus legacy of the Anthropocene: Quantifying residual phosphorus in the biosphere

A defining feature of the Anthropocene is the distortion of the biosphere phosphorus (P) cycle. A relatively sudden acceleration of input fluxes without a concomitant increase in output fluxes has led to net accumulation of P in the terrestrial-aquatic continuum. Over the past century, P has been mined from geological deposits to produce crop fertilizers. When P inputs are not fully removed with harvest of crop biomass, the remaining P accumulates in soils. This residual P is a uniquely anthropogenic pool of P, and its management is critical for agronomic and environmental sustainability. Managing residual P first requires its quantification-but measuring residual P is challenging. In this review, we synthesize approaches to quantifying residual P, with emphasis on advantages, disadvantages, and complementarity. Common approaches to estimate residual P are mass balances, long-term experiments, soil test P trends and chronosequences, with varying suitability or even limitations to distinct spatiotemporal scales. We demonstrate that individual quantification approaches are (i) constrained, (ii) often complementary, and (iii) may be feasible at only certain time-space scales. While some of these challenges are inherent to the quantification approach, in many cases there are surmountable challenges that can be addressed by unifying existing P pool and flux datasets, standardizing and synchronizing data collection on pools and fluxes, and quantifying uncertainty. Though defined as a magnitude, the distribution and speciation of residual P is relatively less understood but shapes its utilization and environmental impacts. The form of residual P will vary by agroecosystem context due to edaphoclimatic-specific transformation of the accumulated P, which has implications for management (e.g., crop usage) and future policies (e.g., lag times in P loading from non-point sources). Quantifying the uncertainty in measuring residual P holds value beyond scientific understanding, as it supports prioritization of monitoring and management resources and inform policy.

Ammonium addition reduces phosphorus leaching in a long-term mineral or organic fertilized calcareous soil during flooding conditions

Organic amendment substitutes mineral fertilizers has been proven to increase the organic matter content of soils, which in turn may induce phosphorus (P) mobilization by triggering the redox reaction. However, under flooded conditions according to local agricultural practices, as one of the factors restricting the decomposition of organic matter, the role ammonium plays in P transformation and leaching from soils with different organic matter remains unclear. To address the knowledge gap, the calcareous soils were collected from a long-term field trial (>13 years) containing two treatments with equal P inputs: a long-term mineral fertilization and a long-term organic amendment. Both long-term mineral fertilized soil and long-term organic amended soil were split into ammonium applications or no ammonium applications. A series of column devices were deployed to create flooded conditions and monitor the P leaching from the collected soils. The K-edge X-ray absorption near-edge structure and sequential extraction method were employed jointly to detect soil P fractions and speciation, and the P sorption/desorption characteristics of soil were evaluated by Langmuir fitting. The results showed a reduction of cumulative leached P from soils by 33.2%-43.3% after ammonium addition, regardless of previous long-term mineral fertilization or organic amendment history. A significant enhancement of soil labile P pool (indicated by the H2O-P fraction and NaHCO3-P fraction) after ammonium addition results in the reduction in soil P leaching. The reduced P sorption capacity coupled with the transformation from hydroxyapatite to β-tricalcium phosphate indicated that the phosphate retention is attributed to the precipitation formation rather than phosphate sorption by soil. The present study highlights that the ammonium addition could affect the phosphate precipitation transformation. This may be attributed to the effect of ammonium addition on the calcium and magnesium ion content and molar ratio in this soil, thereby regulating the form of soil phosphate precipitation. The mechanisms revealed in this study can support developing optimized agricultural management practices to alleviate soil P loss.

Remediation of neonicotinoid-contaminated soils using peanut shell biochar and composted chicken manure: Transformation mechanisms of geochemical fractions

Soil remediation techniques are promising approaches to relieve the adverse environmental impacts in soils caused by neonicotinoids application. This study systematically investigated the remediation mechanisms for peanut shell biochar (PSB) and composted chicken manure (CCM) on neonicotinoid-contaminated soils from the perspective of transformation of geochemical fractions by combining a 3-step sequential extraction procedure and non-steady state model. The neonicotinoid geochemical fractions were divided into labile, moderate-adsorbed, stable-adsorbed, bound, and degradable fractions. The PSB and CCM addition stimulated the neonicotinoid transformation in soils from labile fraction to moderate-adsorbed and stable-adsorbed fractions. Compared with unamended soils, the labile fractions decreased from 47.6% ± 11.8% of the initial concentrations to 12.1 ± 9.3% in PSB-amended soils, and 7.1 ± 4.9% in PSB and CCM-amended soils, while the proportions of moderate-adsorbed and stable-adsorbed fractions correspondingly increased by 1.8-2.4 times and 2.3-4.8 times, respectively. A small proportion (<4.8%) in bound fractions suggested there were rather limited bound-residues after 48 days incubation. The PSB stimulated the -NO2-containing neonicotinoid-degraders, which promoted the degradable fractions of corresponding neonicotinoids by 8.2 ± 6.3%. Degradable fraction of neonicotinoids was the dominant fate in soils, which accounted for 58.3 ± 16.7%. The findings made beneficial theoretical supplements and provided valuable empirical evidence for the remediation of neonicotinoid-contaminated soils.

Soil phosphorus cycling across a 100-year deforestation chronosequence in the Amazon rainforest

Deforestation of tropical rainforests is a major land use change that alters terrestrial biogeochemical cycling at local to global scales. Deforestation and subsequent reforestation are likely to impact soil phosphorus (P) cycling, which in P-limited ecosystems such as the Amazon basin has implications for long-term productivity. We used a 100-year replicated observational chronosequence of primary forest conversion to pasture, as well as a 13-year-old secondary forest, to test land use change and duration effects on soil P dynamics in the Amazon basin. By combining sequential extraction and P K-edge X-ray absorption near edge structure (XANES) spectroscopy with soil phosphatase activity assays, we assessed pools and process rates of P cycling in surface soils (0-10 cm depth). Deforestation caused increases in total P (135-398 mg kg-1 ), total organic P (Po ) (19-168 mg kg-1 ), and total inorganic P (Pi ) (30-113 mg kg-1 ) fractions in surface soils with pasture age, with concomitant increases in Pi fractions corroborated by sequential fractionation and XANES spectroscopy. Soil non-labile Po (10-148 mg kg-1 ) increased disproportionately compared to labile Po (from 4-5 to 7-13 mg kg-1 ). Soil phosphomonoesterase and phosphodiesterase binding affinity (Km ) decreased while the specificity constant (Ka ) increased by 83%-159% in 39-100y pastures. Soil P pools and process rates reverted to magnitudes similar to primary forests within 13 years of pasture abandonment. However, the relatively short but representative pre-abandonment pasture duration of our secondary forest may not have entailed significant deforestation effects on soil P cycling, highlighting the need to consider both pasture duration and reforestation age in evaluations of Amazon land use legacies. Although the space-for-time substitution design can entail variation in the initial soil P pools due to atmospheric P deposition, soil properties, and/or primary forest growth, the trend of P pools and process rates with pasture age still provides valuable insights.

Spatial variation of soil phosphorus in the water level fluctuation zone of the Three Gorges Reservoir: Coupling effects of elevation and artificial restoration

The water level fluctuation zone (WLFZ) is a distinctive and important component of the reservoir ecosystem. Due to periodic inundation, the fraction, spatial distribution, and chemical reactivity of soil phosphorus (P) within the WLFZ can potentially impact the loading of P into reservoir waters. However, a detailed study of this subject is lacking. In this study, the soil P in the WLFZ of the Three Gorges Reservoir, China, was examined using a combination of chemical sequential extraction, 31P NMR, and adsorption experiments. The results of chemical sequential extraction showed that HCl-Pi constituted the largest P pool among all P forms, with a mean concentration of 338 mg/kg. The content of HCl-Pi decreased significantly toward the dam, while the content of Res-P decreased in the opposite direction. The highest contents of most P forms and total P were observed at an elevation of 160 m. 31P NMR measurements showed that NaOH-EDTA Pi detectable in WLFZ soils at 145 m, 160 m, and 175 m elevation consisted mainly of orthophosphate and pyrophosphate, while NaOH-EDTA Po contained phosphate monoesters and phosphate diesters, accounting for 1.4 % to 46.2 % of NaOH-EDTA TP. Adsorption experiments showed that soil P in the WLFZ was a potential P source for reservoir waters, with chemisorption being the dominant mechanism of P sequestration. The adsorption equilibrium concentration of WLFZ soil was lower at higher elevations (>170 m) compared to lower elevations (<150 m), exhibiting a decrease in the average maximum adsorption from 271 mg/kg to 192 mg/kg. Statistical analysis suggested that Ca and Fe content, particle size, elevation, and artificial restoration were key factors affecting the fraction and content of soil P in the WLFZ. Our findings contribute to an improved understanding of the behavior of soil P in the WLFZ of large reservoirs and its potential contribution to the reservoir waters.

Organic/inorganic phosphorus partition and transformation in long-term paddy cultivation in the Pearl River Delta, China

Identification and quantification of different soil phosphorus (P) fractions level are important for improving agricultural productivity and developing sustainable management practices in these agricultural soils under long-term cultivation. However, few studies have been conducted to investigate P fractions level and their transformation in these soils. This study was conducted to characterize P fractions as affected by different paddy cultivation ages (200, 400-yr and 900-yr) among soils of the Pearl River Delta Plain in China. A sequential chemical fractionation scheme and ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) were employed to quantify various P fractions and speciation. Results showed soil easily-labile P, moderately-labile P and non-labile P had a positive relationship with total P (TP) and available P (AP). Analysis with ³¹P NMR spectroscopy revealed that inorganic P including orthophosphate (Ortho-P) and pyrophosphate (Pyro-P) increased with cultivation age, while organic species monoester phosphate (Mono-P) and diester phosphate (Diester-P) decreased. Moreover, acid phosphatase (AcP), neutral phosphatase (NeP), exchangeable Ca and sand contents are the main factors that affected the transformation of soil P composition, and non-labile P (Dil.HCl-Pi) and Pyro-P had significant contribution to soil P availability by affecting P activation coefficient. Therefore, long-term paddy cultivation, influenced by these soil parameters including NeP, AcP, exchangeable Ca and sand, accelerated the transformation of soil organic P/non-labile P to inorganic P.

X-ray absorption near edge structure spectroscopy reveals phosphate minerals at surface and agronomic sampling depths in agricultural Ultisols saturated with legacy phosphorus

Legacy phosphorus (P) soils have received excessive P inputs from historic manure and fertilizer applications and present unique management challenges for protecting water quality as soil P saturation leads to increased soluble P to waterways. We used P K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify and quantify the dominant P minerals in four representative legacy P soils under conventional till and no-till management in Maryland, USA. Various measures of extractable soil P, including water-extractable P (20.6-54.1 mg kg^-1 at 1:10 soil-to-water ratio; 52.7-132.2 mg kg^-1 at 1:100 soil-to-water ratio), plant available P extracted with Mehlich 3 (692-1139 mg kg^-1), and Mehlich 3P saturation ratio (0.54-1.37), were above the environmental threshold values, suggesting the accumulation of legacy P in soils. The quantification of dominant P minerals may provide insights into the potential of legacy P soils to contribute to P release for crop use and soluble P losses. Linear combination fits of XANES spectra identified the presence of four phosphate mineral groups, consisting of (i) calcium-phosphate minerals (11-59%) in the form of fluorapatite, β-tricalcium phosphate, and brushite, followed by (ii) iron-phosphate minerals (12-49%) in the form of ludlamite, heterosite, P sorbed to ferrihydrite, and amorphous iron phosphates, (iii) aluminum-phosphate minerals (15-33%) in the form of wavellite and P sorbed to aluminum hydroxide, and (iv) other phosphate minerals (5-35%) in the form of copper-phosphate (cornetite, 5-18%) and manganese-phosphate (hureaulite, 25-35%). Organic P consisting of phytic acid was found in most soils (13-24%) and was more pronounced in the surface layer of no-till (21-24%) than in tilled (16%) fields. Of the P forms identified with XANES, we conclude that P sorbed to Fe and Al, and Ca-P in the form of brushite and β-tricalcium phosphate will likely readily contribute to the soil WEP pool as the soil solution P is depleted by crop uptake and lost via runoff and leaching.

Phosphorus Acquisition and Utilization in Plants

Tremendous progress has been made on molecular aspects of plant phosphorus (P) nutrition, often without heeding information provided by soil scientists, ecophysiologists, and crop physiologists. This review suggests ways to integrate information from different disciplines. When soil P availability is very low, P-mobilizing strategies are more effective than mycorrhizal strategies. Soil parameters largely determine how much P roots can acquire from P-impoverished soil, and kinetic properties of P transporters are less important. Changes in the expression of P transporters avoid P toxicity. Plants vary widely in photosynthetic P-use efficiency, photosynthesis per unit leaf P. The challenge is to discover what the trade-offs are of different patterns of investment in P fractions. Less investment may save P, but are costs incurred? Are these costs acceptable for crops? These questions can be resolved only by the concerted action of scientists working at both molecular and physiological levels, rather than pursuing these problems independently.

Speciation Evolution of Phosphorus and Sulfur Derived from Sewage Sludge Biochar in Soil: Ageing Effects

Phosphorus (P) and sulfur (S) are usually involved simultaneously in the immobilization of heavy metals in sewage sludge during pyrolysis, and thus their speciation in sewage sludge-derived biochar (SSB) profoundly affects the recycling of the nutrients and the environmental risks of sewage sludge. Here, we investigated the speciation evolution of P and S in SSB induced by ageing processes in soil using X-ray absorption near edge structure spectroscopy. Results showed that Ca-bound compounds like hydroxyapatite dominated the P forms, while over 60% of S existed as reduced inorganic sulfides in the SSB. The stable Ca-associated P species in SSB tended to be transformed gradually into relatively soluble species during ageing in soil. The speciation composition of S in SSB remained almost unaffected when aged in pot soils, whereas about 33.6% of reduced sulfides were transformed into oxidized species after 1-year ageing in field soils. SSB significantly increased the proportion of sulfides and the contents of available P and S in the amended soil but showed relatively weak effects on the speciation distribution of P in the soil because of their similar compositions. These findings provide insights into biogeochemistry of nutrients and behaviors of heavy metals in SSB after its application to the soil environments.

Simultaneous recovery of vivianite and produce short-chain fatty acids from waste activated sludge using potassium ferrate as pre-oxidation treatment

Recovery resources from waste active sludge (WAS) is an effective way to alleviate the predicament of WAS disposal, and it is also conducive to the carbon neutralization of wastewater treatment systems. This study discussed the strategy of WAS anaerobic fermentation after pre-oxidation with potassium ferrate (K₂FeO₄, PF), which can simultaneously recover vivianite and enhance SCFAs production. The results showed that PF pre-oxidation considerably shortened the fermentation time of SCFAs to 2 days, and the main Fe-P mineral was vivianite. The optimal PF dosage of 0.06 g Fe (VI)/g TSS for pre-oxidation WAS resulted in the maximum SCFAs production and vivianite recovery rate of 3698.2 ± 118.98 mg COD/g VSS and 32.39%, respectively. The mechanism analysis showed that the oxidizing properties of PF significantly accelerated the disintegration of tight EPS, release of protein and sludge acidification efficiency. Moreover, the PF strengthened the transfer of P to the solid phase, forming the Fe-P mineral and unsaturated coordination state of phosphate group. Then the key microorganism Geobacter reduced the Fe³⁺ in Fe-P state to Fe²⁺ and combined unsaturated phosphate to form vivianite. This study provides an alternative method for resource recovery and environmentally friendly disposal of WAS and contributes to the carbon neutrality of urban water systems.

Systematic Study of Legacy Phosphorus (P) Desorption Mechanisms in High-P Agricultural Soils

Repeated manure additions containing phosphorus (P) in excess of crop needs have led to many agricultural soils with high levels of soil P (i.e., legacy P), particularly in the Delmarva region (USA). Due to the potential for P release, it is important to gain a better understanding of the mechanisms of P desorption and solubilization. Agricultural soils with high legacy P were collected from the Delmarva Peninsula, and soil P pools were determined using a suite of wet chemical and spectroscopic techniques, including a modified Hedley sequential extraction and X-ray absorption near-edge structure (XANES) spectroscopy. Five different desorption solutions were used to investigate P removal efficiency to assess release mechanisms. The results indicate that sulfate can have a stronger competition for P desorption than silicate, especially in the ditch sample with 21% labile P and 44% P adsorbed to iron and aluminum (via Hedley extraction). Additionally, linear combination fitting results of the ditch sample indicate 10.5% organic P and 73.9% P associated with iron and aluminum. This is an important finding because sulfate is a prevalent ion in sea water, and many agricultural soils with high legacy P in the Delmarva coastal area are threatened by sea level rise and inundation.

Enhanced phosphorus mobility in a calcareous soil with organic amendments additions: Insights from a long term study with equal phosphorus input

The agricultural practice of replacing chemical fertilizers with organic amendments (manure and/or straw) may have consequences for phosphorus (P) loss to the environment. Such a knowledge gap was examined using a ten-year field trial in calcareous soil containing four treatments with the equal annual P input but varied organic amendment combinations as follows: mineral fertilizer only as control (MF), mineral fertilizer coupled with manure (MM), mineral fertilizer coupled with manure and straw (MMS) and mineral fertilizer coupled with straw (MS). The soil P distribution, P fractions and speciation, Fe(III) reduction and P sorption kinetics were investigated using the chemical extraction, K edge X-ray absorption near-edge structure and Langmuir equations. The electronic shuttle capacity of soils and speciation of soil dissolved organic matter (DOM) were also evaluated using electrochemical methods, three-dimensional excitation-emission matrix fluorescence spectroscopy and Fourier transform infrared spectra methods. Results showed that soil Olsen-P and total P increased at depths of 20-40 cm in MM, MMS and MS treatments, suggesting that manure and/or straw addition significantly mobilized P in the soil profile. Manure and/or straw addition also decreased soil maximum P sorption capacity (S_max) and increased the desorption rate at depths of 0-20 cm in soil across treatments. At a depth of 0-20 cm in soil of the MS treatment, the enhanced Fe(Ⅲ) reduction coupled with a decrease of Fe-bound P supports that Fe reduction dominates the mobilization of P. The transformation of Ca bound-P to Al/Fe bound-P in a depth of 0-20 cm in soil of the MM treatment may be due to the high proportion of humic-like substances in the DOM at a depth of 0-20 cm in soil of the MM treatment, which may have caused a slight/microsite acidification. These results can help to develop optimized fertilization practices to effectively mitigate P loss from calcareous soils with manure and/or straw addition.

Flooding-induced inorganic phosphorus transformations in two soils, with and without gypsum amendment

Anaerobic conditions developed during flooding can increase phosphorus (P) losses from soils to waterways. Soil amendment with gypsum (CaSO₄ ·2H₂ O) can effectively reduce flooding-induced P release, but its effectiveness is soil dependent, and the reasons are poorly understood. The objectives of this study were to reveal the possible inorganic P transformations during flooding of two soils (acidic-Neuenberg sandy loam [NBG-SL] and alkaline-Fyala clay [FYL-Cl]), with and without gypsum amendment prior to flooding. Porewater samples collected at 0, 35, and 70 d after flooding (DAF) from soils incubated in vessels were analyzed for dissolved reactive P (DRP); pH; and concentrations of calcium (Ca), magnesium, iron (Fe), manganese, chloride, nitrate, sulfate, and fluoride. Thermodynamic modeling using Visual MINTEQ software and chemical fractionation of soil P were used to infer P transformations. Soil redox potential (Eh) decreased with flooding and favored reductive dissolution of Fe-associated P increasing porewater DRP concentrations. Greater solubility of Ca-P under acidic pH maintained a higher DRP concentration in NBG-SL during early stages of flooding. A subsequent increase in pH with flooding and higher Ca concentration with added gypsum enhanced the stability of Ca-P (β-tricalcium phosphate and octacalcium phosphate), reducing the DRP concentration in gypsum-amended NBG-SL. Stability of Ca-P was less affected with flooding and gypsum amendment in FYL-Cl soil because it had an alkaline pH and inherently higher Ca concentration. The FYL-Cl, with a more rapid decrease in Eh than NBG-SL, became severely reduced, releasing more P and Fe by 70 DAF. These conditions favored vivianite formation in FYL-Cl but not in NBG-SL.

Mobilization of soil phosphate after 8 years of warming is linked to plant phosphorus-acquisition strategies in an alpine meadow on the Qinghai-Tibetan Plateau

Phosphorus (P) is essential for productivity of alpine grassland ecosystems, which are sensitive to global warming. We tested the hypotheses that (1) mobilized 'calcium-bound inorganic P' (Ca-P_i ) is a major source of plant-available P in alpine meadows with alkaline soils after long-term warming, (2) mobilization of Ca-P_i is linked to effective plant carboxylate-releasing P-acquisition strategies under warming, and (3) the mobilization is also related to plant nitrogen (N)-acquisition. We conducted an 8-year warming experiment in an alpine meadow (4635 m above sea level) on the Qinghai-Tibetan Plateau. A significant increase in P concentration in both aboveground and belowground biomass indicates an increased mobilization and assimilation of P by plants under warming. We observed a significant decrease in Ca-P_i , no change in moderately-labile organic P, and an increase in highly resistant organic P after warming. There was no increase in phosphatase activities. Our results indicate that Ca-P_i , rather than organic P was the major source of plant-available P for alpine meadows under warming. Higher leaf manganese concentrations of sedges and forbs after warming indicate that carboxylates released by these plants are a key mechanism of Ca-P_i mobilization. The insignificant increase in Rhizobiales after warming and the very small cover of legumes show a minor role of N-acquisition strategies in solubilizing phosphate. The insignificant change in relative abundance of mycorrhizal fungi and bacteria related to P cycling after warming shows a small contribution of microorganisms to Ca-P_i mobilization. The significant increase in leaf N and P concentrations and N:P ratio of grasses and no change in sedge leaf N:P ratio reflect distinct responses of plant nutrient status to warming due to differences in P-acquisition strategies. We highlight the important effects of belowground P-acquisition strategies, especially plant carboxylate-releasing P-acquisition strategies on responses of plants to global changes in alpine meadows.

Soil acidification enhances the mobilization of phosphorus under anoxic conditions in an agricultural soil: Investigating the potential for loss of phosphorus to water and the associated environmental risk

Soil redox potential (E_H) and pH are key parameters regulating the solubility and fate of phosphorus (P). However, the impact of soil acidification on the redox-induced mobilization and speciation of P in soils under a wide range of E_H values has not been extensively studied. Here, we investigated the mobilization and speciation of P in an acidified agricultural soil at two different pH values (e.g., highly acidic soil; pH = 5.6 and slightly acidic soil; pH = 6.1) compared to the un-acidified soil (control soil; pH = 7.3) under a wide range of E_H condition (+459 to -281 mV). The impacts of E_H/pH-dependent changes of Fe-Mn oxides, and dissolved organic (DOC) and inorganic (DIC) carbon on P mobilization and speciation were also investigated using geochemical and spectroscopic (X-ray absorption near edge structure) techniques. The concentrations of dissolved P under anoxic conditions increased up to 69.3% in the highly acidic soil compared with the control soil. The decrease of the Fe-P fraction, the decrease of Ferrihydrite-P_ads speciation, and the strong linear correlation between the dissolved P and Fe²⁺ (R² > 0.85) supports the finding that enhanced P mobilization under anoxic conditions may be attributed to Fe reduction in the highly acidic soil. The concentration of dissolved Fe and P remained low until pH dropped below 6.35 for P and 6.28 for Fe, while a liner increase was found in dissolved Mn accompanying a general trend of pH decrease. This result suggests that the dissolution of reducible Mn under acidic soil conditions was an important factor for enhancing mobilization of dissolved P under anoxic conditions. This trend was due to the low amount of Mn, indirectly speeding up Fe reduction. These results can help to develop management practices to effectively mitigate P export and protect water resources from diffuse P pollution.

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.

Accessing Legacy Phosphorus in Soils

Repeated applications of phosphorus (P) fertilizers result in the buildup of P in soil (commonly known as legacy P), a large fraction of which is not immediately available for plant use. Long-term applications and accumulations of soil P is an inefficient use of dwindling P supplies and can result in nutrient runoff, often leading to eutrophication of water bodies. Although soil legacy P is problematic in some regards, it conversely may serve as a source of P for crop use and could potentially decrease dependence on external P fertilizer inputs. This paper reviews the (1) current knowledge on the occurrence and bioaccessibility of different chemical forms of P in soil, (2) legacy P transformations with mineral and organic fertilizer applications in relation to their potential bioaccessibility, and (3) approaches and associated challenges for accessing native soil P that could be used to harness soil legacy P for crop production. We highlight how the occurrence and potential bioaccessibility of different forms of soil inorganic and organic P vary depending on soil properties, such as soil pH and organic matter content. We also found that accumulation of inorganic legacy P forms changes more than organic P species with fertilizer applications and cessations. We also discuss progress and challenges with current approaches for accessing native soil P that could be used for accessing legacy P, including natural and genetically modified plant-based strategies, the use of P-solubilizing microorganisms, and immobilized organic P-hydrolyzing enzymes. It is foreseeable that accessing legacy P will require multidisciplinary approaches to address these limitations.

Speciation of Phosphorus from Suspended Sediment Studied by Bulk and Micro-XANES

Mobilization, transformation, and bioavailability of fluvial suspended sediment-associated particulate phosphorus (PP) plays a key role in governing the surface water quality of agricultural catchment streams. Knowledge on the molecular P speciation of suspended sediment is valuable in understanding in-stream PP cycling processes. Such information enables the design of appropriate catchment management strategies in order to protect surface water quality and mitigate eutrophication. In this study, we investigated P speciation associated with fluvial suspended sediments from two geologically contrasting agricultural catchments. Sequential chemical P extractions revealed the operationally defined P fractions for the fluvial suspended sediments, with Tintern Abbey (TA) dominated by redox-sensitive P (PCBD), Al, and Fe oxyhydroxides P (PNaOH) and organic P (POrg) while Ballyboughal (BB) primarily composed of acid soluble P (PDetr), redox-sensitive P (PCBD), and loosely sorbed P (PNH4Cl). The dominant calcareous (Ca) elemental characteristic of BB suspended sediment with some concurrent iron (Fe) influences was confirmed by XRF which is consistent with the catchment soil types. Ca-P sedimentary compounds were not detected using bulk P K-edge XANES, and only P K-edge µ-XANES could confirm their presence in BB sediment. Bulk P K-edge XANES is only capable of probing the average speciation and unable to resolve Ca-P as BB spectra is dominated by organic P, which may suggest the underestimation of this P fraction by sequential chemical P extractions. Notably, µ-XANES of Ca K-edge showed consistent results with P K-edge and soil geochemical characteristics of both catchments where Ca-P bonds were detected, together with calcite in BB, while in TA, Ca-P bonds were detected but mostly as organic complexed Ca. For the TA site, Fe-P is detected using bulk P K-edge, which corresponds with its soil geochemical characteristics and sequential chemical P extraction data. Overall, P concentrations were generally lower in TA, which led to difficulties in Fe-P compound detection using µ-XANES of TA. Overall, our study showed that coupling sequential chemical P extractions with progressively more advanced spectroscopic techniques provided more detailed information on P speciation, which can play a role in mobilization, transformation, and bioavailability of fluvial sediment-associated P.