Water-extractable P Research Articles

Potential phosphorus mobilization from riparian vegetation following freezing

Phosphorus (P) rich runoff from agricultural landscapes is a major contributor to freshwater eutrophication. Vegetated riparian zones are often employed to retain P from field runoff before it enters streams. In cold regions, vegetation has the potential to release P to runoff following freezing; however, it is unclear if this differs with winter frost severity, riparian zone local topography, vegetation type, and/or exposure to flooding/inundation. To explore the vulnerability of riparian vegetation to winter P losses, this study quantified soil and vegetation P concentrations from 8 riparian zones in Canada at different topographic positions within each riparian site (upper/field edge, lower/water edge) in both the fall and spring seasons and related this to observed surface temperatures and water levels throughout the non-growing season. This was complemented by two laboratory mesocosm experiments in which (1) plant samples were subjected to moderate (−40C) and severe (−250C) simulated winter frost treatments to quantify changes in their water extractable P (WEP) content, and (2) mesocosms were inundated with water to determine if dissolved P concentrations in flood water differed when plants were left intact, clipped but left on the soil surface, or harvested. Greater soil and vegetation P concentrations were observed at upper locations (field edge), and this remained consistent following freezing; however, vegetation WEP concentrations increased with greater simulated frost severity. In the field, temperatures were moderated by snow cover and although differences with riparian zone position were apparent between fall and spring collected samples, changes in P pools did not appear to be related to frost severity or inundation in the field. The mesocosm experiment revealed that harvesting vegetation considerably reduced dissolved P concentrations in flood water. This study shows that vegetated riparian zones can act as a source of P to streams during the winter non-growing season, and highlights the potential for riparian vegetation management in reducing P losses from riparian zones in cold climates.

Enhancing phosphorus availability in biochar: Comparing sulfuric acid treatment to biological acidification approaches

AbstractBackgroundThe use of sulfuric acid (SA) to acidify biochars is known to enhance their phosphorus (P) fertilizer value. Potentially, biological approaches such as lowering the pH of biochar by lactic acid co‐fermentation or applying biochar with a nitrification inhibitor (NI) to reduce rhizosphere pH are an alternative to SA.AimThis study aimed to evaluate the two methods for increasing plant P availability from two biochars and compare them with SA‐treated biochars (as a reference) in a pot experiment.MethodsMeat and bone meal biochar (MB‐C) and digestate solids biochar (DS‐C) were bio‐acidified (BA) by lactic acid fermentation with organic waste. The untreated, SA‐treated, BA biochars, and biochars co‐applied with a NI (3,4‐dimethylpyrazolephosphate) were tested in a pot experiment with maize.ResultsThe fermentation reduced the pH of the organic waste biochar mixtures to <4.3 and increased water‐extractable P (WEP) to 30% of total P. The untreated biochars had a mineral fertilizer replacement value of >50% and SA increased replacement values to ≈100%. The application of NI did not affect rhizosphere pH or P uptake. The BA MB‐C increased soil solution P concentration, but P uptake did not significantly increase. The application of the BA DS‐C raised soil pH and reduced plant P uptake and biomass.ConclusionThe untreated biochars showed considerable P fertilizer effectiveness, suggesting that acidification may not always be necessary. Rhizosphere acidification and the bio‐acidification of biochars were not effective in further increasing P uptake, despite higher levels of WEP.

Available and total phosphorus background levels in soils: a calcareous and semi-arid region

It is critical to understand the risk of element pollution in soils by evaluating their background levels. Phosphorus (P) content in agricultural soils needs to be assessed from agronomic and environmental standpoints. The current study intended to calculate the background levels of available and total P in soils. To achieve this goal, 50 sites without human activities were selected. Soils were sampled from the surface and subsurface of each site (100 soil samples). The available P forms in soils were extracted using the water-extractable P (WEP), calcium chloride-extractable P (CCEP), and Olsen-extractable P (OEP) methods. The first two extractants are being used to evaluate P leaching from soils, while the last one is being used as an agronomic indicator. The methods used to calculate background levels were the iterative 2-δ technique (2-δ) and the calculated distribution function (CDF). Results showed that the upper limits of background levels using 2-δ method were 1.45, 0.92, 8.12, and 424.4mgkg-1 for WEP, CCEP, OEP, and total P, respectively. Also, the upper limits of background levels using CDF method were 1.42, 1.15, 12.09, and 447.6mgkg-1, for WEP, CCEP, OEP, and total P, respectively. It can be concluded that using these background levels, which for the first time were calculated for P, would enable us to have an accurate examination of P excess as a result of human activities.

Factors controlling shallow subsurface dissolved reactive phosphorus concentration and loss kinetics from poorly drained saturated grassland soils.

Shallow subsurface pathways dominate dissolved reactive phosphorus (DRP) losses in grassland soils that are: poorly drained, shallow, or have a perched water table in wetter months causing saturation-excess runoff. Saturated conditions can lead to anoxia, which can accelerate phosphorus (P) loss. Two scales of investigation were utilized in this study. First, at the field scale, soil cores were extracted to 2.5m, subdivided and samples extracted using water extractable P (WEP) and sodium-bicarbonate-dithionite extractable P (NaBD-P). Second, at the laboratory scale, detailed incubation studies using field-moist grassland topsoils from sites in Ireland and New Zealand examined the kinetics of WEP under anoxic (WEPanox ) and oxic (WEPox ) conditions with imposed temperature and soil P fertilizer input treatments. Results from soil-core samples showed that redox-sensitive NaBD-P concentrations were depleted where artificial drainage lines were installed (100cm deep), but WEP concentrations available to shallow flow were enriched in topsoil. The laboratory scale incubation experiment investigated the influence of temperature (3vs. 18 °C), anoxia (designed to simulate saturation following a rainfall event), and superphosphate fertilizer (10 to 60kg P ha-1 yr-1 ) on WEP concentrations over 24h in three grassland topsoils (clay, silt, and sandy loam textures). Concentrations increased with fertilizer rate, temperature, and-in two soils-anoxic conditions. This was commensurate with nitrate (NO3 - ) depletion and the reductive dissolution of iron and manganese. The release of P during anoxia was complete within 24h. The results highlighted late winter to spring as the riskiest period for topsoil P losses in shallow subsurface flow due to wet soil conditions, increasing temperatures, and low soil NO3 - concentrations. This knowledge highlights the necessity to consider and refine tests used to assess topsoil P loss risk, where in the landscape P losses are likely, and what strategies can be used to mitigate losses.

Losses of phosphorus, potassium and nitrogen from horse manure left on the ground

ABSTRACT In this five-month Swedish field study, we examined losses of nutrients from horse manure over time, in order to examine how regularly manure should be cleared from paddocks in order to minimise the risk of nutrient leaching. Small heaps of manure (400 g) were placed in open cylinders outdoors and samples (five replicates) were taken on 12 occasions from December 2020 to May 2021. The samples were analysed for weight, dry matter content and concentrations of total nitrogen (N), ammonium N, total phosphorus (P), water-extractable P (WEP), potassium (K) and carbon (C). There was a fast decline in P and K concentrations and a strong correlation between accumulated precipitation and losses from the manure into the soil. The mean reduction in total-P was 11 mg P kg−1 manure dry weight per mm accumulated precipitation. Manure N was retained in the manure over the five-month period. In conclusion, this study demonstrated high mobility of P and K, indicating a need for strategies for rapid removal of manure from paddocks. Daily removal of manure from paddocks used year-round would, approximately, save 1.7 kg P and 5.5 kg K per horse per year, which could be recycled to replace non-renewable mineral fertilisers.

Fate and transformations of dissolved phosphorus forms in runoff: Effect of poultry litter and products extracted with variable water extraction ratios

In many intensive animal production areas, the over-application of manure has resulted in a build-up of soil phosphorus (P) and the creation of legacy P soils that threaten water quality. We investigated dissolved P forms losses in runoff using simulated rainfall in packed soil boxes amended with three poultry litter and products, including raw (unprocessed) litter, granulated litter with the addition of urea, and heated raw litter. These were applied at 3 kg water-extractable P (WEP) ha−1 as determined with three litter-to-water extraction ratios (1:10, 1:100, and 1:200). Over three simulated rainfall events, the amount of dissolved reactive P (DRP) lost was significantly greater in runoff from soils amended with granulated litter (1.09 ± 0.02 kg ha–1) than raw (0.81 kg ha−1) and heated (0.58 kg ha−1) litters. No significant differences in the amount of dissolved unreactive P (DUP) in runoff (0.38 ± 0.07 kg ha−1) were observed among three litter amended soils. The soil test P (i.e., Mehlich 3–P) increased from 6.9 mg kg−1 in control to 10.4–11.6 mg kg−1 in litter amended soils, whereas the total WEP (0.26 ± 0.03 mg kg−1) in soils was similar after three rainfall simulation events. We conclude that (1) an accurate litter-to-water extraction ratio (>1:200) is critical to determine the amount of WEP in manure as it will ensure similar amounts of soluble P application and will result in identical runoff losses of dissolved P, and (2) the granulation and heating of litter created a product that could enhance the use of poultry litter, especially in non-agricultural markets, resulting in sustainably using manure and reducing the risk of P loss to water bodies.

Application of Fourier transform mid-infrared photoacoustic spectroscopy for rapid assessment of phosphorus availability in digestates and digestate-amended soils

Digestate is the anaerobic digestion by-product of biogas production that can be used as a phosphorus (P) fertilizer. To achieve the efficient utilization of digestate as a P fertilizer and evaluate P availability in digestate-amended soils, it is necessary to assess both available P in different digestates and digestate-amended soils. In this study, Fourier transform mid-infrared photoacoustic spectroscopy (FTIR-PAS) combined with multivariate analysis was applied to predict water-extractable P (WEP) in digestates and plant-available P in digestate-amended soils. The plant-available P was determined by the diffusive gradients in thin films (DGT) technique. 45 digestate samples were collected both from laboratory-scale digesters (26 samples) and operating biogas plants (19 samples) in Denmark for WEP determination. Three soils amended with the collected 19 digestate samples from biogas plants (that results to 57 digestate-amended soil samples in total) were deployed for DGT measurement of plant- available P. The WEP predicting model had a coefficient of determination (R2) of 0.80 and a root mean square error of 0.78 g kg−1 while the plant-available P predicting model exhibited an R2 of 0.70 and a root mean square error of 134.09 μg P L−1. Furthermore, regression coefficients with a significant contribution of the plant-available P predicting model were identified, indicating that FTIR-PAS is capable for correlating spectra information with plant-available P related chemical bonds. In conclusion, FTIR-PAS can be used as a faster and non-destructive alternative for the assessment of both WEP in digestates and plant-available P in digestate-amended soils.

Comparison of soil phosphorus index systems for grassland in the cross‐border region of Ireland#

AbstractBackgroundThe use of soil phosphorus (P) tests and index systems provides a guide for agronomic nutrient requirements and is frequently also used to estimate risk of P losses to watercourses. Use of soil testing and management based on the results thereof is mandated in some regions. Several P extraction methods are available which evaluate different P pools and are designed for particular soil types. Further to this, index systems categorising specific ranges of plant‐available P, differ. Hence, translation between different tests and index systems is not straightforward. In cross‐border regions, where hydrologic basins encompass more than one political jurisdiction, different tests and rules are implemented in adjacent lands. This can create disparities in land management, confusion as to what legislation applies and obscures the impacts of best management practices at catchment scale.AimsThe aim of this research was to compare the Morgan's and Olsen soil tests used to quantify plant‐available P and the respective index systems, in a border region of the Republic of Ireland (ROI)–Northern Ireland (NI).MethodsOlsen, Morgans and water extractable P (WEP) were evaluated (n = 1,038). Statistical analysis was conducted to derive conversion equations to translate between the statutory test methods and comparison of the respective index categories was performed.ResultsThe conversion equations compared favourably with previous attempts. A stronger relationship was observed between Morgan P and WEP (R2 = 0.60) than between Olsen P and WEP (R2 = 0.45) (including pH and site as interaction factors). The ROI index system was found to indicate lower levels of plant available P in the soil compared to the NI system, for the same soils.ConclusionsThe differences in categorisation of P availability using either index system creates differences in fertiliser recommendations and also perceived aquatic risks even within small cross‐border catchments. This study points to a wider implication for international cross‐border catchments, suggesting that evaluation of the relationships between adjacent national soil index systems is required to achieve harmonised management of shared waterbodies. Neither index system is preferred, but a combination of soil P tests incorporating both agronomic and environmentally oriented analyses may have utility in future decision support tools.

Contribution of bunker silo effluent discharged via a riparian zone to watershed phosphorus loads

Nutrient losses from agricultural operations are a major contributor to the eutrophication of freshwaters. Although many studies have quantified diffuse nutrient losses, less is known about agricultural point-source contributions, such as bunker silos, to watershed phosphorus (P) loads. This study examined the contributions of a dairy farm bunker silo effluent to watershed soluble reactive P (SRP) and total P (TP) losses. The bunker silo effluent discharged to an adjacent stream via a riparian soakaway for ca. 15 years. Prior to the annual refilling of the bunker silo, flow weighted mean concentrations of SRP (TP) were similar between stream locations up and downstream of the farm. After the bunker silo was refilled, flow-weighted SRP (TP) concentrations in the stream increased by factors of 1.5(2.2) during events and 3.1(2.3) during baseflow. Higher P concentrations occurred in the riparian soils receiving bunker silo effluent (525–3125 mg/kg TP, and 0.1–9.9 mg/kg water extractable P (WEP), compared with 525–939 mg/kg TP, and 0.11–1.43 mg/kg WEP on the opposite side of the stream with no bunker silo effluent. Riparian soils impacted by the bunker silo were near P-saturation, and the riparian zone did little to reduce P transfer in shallow groundwater. The net contributions of bunker silo effluent to annual watershed P losses were 32% (SRP) and 22% (TP). This study highlights the importance of agricultural point sources, and the need to quantify their contributions to watershed P budgets to target P remediation effectively.

Hotspots of Legacy Phosphorus in Agricultural Landscapes: Revisiting Water-Extractable Phosphorus Pools in Soils

Controlling phosphorus (P) losses from intensive agricultural areas to water bodies is an ongoing challenge. A critical component of mitigating P losses lies in accurately predicting dissolved P loss from soils, which often includes estimating the amount of soluble P extracted with a laboratory-based extraction, i.e., water-extractable P (WEP). A standard extraction method to determine the WEP pool in soils is critical to accurately quantify and assess the risk of P loss from soils to receiving waters. We hypothesized that narrower soil-to-water ratios (1:10 or 1:20) used in current methods underestimate the pool of WEP in high or legacy P soils due to the equilibrium constraints that limit the further release of P from the solid-to-solution phase. To investigate P release and develop a more exhaustive and robust method for measuring WEP, soils from eight legacy P fields (Mehlich 3–P of 502 to 1127 mg kg−1; total P of 692 to 2235 mg kg−1) were used for WEP extractions by varying soil-to-water ratios from 1:10 to 1:100 (weight:volume) and in eight sequential extractions (equivalent to 1:800 soil-to-water ratio). Extracts were analyzed for total (WEPt) and inorganic (WEPi) pools, and organic (WEPo) pool was calculated. As the ratios widened, mean WEPi increased from 23.7 mg kg−1 (at 1:10) to 58.5 mg kg−1 (at 1:100). Further, WEPi became the dominant form, encompassing 92.9% of WEPt at 1:100 in comparison to 79.0% of WEPt at 1:10. Four of the eight selected soils were extracted using a 1:100 ratio in eight sequential extractions to fully exhaust WEP, which removed a cumulative WEPt of 125 to 549 mg kg−1, equivalent to 276–416% increase from the first 1:100 extraction. Although WEP concentrations significantly declined after the first sequential extraction, WEP was not exhausted during the subsequent extractions, indicating a sizeable pool of soluble P in legacy P soils. We conclude that (i) legacy P soils are long-term sources of soluble P in agricultural landscapes and (ii) the use of a 1:100 soil-to-water ratio can improve quantification and risk assessment of WEP loss in legacy P soils.

Novel approaches to investigating spatial variability in channel bank total phosphorus at the catchment scale

Phosphorus (P) is often a limiting nutrient that leads to the eutrophication of aquatic systems. While dissolved P forms are the most bioavailable, the form, mobility, transport and fate of P are directly related to its association with fine-grained riverine sediment. Therefore, to implement successful P catchment management strategies it is important to understand the relative contribution of different sediment sources to P loads across the river continuum. While agricultural topsoil and, to a lesser extent, riverbed sediment are important sources of sediment-associated P, channel banks have been shown to be an important sediment source in some catchments. However, comparatively little is known about the P concentration and corresponding spatial variability in channel bank sediment and the associated implications for catchment management. The present study examines the spatial variability of P associated with channel bank profiles within a series of three nested catchments using both non-spatial and spatial statistical methods, where for the latter, a novel spatial approach was used to estimate the spatial averages and variances of P in channel bank sediment along the stream network. Channel bank P concentrations were compared to factors such as catchment scale, stream order, land use, bank exposure and location along the stream network. Concentrations of TP ranged between 129.6 and 1206.9 mg P kg−1 of which the water extractable P (WEP) content ranged from 0.01 to 0.12%. Stream order was found to influence TP concentrations, while land use and catchment scale provided only a moderate influence. This suggested that focussing channel bank sampling strategies at the largest catchment scale would capture key drivers of TP variability provided stream order is sufficiently represented. Whether the bank was had limited vegetation and was exposed and potentially eroding had a slight influence on TP variability in second-order stream banks in the larger of the two nested catchments. However, the slightly lower TP concentrations measured at these sites indicated that banks that are actually eroding may be contributing less TP than the total channel bank TP values measured across the catchments as a whole. The results of an explicitly spatial analysis demonstrated that local channel bank TP averages and TP variances vary along the stream network. Specifically, the most accurate spatial predictor of TP was local TP means with the use of ‘crow flies’ rather than stream network distances. Local TP variances were used to provide optimal designs for future channel bank TP sampling campaigns, given available resources. Throughout, both standard and outlier-resistant statistical analyses were applied to improve interpretation of the study findings.

Soil Phosphorus Speciation and Availability in Meadows and Forests in Alpine Lake Watersheds With Different Parent Materials

In the Lake Tahoe Basin in California and Nevada (USA), managing nutrient export from watersheds into streams and the lake is a significant challenge that needs to be addressed to improve water quality. Leaching and runoff of phosphorus (P) from soils is a major nutrient source to the lake, and P loading potential from different watersheds varies as a function of landscape and ecosystem properties, and how the watershed is managed. In this research, P availability and speciation in forest and meadow soils in the Lake Tahoe Basin were measured at two watersheds with different parent material types. Soils developed on andesitic parent materials had approximately twice as much total P compared to those developed on granitic parent materials. Regardless of parent material, organic P was 79–92% of the total P in the meadow soils, and only 13–47% in the forest soils. Most of the soil organic P consisted of monoester P compounds, but a significant amount, especially in meadow soils, was diester P compounds (up to 30% of total extracted P). Water extractable P (WEP) concentrations were ~10 times greater in the granitic forest soils compared to the andesitic forest soils, which had more poorly crystalline aluminosilicates and iron oxides that retain P and thus restrict WEP export. In the meadow soils, microbial biomass P was approximately seven times greater than the forest soils, which may be an important sink for P leached from upland forests. Results show that ecosystem and parent material are important attributes that control P speciation and availability in the Lake Tahoe Basin, and that organic P compounds are a major component of the soil P and are available for leaching from the soils. These factors can be used to develop accurate predictions of P availability and more precise forest management practices to reduce P export into Lake Tahoe.

Characterization of Different Phosphorus Forms in Flooded and Upland Paddy Soils Incubated with Various Manures.

Phosphorus (P) is an essential nutrient for crop production, and animal manures are rich in P. When using animal manures as alternatives to synthetic fertilizers, it is important to know the kinetics of P release from different animal manures and the forms, amounts, and dynamics of P in manure-treated soils. We chose four types of manure, viz., pig manure (PM), chicken manure (CM), dairy manure (DM), and commercial organic compost (OM), and evaluated the P release rate and availability in water solution and flooded/upland paddy soils. The WEP/total P (TP) and the water-extractable P (WEP) concentrations are highest for OM with the order: OM > PM > CM > DM. An increase in soil Olsen-P concentration was observed for the addition of manure with a varying application rate of P from low to moderate to high. The release capacity of Olsen-P in flooded conditions was higher than that in upland conditions. Under the flooded soil, PM and OM have faster release rates than CM and OM in the upland soil. Moreover, PM significantly increased available P by 29% in the flooded paddy soil while moderately inorganic P increased by 17% in the upland paddy soil. Olsen-P has a significant linear relationship with available P (Resin-P + NaHCO3-Pi; R2 = 0.104; P < 0.01) and moderately inorganic P (NaOH-Pi + HCl-P; R2 = 0.286; P < 0.01). The structural equation model showed that the organic input was beneficial to the conversion of moderately inorganic P to available P. Our results indicate that PM amendment promotes the release of available P in paddy soil.

Amending soils of different pH to decrease phosphorus losses

Context Soils irrigated with wastewater are generally phosphorus (P)-enriched. P losses from these soils may impair surface water quality. However, wastewater applications also alter soil pH and P availability. Aims We investigated if amending soils with aluminium (Al), iron (Fe) or calcium (Ca) sorbents could decrease the potential for P losses despite altering soil pH and potentially increasing soil P availability. Methods Seven soils (pH 5.3–6.9) were incubated with lime, gypsum, hydrotalcite, alum sulfate, ferric sulfate, and ferric chloride at rates of 0:1, 0.25:1, 0.5:1 and 1:1 molar ratios of Al/Fe to P, and 0:1, 0.5:1, 1:1 and 5:3 for Ca to P, respectively. After 21 days pH and water extractable P (WEP) were measured. Key results In most cases the application of Al, Fe and Ca amendments decreased WEP in proportion to the rates applied. However, poor performance was noted when amendments were mismatched to soils altering their pH into the range where high soil P availability was expected. Of the amendments used, alum and iron sulfate were the most cost-efficient. However, even when optimised and applied to critical source areas the estimated cost-effectiveness of these amendments is still poor and may only be effective in the short term. Conclusions and implications We therefore recommend that other strategies such as inversion tillage bringing low P topsoil to the surface (and decreasing the potential for P loss by surface runoff) together with changes in the farm system to extract more P from the topsoil are the only strategies that will decrease the potential for P loss cost-effectively and in the long-term.

Characteristics of solids produced from coal mine drainage and their suitability for phosphorus control in dairy manure management.

One strategy to limiting eutrophication in waterways is to reduce the concentration of water-extractable P (WEP) in land-applied manure. In this study, we investigated the feasibility of using mine drainage residuals (MDR), waste solids produced in large quantities from coal mine drainage treatment, to reduce WEP in dairy manure. Twenty MDRs from treatment systems in Pennsylvania were collected and analyzed to determine concentrations of pollutants that may limit land application. Laboratory dose-response tests were conducted using the selected MDRs to determine the effectiveness and kinetics of WEP reduction, and three field-scale MDR application tests were conducted to demonstrate the process of using MDR to decrease manure WEP. The MDR-manure mixtures investigated in this study do not exceed biosolid land application concentration limits set by the USEPA. Amendment rates of 5-10g MDR L-1 of manure provided significant reductions in WEP. Iron-rich MDR, produced from passive and oxidant treatment of mine drainage, required 1-4 d to reduce WEP to an equilibrium concentration, while Ca-rich materials, produced from lime treatment, required 4-7 d. Three field studies at operating dairy farms confirmed the reduction in WEP when manure was amended with MDR. Unit costs calculated for a 1,900-m3 manure tank treated with 4.4g L-1 MDR were US$2.16 per 1,000 L of manure and $30kg-1 WEP removed. These findings indicate that the WEP of dairy manure is not a fixed chemical parameter and can be modified with amendments such as MDR.

Effect of organic and inorganic phosphorus fertilizers on phosphorus availability and its leaching over incubation time.

The use of organic and inorganic phosphorus (P)fertilizers in agricultural soils is very common, and few studies have been conducted to study the effect of different P sources on relative P extractability (RPE) and leaching using different P extractants and degree of P saturation (DPS), over a long period of time. Thus, this study was conducted to investigate the effect of incubation time and different P sources on RPE, DPS, and to predict the concentration of P leached from soil using different P extractants. In order to achieve these goals, nine sewage sludges (SSs), two biochars, animal manure (AM), poultry manure (PM), wheat residue (WR), diammonium phosphate (DAP), and triple superphosphate (TSP) were added to the soil as much as 100mg P kg-1 in a 163days incubation experiment. On average across all amendments and incubation periods, Mehlich-3 extractable P (M3EP) gave the highest mean RPE (42.9%, SE = 7.1%), with water-extractable P (WEP) the lowest (4.6%, SE = 0.93%), and Olsen-extractable P (OEP) (38.3%, SE = 6.3%) in between. Among SSs and based on average across of all incubation periods, soils treated with Shiraz and Takestan SSs were the least soluble source of P, while the highest soluble source of P were soils treated with Kermanshah and Tehran SSs. The results indicated that soil samples taken 16days following the addition of amendments should reflect agronomic and environmental purposes aiming to assess available and the potential P loss from agricultural soils. The split line model perfectly fitted to the relation between OEP and M3EP (r = 0.93). The DPSs were calculated and the P leaching rate was estimated. Based on OEP, the soils treated with TSP and DAP were at high risk, the medium risk was for soils treated with Kermanshah, Saveh, Tehran, Rasht, Sanandaj, and Isfahan SSs, and PM. Control soil, and soils treated with WRwere at no risk, and the soils treated with Arak, Shiraz, and Takestan SSs, ABC, WBC, and AM were classified as low risk.

Short-Term Effect of the Addition of Rice Husk Gasification Slag on the Movement and Transformation of Phosphorus in Different Soil Types

Rice husk gasification slag (RS) is a type of biochar that is one of the main by-products generated from the production of biomass power with rice husk as the feed. This study aimed to explore the short-term effect of the application of RS on the movement and transformation of fertilizer P in two different soil types through an incubation experiment. The results showed that the RS addition had a significant influence on the diffusive movement of P in soil microsites close to fertilizer placements both in latosolic red soil and fluvo-aquic soil. After 50 d of incubation, most of the WE-P (water-extractable P), AE-P (acid-extractable P), and Olsen-P (available P) were concentrated within 0–5 mm from the fertilization site. WE-P, Olsen-P, and the movement amount of the P in the 0–5 mm soil section were significantly increased at all levels of the RS application in the fertilizer P both in the two soil types. The application of the RS reduced the sorption and precipitation of the fertilizer P in the soil and improved the efficiency of the fertilizer P. The findings presented in this study may be used as references in developing RS applications that reduce losses of fertilizer P and reduce environmental risks.

The Ability to Reduce Soil Legacy Phosphorus at a Country Scale

The build-up of soil phosphorus (P) beyond plant requirements can lead to a long-term legacy of P losses that could impair surface water quality. Using a database of ~450,000 samples collected from 2001-2015 we report the level of soil P enrichment by soil type, land use and region and the time it would take for Olsen P to decline to agronomic targets (20-40 mg L-1) if P fertiliser was stopped. We also modelled the time it would take for water extractable P (WEP), an indicator of P losses in surface runoff, to decline to an environmental target (0.02 mg L-1). Some 63% of the samples were enriched beyond agronomic targets. The area-weighted median time to reach the agronomic target was predicted to occur within a year for 75% of samples but varied up to 11.8 years in some land uses. However, the area-weighted time to reach an environmental target was 26-55 years for the 50th and 75th percentile of areas. This indicates that while an agronomic target can be easily met, additional strategies other than stopping P fertiliser inputs are required to meet an environmental target.

Species and termination method effects on phosphorus loss from plant tissue.

Cover crops are often recommended as a best management practice to reduce erosion, weed pressure, and nutrient loss. However, cover crops may be sources of phosphorus (P) to runoff water after termination. Two greenhouse trials were conducted to determine the effects of cover crop species, termination method, and time after termination on water-extractable P (WEP) release from crop biomass. Treatments were structured in a 3 × 3 × 3 factorial and arranged in a randomized complete block design with six replicates. Treatments included three cover crop species (triticale [× Triticosecale; Triticum × Secale 'Trical'], rapeseed [Brassica napus L. 'Winfred'], and crimson clover [Trifolium incarnatum L.]); three termination methods (clipping, freezing, and herbicide); and three WEP extraction times (1, 7, and 14 d after termination). Rapeseed consistently resulted in the least WEP when exposed to the same method of termination and at the same extraction time as the other species. For both trials, terminating crop tissue via freezing increased concentrations of WEP compared with other termination methods. The WEP release from cover crop tissue increased as the time after extraction increased, but the effect was greater for herbicide- and freeze-terminated cover crops and less for clipping-terminated cover crops. Future studies on WEP release from cover crops should pay close attention to the effects of extraction timing. Producers may be able to reduce P loss from cover crop tissue by selecting cover crop species with low WEP and minimizing the amount of biomass exposed to freezing conditions.

Runoff and Leachate Phosphorus and Nitrogen Losses from Grass‐Vegetated Soil Boxes Amended with Biosolids and Fertilizer

Recent evidence suggests an upward trend in surface water phosphorus (P) concentrations in many segments of Florida, including the upper basin of the St. Johns River, a region that currently receives about two-thirds of the state Class B biosolids land application. Concerns about water quality in this area are encouraging reexamination of the regulations governing biosolids programs. The objectives of this study were (i) to identify and thoroughly characterize the main biosolids sources routinely applied in the region, and (ii) to evaluate runoff and leachate N and P losses from a typical Florida Spodosol amended with biosolids or commercial inorganic fertilizer. Biosolids and inorganic fertilizer were surface applied uniformly at a rate equivalent to ∼114 kg P ha, which corresponded to a typical P load associated with nitrogen (N)-based biosolids application. Soluble reactive P (SRP) was the predominant form of P lost in runoff and leachate. Inorganic P fertilizer increased flow-weighted runoff total P concentrations nearly 60-fold relative to control treatment (0.4 vs. 22 mg P L for control and fertilizer treatments, respectively). With exception of biological P removal (BPR) biosolids, all other tested biosolids yielded flow-weighted runoff P concentrations similar to untreated soils. Cumulative P and N losses (as a percentage of P and N applied) were greater from commercial inorganic fertilizer (∼38% of P and 46% of N) than any biosolids source (3% of P and 6% of N). Results demonstrate the value of water-extractable P (WEP) as an indicator of biosolids P loss potential.