Purpose of Review Phosphorus (P) loss from agricultural land to surface water is a leading cause of water quality deterioration. We reviewed the climate change impacts on sources and transport of P and how they can exacerbate P loss from agricultural soils to waterways. Recent Findings The effects of climate change include extreme precipitation events, increased temperature, elevated atmospheric carbon dioxide (eCO 2 ), and saltwater intrusion induced by sea level rise. Extreme precipitation (EP) events cause accelerated transport of dissolved and particulate P from soils, exacerbated after the application of fertilizers and manures or drought. The unpredictability of EP leads to greater incidental P losses as appropriately timing nutrient applications is more challenging. Increased soil and air temperatures influence soil microbial communities and P-solubilizing microbes, but their effects on P losses are uncertain. Likewise, eCO 2 may increase plant growth, P demand, and soil P cycling, but its impact on P losses is unclear. Saltwater intrusion caused by sea level rise can further mobilize P in high (legacy) P soils and enhance P loss from land to water. Summary Climate change is likely to increase P losses due primarily to increases in extreme precipitation and saltwater intrusion in coastal areas. These impacts will be geographically variable. Current P loss models could be improved by including climate change effects on P sources and transport, the inclusion of legacy P soil data, and accounting for P losses from legacy P soils. Graphical Abstract Climate change effects on phosphorus loss

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.

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, beta-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 beta-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.

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.