Legacy phosphorus management in dairy agroecosystems

Legacy phosphorus management in dairy agroecosystemsPhosphorus (P) is a critical element for life in the biosphere and plays a unique, dual role in crop productivity and freshwater eutrophication risk.Natural P sources include organic material and soil minerals, some of which contain appreciable inorganic P contents (termed "geogenic P").Legacy P is anthropogenically derived P in the environment present at levels exceeding background P concentrations in unfertilized soils (Shober et al. 2025).In agroecosystems, anthropogenic P inputs include organic wastes/manures, fertilizers, crop residues, and pesticides.While crop P uptake is a large removal mechanism, considerable P is recycled onto fields from manure when crops are fed to livestock.Repeated manure applications above crop P removal rates increase legacy P stocks (Sharpley et al., 2013).Soluble and particulate legacy P are subject to transport in surface runoff and erosion, in addition to downward (leaching) and horizontal (interflow) mobilization in soil water flows that can transfer P to subsurface drainage and/or shallow groundwater (King et al., 2015).Soil is the largest legacy P reserve.Since legacy P is a mixture of different inorganic and organic P forms, it is important to measure total P in soil, sediment, and water samples.Total P estimates all P species present, providing more consistent legacy P tracking.Accurate legacy P measurements also support better assessments of P input/output budgets and best practices aimed at increasing P use efficiency.Transport factors (soil type, slope, drainage) and management (tillage, cropping system, P applications, presence/absence of cover crops) affect P fluxes and therefore legacy P stocks (Sharpley et al., 2013).The combination of varying soils, P application rates and management over time can create non-uniform legacy P distribution across fields, creating "P hotspots" that may require different management than the bulk of the field.When field soil test P levels exceed critical levels, additional P application is rarely required for optimal yields, thus potentially reducing farm fertilizer expense.Despite the sound science supporting critical soil test P thresholds, some pro-Abbreviation: PSI, phosphorus site index.