Phosphorus distribution in soil aggregate size fractions in a poultry litter applied soil and potential environmental impacts

Long-term poultry litter (PL) application in pasture soil was found to enhance accumulation of certain phosphorus (P) forms in soil micro-aggregate fractions and have potential impacts on non-point water pollution by wind erosion or run-off. A P fractionation method was utilized to identify P forms in aggregate fractions derived from a long-term PL applied pasture soil. Enrichment of surface soil P by PL application has considerable effects on increasing specifically the labile P forms in surface soil micro-aggregates. Particular concern is of accumulation of water soluble and labile inorganic P forms in micro-aggregate particles susceptible to wind and soil erosion. Continuous application of PL to pasture fields for at least 15 years at a rate of 2.3 T ha− 1 y− 1 resulted in a considerable increase in inorganic P forms including Mehlich-3, water soluble and labile bicarbonate P (NaHCO3–P) forms in most aggregate size fractions. Mehlich-3-P was highest in < 0.053 mm micro-aggregates (223.8 mg kg− 1) and relatively high amounts ranging from 145.5 to 170.4 mg kg− 1 were found in 0.053–1.0 mm micro-aggregates. High levels of water soluble P were found in majority of micro-aggregate particles in PL applied soil, which amounts to 11–18% of total inorganic P in these particles. Bicarbonate–P or labile inorganic P forms were at elevated levels in 0.125–0.25, 0.053–0.125 and < 0.053 mm micro-aggregates, which amounts to approximately 11–22% of total inorganic P in these particles. The PL application also increased the NaOH extractable P (NaOH–P) or Al/Fe bound P in soil aggregate fractions (192–347 mg kg− 1). Notable increase in labile organic P forms in certain micro-aggregate size fractions may be of concern due to ease of P mineralization.