Effects of rainfall and manure application on phosphorus leaching in field lysimeters during fallow season

Transformation and transport of soil phosphorus (P) from chemical fertilizer or manure are affected by agronomic practices (i.e., fertilization, irrigation, and tillage) and numerous abiotic factors (i.e., temperature, drought, and rainfall). Previous studies on the effects of manure application on P loss were mostly conducted during cultivation and often using the laboratory column approach. Validated field observations by integrating P availability with the risk of P loss are few. This study aimed to evaluate the effects of rainfall and manure application on P leaching during fallow season. An in situ field lysimeter experiment was performed. Leachate was collected and analyzed for total P (TP) and dissolved reactive P (DRP). At the end of the lysimeter trial, soils were sampled incrementally to a depth of 40 cm with the following depth intervals: 0-4, 4-10, 10-20, 20-30, and 30-40 cm. Soil water extractable P (P-w) was analyzed and degree of P saturation (DPS) was calculated after oxalate and Mehlich-3 (M3) extractions. Phosphorus loading was found below 40 cm in all treatments and P was most concentrated in the 4-10 cm soil layer. High rainfall enhanced P leaching and consequently led to lower P-w and higher leachate DRP and TP than low rainfall. Furthermore, P leaching was observed regardless of manure application, with the highest leachate DRP (1.83 mg L-1) and TP (7.46 mg L-1) concentrations found at the end of experiment (day 53). Observed P leaching loads during fallow season (53 days) varied between 0.08 and 1.21 kg ha(-1). The thresholds of DPS indicating P leaching were identified at 18.9% DPSM3(Ca) and 12.9% DPSM3(Ca+Mg), respectively. In this study, the DPSM3(Ca) and DPSM3(Ca+Mg) values exceeded the corresponding thresholds in the upper 30-cm soil layer but did not reach the thresholds at 30-40 cm. Nevertheless, the leachate DRP and TP indicated soil P leaching. The results showed that intensive rainfall could significantly increase P leaching from manure application. Moreover, P leaching could occur in fallow season even in the absence of manure input, which should be assessed by not only soil P sorption capacities, but also soil fertilization history and hydrologic conditions. Implementation of soil P level evaluation is critical before spreading manure on land to avoid P pollution. Cost-effective and applicable soil test methods are required to generate environmental indicators to classify agricultural lands for risk of P loss, providing basis to safe manure disposal.