Effect of biogas slurry application rate on colloidal phosphorus leaching in paddy soil: A column study

Colloidal phosphorus (Pcoll) in soil solutions and subsurface drainage has attracted increasing attention as an additional mobile form of P. A column study was conducted to investigate the effect of biogas slurry (BS) on Pcoll leaching in paddy topsoil (0-20 cm) at rate of 0; 225 m3 ha−1 (55 kg N ha−1, 17 kg P ha−1); 673 m3 ha−1 (165 kg N ha−1, 52 kg P ha−1); 1350 m3 ha−1 (330 kg N ha−1, 104 kg P ha−1) of BS. The colloids particles were separated by microfiltration and ultra-centrifugation processes. Dissolved phosphorus (DP) in leachate ranged from 0.04 to 0.09 mg P L−1 and contributed between 56 and 76% to the total P leaching. Pcoll in leachate ranged from 0.002 to 0.03 mg P L−1 in leachate and contributed between 3 and 26% to TP leaching. Results clearly showed that BS increased Pcoll leaching at medium (673 m3 ha−1: 165 kg N ha−1, 52 kg P ha−1) and high application rate of BS (1350 m3 ha−1: 330 kg N ha−1, 104 kg P ha−1). In contrast, the smallest tested BS application of 225 m3 ha−1 (55 kg N ha−1, 17 kg P ha−1) did not affect Pcoll concentrations in leachates. Significant positive correlations was found between Pcoll and colloidal aluminum (Alcoll), colloidal calcium (Cacoll), total organic carbon (TOCcoll), and with pH, indicating that Al/Ca minerals - organic matter may play a major role as carriers for Pcoll leaching and these processes were pH-dependent. This study confirms that high level of BS application could result in the mobilization of Pcoll from paddy topsoil by leaching.