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

Colloidal phosphorus (P-coll) 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 P(coll )leaching in paddy topsoil (0-20 cm) at rate of 0; 225 m(3) ha(-1) (55 kg N ha(-1), 17 kg P ha(-1)); 673 m(3) ha(-1) (165 kg N ha(-1), 52 kg P ha(-1)); 1350 m 3 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. P-coll 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 P-coll leaching at medium (673 m(3) ha(-1): 165 kg N ha(-1), 52 kg P ha(-1)) and high application rate of BS (1350 m(3) ha(-1): 330 kg N ha(-1), 104 kg P ha(-1)). In contrast, the smallest tested BS application of 225 m(3) ha(-1) (55 kg N ha(-1), 17 kg P ha(-1)) did not affect P-coll concentrations in leachates. Significant positive correlations was found between P(coll )and colloidal aluminum (Al-coll), colloidal calcium (Ca-coll), total organic carbon (TOCcoll), and with pH, indicating that Al/Ca minerals - organic matter may play a major role as carriers for P-coll leaching and these processes were pH-dependent. This study confirms that high level of BS application could result in the mobilization of P-coll from paddy topsoil by leaching.