Glyphosate Degradation in Malaysian Sandy Loam Soil Amended with Cow Dung or Rice Husk Ash as Influenced by Soil Moisture Content

摘要: This study investigated the effect of cow dung or rice husk ash amendment on glyphosate degradation in a Malaysian sandy loam soil under three moisture regimes. The control and soils amended with 10 tons ha-1 of either cow dung or rice husk ash were spiked with 5000 µg g-1 of glyphosate. Water was added accordingly to obtain soil moisture content equivalent to either a submerged condition, field capacity or permanent wilting point. Glyphosate degradation was monitored for 65 days and the data obtained was fitted to first order-double exponential decay model (FODED). The results revealed that glyphosate degradation occurred in two-phases; an initial high degradation rate followed by a slow rate representing degradation of labile and non-labile phases, respectively. The rate constants of the labile phases ranged from 0.0063 to 0.0604 µg day-1 while those of non-labile phases were from 0.0077 to 0.0732 µg day-1. The degradation rate was generally higher in the labile phase. Irrespective of moisture content, the degradation data from the cow dung-amended soil fitted the FODED model best (0.042≥ r2≤ 0.909) followed by the rice husk ash-amended soil (0.023≥ r2≤ 0.914) and the control (0.030≥ r2≤ 0.756). Meanwhile, irrespective of the amendments, soils maintained at field capacity had the highest degradation rate in labile phases (k1= 0.0371–0.0604 µg day-1) while at nonlabile phases, soils maintained at permanent wilting point recorded the highest rate (k2= 0.0077–0.0732 µg day-1). The soils maintained at field capacity generally had the lowest glyphosate half-life (11-42 days) followed by soils at permanent wilting point (9 …