Effect of Rice Mill Wastewater on Soil Respiration and Enzyme Activities under Field and Pot Conditions

The physico-chemical characteristics of rice mill wastewater was measured and the effect of rice mill wastewater on respiration and enzyme activities (amylase, invertase, protease and dehydrogenase) of rice cropped soil was investigated under field and pot conditions at 15 days interval for 90 days. For the pot experiment five different concentrations (0, 25, 50, 75 and 100%) of rice mill wastewater was used. The results of the field experiment revealed significant difference in the soil respiration and enzyme activities rates of control and experimental plots from 30 days onwards. After 90 days the soil respiration and enzyme activities were inhibited by about 25-34% in experimental plots, whereas the results of the pot experiment revealed a maximum increase of about 36% in soil respiration and 24-45% in enzyme activities (amylase, invertase, protease and dehydrogenase) in 50% wastewater irrigated soil and a maximum decrease of about 40% in soil respiration and 24-40% in enzyme activities in 100% wastewater irrigated soil. The adverse effects of rice mill wastewater (100%) on soil respiration and enzyme activities were attributed to alkaline pH (8.0) of wastewater with higher contents of phenols (35 mg $^{-1}$ l), silica (58 mg $^{-1}$ l) and sodium (235 mg $^{-1}$ l). Significant increase in soil respiration and enzyme activities at lower concentrations (i.e. 50%) may be due to the fact that, the above parameters being in the diluted form could favour the microflora to boost their activities. On the basis of above findings, we suggest that the rice mill wastewater should be diluted up to 50% before use for agricultural purpose. However, further works on the effect of rice mill wastewater on different crops and soil animals are needed to corroborate the present findings.