Effects of straw returning combine with biochar on water quality under flooded condition

Abstract

Biochar is generally available to absorb nitrogen, phosphorus and other pollutants to improve water quality. However, the feasibility of biochar in improving water quality deterioration after straw returning is still unclear. In this study, pot experiments were conducted to evaluate the effects of straw decomposition on total phosphorus (TP), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N) and potassium permanganate index (CODMn) under CK (no straw returning), ST (straw of 7 t/hm2 returning) and SC (straw of 7 t/hm2 and biochar of 20 t/hm2 returning) conditions. Results showed that straw returning could significantly increase the nitrogen and phosphorus contents in field water. After adding biochar, there were significant differences in TP, NH4+-N, NO3--N and CODMn both in surface water and 0-10 cm soil water in SC treatment compared to ST treatment. The concentration of TP, NH4+-N, NO3--N and CODMn in surface water under SC treatment were always lower than that under ST treatment, and the maximum concentration could decrease by 52.29%, 39.67%, 35.23% and 44.50%, respectively. In 0-10 cm soil water, the concentration of TP, NO3--N and CODMn under SC treatment was always significantly higher than that under ST treatment, and the NH4+-N concentration in SC treatment was gradually higher than that under ST treatment at the middle-late observation period. Results indicate that straw returning combined with biochar can effectively decrease the nitrogen concentration, phosphorus concentration and organic pollutants in surface water, inhibit the diffusion of non-point source pollutant, and reduce the risk of water pollution caused by straw returning.