The Effect of Roots on Soil Denitrification

Abstract

Abstract The effects of plant roots on denitrification were studied using the acetylene inhibition method, both with short‐term anaerobic assays for potential denitrifying activity and with intact plants in soil. Experiments with anaerobic assays demonstrated that denitrification activity was greater in the rhizosphere of both pot and field grown plants. A split‐plate technique showed that potential denitrifying activity decreased rapidly in the first few millimeters away from roots. These effects were shown to be due, at least in part, to differences in available organic matter. A new method was developed for measuring short‐term denitrification rates of soils with intact plants in an aerobic atmosphere. The soil conditions in these assays were comparable to field conditions. When soil NO 3 ‐ concentrations were high, the results confirmed prevailing opinion that denitrification rates are increased in the rhizosphere. In contrast, the planted soils denitrified at significantly lower rates than unplanted soils when NO 3 ‐ concentrations were low. Thus, denitrification may sometimes be reduced in the rhizosphere. These experiments also indicated that increasing NO 3 ‐ concentration increased the ratio N 2 O/(N 2 + N 2 O), but that the roots had no consistent effect on this ratio.