Effect of growing plants on denitrification at high soil nitrate concentrations

The availability of plant rhizosphere C deposits and its influence on microbial denitrification is not clearly defined. Conflicting reports as to the influence of plants and root exudation on denitrification continue to appear in the literature. The results of the authors earlier phytotron study indicated that denitrification was not stimulated in soils planted with corn or wheat compared to unplanted soils. Lower nitrate concentrations in the planted soils, however, may have led to misinterpretation of this data. A second study was conducted, to evaluate the effect of actively growing plants on denitrification where the NO37 content of planted soils was maintained similar to unplanted soils. Simultaneously the C fixed by corn (Zea mays) and the fate of fertilizer N applied to the soil during the growing season were quantified. The corn was grown in a phytotron under a continuous supply of 14CO2 in 15N fertilized soils to which 15N-NO3- was added periodically during the growing season. The results of these studies showed that denitrification was not stimulated in soils planted with corn during active plant growth phase even when soil NO3- was relatively high. Denitrification was, however, greater in corn planted than unplanted soil when the recoverable root biomass began to decrease. Less N was immobilized and net 15N immobilization was lower in planted soils than in unplanted soils. As denitrification was lower in planted soils during the time of active plant growth, the study suggests that root exudates did not stimulate either process