DEPOSITION OF NITROGEN INTO THE RHIZOSPHERE BY WHEAT ROOTS

Previous research has demonstrated that plants contribute significant amounts of carbon to soil by exudation processes but concurrent N deposition has not been quantified. In the present study, wheat was grown in soil columns under controlled conditions to measure the release of N from roots during a growing season and to determine the influence of soil N fertility on that process. Rhizodeposition of N was measured by pulse-labelling shoots with 15N-labelled gaseous NH3 and determining the accumulation of 15N in the soil at three times (after 6 and 9 weeks and at maturity). Net deposition of N over the entire growing season amounted to 26 and 76 mg (3 plants)-1 in low-N and high-N fertility treatments, respectively. These amounts represented 18 and 33%, respectively, of the total N yield of the wheat plants. The enhanced release of N under high soil-N fertility was attributed to greater translocation of N to roots in response to excessive accumulations in the shoot and to diminished reabsorption of exuded N by the plant. Because of very high concentrations of soluble N, deposition observed in the high-N treatment probably overestimates amounts under normal field conditions. After removal of roots, soil from the various sampling times and fertility treatments was incubated at 30°C to determine mineralization of deposited N. After 76 days, 36% of the root-derived N originally present in organic form was mineralized to plant-available N. indicating that (he material deposited was highly labile. Up to 32% of all N mineralized was derived from the decomposition of materials released from the roots. The results provide quantitative evidence of appreciable flux of N from wheat roots into soil. © 1990.