Root-induced nitrogen mineralization and its availability to plants

A greenhouse experiment was conducted to compare maize (Zea mays L.) and dhaincha (Sesbania aculeata L.) in terms of biomass accumulation (especially root proliferation) and efficiency of N uptake from potentially mineralizable pool with emphasis on root-induced N mineralization. Pool of potentially mineralizable N was developed by incubating the clay loam soil with sucrose and 15N-labelled ammonium sulphate with the objective to differentiate between the sources of plant N. After 6 weeks of incubation, 55% of the mineralizable N was derived from the added ammonium sulphate and 45% from the native soil N. Plants grown in 15N-labelled potted soil were harvested at 3, 4, and 5 weeks after sowing and studied for water content and partitioning of biomass, N and 15N into root and shoot portions. Accumulation of dry matter in root and shoot portions was 7 and 28 times higher, respectively, in maize as compared to Sesbania. Water content of the shoot portions was almost similar in both crops, while roots of Sesbania contained significantly higher water content. Higher root proliferation in maize led to significantly more accumulation of dry matter and N as compared to Sesbania. Efficiency of N utilization (biomass produced per unit N in plants) was also higher in maize as compared to Sesbania. However, roots of the latter were more efficient in extracting N from soil as determined by amount of N taken up per unit root mass. Since, roots of Sesbania did not show nodules, entire plant N was ascribed to native soil N or from the recently developed pool of potentially mineralizable N. The differences in the uptake of N by the two crop types were attributed to i) root proliferation and thus the access to available N and ii) root-induced changes in the N mineralization potential that were assumed to result from differences in quality and quantity of rhizodeposits.