Abstract
Plant-induced solubilization of soil nitrogen (N) is a key process for plants to utilize the recalcitrant form of N. To evaluate its contribution to plant uptake, the factors affecting the contribution and the forms of N solubilized by plants, we analyzed the results of a pot experiment in which komatsuna (Brassica rapa L. var. peruviridis) was grown in 3 different soils applied with 5 types of sewage sludge as a source of N for plants. The amount of N solubilized by plants, which was defined experimentally as the difference in the amount of solubilized N between the planted and unplanted treatments, varied with the soil types more than the types of sewage sludge. This accounted for 30% (Arenosol), 15% (Fluvisol) and 1.6% (Andosol) of the amount of N uptake on average. These percentages were high when the level of soil soluble N after the experiment was below approximately 30 mg kg-1. Sequential analyses of insoluble N in soil after the experiment indicated the occurrence of plant-induced solubilization of both bio-soluble and acid-soluble N in many of the Arenosol and Fluvisol treatments and that of acid-soluble N in the Andosol treatments. The plant-induced solubilization in the Andosol resulted in the accumulation of more labile bio-soluble N rather than enhanced plant uptake. For komatsuna grown in fertilized soil, the depletion of soluble N in the root zone seems to be important for the increase in the contribution of the plant-induced solubilization to uptake but not for the occurrence of the solubilization.
