Competition for nitrogen during decomposition of plant residues in soil: Effect of spatial placement of N-rich and N-poor plant residues

The distance between “hot spots” for N-mineralization (N-rich clover residues) and for N-immobilization (high C-to-N straw) in soil was experimentally manipulated to investigate its effect on the competition between plant roots and microorganisms for mineralized N. The experiment demonstrated that plant roots were reasonably competitive, resulting in deprivation of the N-supply to the microorganisms growing on the straw material, but this was totally dependent on the distance between the N-rich and the N-poor sites in the soil. The critical distance was somewhere between 3 and 6 mm, above which plant roots outcompeted the microorganisms more or less completely. Our study illustrates an important mechanism by which plant roots can interfere with microbial N-transformations in soil. It may be the mechanism responsible for an often alleged “stimulation” of N-mineralization by plant roots. The mineralization rate of clover- and straw-C was measured in planted as well as unplanted soil. The presence of plant roots retarded the straw-C mineralization significantly, but not clover C.