Soil inorganic N availability: Effect on maize residue decomposition

The effect of soil inorganic N availability on the decomposition of maize residues was tested under aerobic conditions in soil samples incubated for 125 days at 15 degrees C. Carbon residue were ground maize shoots applied at 4 g dry matter kg(-1) soil. The C-amended soils contained five initial inorganic N concentrations (10, 30, 60, 80 and 100 mg N kg(-1) soil). Gross N immobilization was calculated with a N-15 tracer, using changes in both the inorganic and organic N-15 pools. Inorganic N remained available in those soils having the three highest initial N concentrations. In this case the rates of C mineralization and N immobilization were similar. Soil inorganic N completely disappeared at the beginning of C decomposition in the soil samples with the two lowest N contents, resulting in a marked decrease of C mineralization rate compared to the three highest N contents. Gross N immobilization amounted to 39 mg N g(-1) added C after 40 days (end of the net immobilization period) for the three highest N concentrations, indicating that there was no luxury N consumption by the soil microflora. N immobilization was much lower in the two lowest-N treatments because decomposition was slow and microbial N immobilization per unit of mineralized C was reduced. The ratio N immobilized:C mineralized also decreased in all treatments during decomposition due to changes in microbial N demand with time or increasing contributions from other sources of N, such as biomass-N recycling, to microbial N assimilation.