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°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 15N tracer, using changes in both the inorganic and organic 15N 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.