SOIL INORGANIC N AVAILABILITY - EFFECT ON MAIZE RESIDUE DECOMPOSITION

The effect of soil inorganic N availability on the decomposition of ma ize residues was tested under aerobic conditions in soil samples incub ated for 125 days at 15 degrees C. Carbon residue were ground maize sh oots applied at 4 g dry matter kg(-1) soil. The C-amended soils contai ned 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 tra cer, using changes in both the inorganic and organic N-15 pools. Inorg anic N remained available in those soils having the three highest init ial 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 l owest N contents, resulting in a marked decrease of C mineralization r ate compared to the three highest N contents. Gross N immobilization a mounted to 39 mg N g(-1) added C after 40 days (end of the net immobil ization period) for the three highest N concentrations, indicating tha t there was no luxury N consumption by the soil microflora. N immobili zation was much lower in the two lowest-N treatments because decomposi tion 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 dema nd with time or increasing contributions from other sources of N, such as biomass-N recycling, to microbial N assimilation.