IMMOBILIZATION AND REMINERALIZATION OF NITRATE DURING GLUCOSE DECOMPOSITION AT 4 RATES OF NITROGEN ADDITION

In this study we investigated how N application rate influenced N immobilization and its subsequent remineralization. By incubating soil for 84 days in the laboratory and using C-14-glucose as substrate and four concentrations of N-15-labelled fertilizer addition, we followed changes in labelled and unlabelled inorganic N, soil microbial biomass C and N, and organic soil residues. Sufficient replicates were incubated to enable 8 sampling dates. During the decomposition of glucose, nitrate was rapidly immobilized in all N-treatments between days 1 and 3. Immobilization increased with increased N addition, suggesting that at the low-N level the microflora could use more N if it was supplied. A small positive (apparent) added N interaction (ANI) occurred. ANI was related to the N-concentration and was no longer evident after day 28. The decrease in residual organic C-14 followed first-order rate kinetics. Our results showed that there were small but significant differences in the C-14 remaining in soil between the two high-N treatments and the two low-N treatments. More, but only slightly more, CO2 was evolved after 84 days in the latter than in the former. This was probably the result of an interaction between microbial growth and N availability. After the immobilization of N, different amounts of N were mineralized. The higher the addition of N the higher the amount of N determined as labelled nitrate or remaining as soil biomass N-15 at the end of incubation. Possible explanations for the differences found between treatments are discussed. High C-to-N ratios were determined in the low-N treatments for labelled biomass and for organic labelled residuals No single mechanism is proposed to account for the large differences in C-to-N ratio of the microbes (6-26). Several factors which could be involved are discussed.