CARBON AND NITROGEN CYCLING THROUGH SOIL MICROBIAL BIOMASS AT VARIOUS TEMPERATURES

C and N cycling were examined in a soil incubated at 4, 12, 20 or 28-degrees-C for 140 days. Before incubation the soil was amended with (KNO3)-N-15, and either glucose-C-14 or holocellulose-C-14. The kinetics of tracer and non-tracer C and N from the biomass, mineralized-C and inorganic-N were measured. C and N behaviours in soil were influenced by temperature, substrate and substrate-temperature interactions. Labelled-C mineralization rates after 140 days ranged from 41 to 58% for glucose and from 34 to 65% for holocellulose. Maximal immobilization was 21.8-31.6 mg N kg-1 soil for holocellulose and 24.3-33.5 mg N kg-1 soil for glucose. Re-mineralization began earlier with glucose and at higher temperatures: 6-23% of immobilized-N were re-mineralized for glucose and 0-19% for holocellulose. More labelled C and N were incorporated into the microbial biomass from both carbon sources at lower temperatures. The biomass turnover was highly influenced by temperature: 40-46% of labelled C or N incorporated in the biomass remained in this compartment at 20-28-degrees-C, while corresponding values at 4-12-degrees-C were only 0-40%. Organic-C-14 mineralization and immobilization rate constants were influenced by temperature, the different trends depending on the carbon source. Thus an overall temperature coefficient (Q-10) could not be determined for these complex transformations. Variations in the rate constant with temperature were described using polynomial regressions.