SIMULATION OF CARBON AND NITROGEN TRANSFORMATIONS IN SOIL - MICROBIAL BIOMASS AND METABOLIC PRODUCTS

The mineralization of C and N in soils involves the dynamic behaviour of the microbial biomass. This behaviour was reproduced in a simulation model which was used to study microbial growth and mineralization following soil amendments with labelled glucose and crop residues. Temporal trends of simulated microbial C and N associated with these amendments were consistent with those of labelled C and N estimated from chloroform fumigation (SD < 10% of estimated values). Closer agreement between simulated and estimated values of labelled microbial C were obtained when nonfumigated controls were omitted from the estimated values. Closer agreement between simulated and estimated values of labelled microbial N were obtained when k(n) was allowed to change with mineralization ratios of C and N following fumigation. Agreement between simulated and estimated values of unlabelled microbial N was improved by using higher k(n) than that used for labelled microbial N. In the model, the time-course of labelled C and N mineralization arose from microbial consumption of the substrate amendment during the earlier part of incubation, and from microbial consumption of the products of microbial inactivation during the later part. Resolution of the microbial biomass into labile and resistant components allowed the model to simulate changes in active vs quiescent biomass fractions. Changes in these fractions allowed the reproduction of rapid changes in specific respiration activity from > 1.0 to <0.01 g g-1 day-1 that were caused by changes in substrate availability.