CARBON AND NITROGEN MINERALIZATION FROM 2 SOILS OF CONTRASTING TEXTURE AND MICROAGGREGATE STABILITY - INFLUENCE OF SEQUENTIAL FUMIGATION, DRYING AND STORAGE

Two top soils (an Alfisol and a Vertisol) of contrasting cation exchange capacity, microporosity, and microaggregate stability were sampled from the same climatic region and were incubated for 4 weeks with C-14-labelled plant material. Each soil was then subjected to combinations of two of the following treatments: (1) drying (40-degrees-C), remoistening and incubation for 10 days at 25-degrees-C, (2) fumigation with chloroform vapour and incubation, and (3) storage (4-degrees-C) and incubation. The amounts of CO2 and (CO2)-C-14 evolved and inorganic N released during each incubation period were measured. Also, during drying and after remoistening of soils, concentrations of biomass C and C-14 were monitored using a fumigation extraction technique. Biomass C and C-14 decreased by 26-30% during desiccation and increased to 77-84% of untreated control soils during incubation after rewetting. The relative decline in biomass during soil drying was of similar magnitude for both soils and for C-14-labelled and total biomass C, indicating that factors other than soil properties had determined the extent of decline. The evolution of microbial populations exposed frequently to high temperatures and to extreme desiccation in the natural field environment has been proposed to explain the similar responses of biomass C to the imposed drying regime. Previous fumigation-incubation of soils to destroy the majority of the microbial biomass had little effect on the sizes of the C mineralization flushes obtained when the soils were subsequently dried, rewetted and incubated. The specific activities of the CO2-C flushes after drying were much lower than those from fumigated or stored control soils respectively. This was especially evident for CO2-C flushes from the well-aggregated Vertisol. From the magnitude of the flushes of CO2 and (CO2)-C-14 after the various combinations of treatments, and from their specific activities, we have deduced that microbial cells killed by soil desiccation had made only a minor contribution to the C and N mineralization flushes after soil rewetting and incubation. The larger contribution had come from other sources, the relative importance of which appears to be influenced by soil characteristics, possibly cation exchange capacity and microporosity.