Microbial biomass responses to seasonal change and imposed drying regimes at increasing depths of undisturbed topsoil profiles

Concentrations of microbial biomass C and extractable ninhydrin-reactive N in increasing depth layers of an Alfisol topsoil were determined periodically over 13 months for soils under two contrasting tillage regimes. Concentrations decreased rapidly with depth to 10 cm, halving approximately 2.5 cm with each. Organic C and total N concentrations also decreased with depth of topsoil, those of the surface layer (0-2.5 cm) being 1.5-2.1 times and 1.3-2.4 times those respectively of the 2.5-7.5 cm layer. Tillage management affected the relative concentrations of biomass C, total organic C and total N in surface and lower depths of topsoil, but only through its influence on surface soil concentrations. Respective concentrations at 0-2.5 cm depths for soils under a direct drill tillage regime were on average 24, 28 and 50% higher than for soils subjected annually to district cultivation practices. Highest values for microbial biomass C were recorded for moist Winter-sampled soils, and the lowest in soils sampled at times of severe desiccation after prolonged hot dry periods in Summer. Seasonal trends in biomass contents were not significantly influenced by tillage practice. Averaged for both tillage regimes, the seasonal decrease in biomass C was 28% for the surface soils and 23% for the subsurface (2.5-7.5 cm) soils. Biomass C concentrations of soils sampled dry from the field, moistened, and assayed without prior incubation were about 10% higher than those of the same samples incubated moist for 2 weeks before assay. We suggest that severe soil desiccation was likely the main cause of the biomass C decline, and that the decline may be underestimated, due (1) in the case of soils moistened without incubation before assay, to restricted hydrolysis-deamination of killed cell protein under the field dry conditions, but not under assay conditions, and (2) in the case of soils moistened and incubated before assay, to growth of cells on substrates from killed cells and non-biomass sources during the preincubation period. In contrast to soil cores sampled seasonally from the field, biomass C concentrations at different depths of the Alfisol topsoil were not significantly affected by controlled drying of undisturbed cores under constant or fluctuating temperature regimes. Neither were the inorganic N contents of the total cores, nor the C and N mineralization activities of soils sampled from different core depths, influenced by the imposed drying treatments, which resulted in gradual and graded losses of soil moisture. During soil drying, nitrate-N accumulated in the surface layer of the cores, but became redistributed on remoistening of the cores. Thus gradual drying of intact soil cores at moderate temperatures did not per se significantly influence the availability for decomposition of organic substrates, which has implications for modelling C and N mineralization in natural environments with intermittent soil drying.