Influence of gap size and soil properties on microbial biomass in a subtropical humid forest of north-east India

We examined the effects of treefall gap size and soil properties on microbi al biomass dynamics in an undisturbed mature-phase humid subtropical broadl eaved forest in north-east India. Canopy gaps had low soil moisture and low microbial biomass suggesting that belowground dynamics accompanied changes in light resources after canopy opening. High rainfall in the region cause s excessive erosion/leaching of top soil and eventually soil fertility decl ines in treefall gaps compared to understorey. Soil microbial population wa s less during periods when temperature and moisture conditions are low, whi le it peaked during rainy season when the litter decomposition rate is at i ts peak on the forest floor. Greater demand for nutrients by plants during rainy season (the peak vegetative growth period) limited the availability o f nutrients to soil microbes and, therefore, low microbial C, N and P. Weak correlations were also obtained for the relationships between microbial C, N and P and soil physico-chemical properties. Gap size did influence the m icrobial nutrients and their contribution to soil organic carbon, total Kje ldhal nitrogen and available-P. Contribution of microbial C to soil organic carbon, microbial N to total nitrogen were similar in both treefall gaps a nd understorey plots, while the contribution of microbial P to soil availab le-P was lower in gap compared to the understorey. These results indicate t hat any fluctuation in microbial biomass related nutrient cycling processes in conjunction with the associated microclimate variation may affect the p attern of regeneration of tree seedlings in the gaps and hence be related w ith their size.