Microbial biomass associated with water-stable aggregates in forest, savanna and cropland soils of a seasonally dry tropical region, India

The distribution of microbial biomass associated with water-stable macro- and microaggregates in soils was studied along a forest-savanna-cropland gradient. In both macro- and microaggregates, mean microbial biomass C, N and P were maximum in forest and minimum in cropland soils. At all the sites microbial C was greater in the macroaggregate than the microaggregate size class, whereas microbial biomass N and P were greater in the microaggregates. The microbial C-to-nutrient ratios were lower for the microaggregates compared to macroaggregates. It is possible that these two size classes support two different types or components (fungi-dominated and bacteria-dominated) of the food web in soil. There was a decrease in the amount of microbial C, N and P in both macro- and microaggregates along the forest-savanna-cropland gradient, which was due to the reduction of total soil organic matter. Total soil organic C and total N showed significant positive relations with both macro- and microaggregate microbial C and N, indicating that the latter are limited by the quantity of soil organic matter.