Assessment of Soil Specific Enzyme Activities in Aggregates Size Fractions: a Case Study from Subtropical Agro-ecosystem

Soil enzyme activities are closely associated with soil organic carbon (SOC) or microbial biomass carbon (MBC). High correlation between SOC and MBC masks their individual effects on enzyme activities. Expressing soil enzymes activity relative to soil organic carbon (SOC) or Microbial Biomass Carbon (MBC) would normalize the differences in SOC/MBC. A long term subtropical rice-wheat system was selected to study the variation in soil-specific enzyme activity (per unit SOC and MBC) in soil aggregates under integrated nutrient management. Soil organic carbon, microbial biomass carbon and the enzyme activities like acid phosphatase (ACP), alkaline phosphatase (AKP), dehydrogenase (DHA), fluorescien diacetate hydrolysing capacity (FDA) and urease were measured in three aggregate size fractions viz., >2000, 2000–250 and <250 µm. Integrated nutrient management practices significantly increased the specific enzyme activities expressed in relation to SOC and MBC. Aggregate size fractions of 2000–250 µm size exhibited higher specific enzyme activities, when expressed in terms of SOC. Enzyme activity (per unit MBC) in aggregates also followed the same pattern with an exception to dehydrogenase activity which was almost uniform across all the aggregates size fractions. Aggregates of larger size do not always possess higher enzymes activity; even smaller macroaggregates were able to retain stable extracellular enzyme activity per unit of SOC or MBC. This study opens an alley to express soil enzymatic activities relative to SOC or MBC level in the soils, rather than expressing absolute activities.