Effects of maize roots on aggregate stability and enzyme activities in soil

Soil aggregation and microbial activities within the aggregates are important factors regulating soil carbon (C) turnover. A reliable and sensitive proxy for microbial activity is activity of extracellular enzymes (EEA). In the present study, effects of soil aggregates on EEA were investigated under three maize plant densities (Low, Normal, and High). Bulk soil was fractionated into three aggregate size classes (> 2000 gm large macro aggregates; 2000-250 mu m small macroaggregates; < 250 mu m microaggregates) by optimal-moisture sieving. Microbial biomass and EEA (beta-1,4-glucosidase (BG), beta-1,4-N-acetylglucosaminidase (NAG), L-leucine amino peptidase (LAP) and acid phosphatase (acP)) catalyzing soil organic matter (SOM) decomposition were measured in rooted soil of maize and soil from bare fallow. Microbial biomass C (C-mic) decreased with decreasing aggregate size classes. Potential and specific EEA (per unit of Cmic) increased from macro-to microaggregates. In comparison with bare fallow soil, specific EEA of microaggregates in rooted soil was higher by up to 73%, 31%, 26%, and 92% for BG, NAG, acP and LAP, respectively. Moreover, high plant density decreased macroaggregates by 9% compared to bare fallow. Enhanced EEA in three aggregate size classes demonstrated activation of microorganisms by roots. Strong EEA in microaggregates can be explained by microaggregates' localization within the soil. Originally adhering to surfaces of macroaggregates, microaggregates were preferentially exposed to C substrates and nutrients, thereby promoting microbial activity.