Biotic relationships to aggregation in a Vertisol under long-term no-till, stubble retention and nitrogen fertilisation

Soil aggregation is critical for high-clay Vertisols to facilitate water movement, aeration, nutrient cycling, microbial activity, and hence soil productivity. Both abiotic and biotic factors affect aggregation, although the effect of biotic factors on aggregation remains unclear in subtropical Vertisols managed under no-till (NT), stubble retained (SR) and N fertilisation practice. Using a 54-years old cereal cropped NT experiment in subtropical semi-arid region, we examined the biotic effects on soil aggregate-size distribution in a smectitic Vertisol (65 % clay) at 0-0.1 m depth. The experiment consisted of NT versus conventional till (CT), SR versus stubble burned (SB), and N fertiliser rates (0, 30, and 90 kg N ha(-1); N0, N30, and N90, respectively) arranged in a randomised complete block design, with four replications (48 plots). NT generally had no significant effect on biotic factors and aggregate mean weight diameter (MWD). SR had significantly higher microbial activity (as assessed by cumulative CO2 production over 140 days), C-acquiring and N-acquiring enzyme activities (beta-glucosidase, cellobiohydrolase, N-acetyl-beta-D-glucosaminidase), easily extractable and total glomalin-related proteins, microbial biomass C, large- (>2 mm) and medium aggregates (1-2 mm), and MWD (1.23 +/- 0.06 mm in SR and 1.02 +/- 0.07 mm in SB (P < 0.02)). In addition, N fertiliser rates had significantly higher soil organic C and mineral associated organic N but lower C:N ratio. Thus, soil organic C and N, microbial biomass C, glomalin, and microbial and enzyme activities enhanced by SR, and lower C:N ratio of the organic matter following N fertilisation improved MWD in this subtropical Vertisol.