Changes in microbial community and soil organic matter mediated by macroaggregate pore structure across soil-landscapes in a hilly watershed

Heterogeneous soil-landscapes with varying land uses and land covers are common in the hilly areas of the lower Yangtze valley of China. Variations of soil quality, mainly driven by soil organic matter and soil aggregation, across these soil-landscapes impacts the development of agro-industries in rural areas. How the development of macroaggregates (MAC) and their pore structure in relation to soil organic matter and microbial community varies with soil-landscapes (disturbed and undisturbed) remained unclear. Both bulk samples and undisturbed cores were collected from topsoil (0-15 cm) respectively on forestland (FL) under vegetation conservation on the hill slope, orchard (OR) and upland cropland (UL) on the slope and paddy fields (PF) in the basin in a small watershed from suburb Nanjing of China. Soil organic carbon (SOC) pools and microbial phospholipid fatty acids (PLFAs) as well as basic physico-chemical properties were measured while size fractionation of water-stable aggregates were performed. Further, the pore structure of the macroaggregate samples were analyzed with X-ray micro-computed tomography (X-ray μCT). Compared to FL, topsoil SOC was lower by 54%-70%, soil aggregate stability by 41-67% and total PLFAs by 14%-42% under the disturbed agricultural soil-landscapes. The mass fraction of macroaggregates was lower in OR, UL and PF, by over 44%. The total porosity of the macroaggregates, estimated by the μCT images, was lower by 17% and 33% under UL and OR though unchanged under PF. A similar trend was found for the connected porosity and total throat area. To note, both SOC and microbial PLFAs of the macroaggregate samples were significantly positively correlated to total porosity, connected porosity and total throat area, across the landscapes. Overall, soil quality was seen profoundly reduced in the disturbed soil-landscapes under agricultural activities although PF was shown most close to FL in the context of organic carbon stabilization and microbial biomass conservation. Thus, improving rice paddy management through soil organic matter conservation and macroaggregation could contribute to sustaining local soil quality for better agricultural development in the hilly rural area.