Soil organic carbon storage, microbial abundance and pore structure characteristics of macroaggregates across a soil-landscape sequence in a subtropical hilly watershed

Mosaic soil-landscapes with varying landforms and land covers are common in hilly areas of the lower Yangtze Valley in China. Soil-health variations, primarily driven by soil organic matter (SOM) and aggregation, across these soil-landscapes affect agro-industrial development in rural areas. Macroaggregates, through their pore structure, SOM storage and microbial community habitat, the variation among soil-landscape remains unaddressed. In this study, undisturbed topsoils (0–15 cm) were collected from forestland (FL) on hillslope, orchards (OR) and uplands (UL) on downland, and paddy fields (PF) in basin of a small watershed in Nanjing suburb. Soil organic carbon (SOC) pools and microbial phospholipid fatty acids (PLFAs) were measured while water-stable aggregates were size-fractionated. Macroaggregate samples were analyzed for micro-pore structure with X-ray micro-computed tomography (X-ray μCT). Compared with FL, topsoil SOC was lower by 54 %-70 %, mean weight diameter by 41–67 %, and total PLFAs by 14 %-42 % in agriculture landscapes (OR, DL and PF). Meanwhile, macroaggregates mass proportion was more than 44 % lower in OR, UL and PF than in FL. The macroaggregate total porosity was lower by 17 % and 33 % in UL and OR, respectively, although it was unchanged in PF. The connected, elongated and meso- porosities as well as total throat area exhibited a similar trend. Except for pore number but total, connected, elongated and meso- porosities, as well as total throat area were significantly positively correlated with SOC or microbial PLFAs in macroaggregate. The results indicated that the agricultural activity altered the pore structure of macroaggregates, potentially reducing the functional services of macroaggregates such as SOC preservation and microbial biomass conservation. Furthermore, the positive correlation between SOC content, microbial abundance and macroaggregate porosity indicated enhancing the aggregate structure was essential for improving the aggregate function, which was crucial for comprehending the mechanism of soil-landscapes affecting soil aggregation and function.