Soil aggregation is more important than mulching and nitrogen application in regulating soil organic carbon and total nitrogen in a semiarid calcareous soil

Evaluating soil aggregation and microbial activities within soil aggregates contributes to understanding carbon (C) and nitrogen (N) cycling. Here we examined soil aggregate distribution, C and N pools, and extracellular enzymatic activities (EEAs) in soil aggregates after 16-year mulching (CT, no mulch; RF, plastic-mulched ridges and straw-mulched furrows; SM, straw mulch) and N fertilization (0, 120, and 240 kg ha⁻¹). RF and SM significantly increased macroaggregate formation and aggregate stability (MWD, mean weight diameter) but N rate did not. Mulching had similar effects on aggregate-associated SOC (soil organic C) and TN (total N), with the order SM > RF > CT in macroaggregates and macroaggregate-occluded microaggregates. N input significantly increased TN in most cases, whereas its effect on SOC was only significant in SM. Notably, the majority of SOC and TN was isolated in the macroaggregate-occluded silt and clay fractions. SOC, TN, microbial biomass C (MBC), and microbial biomass N (MBN) decreased as aggregate-size decreased, whereas C- and N-acquiring enzymes varied greatly across aggregate fractions. Mulching had greater effects than N-fertilization on soil C and N pools and EEAs, whilst SM performed more beneficial effects than RF on SOC, TN, MBC, MBN, and EEAs. MBC rather than SOC was associated with MWD in bulk soil, while significant relations between MWD and SOC were observed in macroaggregates and macroaggregate-occluded microaggregates. Partial least squares path modeling illustrated that soil aggregation was the most important factor affecting SOC and TN, followed by mulching and N addition. Regression analysis further revealed that α-glucosidase and leucine aminopeptidase were major variables mediating SOC and TN dynamics at the aggregate scale. This study highlights the importance of macroaggregate-occluded microaggregate fraction sensitively evaluating soil C and N dynamics, and straw mulch can effectively increase soil aggregation and stabilization of C and N in semiarid areas with infertile soils.