Depth-dependent effects of aggregate-associated organic, inorganic carbon, and stoichiometry on soil structural stability following farmland abandonment

• Aggregate-associated SIC first decreased, then increased as abandonment progressed. • Farmland abandonment improved aggregate stability, especially in topsoil (0–30 cm). • Aggregate-associated SOC and stoichiometry strongly increased topsoil stability. • Aggregate-associated SIC decreased topsoil but increased subsoil aggregate stability. • Subsoil aggregate stability negatively affected by aggregate-associated TP. Vegetation restoration through farmland abandonment is an effective strategy for enhancing soil quality and aggregate stability in fragile ecosystems. However, the long-term effects of abandonment on soil aggregate stability, particularly associated with soil inorganic carbon (SIC) and ecological stoichiometry across different soil depths, remain unclear in semi-arid regions. We investigated the effects of farmland abandonment on soil aggregate stability and associated carbon and nutrient dynamics across soil depths of 0–50 cm using a chronosequence approach spanning five abandonment stages (0, 6, 15, 25, and 45 years). Initially, SIC in aggregates decreased by 12–37 % after 6 years but subsequently increased as abandonment progressed. Both soil organic carbon (SOC) and total nitrogen in the 0–20 cm depth increased by 39–143 % and 32–180 %, respectively. The C:P and N:P ratios within aggregates increased with abandonment time but decreased with soil depth, while the C:N ratio remained stable. Soil aggregate stability significantly improved after abandonment, with mean weight diameter in the topsoil (0–30 cm) increasing by up to 118 % after 45 years. In the topsoil, aggregate-associated N:P and C:P ratios had the largest positive effect on stability, followed by aggregate-associated SOC , whereas aggregate-associated SIC negatively affected stability. In contrast, in the subsoil (30–50 cm), aggregate-associated SIC positively influenced stability, while aggregate-associated nutrients, particularly total phosphorus, had a negative effect. These findings highlight the depth-dependent and contrasting roles of aggregate-associated organic and inorganic carbon forms in soil restoration processes, providing valuable insights into ecosystem restoration strategies in semi-arid regions.