Effects of Subsoiling Depth on Soil Aggregate Stability and Carbon Storage in a Clay-Loam Soil

Subsoiling can affect the stability of soil aggregates and soil organic carbon (SOC) sequestration. However, the effects of subsoiling depth on soil aggregation and SOC storage in different aggregates remain largely unknown. In this study, we evaluated the impact of subsoiling in the North China Plains (NCP) on soil aggregate stability and aggregate-associated SOC. Tillage practices included conventional tillage to a depth of 25 cm (CT25) and subsoiling with three depths of 30 cm (SS30), 35 cm (SS35), and 40 cm (SS40). SS35 and SS40 had significantly higher proportions of macro-aggregates in the 0–20-cm soil than CT25 (p < 0.05). Mean weight diameter of wet aggregates in the 0–20 cm is significantly higher (p < 0.05) under SS30, SS35, and SS40 by 39.5%, 57.7%, and 54.1% compared with CT25. The percentage of aggregate destruction was 22.2% and 14.9% lower with SS35 and SS40 than with SS30, respectively. The SOC storage in SS35 and SS40 was significantly higher by 12.7%, 18.5%, and 16.5%, compared with CT25 in the 0–40-cm soil layer. SS35 increased the number of soil macro-aggregates in the 0–20-cm soil and the concentration of SOC in the 0–40-cm soil and saved fuel and power costs compared with SS40. Therefore, subsoiling to a 35-cm depth is a promising tillage practice for enhancing the soil quality of cropped lands in the NCP.