Depth-dependent patterns in soil organic C, enzymatic stochiometric ratio, and soil quality under conventional tillage and reduced tillage after 55-years

Interest in managing agroecosystems for improving soil health has driven the application of conservation practices, such as reduced tillage. However, our understanding remains limited regarding changes in soil organic carbon (SOC), microbial resource limitations, and soil ecosystem multifunctionality across the soil profile (not just in the upper layers) following long-term reduced tillage as compared to conventional tillage. This study aimed to compare the impacts of reduced tillage (RT) and conventional tillage (CT) on SOC storage, soil enzyme activities, and ecosystem functionality in a 90 cm soil profile. Soil cores were collected from four different soil depths (0–10, 10–30, 30–50, 50–90 cm) in a 55-year field experiment (crop rotation primarily featured cereals) in central Germany comparing RT (rotary harrow, 5–8 cm depth) and CT (mouldboard plough, 25 cm depth). Results showed that RT increases SOC content by 24 % at 0–10 cm depth but decreases SOC content by 22 % at 10–30 cm depth relative to CT. The enhanced SOC content under RT further improved soil ecosystem multifunctionality by a factor of 2.8 at 0–10 cm compared to CT. Notably, higher available nitrogen (N) content increased vector length at 0–10 cm depth under RT, related to the higher production and release of C-acquisition enzymes. In contrast, the higher exudation of N-acquiring enzymes found at 10–50 cm depth indicated N limitation for microorganisms under RT. At this depth, CT alleviated microbial N limitation. Moreover, RT lowered SOC stock by 24 % compared to CT at 10–30 cm, resulting in an 11 % decrease in SOC across the entire 0–90 cm soil profile. In conclusion, while reduced tillage improved soil ecosystem functionality in the 0–10 cm, it may not enhance SOC sequestration, highlighting the importance of considering the whole soil profile when comparing the SOC sequestration potential of different tillage practices.