The effect of no-till on organic C storage in Chinese soils should not be overemphasized: A meta-analysis

No-till (NT) has been widely regarded as a potential option to enhance soil organic carbon (SOC) sequestration and thus mitigate climate change. However, recent studies have shown that previous estimates of SOC storage under NT seem to be overestimated due to shallow sampling and improper SOC accounting. In China, the overall trend and magnitude of changes in SOC storage in response to NT adoption remain inconclusive. Here we use a meta-analysis to assess the relative change in SOC storage (profiles ≥20 cm) based on 95 comparisons between NT and conventional tillage (CT) from 57 experimental sites (≥3 years) covering various cropping systems, soil types, and climatic regions of China. The results revealed that compared to CT, adopting NT led to SOC accumulation in the upper 20 cm layer and SOC depletion in the 30-40 cm layer. On average, NT improved SOC stocks over CT by 3.8% [95% confidence interval (CI): 1.4%-6.3%; P = 0.005] and 5.1% (95%CI: 2.5%-7.7%; P < 0.001) as represented by the equivalent soil mass (ESM) and fixed-depth (FD) approaches, respectively. The relative change in SOC storage rate under NT over tilled soil for the top 30 cm layer was therefore significantly higher (Mean: 0.300 Mg ha−1 yr−1, 95%CI: 0.054-0.547; P < 0.001) calculated by FD approach than that by ESM (Mean: 0.141 Mg ha−1 yr−1, 95%CI: −0.102-0.384; P = 0.100). Further analysis suggested that the effect of NT on SOC storage was significant with residue retained‎, double cropping systems, and in the coarse textured soils. We conclude that although NT was an effective agronomy option for soil conservation in China, the potential for mitigation climate change through increased SOC sequestration by NT should not be amplified.