Factors shaping soil organic carbon stocks in grass covered orchards across China: A meta-analysis

Orchard grass coverage has been widely adopted to increase fruit yield by improving soil fertility. However, the impact of the environment on the changes in soil organic carbon (SOC) stocks consecutive to orchard grass coverage remain poorly quantified at a large scale. The present study aimed to examine the responses of SOC stocks to grass coverage at a soil depth of 0-30 cm in orchards compared to clean tillage. A total of 342 observations across China from 139 peer-reviewed publications were subjected to meta-analysis. Aggregated boosted tree analysis was performed, evaluating the determinants of SOC stocks, such as plant traits (e.g., fruit tree type, grass type, orchard age, and grass age), edaphic variables (e.g., initial SOC and nitrogen concentration, soil pH, and soil clay content), climatic factors (e.g., mean annual precipitation (MAP) and mean annual temperature (MAT)), and management practices (e.g., grass source, grass growing mode, fertilization, grass mowing, placement of mowed residues, and irrigation). On average, orchard grass coverage significantly enhanced SOC stocks by 21.47% (percentage change) compared to clean tillage. Biotic and abiotic factors influenced this increase in SOC stocks following grass coverage in orchards to different extents. Grass age and soil clay content were the main determinants driving the variation in the SOC stocks following grass coverage in orchards. Thus, we propose an efficient way to optimize C sequestration in grass covered orchards, regarding plant traits, climatic factors, edaphic variables, and management practices. Longer than 12 months of surface grass coverage with cultivated grass species in mature deciduous fruit orchards (≥5 years) efficiently increased SOC stocks. This is particularly the case for acidic (pH < 6.5) soils with low C content (SOM < 15 g kg^-1) in areas with suitable rainfall and temperature conditions (MAP ≥ 400 mm, MAT ≥ 10 °C). Collectively, this meta-analysis identified orchard grass coverage as a promising practice for significantly increasing SOC stocks at 0-30 cm across large geospatial locations in China.