Green Manure Mediated Improvement in Saline Soils in China: A Meta-Analysis

The application of green manure is a traditional and valuable practice to improve the fertility of saline soil. However, the impact of environmental factors, green manure types and returning methods on the changes in soil fertility and soil salinity remain poorly quantified at a large scale. In the present study, we conducted a meta-analysis to generate a comprehensive evaluation of the effects of green manure on soil organic carbon (SOC), soil salt content, and soil nutrients compared to bare soil in China. The results showed that compared with bare soil, green manure planting could significantly increase the SOC content of saline soil, reduce salt content, and improve the soil total nitrogen (N), soil available phosphorus (P) and soil available potassium (K) contents. On average, green manure significantly enhanced SOC by 34.82% (percentage change), soil total N by 32.23%, soil available P by 34.34% and soil available K by 17.43%, while reducing soil salt content by 47.75%, compared to bare soil. In areas with a mean annual temperature (MAT) of <10 °C or a mean annual precipitation (MAP) of 200–400 mm, green manure had the largest increase in SOC, soil total N, soil available P, and soil available K. The smallest increases were observed in areas with an MAT above 15 °C and MAP greater than 800 mm. Green manure types influenced the improvement effect of green manure on saline soil. Green manure mixtures were more conducive to increases in SOC, while the increases in soil total N resulting from mixed green manure were lower in comparison to those from both legumes and non-legumes. In addition, the initial salt content, experimental years, and returning method influenced the improvement effect of green manure on saline soil. Therefore, this meta-analysis identified green manure as a promising practice for significantly improved saline soil in China.