Grass cover increases soil microbial abundance and diversity and extracellular enzyme activities in orchards: A synthesis across China

The losses of soil organic carbon (SOC) in orchards are more intensive than those of cereal production and result from poor orchard management. Grass cover in orchards is an optimal approach to enhance the sequestration of SOC that has been shown to improve the biological properties of soil. China has the largest area of orchards and the highest production of fruit in the world. However, a quantitative assessment of soil biological properties in orchards in response to grass cover across China is lacking. In this study, a meta-analysis was conducted based on data collected from 103 peer-reviewed publications to evaluate the effect of grass cover on microbial abundance and enzyme activities, which were primarily invertase, urease, phosphatase and polyphenol oxidase, in the 0–40 cm soil layer and to assess the spatial and temporal variations of those parameters in orchards. The results indicated that the grass cover increased the microbial biomass carbon; abundances of bacteria, fungi, and actinobacteria; and microbial diversity (Shannon index) by 52.6 %, 61.2 %, 78.7 %, 47.3 %, and 9.4 %, respectively, compared with orchards without grass cover. The grass had been removed by clean tillage. A grass cover also increased activities of invertase, urease, acid phosphatase, alkaline phosphatase, catalase, and cellulase by 26.0 %, 27.0 %, 15.1 %, 26.6 %, 11.9 %, and 71.0 %, respectively. These changes varied with the traits of cover grass, such as grass source and nitrogen fixation; climatic conditions, such as mean annual temperature and precipitation; edaphic variables, such as soil pH, soil texture, and soil sampling season; and management practices, such as orchard age, duration of grass cover, and grass sowing mode. Additionally, we provided a scientific basis to bridge the knowledge of soil nutrients, soil microbes, enzyme activities, SOC contents, and fruit yield in grass-covered orchards. Therefore, given the traits of cover grass, climate, soil, and managerial conditions, the data from this study has valuable implications for the site-specific management of grass coverage and sustainable orchard production.