Impact of grass cover on the soil physicochemical properties in China's orchards: A meta-analysis

Grass cover in orchards has been proposed as a sustainable management system with greater ecological and economic benefits than open pastures. However, few studies have compared the ecological impact of grass-covered orchards with that of traditional tilled systems. Therefore, this meta-analysis was focused on evaluating the ecological benefits of grass cover in orchards on the soil physicochemical properties under different establishment modes and construction period in China. In total, 492 paired data points were extracted from 29 references, and effect values (ES) were calculated to show the effect of grass cover on soil physicochemical properties. The results showed that grass cover in orchards improved soil physicochemical properties by decreasing the soil bulk density (ES: -0.0659) of three soil layers (0-20 cm, 20-40 cm and 40-60 cm depth) and by decreasing soil pH in the alkaline areas (ES: -0.0219) of 0-20 cm and 20-40 cm soil layers. Grass cover in orchards also significantly improved the nutrient contents of the topsoil layer (0-20 cm depth), having the greatest effect on the total nitrogen content (ES: 0.1416), followed by that on the total phosphorus content (ES: 0.1002), and the weakest effect was observed on the total potassium content (ES: 0.0832). Grass cover in orchards significantly increased the soil available nitrogen content at all three soil depths (ES: 0.1608) and the soil available phosphorus content at the 0-40 cm soil depth (ES: 0.0788). However, no significant effect on the soil available potassium content was detected. Grass cover in orchards significantly increased soil organic matter content at all three soil depths (ES: 0.3734; P < 0.01), with the greatest effect occurring in the 40-60 cm soil layer (ES: 0.7972). Subgroup analysis of available nutrients in the 0-20 cm soil layer showed that 2-3 years after grass covering in orchards greatest promoting on the soil available nitrogen (ES: 0.2967) and phosphorus contents (ES: 0.1815), while the soil available potassium content decreased with construction period. In summary, grass cover in orchards has various effects on soil physicochemical properties, and these effects are closely related to soil layer depth, construction period, plant species and geographical region. Moreover, long-term monitoring is required to confirm the ecological influence of diverse types of grasses on soil properties in orchards.