Collaborative restoration between vegetation and soil attributes in degraded grassland of different grassland types

<p indent="0mm">Grasslands cover approximately 40% of the land in China and provide numerous ecosystem services, such as forage and food production, water supply and regulation, erosion control, carbon sequestration, and climate mitigation. However, over 90% of grasslands have been degraded due to overgrazing, reclamation to cropland, mineral exploration, and climate change. This degradation of grasslands has resulted in serious environmental, social, and economic impacts. Over the past two decades, a variety of restoration approaches have been underscored to restore biodiversity, ecosystem functioning, and services of degraded grasslands in China. Several syntheses have documented the restoration outcomes of grazing exclusion and land abandonment referring to degraded grasslands in China. However, the factors controlling the recovery of ecosystem attributes and the overall effectiveness of these restoration actions compared to native grasslands remain poorly documented. The natural and undisturbed grasslands assemble with high species diversity, long-lived perennial plants, and well-developed belowground structures, hence they can be used as references for grassland restoration. Based on data from 380 publications regarding ecological restoration experiments at 140 sites, we constructed a restoration database of degraded grasslands and a natural grassland database for four grassland types in China: alpine meadow, meadow steppe, typical steppe, and desert grassland. We performed a meta-analysis to evaluate the effectiveness of ecological restorations relative to degraded and natural reference grasslands for 18 vegetation and soil variables. Vegetation variables included plant height, abundance, species richness, aboveground biomass, belowground biomass, and the biomasses of grass, forb and legume plants. Soil variables included soil pH, moisture content, the contents of soil organic carbon, total nitrogen and phosphorus, microbial biomass carbon and nitrogen, bacteria and fungi, and soil organic carbon storage. We further explored the influence of restoration approach, climatic conditions, soil properties, and the duration of restoration programs on the recovery of ecosystem attributes. Overall, our results demonstrated that the recovery of biodiversity and soil properties was substantially slower than that of biomass production and plant community structure across different grassland types. Ecological restorations showed larger enhancements in plant community structure and biomass production compared with plant diversity and soil properties. Specifically, aboveground biomass recovered faster than belowground biomass. However, most plant and soil attributes did not fully recover to the levels observed in the natural reference grasslands. Soil moisture, organic carbon, and total nitrogen contents improved to a greater extent in topsoil than in subsoil. Passive and active restorations generally yielded similar outcomes across different grassland types. Moreover, the effectiveness of grassland restoration can be substantially modulated by climatic conditions, soil properties, and the duration of restoration programs. There was collaborative restoration between plant productivity, soil moisture, and soil nutrient contents. The restoration of plant diversity in typical steppe and desert grassland benefited more from the improved soil nutrient contents compared to alpine meadow and meadow steppe. Our findings have important implications for the restoration of plant productivity, community structure, diversity, and ecosystem carbon sink function of degraded grassland ecosystems in China.