Formation and regulation mechanisms of grassland productivity: A review

<p indent="0mm">Grasslands cover approximately 40% of the global land area (excluding the Antarctic and Greenland), playing a fundamental role in supporting food, water, and other ecological services for one-third of the global population. However, grassland ecosystems have suffered from severe degradation due to overgrazing and climate change, constraining their sustainable production. In China, grasslands are the second largest terrestrial biome, approximately 90% of which is degraded. Improving grassland productivity and its stability are among the top challenges nowadays. Here, we review the mechanisms of grassland productivity formation and regulation at four levels: the dependence of grassland community productivity on plant diversity, plant-soil interactions regulating primary productivity, the transition from primary to secondary productivity, and the trade-off between production functions and carbon sink functions. The stability of grassland primary productivity involves multi-dimentional interactions between biotic and abiotic factors, including plant community structure, soil microorganisms, climate change, soil nutrients, etc. Increasing evidence shows that high biodiversity can benefit ecosystem primary productivity and stability. However, the relationship between biodiversity and productivity is largely constrained by abiotic resources and climate change. In temperate grasslands, water and nitrogen are the two primary limiting factors. Under climate change, precipitation variability and nitrogen deposition have significantly increased, whose effects are vital to consider in water and nutrient management. Additionally, recent evidence shows that the interactions between soil microorganisms and plants can significantly impact primary producitivity and stability. The transition from primary to secondary productivity is influenced by grazing intensity and supplementary feeding during the grazing season. The grazing optimization hypothesis postulates that the primary productivity peaks at the medium grazing intensity. However, the optimal points for the primary production is not necessarily that for carbon sinks. The trade-off between grassland productivity and carbon sink functionality are affected by different management measures and climate fluctuations because of large spatial and temporal heterogeneity. Currently, multidimensional and multisite networks are still lacking, suppressing the understanding of mechanisms that regulate grassland productivity on large scales. Therefore, we summarize the following research opportunities in the future. First, plant community structure is among the top factors determining grassland productivity and stability. Yet the community structure, which maintains relatively high productivity and stability, is region-dependent. Revealing the spatial pattern of optimal community structures in different grassland ecosystems would be necessary. On top of that, exploring techniques of reseeding and fertilization to enhance primary productivity through plant-microbe interactions would facilitate grassland restoration. Additionally, grazing is the most common grassland management practice, affecting multitrophic biodiversity and productivity. It is critical to clarify the role of grazing in maintaining the stability of grassland productivity through plant-animal interactions. Meanwhile, livestock produces approximately 14.5% of anthropogenic greenhouse gas emissions (CO₂-equivalent), whereas grassland vegetation and soils are vital carbon reservoirs and sinks. In the era of climate change, studying the synergetic mechanisms of grassland productivity and carbon sink potential is a critical issue. To achieve the research goals, interdisciplinary approaches that integrate grassland science, animal husbandry, biochemistry, genomics, ecology, and other related disciplines are vital in studying the mechanisms of grassland productivity in response to climate change and management.