Grazing regulation of phosphorus cycling in grassland ecosystems: Advances and prospects

Abstract

Grassland ecosystems, which cover a total area of 52.5 million square kilometers and account for 40.5% of the global land surface, are the largest biomes in the world. Grazing is the most dominant and direct land use type in grasslands and a predominant driver regulating multiple ecosystem functions and services, such as primary production, nitrogen (N) and phosphorus (P) cycling, carbon sequestration, forage and food production, and water and soil erosion control etc. Many studies have demonstrated that primary production of grasslands is often limited by water, N and P availability, which interact with grazing to influence the maintenance of ecosystem functions and services. However, compared with numerous studies on the effects of grazing on N cycling and plant water use, fewer studies have explored how grazing influence P cycling of grasslands, particularly the internal cycling of P among main pools within ecosystems. Here we reviewed the advances in grazing effects on soil and plant P pools and biological processes of P cycling. Our review is mainly focused on: (1) Grazing effects on internal and external cycles of P in grassland ecosystems; (2) grazing effects on soil P vertical distribution, and P inputs and outputs mediated by soil erosion and spatial heterogeneity; and (3) mechanisms underlying grazing-induced changes in plant P content and P pools across different levels of organization (individual, species, functional group and community). Finally, we proposed a unified framework synthesizing the influence of grazing on P pools and key biological processes of P cycling in grassland ecosystems. We also proposed eight key questions that need to be addressed in future studies: (1) How do the dominant P forms (e.g., nucleic acids, phospholipids, cytoplasmic inorganic P, cytoplasmic organic P, and polyphosphate) and concentration within soil microbial biomass vary with grazing intensity? How does the role of microorganisms as sink and source of P change with increasing grazing intensity? (2) How does grazing affect enzyme activities, including phosphomonoesterases, phosphodiesterases, triphosphoric monoester hydrolases, and enzymes acting on phosphoryl-containing anhydrides and on P–N bonds, and thereby P mineralization in soil? (3) How does grazing regulate the biological processes of P transformation in plant-soil-microbial P cycle? (4) To what degree molecular approaches and omics analyses can improve our understanding of the important role of plants and rhizosphere microorganisms in P cycling? (5) How does grazing affect the relationship between plant diversity and arbuscular mycorrhizal fungi (AMF) diversity, and thereby plant P uptake and utilization? (6) How do grazing animals differ in their effects on the pools and pathways of P cycling due to the interspecific differences in diet and foraging preference? (7) How do other herbivores in terms of grasshoppers, rodents, earthworms, and plant feeding nematodes influence the biological processes of P cycling? (8) How does grazing affect the N:P stoichiometry of key ecosystem compartments, and hence the coupled N and P cycles.