Plant α-and β-diversity, and soil microbial stoichiometry co-regulate the alterations in ecosystem multifunctionality in response to grazing and N addition in a typical steppe

Despite their significance, how interactions of plant diversity at multiple spatial scales and soil microbial stoichiometry alter a series of ecosystem functions (multifunctionality, EMF) in response to anthropogenic nitrogen (N) input and herbivores are poorly known. We conducted a 17-year sheep grazing experiment with 6-year N addition to explore the impacts of grazing (0, 2.7, 5.3 and 8.7 sheep ha⁻¹) and N addition (N0, N5, N10 and N20, i.e., 0, 5, 10 and 20 g N m⁻² yr⁻¹, respectively) on grassland functions and EMF via changes in plant α-and β-diversity, and carbon to nitrogen ratio (C:N) of soil microbes in a typical steppe. The results show that grazing or N addition alone significantly affected EMF with a treatment order of 2.7 and 8.7 sheep ha⁻¹ > 0 and 5.3 sheep ha⁻¹ for grazing intensity or N5 > N10 and N20 > N0 for N addition, which resulted in a significant higher EMF in the combination treatment of 2.7 sheep ha⁻¹ and 5 g N m⁻² yr⁻¹. Plant α-and β-diversity, and soil microbial C:N were the predominant drivers of changes in EMF. Grazing reduced EMF indirectly by decreasing the plant β-diversity. N addition promoted EMF indirectly by decreasing plant α-diversity. In addition, lower plant α-diversity enhanced EMF indirectly by increasing soil microbial C:N. Our results suggest that the negative effects of herbivore on EMF were stronger at larger spatial scales compared to the smaller local communities, while N addition could maintain a higher level of EMF at smaller scales rather than at the larger ones. Our results highlight that multiple spatial scales should be considered to fully unravel the effects of herbivore and eutrophication on ecosystem functions. Our results also demonstrate the important role of soil microbe in maintaining higher grassland multifunctionality, thus we should include the soil microbial functions (i.e., C and N transformation) in further studies. Our results suggest that grazing at a low grazing intensity of 2.7 sheep ha⁻¹ with a low N supplementation of 5 g N m⁻² yr⁻¹ could maintain the most important ecosystem functions. Our work provides important insight into grassland conservation and management, aiming to maintain the capacity of grasslands to sustainably supply ecological and productive functions.