Grazing exclusion alters ecological stoichiometry of plant and soil in degraded alpine grassland

Abstract

Grazing exclusion is one of the most effective practices to recover the degraded alpine grasslands in Tibetan Plateau. In addition, ecological stoichiometry serves as a key regulator of ecosystem function and is also closely related to nutrient process. However, knowledge about the plant-soil interactions of stoichiometry throughout the growing season is still poorly understood. In this study, we examined how grazing exclusion affected the change of C-N-P stoichiometry of plant-soil by carrying out monthly sampling in three sites. The sites were: 9 years grazing exclusion, GE9; 5 years grazing exclusion, GE5; and free grazing, FG in degraded alpine meadow. Plant and soil samples were collected five times during the growing season (May-September) to analyze plant biomass and ecological stoichiometry. Across plots, profound fencing changed plant biomass and ecological stoichiometry. Specifically, increases in green, litter and root biomass were observed under grazing exclusion. Fencing increased plant C content, C:N, C:P, N:P ratios but declined N and P contents. In contrast to the plant, there was an increasing concentrations of soil C, N and P. Our study highlighted, under grazing exclusion, grasslands advanced soil C not only by greater biomass input but also higher plant C content. Greater soil N was mainly fixed by plant production, while enhancement of soil P was mainly due to plant production recovery to stop water erosion. With a continuous improvement of nutrient pools under short-term (< 9 years) grazing exclusion, our results showed that a long-term fencing practice effect ought to be monitored to recovery degraded alpine grassland. These findings also indicated that ecological stoichiometry provides a new insight for better understanding of the mechanisms of grazing exclusion effects on nutrient process.