Effects of Long-Term Grazing Exclosure and Plateau Pika Disturbance on Biochemical Cycling in an Alpine Meadow, West China

Abstract

Grazing exclusion and excessive disturbance by plateau pika can change grassland carbon (C), nitrogen (N) and phosphorus (P) cycles, with their effect being particularly pronounced in areas affected by overgrazing. Specifically, grazing exclusion mitigates soil disturbance and vegetation damage caused by grazing in such regions, while plateau pika of a high population density can further regulate nutrient cycling processes through behaviors like burrowing and herbivory. These two sets of changes also exert impacts on net ecosystem carbon exchange (NEE). However, the regulatory role of such nutrient cycle changes and NEE in response to grassland degradation remains largely unknown. Here, we conducted a 10-year fenced experiment with three treatments including regular grazing (CK), fenced without (FN) pika and with pika (FP) disturbance to explore the linkage between NEE and plant-soil C:N:P stoichiometry in long-term (up to 10 years) grazing-excluded and plateau pika-disturbed alpine meadows. Results showed that compared with fenced meadows devoid of pika disturbance, the regularly grazed meadows had significantly reduced microbial biomass carbon, above-ground biomass and NEE, but no significant differences in the elemental concentrations and ratio in leaves, roots, or soil. Plateau pika disturbance further reduced leaf N and P, soil C, N, and P, above-ground biomass and NEE, while increasing leaf C:N and C:P ratios, compared to fenced meadows without plateau pika. In contrast, P concentrations in roots were reduced significantly in the enclosed pika-active meadows, together with the pre-existing reduction in leaves and soil, resulting in increased leaf C:N, C:P, N:P, root N:P, and microbial C:N, and C:P, compared to the regularly grazed meadows. Leaf element concentrations and ratios have a direct effect on NEE, and soil element concentrations and their ratios have an indirect effect on NEE by affecting above-ground biomass and microbial element concentrations. Our results emphasize that above- and below-ground C:N:P stoichiometry strongly regulated NEE, and incorporating C:N:P stoichiometry into next generation Earth system models may improve predictions of climate-grassland feedbacks in the Anthropocene. Additionally, targeted rodent control may be necessary to restore grassland balance in post-enclosure pika-disturbed areas.