Changes in Plant Functional Groups, Litter Quality, and Soil Carbon and Nitrogen Mineralization With Sheep Grazing in an Inner Mongolian Grassland

This study reports on changes in plant functional group composition, litter quality, and soil C and N mineralization dynamics from a 9-year sheep grazing study in Inner Mongolia. Addressed are these questions: 1) How does increasing grazing intensity affect plant community composition? 2) How does increasing grazing intensity alter soil C and N mineralization dynamics? 3) Do changes in soil C and N mineralization dynamics relate to changes in plant community composition via inputs of the quality or quantity of litter? Grazing plots were set up near the Inner Mongolia Grassland Ecosystem Research Station (IMGERS) with 5 grazing intensities: 1.3, 2.7, 4.0, 5.3, and 6.7 sheep ha−1·yr−1. Plant cover was lower with increasing grazing intensity, which was primarily due to a dramatic decline in grasses, Carex duriuscula, and Artemisia frigida. Changes in litter mass and percentage organic C resulted in lower total C in the litter layer at 4.0 and 5.3 sheep ha−1·yr−1 compared with 2.7 sheep ha−1·yr−1. Total litter N was lower at 5.3 sheep ha−1·yr−1 compared with 2.7 sheep ha−1·yr−1. Litter C:N ratios, an index of litter quality, were significantly lower at 4.0 sheep ha−1·yr−1 relative to 1.3 and 5.3 sheep ha−1·yr−1. Cumulative C mineralized after 16 days decreased with increasing grazing intensity. In contrast, net N mineralization (NH+4 + NO-3) after a 12-day incubation increased with increasing grazing intensity. Changes in C and N mineralization resulted in a narrowing of CO2-C:net Nmin ratios with increasing grazing intensity. Grazing explained 31% of the variability in the ratio of CO2-C:net Nmin. The ratio of CO2-C:net Nmin was positively correlated with litter mass. Furthermore, there was a positive correlation between litter mass and A. frigida cover. Results suggest that as grazing intensity increases, microbes become more C limited resulting in decreased microbial growth and demand for N