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

      Barger, Nicole N.; Ojima, Dennis S.; Belnap, Jane; Shiping, Wang; Yanfen, Wang; Chen, Zuozhong (Society for Range Management, 2004-11-01)
      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.
    • Litter Decomposition in Semiarid Grassland of Inner Mongolia, China

      Liu, Ping; Huang, Jianhui; Han, Xingguo; Sun, Osbert J. (Society for Range Management, 2009-07-01)
      Long-term overgrazing has significantly changed plant species composition in rangeland ecosystems, and this change may alter ecosystem functioning remarkably. In this study, decomposition rates and nutrient dynamics of pure litter (leaf, stem, or root litter) and 11 litter mixtures (from two to five litter components), including nine aboveground litter mixtures and two root litter mixtures, of five common plant species in degraded semiarid rangelands of northern China were studied for 1 yr. We found that fine root litters generally decomposed faster and had faster nutrient turnover rates than leaf and stem litters. The decomposition rates of leaves and stems were significantly and positively correlated with initial litter nitrogen (P < 0.01) and phosphorus contents (P < 0.05), and the decomposition rates of fine roots were significantly and negatively correlated with initial litter carbon:nitrogen ratios (P < 0.05). Nonadditive effects were found in six out of the nine aboveground litter mixtures (three positive and three negative). There were only additive effects on decay rates and nutrient fluxes in the two root litter mixtures. The occurrence and direction of nonadditive effects were dependent on the properties of component litters and had no obvious correlations with litter diversity. Negative mixing effects on nutrient immobilization can facilitate the release of some important nutrients during litter decomposition processes, and further help to accelerate nutrient cycling in such semiarid rangeland ecosystems. Our results indicate that change of plant species composition by overgrazing may slow down the mass loss rates, but may not necessarily impact the release of some nutrients. 
    • Stocking rate and grazing season modify soil respiration on the Loess Plateau, China

      Chen, J.; Hou, F.; Chen, X.; Wan, X.; Millner, J. (Society for Range Management, 2015-01)
      The influence of sheep grazing on carbon cycling in the midarid steppe of the Loess Plateau, Gansu, China, was investigated by measuring the CO2 exchange rate in pastures with a 7-year history of zero, light, moderate, and heavy grazing. Farming systems in the area are characterized by heavy grazing pressure and low vegetation productivity. The effect of stocking rate on soil respiration (Rs) was determined using field trials to investigate factors influencing spatial and temporal variation in Rs in August 2008 on two sites: summer grazed and winter grazed. Soil respiration is an important component of the carbon cycle in rangeland. Measurements included daily Rs, soil temperature, soil moisture, and root biomass. Daily Rs was also measured in late April and middle December 2008. The response of Rs to increasing stocking rate in August 2008 differed with site; on the summer-grazed site increased stocking rates reduced Rs (P &lt; 0.02), whereas on the winter-grazed site, stocking rate had little effect on Rs. Path analysis revealed that on the summer-grazed site, soil moisture had the greatest influence on Rs, whereas on the winter site soil temperature was the most important factor; stocking rate had the least influence on Rs at both sites. The results highlight the importance of environmental variability in determining the effects of grazing on Rs. © 2015 Society for Range Management. Published by Elsevier Inc. All rights reserved.
    • Sustainability of Inner Mongolian grasslands: application of the Savanna model

      Christensen, L.; Coughenour, M. B.; Ellis, J. E.; Chen, Z. (Society for Range Management, 2003-07-01)
      The sustainability and resilience of an Asian typical steppe grazing ecosystem was assessed by determining thresholds and stable states with an ecosystem simulation model. This analysis used the Savanna model to simulate spatial climate, vegetation, and livestock grazing dynamics, at 14 different stocking rates (5.5-59.8 AUY km-2). Grazing effects on vegetation were assessed, including effects on primary production, vegetation composition, and root biomass. Simulations were run for 100 years: 50 years to examine sustainability and 50 years to examine resilience of the system. Results showed that a grazing intensity (1-g/u; g = biomass in grazed area, u = biomass in ungrazed area) of 0.49 was sustainable for this particular system. This region was resilient to grazing up to the intensity of 0.49, where the system remained dominated by herbaceous production. Grazing intensities higher than 0.49, in combination with low precipitation events, resulted in decreased herbaceous net primary production and root biomass, and increased shrub net primary production and root biomass. Herbaceous vegetation was unable to gain a competitive advantage over shrubs in areas where grazing intensities were above 0.49; consequently, the system shifted to a stable shrub-dominated state that could not return its original composition even without further grazing.