Abstract
In the last several decades, overgrazing has led to various changes in the plant communities, soil nutrients and soil microbial communities in alpine Kobresia meadows, which contain various plant communities coexisting on the Qinghai–Tibet Plateau. Investigating the variations in the biomass and concentration of nutrients in the plant–soil system in these communities may improve understanding of the biochemical responses and adaptation strategies they use to resist disturbances due to overgrazing. We therefore assessed 12 factors across four grazing intensities in alpine Kobresia meadows to explore the following three questions. (1) What the responses are in alpine Kobresia meadows to overgrazing. (2) How they affect plant–soil systems in alpine Kobresia meadows under overgrazing. (3) What factors can be used to evaluate the effects of overgrazing on the ecosystem health status of alpine Kobresia meadows. The results gave the following answers to the above questions. (1) Overgrazing caused the total aboveground biomass to decrease from 333.2 ± 17.4 g/m2 to 217.4 ± 30.2 g/m2, the coverage of plant functional groups of Gramineae and Cyperaceae to decrease from 74.2 ± 3% to 22.5 ± 1.9%, and the total belowground biomass to increase from 4028.5 ± 7.3 g/m2 to 6325.6 ± 24.8 g/m2. (2) Overgrazing resulted in variations in plant–soil systems at three levels. The concentrations of carbon (C) in soil nutrients and plant communities, explained 50.9% of the variation of biomass in plant functional groups; the concentration of soil available nutrients, explained 22.2% of the variation; and the ratio of C and N in shoots and soil total N, explained 11.0% of the variation. (3) The variations in C/N stoichiometry in total soil nutrients and soil microorganisms were 3.4–8.4% and 2.0–3.0%, respectively, and the load of (ammonium-nitrogen (NH4+-N) + nitrate-nitrogen (NO3–-N)) to growth of roots tissue increased from 84.1 ± 5.0 g/m2/(mg/kg) via 99.0 ± 1.3 g/m2/(mg/kg) to 86.1 ± 2.1 g/m2/(mg/kg) at 0 to 40 cm soil in an alpine meadow with grazing intensities rising. Overgrazing would thus increase the deficit of those two kinds of inorganic N on roots growing by 11.4%, 17.7% and 2.4% as grazing rates increased by 93.3%, 126.7% and 213.3%, respectively, compared to a meadow grazed at the lowest rate in the research. We concluded that the alpine meadow changed its distribution of biomass in the plant community, which increased the limiting nutrient deficit on production and altered the concentration and ratio of C and N. This destroyed the original balance to enable the plant community to resist overgrazing. Plot “KH”—a pasture with a grazing intensity next to the lowest one—was the key state in which persistent overgrazing could increase the limiting nutrient load on plant community production, change the dominant position of functional plant groups and species, and lead to plant community degradation. Using ratio of Gramineae to Cyperaceae or Kobresia humilis to K. pygmaea to monitor plant community succession could indirectly estimate these limiting nutrients deficit and balance, and their strategy for incorporating matter into roots and shoots. However how to use those outward characteristics to assess the ecosystem health requires further studies.
Highlights
Alpine Kobresia meadows are the typical and primary pasture of the Qinghai–Tibet plateau, occupying more than 35% of its total area
This has resulted in about 90% of Kobresia meadows on the Qinghai–Tibet Plateau deteriorating into other plant communities, forming diverse plant communities coexisting in the same climate and topography, or succeeding into disturbed disclimax stage [3]
The balance of material and energy in plant–soil systems are partially determined by the ability of plant species in tolerating resource limitation in a particular environment and specific ecological processes, while they can be measured based on inputs by plants to the environment and outputs of heterotrophic respiration and erosion [4,5]
Summary
Alpine Kobresia meadows are the typical and primary pasture of the Qinghai–Tibet plateau, occupying more than 35% of its total area. Plateau [2] has led to various alteration in plant community structure, soil nutrient concentrations and soil microbial community composition and structure. This has resulted in about 90% of Kobresia meadows on the Qinghai–Tibet Plateau deteriorating into other plant communities, forming diverse plant communities coexisting in the same climate and topography, or succeeding into disturbed disclimax stage [3]. Plant community structure and composition as well as soil nutrient concentrations are the most important factors that change in response to overgrazing in alpine Kobresia meadows, for ecosystems have the ability to adjust their structures and contents of material and energy by altering those factors referred above as a response to disturbance. Gaining knowledge on adjustment in plant–soil composition and structure as well as their nutrient concentrations and allocation can reveal the mechanisms of biochemical reactions and dynamic ecosystem adaptions [8,9]
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