| 摘 要： |
A short-term in situ (15) N labeling experiment was conducted to investigate whether the N uptake and preference for different forms of available soil N for dominant plant species and soil microorganisms relate to the plant community productivity change at the no degradation stage, early stage of degradation, and late stage of degradation in an alpine meadow on the Qinghai-Tibetan Plateau. At the early stage of degradation in the alpine meadow, aboveground net primary productivity decreased, while belowground net primary productivity increased. As a result, the total net primary productivity was unchanged at the early stage of degradation. Both aboveground and belowground net primary productivity significantly decreased at the late stage of degradation compared with the non-degraded meadows. Plants and microorganisms mainly absorbed inorganic N and preferred NH4+ at the non-degraded meadows where available soil N (the total concentration of exchangeable NH4+, NO3-, and dissolved organic N) was maintained at a high level of 60.9 mu g N g(-1) dry soil, indicating an N-use chemical niche overlap. Plants and microorganisms showed a niche differentiation at the early stage of degradation where available soil N decreased to a medium level of 44.6 mu g N g(-1) dry soil; plants preferred NO3-, while microorganisms took up more NH4+. In contrast, microorganisms increased their uptake of organic N, while plants assimilated more inorganic N, indicating that plants and microorganisms showed a niche differentiation where available soil N decreased to a low level of 26.6 mu g N g(-1) dry soil at the late stage of degradation. The higher N uptake (30% increase of N uptake compared with non-degraded meadows) of dominant plant species and niche differentiation in using available soil N between plants and microorganisms are two mechanisms maintaining the total community net primary productivity, even when available soil N decreased at the early stage of degradation. Plants and microorganisms also showed a niche differentiation when available N declined further at the late stage of degradation. However, the N uptake by dominant plant species greatly declined at the late stage of degradation (76% reduction of N uptake compared with non-degraded meadows), which might explain the community net primary productivity reduction (78% lower compared with non-degraded meadows). |
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