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While it is established that dominant plant species of alpine meadows showed differential preference for N forms (ammonia, nitrate, and amino acids) under various degradation stages, the perseverance of the N-uptake preference and its affecting factors remains unknown. This is an important consideration because it determines efficacy of nutrient additions for restoration of degraded alpine meadows. An indoor pot experiment was conducted to investigate the plasticity and determinants of different plant species' N-uptake preference using 15N-labeled inorganic N (15NH4+ and 15NO3−) and one of dual-labeled (13C-15N) amino acid (glycine). In the experiment, dominant species of alpine meadow from specific degradation status were planted in soils of alpine meadows with three different degradation status. Most species preferred to uptake nitrate in all soils, except the Kobresia humilis, Carex moorcroftii, and Aster flaccidus planted in the soil of severely degraded alpine meadow (SD-soil) that take up more ammonia. The relative abundance of different available N forms directly affects the N-uptake preferences of all species. The partial correlations between percentage uptake and availability of various N forms were different with the zero-order correlations when either soil moisture or pH was controlled. Differences in soil moisture and pH among the three alpine meadows affects the N uptake preference of the nine species through their impacts on the relative abundance of different available N forms. In conclusion, the differences in soil moisture and pH among soils of alpine meadows under different degradation statuses influence the relative abundance of various available N forms, thereby affecting the plant N uptake. [ABSTRACT FROM AUTHOR]

Understanding the plant–soil feedbacks (PSFs) of dominant species of alpine meadow under different degradation status could provide insights into sustainable restoration. The direction, strength, and influencing factors of dominant species' PSFs in nondegraded (Intact), moderately degraded (MD), and severely degraded (SD) alpine meadows were examined in a two-phase PSFs experiment. Species of Intact exhibited neutral conspecific PSFs, whereas those of MD and SD exhibited negative conspecific PSFs. The species of MD demonstrated neutral heterospecific PSFs to those of Intact, whereas that of SD negatively feedbacked to those of Intact and MD. The NO3−-N and NH4+-N of soil conditioned by the species of Intact were 66% and 58% higher than the control (mixture soil conditioned by all species); but they were 37% and 32% lower in soil conditioned by the dominant species of SD. The relative abundance of soil fungal pathotrophs was 57% and 74% higher in soil conditioned by the dominant species of MD and SD than in Intact soil. The conspecific and heterospecific PSFs of all species positively correlated with the plant conditioning and degradation induced changes of difference in NO3−-N and NH4+-N and negatively correlated with the difference in relative abundance of pathotrophs. Soil microorganisms and nutrients explained most of the variation in conspecific (43%) and heterospecific PSFs (60%). Our results indicated that the N addition would facilitate the sustainable restoration of degraded alpine meadows because the addition of available N could drive the heterospecific PSFs toward more positive. [ABSTRACT FROM AUTHOR]

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 μg 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 μg 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 μg 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). [ABSTRACT FROM AUTHOR]

Different lowercase letters indicate significant differences in soil water content, exchangeable HT ht , HT ht , dissolved organic C (DOC) and DON, and the ratio of exchangeable HT ht to HT ht among degradation stages at I p i < 0.05. Different uppercase letters indicate significant differences between exchangeable HT ht and HT ht at I p i < 0.05 at the same degradation stage. [Extracted from the article]