Evapotranspiration and Its Partitioning in Alpine Meadow of Three-River Source Region on the Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau (QTP) is generally considered to be the water source region for its surrounding lowlands. However, there have only been a few studies that have focused on quantifying alpine meadow evapotranspiration (ET) and its partitioning, which are important components of water balance. This paper used the Shuttleworth–Wallace (S–W) model to quantify soil evaporation (E) and plant transpiration (T) in a degraded alpine meadow (34°24′ N, 100°24′ E, 3963 m a.s.l) located at the QTP from September 2006 to December 2008. The results showed that the annual ET estimated by the S–W model (ETSW) was 511.5 mm (2007) and 499.8 mm (2008), while E estimated by the model (ESW) was 306.0 mm and 281.7 mm for 2007 and 2008, respectively, which was 49% and 29% higher than plant transpiration (TSW). Model analysis showed that ET, E, and T were mainly dominated by net radiation (Rn), while leaf area index (LAI) and soil water content at a 5 cm depth (SWC5cm) were the most important factors influencing ET partitioning. The study results suggest that meadow degradation may increase water loss through increasing E, and reduce the water conservation capability of the alpine meadow ecosystem.

Alpine meadow degradation depresses soil nitrogen fixation by regulating plant functional groups and diazotrophic community composition

Aims Biological nitrogen fixation (BNF), a function performed by diazotrophic microbes, plays an essential role in nitrogen (N) bioavailability in terrestrial ecosystems. However, little is known about the effects of degradation on soil BNF and diazotrophic communities in alpine meadow. Methods We investigated the changes in soil BNF and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau (non-degraded, lightly degraded, moderately degraded, and severely degraded meadows) using real-time quantitative PCR and amplicon sequencing. Results Soil BNF rates decreased significantly along the meadow degradation gradient with a range of 17.34–79.84 nmol C2H4 g− 1 dry soil d− 1 across all sites. The highest BNF was observed in the non-degraded meadow and was 1.5–4.6-fold higher than that in degraded meadows. Meadow degradation significantly reduced the gene abundance of nifH and the Shannon and Chao1 diversity indices of diazotrophs, accompanied by a decrease in plant biomass, soil moisture, and nutrient content (C, N component). Soil BNF potential was closely correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, Rhizobiales, and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, and both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributed simultaneously to the BNF. The plant functional groups, especially the sedges family, were the primary drivers for soil BNF rates via mediating soil moisture, nutrient level (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Conclusions Our results reveal the underlying mechanism of changes in soil BNF during alpine meadow degradation, emphasize the importance of plant functional groups in shaping the diazotrophic community and BNF potential, and provide insights for the restoration of degraded meadow ecosystems.

Soil nutrients dynamics and the evolution of multi-decadal degradation in alpine wetlands of the Qinghai-Tibetan Plateau

<p>The Qinghai-Tibetan Plateau (QTP) is the highest plateau on earth and has a large area of alpine swampy meadows. In the past few decades, overgrazing and climate change have caused severe desiccation and degradation of the alpine wetlands. The remote sensing technology has been used to assess the wetland shrinkage. However, changes in soil nutrients associated with the duration of alpine wetland degradation are poorly known. We took soil samples in three swampy meadows of the QTP, one terrain was relatively flat and the other two were with hummocks and hollows. Decadal gradients of degradation from nondegraded swampy meadows to degraded meadows were selected. The contents of soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) were analyzed. The SOC, TN, and TP contents loss in degraded swampy meadows occurred mainly during the first decade. The soil nutrients loss was highly affected by the geomorphic characteristics of the wetland area. After degradation, the SOC, TN, and TP contents decreased at exponential rates on the flat terrain site. The top layer SOC, TN, and TP contents of the degraded about 30 years area were 24.76±0.91, 2.22±0.07 and 0.45±0.01 (mean ± SE) g kg<sup>-1</sup>, respectively; and the SOC, TN, and TP contents decrease were 75%, 72% and 56% that of the nondegraded swampy meadows, respectively. On one hummock-hollow sites the top layer SOC, TN, and TP contents of the degraded for about 30 years area were 61.22±11.94, 4.09±0.73 and 0.44±0.05 (mean ± SE) g kg<sup>-1</sup>, respectively; and the SOC, TN, and TP contents decrease were 45%, 52% and 46% that of the nondegraded swampy meadows, respectively. The soil nutrients decomposition rate of hummock-hollow sites was much lower than the flat terrain site. Large and tough hummocks in swampy meadow degradation sites can resist environment erosion and stabilize soil nutrients content at high levels.</p>