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Glaciers are among the least explored environments on Earth, especially from a perspective of nutrient stoichiometry. Here, we documented and compared the nutrient concentrations and stoichiometric ratios in six distinct habitats of a glacier terminus in the source area of the Yangtze River, including surface ice (SI), basal ice (BI), basal sediment (BaS), newly exposed forefront soil close to glacial terminus (TS), soil at increasing distances from glacier terminus (DS), and soil with well-developed vegetation (VS). The results showed that SI had higher dissolve organic carbon (DOC) and nitrogen (N) concentrations as well as higher C:P (carbon:phosphorus) and N:P ratios than BI. However, BI had higher soluble reactive P (SRP) than SI. In addition, both SI and BI had very high C:P and N:P ratios, suggesting P-limitation. For sediment/soil along the chronosequence, total N (TN) and organic carbon (SOC) concentrations were lower in BaS, TS, and DS than in VS. Moreover, SRP concentration was higher in BaS and VS than in TS and DS. These nutrient patterns could be explained by differences in biotic influence in soil development or by changes in soil physical properties. With regard to nutrient stoichiometry, BaS, TS, and DS had low SOC:TN ratios but high dissolved inorganic N (DIN):SRP ratios. Considering both nutrient concentrations (high SOC and N but very low SRP) and stoichiometric ratios (low SOC:TN but high DIN:SRP), our data support an interpretation that P-limitation is dominant along the entire chronosequence following deglaciation. Considering that glaciers are experiencing severe retreat, this study can further our understanding and prediction of global biogeochemical cycles under future climate change. [ABSTRACT FROM AUTHOR]

Aims: Grasslands are experiencing severe degradation globally, impacting aboveground vegetation and soil properties. The influences of grassland degradation on bacterial communities in soil are not well-understood. Methods: The normalized difference vegetation index (NDVI) was calculated to represent grassland status and indicate grassland degradation (decreasing NDVI). Soil ph, bacterial communities, as well as nutrient and organic carbon concentrations were measured. Results: Bacterial alpha diversity had negative relationships with soil moisture, soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP). Bacterial community structure was significantly associated with NDVI, the change rate of NDVI, moisture, ph, SOC, TN, as well as soil C:N and C:P ratios. Bacterial phyla were differentially related with these environmental variables. Moreover, network analysis showed that the network of soil bacteria had strong cooperation relationships (positive correlations between taxa) and was grouped into three modules. According to modularity, 71 keystone taxa were detected as network connectors and module hubs. All the modules had close relationships with environmental variables, and many keystone taxa were negatively associated with soil moisture and SOC. Conclusions: These results suggest that grassland degradation might be responsible for shifts in soil bacterial communities in terms of alpha diversity and community structure through changes in soil moisture, SOC, nutrients, and C:nutrient ratios. Moreover, in the network analyses, the strong co-occurrence relationships between taxa as well as close relationships between environmental variables and module structures and keystone taxa suggest low stability and high vulnerability of bacterial communities to the influences of grassland degradation. [ABSTRACT FROM AUTHOR]

Grassland degradation, a process of retrogressive succession, is an extensive global threat, especially on the Qinghai-Tibet Plateau. However, the influences of grassland degradation on carbon (C): nitrogen (N): phosphorus (P) ratios of streamwater and biofilms, as well as their implications for nutrient limitation, are not well studied. Assessing nutrient limitation and stoichiometry could facilitate understanding and mitigating the effects of nutrient input variations on stream ecosystems. Using a "space for time" substitution, contemporary grassland status quantified by landscape-level normalized difference vegetation index (NDVI) were used to study potential impacts of grassland degradation on nutrient cycling in streams feeding Qinghai Lake (China). C, N, and P concentrations and ratios of streamwater and benthic biofilms were assessed. The results showed that grassland degradation was associated with lower DOC, TN, DOC:TP, and TN:TP of streamwater, especially in September. DOC:TN:TP ratios of streamwater demonstrated relatively high variability with an average of 387:169:1 (molar). C, N, P biomass per unit area of stream biofilm also demonstrated considerable variability but their ratios were relatively constrained, with an average of 471:32:1. Moreover, biofilm C, N, and P concentrations and ratios were not closely associated with streamwater nutrients or grassland degradation. The complex chemical and biological composition of stream biofilms may make their stoichiometry difficult to interpret. However, the non-isometric relationships between biofilm C and P, as well as the large imbalance between C:N:P ratios of streamwater and biofilms, suggest that stream biofilms in this catchment are sensitive to P variation and potentially limited by DOC and P. By differentially impacting DOC and nutrient supplies, grassland degradation may alleviate P limitation but aggravate carbon limitation in stream biofilms. [ABSTRACT FROM AUTHOR]

Grassland is among the largest terrestrial biomes and is experiencing serious degradation, especially on the Qinghai–Tibet Plateau (QTP). However, the influences of grassland degradation on microbial communities in stream biofilms are largely unknown. Using 16S rRNA gene sequencing, we investigated the bacterial communities in stream biofilms in sub-basins with different grassland status in the Qinghai Lake watershed. Grassland status in the sub-basins was quantified using the normalized difference vegetation index (NDVI). Proteobacteria, Bacteroidetes, Cyanobacteria, and Verrucomicrobia were the dominant bacterial phyla. OTUs, 7,050, were detected in total, within which 19 were abundant taxa, and 6,922 were rare taxa. Chao 1, the number of observed OTUs, and phylogenetic diversity had positive correlations with carbon (C), nitrogen (N), and/or phosphorus (P) in biofilms per se. The variation of bacterial communities in stream biofilms was closely associated with the rate of change in NDVI, pH, conductivity, as well as C, N, P, contents and C:N ratio of the biofilms. Abundant subcommunities were more influenced by environmental variables relative to the whole community and to rare subcommunities. These results suggest that the history of grassland degradation (indicated as the rate of change in NDVI) influences bacterial communities in stream biofilms. Moreover, the bacterial community network showed high modularity with five major modules (>50 nodes) that responded differently to environmental variables. According to the module structure, only one module connector and 12 module hubs were identified, suggesting high fragmentation of the network and considerable independence of the modules. Most of the keystone taxa were rare taxa, consistent with fragmentation of the network and with adverse consequences for bacterial community integrity and function in the biofilms. By documenting the properties of bacterial communities in stream biofilms in a degrading grassland watershed, our study adds to our knowledge of the potential influences of grassland degradation on aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Nitrogen (N) and phosphorus (P) are key growth‐limiting nutrients for organisms and their absolute and relative supplies regulate the structure and function of ecosystems. Landcover changes lead to modifications of terrestrial biogeochemistry, consequently influencing aquatic nutrient conditions. This study sought to evaluate the potential impacts of grassland degradation on nutrient availability and nutrient limitation in the Qinghai Lake (China) and its inflow streams. We sampled nutrient concentrations and tested stream nutrient limitation by conducting nutrient diffusing substrata (NDS) bioassays in streams flowing through subbasins with different grassland status. To test nutrient limitation and the responses of lake phytoplankton to stream inflows, bioassays were conducted by adding different nutrients (N, P, and joint NP) as well as water from different streams to lake water with phytoplankton, respectively. In general, N concentrations as well as N:P ratios decreased while P concentrations increased with decreased normalized difference vegetation index (NDVI, an index of vegetation status), especially in September, suggesting that grassland degradation (low NDVI) has the potential to differentially decrease N availability and increase P availability in streams. Consistent with this, relative responses (RR) of stream periphyton to P and combined NP enrichments in the NDS bioassays decreased with stream P concentrations while increased with stream water N:P ratios. Lake phytoplankton responded strongly to P and combined NP addition indicating strong P‐limitation of lake phytoplankton. RR of lake phytoplankton to stream water decreased with nitrate concentration and N:P ratios in stream water and increased with the concentrations of ammonium, total phosphorus, and soluble reactive phosphorus, indicating that stream water with higher P but lower N and N:P from degraded subcatchments is associated with increased impact on P‐limited Lake phytoplankton. Overall, this study suggests that grassland degradation has the potential to differentially influence the nutrients delivered to streams with substantial increases in P but decreases in N and N:P, alleviating P limitation of stream periphyton and, ultimately, stimulating P‐limited phytoplankton growth in the lake. [ABSTRACT FROM AUTHOR]