Yanfen Wang

Effects of warming and grazing on soil N availability, species composition, and ANPP in an alpine meadow

Uncertainty about the effects of warming and grazing on soil nitrogen (N) availability, species composition, and aboveground net primary production (ANPP) limits our ability to predict how global carbon sequestration will vary under future warming with grazing in alpine regions. Through a controlled asymmetrical warming (1.2/1.7 degrees C during daytime/nighttime) with a grazing experiment from 2006 to 2010 in an alpine meadow, we found that warming alone and moderate grazing did not significantly affect soil net N mineralization. Although plant species richness significantly decreased by 10% due to warming after 2008, we caution that this may be due to the transient occurrence or disappearance of some rare plant species in all treatments. Warming significantly increased graminoid cover, except in 2009, and legume cover after 2008, but reduced non-legume forb cover in the community. Grazing significantly decreased cover of graminoids and legumes before 2009 but increased forb cover in 2010. Warming significantly increased ANPP regardless of grazing, whereas grazing reduced the response of ANPP to warming. N addition did not affect ANPP in both warming and grazing treatments. Our findings suggest that soil N availability does not determine ANPP under simulated warming and that heavy grazing rather than warming causes degradation of the alpine meadows.

Determinants influencing seasonal variations of methane emissions from alpine wetlands in Zoige Plateau and their implications

To understand the seasonality of methane flux from alpine wetlands in Zoige Plateau, 30 plots were set to measure the methane emissions in the growing and nongrowing seasons in three environmental types: dry hummock (DH), Carex muliensis (CM), and Eleocharis valleculosa (EV) sites. There were clearly seasonal patterns of methane flux in different environmental types in the growing and nongrowing seasons. Mean methane emission rate was 14.45 mg CH4 m(-2) h(-1) (0.17 to 86.78 mg CH4 m(-2) h(-1)) in the growing season, and 0.556 mg CH4 m(-2) h(-1) (0.002 to 6.722 mg CH4 m(-2) h(-1)) in the nongrowing season. In the growing season, the main maximum values of methane flux were found in July and August, except for a peak value in September in CM sites. In the nongrowing season, the similar seasonal variation pattern was shared among all the three sites, in which the methane emissions increased from February to April. In the growing season, the determining factors were surface temperatures (r(2) = 0.55, P < 0.05), standing water depths (r(2) = 0.32, P < 0.01) and plant community heights (r(2) = 0.61, P < 0.01), while in the nongrowing season, ice thickness (r(2) = 0.27, P < 0.05; in CM and EV sites) was found most related to flux. In our understanding, the seasonality of methane emissions in our study areas was temperature- and-plant-growth-dependent, and the water table position was also very important to shape the temperature- and-plant-growth-dependent seasonal variation of flux with its vigorous variations in alpine wetland ecosystems. Different environmental types within the wetland also influenced the seasonal pattern of methane flux. For an accurate estimate of the global methane source strength of alpine wetlands, the pronounced seasonal or even temporal variability in methane emission from alpine wetlands should be taken into consideration.

Spatial variations on methane emissions from Zoige alpine wetlands of Southwest China.

This study was aimed to understand the spatial variation of CH(4) emissions from alpine wetlands in Southwest China on a field-scale in two phenological seasons, namely the peak growing season and the spring thaw. Methane emission rates were measured at 30 plots, which included three kinds of environmental types: dry hummock, Carex muliensis and Eleocharis valleculosa sites. There were highly spatial variations of methane emissions among and within different environmental types in both phenological seasons. Mean methane emission rates ranged from 1.1 to 37.0 mg CH(4) m(-2) h(-1) in the peak growing season and from 0.004 to 0.691 mg CH(4) m(-)(2) h(-1) in the spring thaw. In the peak growing season, coefficients of variation (CV) averaged 38% among environmental types and 64% within environmental types; while in the spring thaw, CV were on the average 61% among environmental types and 96% within environmental types. The key influencing factors were the standing water table and the plant community height in the peak growing season, while in the spring thaw, no significant correlations between factors and methane emissions were found.

Delayed spring phenology on the Tibetan Plateau may also be attributable to other factors than winter and spring warming

In their recent paper “Winter and spring warming result in delayed spring phenology on the Tibetan Plateau,” Yu et al. (1) reported an interesting but unexpected result that spring phenology initiated retreating in the mid-1990s, despite continued warming for grasslands (both steppe and meadow) on the Tibetan Plateau, and shortening the length of the growing season of the steppe together with an advancing end. Although we have not observed the same phenomenon in our own many years of field studies on the eastern edge of the Tibetan Plateau, we believed that there were indeed some complicated yet poorly understood dynamics and processes in the phenology on the Tibetan Plateau. However, we believe their causes should include factors such as grassland degradation, thawing–freezing processes, climate warming, and their combined effect rather than a sole factor of climate change, like winter and spring warming.

Warming and increased precipitation have differential effects on soil extracellular enzyme activities in a temperate grassland.

Few studies have conducted the responses of soil extracellular enzyme activities (EEA) to climate change, especially over the long term. In this study, we investigated the six-year responses of soil EEA to warming and increased precipitation in a temperate grassland of northern China at two depths of 0-10 and 10-20 cm. These extracellular enzymes included carbon-acquisition enzymes (β-glucosidase, BG), nitrogen-acquisition enzymes (N-acetylglucosaminidase, NAG; Leucine aminopeptidase, LAP) and phosphorus-acquisition enzymes (acid and alkaline phosphatases). The results showed that warming significantly increased acid phosphatase at the 0-10 cm depth and NAG at the 10-20 cm depth, but dramatically decreased BG and acid phosphatase in the subsurface. In contrast, increased precipitation significantly increased NAG, LAP and alkaline phosphatase in the surface and NAG, LAP and acid phosphatase in the subsurface. There was a significant warming and increased precipitation interaction on BG in the subsurface. Redundancy analysis indicated that the patterns of EEA were mainly driven by soil pH and NH(4)(+)-N and NO(3)(-)-N in the surface, while by NH(4)(+)-N and microbial biomass in the subsurface. Our results suggested that soil EEA responded differentially to warming and increased precipitation at two depths in this region, which may have implications for carbon and nutrient cycling under climate change.

Responses of soil respiration and its components to drought stress

PurposeClimate change is likely to increase both intensity and frequency of drought stress. The responses of soil respiration (Rs) and its components (root respiration, Rr; mycorrhizal respiration, Rm; and heterotrophic respiration, Rh) to drought stress can be different. This work aims to review the recent and current literature about the variations in Rs during the period of drought stress, to explore potential coupling processes and mechanisms between Rs and driving factors in the context of global climate change.Results and discussionThe sensitivity of soil respiration and its components to drought stress depended on the ecosystems and seasonality. Drought stress depressed Rs in mesic and xeric ecosystems, while it stimulated Rs in hydric ecosystems. The reductions in supply and availability of substrate decreased both auto- and heterotrophic respirations, leading to the temporal decoupling of root and mycorrhizal respiration from canopy photosynthesis as well as C allocation. Drought stress also reduced the diffusion of soluble C substrate, and activities of extracellular enzymes, consequently, limited microbial activity and reduced soil organic matter decomposition. Drought stress altered Q10 values and broke the coupling between temperature and soil respiration. Under drought stress conditions, Rm is generally less sensitive to temperature than Rr and Rh. Elevated CO2 concentration alleviated the negative effect of drought stress on soil respiration, principally due to the promotion of plant C assimilation subsequently, which increased substrate supply for respiration in both roots and soil microorganisms. Additionally, rewetting stimulated soil respiration dramatically in most cases, except for soil that experienced extreme drought stress periods. The legacy of drought stress can also regulate the response of soil respiration rate to rewetting events in terrestrial ecosystems through changing abiotic drivers and microbial community structure.Conclusions and perspectivesThere is increasing evidence that drought stress can result in the decoupling of the above- and belowground processes, which are associated with soil respiration. However, studies on the variation in rates of soil respiration and its components under different intensities and frequencies of drought stress over the ecosystems should be reinforced. Meanwhile, molecular phylogenetics and functional genomics should be applied to link microbial ecology to the process of Rs. In addition, we should quantify the relationship between soil respiration and global change parameters (such as warming and elevated [CO2]) under drought stress. Models simulating the rates of soil respiration and its components under global climate change and drought stress should also be developed.

Inter-Annual Variations of Methane Emission from an Open Fen on the Qinghai-Tibetan Plateau: A Three-Year Study

The study aimed to understand the inter-annual variations of methane (CH4) emissions from an open fen on the Qinghai-Tibetan Plateau (QTP) from 2005 to 2007. The weighted mean CH4 emission rate was 8.37±11.32 mg CH4 m−2 h−1 during the summers from 2005 to 2007, falling in the range of CH4 fluxes reported by other studies, with significant inter-annual and spatial variations. The CH4 emissions of the year of 2006 (2.11±3.48 mg CH4 m−2 h−1) were 82% lower than the mean value of the years 2005 and 2007 (13.91±17.80 mg CH4 m−2 h−1 and 9.44±14.32 mg CH4 m−2 h−1, respectively), responding to the inter-annual changes of standing water depths during the growing season of the three years. Significant drawdown of standing water depth is believed to cause such significant reduction in CH4 emissions from wetlands in the year 2006, probably through changing the methanogen composition and decreasing its community size as well as activating methanotrophs to enhance CH4 oxidation. Our results are helpful to understand the inter-annual variations of CH4 emission and provide a more reasonable regional budget of CH4 emission from wetlands on the QTP and even for world-wide natural wetlands under climate change.

Intermediate grazing intensities by sheep increase soil bacterial diversities in an Inner Mongolian steppe

Ungulate grazing is known to play a crucial role in regulating nutrient cycling and controlling plant community structure and productivity in grassland ecosystems. However, little is known about the effects of grazing intensities on soil bacterial community structure and diversity, particularly at the long-term scale. In this study, we measured plant biomass and diversity, soil characteristics and bacterial community structure, and diversity in a 16-year field experiment that had four grazing intensity treatments (non-grazed, CK; low-intensity grazing (LG), moderate-intensity grazing (MG), and high-intensity grazing (HG)) in an Inner Mongolian typical grassland. The CK, LG, MG, and HG sites were grazed by 0.00, 1.33, 4.00, and 6.67 sheep ha−1, respectively. Bacterial community structure and diversity under grazing intensity treatments were assessed with PCR amplification of DNAs extracted from soils and denaturing gradient gel electrophoresis (DGGE) separation. The results showed that the CK soil had higher moisture, organic C, NH4+–N, and NO3−–N concentrations than grazed soils, and the HG treatment had the lowest plant biomass and diversity across all the treatments. Principal component analysis of DGGE patterns showed that the LG and MG treatments were different from the CK and HG treatments. In addition, soil bacterial diversities in the LG and MG treatments were significantly higher than those in the other treatments. The relationships between environmental variables and soil bacterial community structure were assessed using redundancy analysis, and we found that soil moisture content, Artemisia frigida biomass, and pH were the best indicator of the changes in soil bacterial community structure among all the treatments. Overall, our results indicated that intermediate grazing intensities (LG and MG) increased soil bacterial diversities, and along with previous studies in this area, we suggested the MG treatment was the most suitable management practice in the Inner Mongolian steppe, not only supporting greater livestock amounts but also harboring greater bacterial diversity.

Effect of grazing intensities on the activity and community structure of methane-oxidizing bacteria of grassland soil in Inner Mongolia

The effects of different grazing intensities on in situ methane flux and the structure and diversity of the methanotrophic community are measured in the typical grassland of Inner Mongolia. Four grazing intensity sites founded in 1989, control (CK), low-intensity grazing (LG), middle-intensity grazing (MG) and heavy-intensity grazing (HG), were selected. Group-specific PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) of 16S rRNA genes for the type I and type II methanotrophs was used to characterize the composition of the methanotrophic community. DGGE patterns were further analyzed using the method of the Shannon-wiener index H and non-metric multi-dimensional scaling (MDS). The results showed that there were no significant differences in methane flux among different sites, yet methanotrophic communities showed significant differences. MDS analysis showed that type I methanotroph community composition at the CK site were significantly different from the three other sites. For type II methanotrophic community composition, it was similar between CK and HG site, and between LG and MG site, while that at the former two sites were significantly different from latter two ones. Additionally Shannon indices of type II methanotrophs were higher at the LG and MG sites than two other sites. Though grazing intensities had an impact on the structure of the methanotrophic community, management-induced changes in the structure of methanotrophic community did not reflect methane consumption capacity across sites. These results suggest that methane consumption is a complex process in soil, and we should be cautious when speculating on the change of methane consumption rates based on a change of methanotrophic community structure.

Community Structure, Abundance, and Activity of Methanotrophs in the Zoige Wetland of the Tibetan Plateau

The Zoige wetland of the Tibetan Plateau is a high-altitude tundra wetland and one of the biggest methane emission centers in China. In this study, methanotrophs with respect to community structure, abundance, and activity were investigated in peat soils collected in the vicinity of different marshland plants that dominate different regions of the wetland, including Polygonum amphibium, Carex muliensis, and Eleocharis valleculosa (EV). 16S rRNA gene and particulate methane monooxygenase gene (pmoA) clone library sequence data indicated the presence of methanotrophs with two genera, Methylobacter and Methylocystis. Methylococcus, like pmoA gene sequences, were also retrieved and showed low similarity to those from Methylococcus spp. and thus indicates the existence of novel methanotrophs in the Zoige wetland. Quantitative polymerase chain reaction (qPCR) assays were used to measure the abundance of methantrophs and detected 107 to 108 of total pmoA gene copies per gram dry weight of soil in the three marshes. Group-specific qPCR and reverse transcriptase qPCR results found that the Methylobacter genus dominates the wetland, and Methylocystis methanotrophs were less abundant, although this group of methanotrophs was estimated to be more active according to mRNA/DNA ratio. Furthermore, EV marsh demonstrated the highest methanotrophs abundance and activity among the three marshes investigated. Our study suggests that both type I and type II methanotrophs contribute to the methane oxidation in the Zoige wetland.