Leymus Chinensis Steppe Research Articles

High resistance to spring drought of grasslands under different degradation levels on a Leymus chinensis steppe

High resistance to spring drought of grasslands under different degradation levels on a Leymus chinensis steppe

Optimized soil bacterial structure following grazing exclusion promotes soil nutrient cycling and plant growth

Grazing exclusion is a common grassland restoration strategy, and the effectiveness on optimizing plant species community, increasing vegetation diversity and biomass, and improving soil fertility, has been widely documented in literatures. Soil bacterial community, as one of important ecological components, closely relates to plants and soil features. However, there is no definite conclusion of grazing exclusion effects on various ecological components, especially on soil bacterial community. To clarify this question, we compared the soil bacterial community characteristics and their corresponding vegetation characteristics and soil properties on one 40 years-grazing exclusion (GE) and three free grazing (FG) areas within a Leymus chinensis steppe of Inner Mongolia, China. Our results showed that grazing exclusion could significantly increase the soil bacterial abundance, and optimize the soil bacterial structure, with the synergistic increase of the soil bacterial diversity. The abundance of Euryarchaeota, Verrucomicrobia, Nitrospirae, Proteobacteria, Elusimicrobia, Latescibacteria, and Microgenomates phyla, some taxa of which involved in the N and P cycles, increased significantly following grazing exclusion, while the abundance of Chloroflexi, Firmicutes, Thaumarchaeota, and Pacearchaeota phyla which are often considered to contain many pathogens, decreased significantly. The bacterial phyla increased by grazing exclusion have a significant positive correlation with vegetation biomass and diversity, and soil alkali-hydrolyzable nitrogen content, total nitrogen content, total phosphorus content, organic matter content and water content. The reducing of pathogenic abundance after grazing exclusion provides a healthier environment for plant growth. This work indicates that grazing exclusion optimizes the soil bacterial structure to promote the growth of plants in more sufficient soil nutrients and healthier soil environments, making the enclosed grassland soil ecology form a virtuous circle.

Effects of artificial restoration on vertical distribution of soil carbon storage following revegetation in a semiarid grassland of North China

AbstractArtificial restoration is an important strategy to restore plant communities and soil nutrients in degraded grassland ecosystems. Despite that research has been extensive on the impacts of vegetation restoration on soil carbon storage, little work has been tried to examine the impacts of artificial restoration on the vertical distribution of soil organic carbon (SOC) storage following revegetation in grassland ecosystems. In this paper, the responses of plant species diversity, litter biomass (LB), aboveground biomass (AGB), the relative biomass of three dominant plant species, and the belowground biomass (BGB) were quantified under five different restoration regimes (natural recovery, harrowing, harrowing plus fertilization, harrowing plus irrigation, harrowing plus fertilization and irrigation), to explore the direct and indirect effects of artificial restoration, mediated by changes in the plant properties following revegetation, on SOC storage in Leymus chinensis steppe, North China. We found that artificial restoration greatly facilitated the recovery of Leymus chinensis but lowered the plant diversity. Irrigation treatment, particularly harrowing plus irrigation, was associated with both higher BGB and LB, which had positive effects on SOC storage at the 30–60 cm soil layer when compared with natural restoration. In addition, artificial restoration had positive but not significant effects on SOC storage in the surface soil layer (0–10 cm) mediated directly by changes in BGB, while it exerted negative indirect effects on SOC storage at the 10–30 cm soil layer through low level of plant species diversity. The loss of two dominant species (Stipa krylovii and Cleistogenes squarrosa) could greatly impact SOC storage not only due to lowered species diversity but also the reduced quality of litter input into soil. It is therefore proposed that maintaining high levels of plant species diversity could help sustain higher soil carbon storage through producing high‐quality root and litter. Our findings from the 9‐year restoration experiment suggested that natural restoration is a sustainable grassland restoration regime to conserve both plant diversity and soil nutrients over short timescales in semi‐arid grasslands in North China. In the long term, SOC storage can be substantially enhanced by artificial restoration, especially under treatments that include irrigation.

Nitrogen and Phosphorus Fertilization Reshapes the Abundance and Structure of Ammonia Oxidizers on a Leymus chinensis Steppe in Northern China

Although nitrogen (N) and phosphorus (P) fertilization are important for maintaining the health and productivity of Leymus chinensis steppe, their impact on the abundance and community structure of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in this ecosystem is still unclear. We used the amoA gene as a molecular marker to monitor changes in AOA and AOB under low and high N and P fertilization and applied the resin-core technique to measure soil N mineralization. We found that the abundance of AOA based on amoA gene copies, ranging from 7.51 × 107 to 1.23 × 108 copies g−1 soil, was higher than that of AOB. Fertilization, especially high N and P, increased the number of amoA copies associated with AOB. AOA and AOB communities were dominated by Crenarchaeota and Proteobacteria, respectively. Fertilization decreased the relative abundances of Thaumarchaeota in the AOA community and Proteobacteria in the AOB community and increased the relative abundance of Ammonia_oxidizing_bacteria_ensemble. In N and P fertilization treatments, soil N transformation was closely related to AOB but not AOA. Soil ammonium N and nitrate N reshape the structure of ammonia oxidizers of AOB but not those of AOA on L. chinensis steppes.

Erratic precipitation and clipping frequency reshape the community structure and species stability of Leymus chinensis steppe

Leymus chinensis steppe is subject to degradation under the combined stresses of climate change and human disturbance. This study aimed to determine the independent and compound effects of precipitation and clipping frequency on the community structure and species temporal stability of L. chinensis steppe. Six treatments were applied in a 9-year field study, comprising three precipitation regimes (precipitation reduced by 50%, R1; unchanged from ambient precipitation, R2; and increased by 50%, R3) and two clipping frequencies (clipping once or twice per year, C1 and C2). We evaluated the soil hydrothermal status, dry matter, importance value and temporal stability of five important species (L. chinensis, Artemisia eriopoda, Cleistogenes squarrosa, Potentilla tanacetifolia, and Stipa krylovii) in the L. chinensis community. The results showed: Change in the soil volumetric water content (±2.4%) altered the competitive capability of L. chinensis relative to its associated species. Drought was beneficial to S. krylovii, wet status was favorable to C. squarrosa and P. tanacetifolia, whereas drought and a wet regime both reduced the importance value and temporal stability of L. chinensis. Erratic precipitation, rather than clipping frequency, decreased the temporal stability of L. chinensis. Net primary productivity and dry matter of L. chinensis were strongly correlated under drought or wet regimes. Restoration of the key species L. chinensis and controlling grazing intensity in severe drought or wet conditions are recommended strategies for sustainable management of L. chinensis steppe.

The short-term impacts of soil disturbance on soil microbial community in a degraded Leymus chinensis steppe, North China

The synergy between vegetation and soil is a fundamental driver of ecosystem restoration, yet the short-term impacts of soil disturbance on soil microbes and the relations between vegetation and soil during vegetation recovery remain unclear. We compared the responses of plants, soil physicochemical properties, and microbial communities after seven years of application of soil disturbance techniques (harrowing, HA; shallow ploughing, SP; and natural recovery, NR, the control) in a degraded Leymus chinensis steppe. Our results showed that bacterial community was more stable and did not change significantly under soil disturbance, but the fungal community was more sensitive to soil disturbance and was closely related to soil water content, bulk density, and relative aboveground biomass of dominant plant species. The results of this study suggest that soil disturbance techniques directly (mainly via changes in soil physical properties) and indirectly (mainly via changes in plant community composition) affected soil fungal community composition. Data show that SP largely facilitated the recovery of L. chinensis but decreased plant richness and diversity, resulted in a slower rate of soil microbial and soil quality restoration relative to the other treatments. It is also clear that HA and NR exerted positive effects on the recovery of the plant and soil microbial communities across degraded Leymus chinensis steppe over short timescales. Our results provide scientific guidance for more effective management of degraded grassland, and suggest that above- and belowground communities should be considered to be precise indicators of grassland ecosystem restoration. The long-term effects of soil disturbance techniques on soil microbial communities require further study.

Effects of organic fertilizer on soil nutrient status, enzyme activity, and bacterial community diversity in Leymus chinensis steppe in Inner Mongolia, China.

The long-term impact of human exploitation and environmental changes has led to a decline in grassland productivity and soil fertility, which eventually results in grassland degradation. The application of organic fertilizer is an effective improvement measure; however, it is still not fully understood how the addition of organic fertilizer influences grassland soil fertility and plant composition. A set of experiments were conducted in Inner Mongolia in China to reveal the tradeoff between steppe plants and soil microorganisms and the eco-physiological mechanisms involved, and how the addition of vermicompost and mushroom residues affect microbial diversity, enzyme activities, and the chemical properties of soil in degraded Leymus chinensis grassland. Organic fertilizer improved the soil nutrient status and shaped distinct bacterial communities. Compared with the control the available phosphorus (AP) and available potassium (AK) contents were highest under treatments a3 and b3, and the aboveground biomass was highest under the b3 treatment. Soil sucrase activities increased by 7.88% under the b3 treatment. Moreover, the richness index significantly increased by 7.07% and 7.23% under the a1 and b2 treatments, respectively. The most abundant Actinobacteria and Proteobacteria were detected in the organic fertilizer treatment. A linear discriminant analysis effect size (LEfSe) indicated that the bacterial community was significantly increased under the b3 treatment. A canonical correspondence analysis (RDA) and spearman correlation heatmap confirmed that total P (TP) and urease were the key driving factors for shaping bacterial communities in the soil. Our results indicated that the application of large amounts of vermicompost and mushroom residues increased the availability of nutrients and also enhanced the biodiversity of soil bacterial communities in L. chinensis grasslands, which will contribute to the sustainable development of agro-ecosystems.

An improved phenology model for monitoring green-up date variation in Leymus chinensis steppe in Inner Mongolia during 1962–2017

Phenology is an important indicator of the biological responses to climate change, and previous studies have reported divergent phenological responses of grasslands in Inner Mongolia from the analyses of in-situ measurements and satellite observations; thus, robust phenology models are required to understand and simulate the relationship between ecosystems and climate change. However, phenology models for natural grasslands in which heating accumulation, chilling requirements, precipitation effects and other climate factors are introduced at the same time are relatively deficient; as a result, the accuracy of the herbaceous plant phenology model and its universality across different species and regions are restricted. We proposed an improved phenology model (unified model combined with precipitation driving, UMPD) that reflects the combined effect of temperature and precipitation at the forcing unit level. Green-up evaluation based on satellite observations shows that the UMPD phenology model achieves better accuracy than previous models (RMSE < 10 days), and the simulated spatial distribution of green-up date change is more consistent with the ground observation, which means that the inclusion of the precipitation driving effect in the temperature-based phenology model improves model applicability at a regional scale and under complex climatic conditions. Based on the UMPD model and its parameters, spatial and temporal changes in green-up dates for Leymus chinensis steppe in Inner Mongolia from 1962 to 2017 were simulated, and the results showed an overall advancing trend with obvious spatial differences and complex stage characteristics. We demonstrate the feasibility of combining temperature and precipitation driving effects to develop and refine phenology models, and the improved model provides wide potential for characterizing the spatiotemporal patterns of grassland green-up date variations.

Soil bacterial community shifts driven by restoration time and steppe types in the degraded steppe of Inner Mongolia

Degraded grassland can be restored through management practices such as fencing to protect land from overgrazing. However, little is known about the dynamics of underground microbial communities and their associated ecological function during the restoration process. In this study, we investigated changes in the soil bacterial community composition with varying restoration time in the Leymus chinensis steppe. We also studied the differences in the soil bacterial community composition between a Leymus chinensis steppe and a Stipa grandis steppe at the same restoration time in the Inner Mongolia of China. Results showed that restoration of the degraded steppe through fencing significantly changed the bacterial community composition through the restoration time. The relative abundances of Chloroflexi and Cyanobacteria decreased significantly with restoration. However, the relative abundances of copiotrophic Proteobacteria and Bacteriodetes increased. Significant changes for many dominant taxa occurred between 7 and 13 years of restoration time in the Leymus chinensis steppe. Shifts in the composition of bacterial communities with fencing and grazing were driven by multiple factors, including soil moisture, pH, nutrients (organic carbon and nitrogen, etc.). Network analysis of soil bacterial community compositions showed that the three largest modules in co-occurrence networks were separated by fencing, grazing and steppe type. This study revealed the successions of underground microbial communities associated with steppe restoration management over time.

Seasonal changes in soil water repellency of different land use types in Inner Mongolia grassland

Soil water repellency (SWR) is a physical phenomenon in which the rate of wetting is restricted and water beads are formed on the soil surface where the soil is called water-repellent soil. SWR analysis is necessary to interpret nonuniform wetting and preferential flow for site-specific ecosystems. This study aims to investigate the effects of soil water conditions and land use types on SWR in Leymus chinensis steppe soil in Xilinhot, Inner Mongolia, China. The main objective is to determine how SWR in soils in Inner Mongolia is influenced by different grassland utilization types, including no grazing since 1979 and 1999, winter grazing, and continuous grazing, by using water drop penetration time (WDPT). Results show that a slightly hydrophobic or hydrophilic soil behavior was often observed, and SWR was most evident in the months with the highest evaporation despite heavy rainfall. Changes in SWR in different seasons were affected by grazing intensity. The highest number of hydrophobic soil samples was found in the continuous grazing sites, followed by that in the winter grazing sites. Few hydrophobic soil samples were detected in the ungrazed sites since 1979, and the lowest number of samples was observed in the ungrazed sites since 1999. The SWR of different particle size classes decreased with increasing particle size. Therefore, SWR was significantly correlated with soil particle size class at 0–0.05 and 0.05–0.1mm. These results emphasized the need to consider SWR in soil utilization because grazing intensity affects the soil repellent properties in steppe soil and provided a basis for the development of grassland management.

Response of carbon dioxide exchange to grazing intensity over typical steppes in a semi-arid area of Inner Mongolia

The eddy covariance technique was used to measure the CO2 flux over four differently grazed Leymus chinensis steppe ecosystems (ungrazed since 1979 (UG79), winter grazed (WG), continuously grazed (CG), and heavily grazed (HG) sites) during four growing seasons (May to September) from 2005 to 2008, to investigate the response of the net ecosystem exchange (NEE) over grassland ecosystems to meteorological factors and grazing intensity. At UG79, the optimal air temperature for the half-hourly NEE occurred between 17 and 20 °C, which was relatively low for semi-arid grasslands. The saturated NEE (NEEsat) and temperature sensitivity coefficient (Q 10) of ecosystem respiration (RE) exhibited clear seasonal and interannual variations, which increased with canopy development and the soil water content (SWC, at 5 cm). The total NEE values for the growing seasons from 2005 to 2008 were −32.0, −41.5, −66.1, and −89.8 g C m−2, respectively. Both the amounts and distribution of precipitation during the growing season affected the NEE. The effects of grazing on the CO2 flux increased with the grazing intensity. During the peak growth stage, heavy grazing and winter grazing decreased NEEsat and gross primary production (45 % for HG and 34 % for WG) due to leaf area removal. Both RE and Q 10 were clearly reduced by heavy grazing. Heavy grazing changed the ecosystem from a CO2 sink into a CO2 source, and winter grazing reduced the total CO2 uptake by 79 %. In the early growing season, there was no difference in the NEE between CG and UG79. In addition to the grazing intensity, the effects of grazing on the CO2 flux also varied with the vegetation growth stages and SWC.

Relationships between soil nutrients and plant functional traits in different degradation stages of Leymus chinensis steppe in Nei Mongol, China

Relationships between soil nutrients and plant functional traits in different degradation stages of Leymus chinensis steppe in Nei Mongol, China

Ramet Recruitment from Different Bud Types along a Grassland Degradation Gradient in Inner Mongolia, China

Since the contribution of total belowground bud bank and different bud types to community regeneration has rarely been explored, the vegetative offspring recruited from different belowground bud types was investigated in four plant communities along a grassland degradation gradient in northeastern China (Inner Mongolia). This gradient, between 1000 and 1500 m a.s.l., has been caused by overgrazing. It is a Leymus chinensis steppe which occupies about 3.0×10 5 ha. Recruitment from tiller buds was dominant (>80%) in determining the total vegetative offspring density along the whole grassland degradation gradient. However, the proportional contribution of tiller-ramets to total ramet recruitment was significantly greater ( P <0.05) during earlier than later stages of grassland degradation, while that of rhizome-ramets showed an opposite pattern. While the percentage contribution and density of root-derived ramets to total ramet density increased significantly (P <0.05) during the late stages of grassland degradation, those of bulb-ramets kept relatively constant along the whole grassland degradation gradient. The relative contribution of hemicryptophytes (i.e., Achnatherum sibiricum, Cleistogenes squarrosa, Festuca ovina, Koeoleria cristata, Poa annua, Stipa grandis) to total plant species richness decreased, while that of geophytes (i.e., Agropyron cristatum, Carex korshinskyi. Leymus chinensis, Allium anisopodium, A. bidentatum, A. tenuissimum, Astragalus galactites, Cymbaria dahurica, Iris tenuifolin, Potentilla acaulis, P. bifurca , Pulsatilla turczaninovii, Serratula chinensis, Thalictrum aquilegifolium ) increased with the increases of grassland degradation. Our results showed that as grassland degradation increased, changes in the proportion of tiller-, rhizome- and root-derived ramets with respect to total ramet density determined in turn changes in the proportion of hemicryptophytes and geophytes in the study plant communities.

Rate-specific responses of prokaryotic diversity and structure to nitrogen deposition in the Leymus chinensis steppe

Serious nitrogen (N) deposition in terrestrial ecosystems causes soil acidification and changes the structure and function of the microbial community. However, it is unclear how these changes are dependent on N deposition rates, other factors induced by N (e.g., pH), and their interactions. In this study, we investigated the responses of soil prokaryotic community structure and stability after a 13-year N addition in the semi-arid Leymus chinensis steppe in Inner Mongolia, China. Our results demonstrated that the prokaryotic community structure changed at the low N addition rate of 1.75 g N m−2 yr−1; however, dramatic changes in microbial abundance, respiratory quotient, and prokaryotic diversity occurred at N addition rates of more than 5.25 g N m−2 yr−1 when the soil pH dropped below 6.0. The two patterns indicated the difference in driving forces for different microbial properties. The N-driven and pH-driven processes are likely the most important mechanisms determining the responses of bacterial community to N. Some copiotrophic/oligotrophic bacteria, e.g., Proteobacteria and Acidobacteria, changed their relative abundances with the N addition continuously even at a low rate, indicating that they were more sensitive to N directly. Some bacterial groups significantly changed their relative abundance at a high N addition rate when pH dropped below 6.0, e.g., Verrucomicrobia and Armatimonadetes, indicating that they were more sensitive to pH below 6.0. N addition altered the prokaryotic community structure through enrichment of copiotrophic bacteria (species adjustment) at low N addition rates and through enrichment of nitrophilous taxa and significant loss of diversity at high N rates. The results also demonstrated that a high N addition diminished the stability of the prokaryotic community structure and activity through reduction in species diversity and bacterial interaction. Overall, this study supported the hypothesis that the responses of prokaryota to N were dependent on deposition rates, and N-driven and pH-driven processes were the important mechanisms to control the shift of the prokaryotic community.

Simulated water fluxes during the growing season in semiarid grassland ecosystems under severe drought conditions

summary To help improve understanding of how changes in climate and land cover affect water fluxes, water budgets, and the structure and function of regional grassland ecosystems, the Grassland Landscape Productivity Model (GLPM) was used to simulate spatiotemporal variation in primary water fluxes. The study area was a semiarid region in Inner Mongolia, China, in 2002, when severe drought was experienced. For Stipa grandis steppe, Leymus chinensis steppe, shrubland, and croplands, the modeled total, daily and monthly averaged, and maximum evapotranspiration during the growing season and the modeled water deficits were similar to those measured in Inner Mongolia under similar precipitation conditions. The modeled temporal variations in daily evaporation rate, transpiration rate, and evapotranspiration rate for the typical steppes also agreed reasonably well with measured trends. The results demonstrate that water fluxes varied in response to spatiotemporal variations in environmental factors and associated changes in the phenological and physiological characteristics of plants. It was also found that transpiration and evapotranspiration (rather than precipitation) were the primary factors controlling differences in water deficit among land cover types. The results also demonstrate that specific phenomena occur under severe drought conditions; these phenomena are considerably different to those occurring under normal or well-watered conditions. The findings of the present study will be useful for evaluating day-scale water fluxes and their relationships with climate change, hydrology, land cover, and vegetation dynamics. 2014 Published by Elsevier B.V.

China Forest-Steppe Ecotone Database

The China Forest Steppe Ecotone Database (GIVD ID AS-CN-001) contains releves of the forest-steppe ecotone, typical steppe and desert steppe in central part of Inner Mongolia, China as well as of the Zamin Ude region in Mongolia. Releves of the forest-steppe ecotone were recorded during the 1990s. The communities in the forest-steppe ecotone are classified in a phytocoenological way. 12 major types of forest, shrubland, meadow, fen, open woodland and steppe are differentiated and described according to 133 releves. Due to limited releves, the plant communities are named by their dominant species: 1. Quercus mongolicawoodland; 2. Betula platyphylla-woodland; 3. Betula dahurica-woodland; 4. Populus davidiana-woodland; 5. Picea meyeri-woodland; 6. Pinus tabulaeformis-woodland; 7. Ostryopsis davidiana-shrubland; 8. Polygonum viviparum-meadow; 9. Ranunculus japonica-fen; 10. Stipa baicalensis-steppe; 11. Leymus chinensis-steppe; 12. Ulmus pumila-open woodland. Other plant community types with less than 5 releves are Larix principis-ruprechtii-woodland, Pinus Betula fruticosa-scrub, Stipa krylovii-steppe, Filifolium sibiricumsteppe and Thymus serphyllum-steppe. During the 2000s, we extended our survey further to the dryer region, including typical steppes and desert steppes in central Inner Mongolia of China, including the huge sandy sheets, Otindag and Mu Us. Besides releve records, we systematically collected 344 soil profiles, with C, N content measured for most of the profiles, as well as grain sizes for all the profiles.

The fluxes of CO 2 from grazed and fenced temperate steppe during two drought years on the Inner Mongolia Plateau, China

The CO 2 flux was measured by the eddy covariance method on a temperate Leymus chinensis steppe over a period of 17 months spanning two consecutive growing seasons. The amount of precipitation was nearly normal, but it was low in the early and high in the late growing period in 2006. In the 2007 growing season, the amount of precipitation was about 45% less than the multi-year average and more evenly distributed. Comparisons were made between a moderately grazed site and a 28-year-old fenced site. The maximum instantaneous CO 2 release and uptake rates were 0.12 (May) and − 0.11 mg CO 2 m −2 s −1 (July) at the fenced site, and 0.11 and − 0.16 mg CO 2 m −2 s −1 (both in July) at the grazed site. In both growing seasons, the grazed site always had a higher daily uptake rate or lower release rate than the fenced site. The grazed site was a CO 2 sink during the growing season of 2007 and a CO 2 source in the growing season of 2006, whereas the fenced site was a CO 2 source in both seasons. Lower precipitation decreased CO 2 loss during the growing season more in the grazed site than in the fenced site, mainly because of depression of total ecosystem respiration ( R e) in the former and stimulation in the latter. During the dormant season (from October to April), the fenced and grazed sites released 60.0 and 32.4 g of C per m 2, respectively. Path analysis showed that temperature had the greatest effect on daily variation of ecosystem CO 2 exchange during the growing seasons at the two study sites. The results suggest that decrease of precipitation and/or increase of temperature will likely promote C loss from L. chinensis steppes, whether fenced or grazed, and that a grazed site is more sensitive.

Effects of different land use patterns on nifH genetic diversity of soil nitrogen-fixing microbial communities in Leymus Chinensis steppe

Biological nitrogen fixation through prokaryotic microbe is an important source of nitrogen been input into many natural ecosystems. In this study the active diazotrophic community was investigated in the three treatments of mowed, grazed and enclosed Leymus chinensis steppes in Hulunbeier grassland of Inner Mongolia by using approaches of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR–DGGE) and sequence analysis. The community structure and diversity of the bacterial groups from the different samples was further analyzed by using different techniques, such as statistical analysis and diversity index evaluation of the band patterns etc. The results showed that grazing activity significantly reduced the number of species and quantities of nitrogen-fixing microorganisms, as well as the nifH gene diversity. However, enclosed plots had the lowest diversity of nifH gene. While the highest one found in mowing plots. A total of 30 sequences representing 25 different sequence types were recovered from the DGGE gels after phylogenetic constructions. The results also revealed that most sequences were coming from Alphaproteobacteria of Proteobacteria, and characterized by sequences of members of Rhodobacter, Bradyrhizobium, Mesorhizobium, Rhodopseudomonas, Xanthomonas, Azospirillum, Gluconacetobacter, Methylobacterium and Methylocystis. Symbiotic nitrogen-fixers existed in grazing, mowing and enclosed plots accounted for 21.4%, 47.3% and 31.3% respectively in their dominant nitrogen-fixing bacteria. The result of principal components analysis showed that the influence of different land use patterns on nitrogen-fixing microbial communities composition can be ordered as grazing plots > enclosed plots > mowing plots. Nitrogen-fixing microbial communities in L. chinensis steppe were significantly ( P < 0.05) influenced by the levels of nitrate nitrogen, total phosphorus, available phosphorus contents and pH value when canonical correspondence analysis was employed to identify relationship between nifH gene and soil physicochemical factors under different land use patterns. The result obtained from correlation analysis showed that there was a significant ( P < 0.05) negative relationship between the nitrate nitrogen and the total phosphorus content, furthermore, the available phosphorus content was strongly correlated ( P < 0.01) with the pH value.

Annual methane uptake by temperate semiarid steppes as regulated by stocking rates, aboveground plant biomass and topsoil air permeability

AbstractOvergrazing‐induced degradation of temperate semiarid steppes may affect the soil sink for atmospheric methane (CH4). However, previous studies have primarily focused on the growing season and on single grazing patterns. Thus, the response of annual CH4 uptake by steppes compared with various grazing practices is uncertain. In this study, we investigated the effects of grazing on the annual CH4 uptake by two typical Eurasian semiarid steppes (the Stipa grandis steppe and the Leymus chinensis steppe) located in Inner Mongolia, China. The CH4 fluxes were measured year‐round using static chambers and gas chromatography at 12 field sites that differed primarily in grazing intensities. Our results indicated that steppe soils were CH4 sinks throughout the year. The annual CH4 uptake correlated with stocking rates, whereas the seasonality of CH4 uptake was primarily dominated by temperature. The annual CH4 uptake at all sites averaged 3.7±0.7 kg C ha−1 yr−1 (range: 2.3–4.5), where approximately 35% (range: 23–40%) occurred during the nongrowing season. Light‐to‐moderate grazing (stocking rate≤1 sheep ha−1 yr−1) did not significantly change the annual CH4 uptake compared with ungrazed steppes, but heavy grazing reduced annual CH4 uptake significantly (by 24–31%, P&lt;0.05). These findings imply that easing the pressure of heavily grazed steppes (e.g. moving to light or moderate stocking rates) would help restore steppe soil sinks for atmospheric CH4. The empirical equations based on the significant relationships between annual CH4 uptake and stocking rates, aboveground plant biomass and topsoil air permeability (P&lt;0.01) could provide simple approaches for the estimation of regional CH4 uptake by temperate semiarid steppes.

Effects of cloudiness change on net ecosystem exchange, light use efficiency, and water use efficiency in typical ecosystems of China

As a weather element, clouds can affect CO 2 exchange between terrestrial ecosystems and the atmosphere by altering environmental conditions, such as solar radiation received on the ground surface, temperature, and moisture. Based on the flux data measured at five typical ecosystems of China during mid-growing season (June–August) from 2003 to 2006, we analyzed the responses of net ecosystem exchange of carbon dioxide (NEE), light use efficiency (LUE, defined as Gross ecosystem photosynthesis (GEP)/Photosynthetically active radiation (PAR)), and water use efficiency (WUE, defined as GEP/Evapotranspiration (ET)) to the changes in cloudiness. The five ecological sites included Changbaishan temperate mixed forest (CBS), Dinghushan subtropical evergreen broad-leaved forest (DHS), Xishuangbanna tropical rainforest (XSBN), Inner Mongolia semi-arid Leymus chinensis steppe (NMG), and Haibei alpine frigid Potentilla fruticosa shrub (HB). Our analyses show that cloudy sky conditions with cloud index ( k t) values between 0.4 and 0.6 increased NEE, LUE, and WUE of the ecosystems at CBS, DHS, NMG and HB from June to August. The LUE of tropical rainforest at XSBN was higher under cloudy than under clear sky conditions, but NEE and WUE did not decrease significantly under clear sky conditions from June to August. The increase in GEP with increasing diffuse radiation received by ecosystems under cloudy skies was the main reason that caused the increases in LUE and net carbon uptake in forest ecosystem at CBS, DHS, and alpine shrub ecosystem at HB, compared with clear skies. Moreover, for the ecosystem at CBS, DHS, and HB, when sky condition became from clear to cloudy, GEP increased and ET decreased with decreasing VPD, leading to the increase in WUE and NEE under cloudy sky conditions. The decrease in R e with decreasing temperature and increase in GEP with decreasing VPD under cloudy skies led to the increase in LUE, WUE, and net carbon uptake of semi-arid steppe at NMG, compared to clear skies. These different responses among the five ecosystems are attributable to the differences in canopy characteristics and water conditions. From June to August, the peaks of the k t frequency distribution in temperate ecosystems (e.g., CBS, NMG, and HB) were larger than 0.5, but they were smaller than 0.4 in subtropical/tropical forest ecosystems (e.g., DHS and XSBN). These results suggest that the pattern of cloudiness during the years from 2003 to 2006 in the five ecosystems was not the best condition for their net carbon uptake. This study highlights the importance of cloudiness factor in the prediction of net carbon absorption in the Asia monsoon region under climate change.