Grassland Carbon Cycling Research Articles

A long-term high-resolution dataset of grasslands grazing intensity in China

Grazing is a significant anthropogenic disturbance to grasslands, impacting their function and composition, and affecting carbon budgets and greenhouse gas emissions. However, accurate evaluations of grazing impacts are limited by the absence of long-term high-resolution grazing intensity data (i.e., the number of livestock per unit area). This study utilized census livestock data and a satellite-based vegetation index to develop the first Long-term High-resolution Grazing Intensity (LHGI) dataset of grassland in seven pastoral provinces in western China from 1980 to 2022. The LHGI dataset effectively captured spatial variations in grazing intensity, with validation at 73 sites showing a correlation coefficient (R2) of 0.78. The county-level validation showed an averaged R2 values of 0.73 ± 0.03 from 1980 to 2022. This dataset serves as a vital resource for estimating grassland carbon cycling and livestock system CH4 emissions, as well as contributing to grassland management.

Impact of naphthalene on soil fauna diversity: Consequences for ecosystem functioning and carbon cycling in grasslands

Soil fauna, key indicators of grassland ecosystem health, face significant threats from the vast amounts of chemical pollutants released by human activities. These organisms are crucial for fundamental processes such as litter decomposition and carbon cycling. However, the response of soil animal communities and their functional dynamics to environmental chemical stress remains poorly understood. To investigate this, we conducted a naphthalene addition and litterbag experiment in the Bashang Grassland, located on the southern Inner Mongolia Plateau, to analyze its effects on soil fauna and litter decomposition. The investigated soil fauna groups included Arachnida (Arachnids), Insecta (Insects), Collembola (Springtails), Enoplea (enopleans), and Chromadorea (chromadoreans). The litter composition included Leymus chinensis, Stipa krylovii, Artemisia, Saussurea amara, and Artemisia giraldii. The findings indicated that a high diversity of soil fauna enhanced the decomposition of litter and the release of organic carbon from the litter. While the addition of naphthalene decreased both the abundance of soil fauna and the rate of litter decomposition, it specifically increased the relative proportion of Collembola (Springtails). Microfauna and mesofauna (0.1–2.0 mm) contributed more to the loss of litter mass and the release of organic carbon from the litter than macrofauna (2.0–4.0 mm). The effects of diverse soil fauna on litter decomposition and soil organic carbon accumulation were diminished by naphthalene exposure. This study further reveals the relationship between composite soil fauna diversity, litter decomposition, and soil organic carbon under chemical stress, highlighting the threat of environmental pollution to ecosystem health. It also enriches our understanding of the complexity of grassland ecosystems, especially in response mechanisms under chemical pollution stress.

Response of Grassland Vegetation Growth to Drought in Inner Mongolia of China from 2002 to 2020

Drought poses a significant environmental risk and can deeply affect the growth of grasslands. However, there is still uncertainty regarding the precise impact of varying levels of drought on grassland growth. To address this gap, we utilized several key indicators, including the Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), Global Orbiting Carbon Observatory-2-based Solar-induced Chlorophyll Fluorescence (GOSIF), and Gross Primary Productivity (GPP), in conjunction with drought indices (the Standardized Precipitation Evapotranspiration Index (SPEI) and soil moisture (SM). Our study aimed to comprehensively assess the consistency of spatiotemporal patterns in grassland vegetation and its responsiveness to different drought levels in the Inner Mongolia region from 2002 to 2020. The results indicated that NDVI, EVI, GOSIF, and GPP in grassland vegetation across Inner Mongolia exhibited significant increasing trends from 2002 to 2020. Specifically, NDVI, EVI, GOSIF, and GPP all displayed consistent spatial patterns, with 25.83%, 21.18%, 22.65%, and 48.13% of the grassland area showing significant increases, respectively. Drought events, as described through SPEI and SM, from June 2007 to September 2007 and June 2017 to July 2017 were selected to evaluate the response of grassland vegetation to drought. The drought events of 2007 and 2017 resulted in reductions in NDVI, EVI, GOSIF, and GPP relative to the multi-year average (2002–2020). GOSIF exhibited a more intense response to drought, suggesting that GOSIF may reflect the inhibition of water stress on grassland photosynthesis better than NDVI and EVI for the drought in 2007 and 2017. The reductions in NDVI, EVI, GOSIF, and GPP in grassland increased significantly across different drought levels, with the sharpest reductions observed during extreme drought. Under the severe and extreme drought events, the most substantial reductions in NDVI, EVI, GOSIF, and GPP were observed in the temperate steppe (TS). Moreover, the effects of different drought severity levels within the same grassland type varied, with the most significant reductions in NDVI, EVI, GOSIF, and GPP observed during extreme drought. Our results provide new perspectives for developing and implementing effective strategies to address grassland carbon cycling management and climate change in Inner Mongolia.

Observed impacts of large wind farms on grassland carbon cycling

Observed impacts of large wind farms on grassland carbon cycling

The grassland carbon cycle: Mechanisms, responses to global changes, and potential contribution to carbon neutrality.

Grassland is one of the largest terrestrial biomes, providing critical ecosystem services such as food production, biodiversity conservation, and climate change mitigation. Global climate change and land-use intensification have been causing grassland degradation and desertification worldwide. As one of the primary medium for ecosystem energy flow and biogeochemical cycling, grassland carbon (C) cycling is the most fundamental process for maintaining ecosystem services. In this review, we first summarize recent advances in our understanding of the mechanisms underpinning spatial and temporal patterns of the grassland C cycle, discuss the importance of grasslands in regulating inter- and intra-annual variations in global C fluxes, and explore the previously unappreciated complexity in abiotic processes controlling the grassland C balance, including soil inorganic C accumulation, photochemical and thermal degradation, and wind erosion. We also discuss how climate and land-use changes could alter the grassland C balance by modifying the water budget, nutrient cycling and additional plant and soil processes. Further, we examine why and how increasing aridity and improper land use may induce significant losses in grassland C stocks. Finally, we identify several priorities for future grassland C research, including improving understanding of abiotic processes in the grassland C cycle, strengthening monitoring of grassland C dynamics by integrating ground inventory, flux monitoring, and modern remote sensing techniques, and selecting appropriate plant species combinations with suitable traits and strong resistance to climate fluctuations, which would help design sustainable grassland restoration strategies in a changing climate.

Partitioning evapotranspiration and carbon flux in ungrazed and grazed tallgrass prairie

The effect of cattle grazing on water and carbon cycling in grasslands is not fully understood. The goal of this study was to evaluate how grazing affects carbon and water cycles in a tallgrass prairie. Eddy covariance measurements were carried out from 2003 to 2005 growing seasons (from May to September) in Manhattan, Kansas, U.S., in grazed (GR) and ungrazed (UG) tallgrass prairie paddocks. Net ecosystem CO2 exchange (NEE) was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco) using the relationship between the air temperature and nighttime NEE data. Evapotranspiration (ET) was partitioned into transpiration (T) and evaporation (E) using an approach based on the concept of underlying water use efficiency. Our results revealed that grazing reduced maximum aboveground biomass (AGB) and green leaf area index (GLAI) by 22.5% and 24.3%, respectively. Grazing also reduced GPP, NEE and T in the middle of the growing seasons but enhanced these flux components at the end of the growing seasons compared to the UG paddock. Grazing reduced the growing season cumulative NEE by 11.9% and enhanced ET by 3.3% compared to the UG paddock. The reduction in NEE at UG was associated with an increase in Reco (9.4%). In turn, changes in ET in response to grazing were driven more by the increase in E (26.6%) than by the decrease in T (5.9%). Despite the reduction in AGB and GLAI caused by grazing, similar cumulative GPP values at the canopy scale at both paddocks suggest that there are compensatory mechanisms that maintain similar CO2 assimilation at UG and GR. Our findings suggest that the grazing regime adopted in this tallgrass prairie impacts the carbon cycle more strongly than the water cycle, given the higher reduction in NEE compared to the slight increase in ET on GR compared to UG. These results can be useful to outline strategies to improve management practices aimed at improving the efficiency of rangelands.

Precipitation and land use alter soil respiration in an Inner Mongolian grassland

Grasslands are facing the threat of climate change and intensive land use. Soil respiration (Rs) in grassland ecosystems can be potentially altered by changes in precipitation and land use. We aimed to quantify the impact of changes in precipitation and common land use practices in an Inner Mongolia grassland, i.e., mowing and grazing, on soil respiration. We performed an in situ experiment with altered precipitation (+ 50%, ambient, and -50%) and land use (control or fencing, mowing, and grazing) to explore their impacts on soil respiration and its autotrophic (Ra) and heterotrophic (Rh) components. Altered precipitation had stronger impacts on abiotic and biotic drivers than land use, leading to stronger impacts on Rs and its components. Over the 3-year experiment, Rs, Ra and Rh decreased by 36%, 42% and 33% with reduced precipitation and increased by 29%, 36% and 25% with increased precipitation, respectively. Grazing and mowing caused relatively small decreases in Rs compared to fencing (generally < 10%). However, precipitation and land use interactively impacted abiotic and biotic drivers and thus Rs. The decrease in Rs with reduced precipitation was greater with grazing (38%) and mowing (37%) than with fencing (32%). Rs and its components may decrease under the projected decrease in precipitation and may further decrease with grazing and mowing compared to fencing. Therefore, land use should be considered when predicting grassland carbon cycling in response to future precipitation changes.

Nitrogen increases early‐stage and slows late‐stage decomposition across diverse grasslands

Abstract To evaluate how increased anthropogenic nutrient inputs alter carbon cycling in grasslands, we conducted a litter decomposition study across 20 temperate grasslands on three continents within the Nutrient Network, a globally distributed nutrient enrichment experiment We determined the effects of addition of experimental nitrogen (N), phosphorus (P) and potassium plus micronutrient (Kμ) on decomposition of a common tree leaf litter in a long‐term study (maximum of 7 years; exact deployment period varied across sites). The use of higher order decomposition models allowed us to distinguish between the effects of nutrients on early‐ versus late‐stage decomposition. Across continents, the addition of N (but not other nutrients) accelerated early‐stage decomposition and slowed late‐stage decomposition, increasing the slowly decomposing fraction by 28% and the overall litter mean residence time by 58%. Synthesis. Using a novel, long‐term cross‐site experiment, we found widespread evidence that N enhances the early stages of above‐ground plant litter decomposition across diverse and widespread temperate grassland sites but slows late‐stage decomposition. These findings were corroborated by fitting the data to multiple decomposition models and have implications for N effects on soil organic matter formation. For example, following N enrichment, increased microbial processing of litter substrates early in decomposition could promote the production and transfer of low molecular weight compounds to soils and potentially enhance the stabilization of mineral‐associated organic matter. By contrast, by slowing late‐stage decomposition, N enrichment could promote particulate organic matter (POM) accumulation. Such hypotheses deserve further testing.

Shifts in plant composition mediate grazing effects on carbon cycling in grasslands

Abstract Carbon cycling in grasslands can be impacted by livestock grazing, partially as an indirect result of herbivory‐induced compositional shifts in the plant community. However, the underlying mechanisms of how these shifts impact carbon cycling are not well‐documented. We conducted a long‐term grazing experiment with four sheep stocking rates in the semi‐arid grasslands of Inner Mongolia, China, to examine grazing effects on the ratio of C3 to C4 species (C3:C4), shoot biomass, root biomass, root:shoot, soil respiration, soil C, soil N and soil C:N between 2014 and 2018. We explored the responses of these carbon metrics to C3:C4 under different grazing treatments and the mechanisms driving grazing‐induced carbon loss using structural equation models. Livestock grazing directly shifted plant community composition (i.e. increasing C3:C4) and reduced vegetation carbon (i.e. shoot biomass), whereas grazing effects on below‐ground carbon were mediated by the interactions of the soil profile (i.e. depth dependence) and year‐to‐year variation (e.g. rainfall regulation). Grazing‐induced increases in C3:C4 suppressed soil carbon loss by inhibiting the rate of soil respiration. Furthermore, grazing intensity indirectly altered these relationships. Specifically, C3:C4 was positively related to shoot biomass and negatively associated with root:shoot, soil C and soil N, whereas these relationships were only significant in no‐grazed plots. Meanwhile, soil respiration was negatively associated with C3:C4, soil C, soil N and soil C:N, but a positive relationship with shoot biomass; these relationships were significant only in grazed plots. Synthesis and applications. Our study highlights the importance of the functional linkages between community characteristics and ecosystem processes, that is, shifts in plant community composition play a key role in regulating grassland carbon cycling. These findings provide a useful field‐observed resource for model development and could improve the guidelines for livestock management and policies regarding climate mitigation.

Grazing exclusion enhanced net ecosystem carbon uptake but decreased plant nutrient content in an alpine steppe

Grazing exclusion is widely conducted to restore degraded grasslands. The recovery status can be indicated by both nutrient and carbon cycling. However, the effect of grazing exclusion on net ecosystem CO2 exchange and mineral nutrients in pastures on the Tibetan Plateau remains unclear, due to a lack of in situ investigations. Here, we conducted an in situ investigation of the grazing exclusion effect on net ecosystem CO2 exchange and foliar mineral elements in an alpine steppe on the central Tibetan Plateau. Our results show that seven years of grazing exclusion enhanced net CO2 uptake by 17% during the growing season via increased gross ecosystem photosynthesis, rather than unchanged ecosystem respiration. Grazing exclusion reduced the sensitivity of ecosystem respiration to soil surface temperature by 16–17% at both diurnal and seasonal scales. Grazing exclusion, therefore, may be an effective way to enhance carbon sequestration under the background of a warming climate. We further found that grazing exclusion significantly decreased the soil available nitrate-nitrogen concentration by 50% and foliar mineral concentrations such as nitrogen, calcium, sulfur, iron, and manganese. Grazing exclusion increased the foliar carbon:nitrogen ratio by 16% and decreased the foliar nitrogen:phosphorus ratio by 15%. These results indicate that grazing exclusion enhanced net ecosystem CO2 uptake but aggravated mineral nutrient limitation of plant growth. Our study highlights that grazing exclusion influences alpine grassland carbon cycling by directly preventing grazing and by indirectly altering the status of plant nutrients. A quantitative distinction of such direct and indirect effects based on long-term and continuous in situ field observations is critical for revealing the mechanism underlying carbon-nutrient cycling evolution in the restoration of degraded alpine grasslands.

Drought and heat wave impacts on grassland carbon cycling across hierarchical levels.

Droughts and heat waves are increasing in magnitude and frequency, altering the carbon cycle. However, understanding of the underlying response mechanisms remains poor, especially for the combination (hot drought). We conducted a 4-year field experiment to examine both individual and interactive effects of drought and heat wave on carbon cycling of a semiarid grassland across individual, functional group, community and ecosystem levels. Drought did not change below-ground biomass (BGB) or above-ground biomass (AGB) due to compensation effects between grass and non-grass functional groups. However, consistently decreased BGB under heat waves limited such compensation effects, resulting in reduced AGB. Ecosystem CO2 fluxes were suppressed by droughts, attributed to stomatal closure-induced reductions in leaf photosynthesis and decreased AGB of grasses, while CO2 fluxes were little affected by heat waves. Overall the hot drought produced the lowest leaf photosynthesis, AGB and ecosystem CO2 fluxes although the interactions between heat wave and drought were usually not significant. Our results highlight that the functional group compensatory effects that maintain community-level AGB rely on feedback of root system responses, and that plant adjustments at the individual level, together with shifts in composition at the functional group level, co-regulate ecosystem carbon sink strength under climate extremes.

Soil carbon release responses to long-term versus short-term climatic warming in an arid ecosystem

Abstract. Climate change severely impacts the grassland carbon cycling by altering rates of litter decomposition and soil respiration (Rs), especially in arid areas. However, little is known about the Rs responses to different warming magnitudes and watering pulses in situ in desert steppes. To examine their effects on Rs, we conducted long-term moderate warming (4 years, ∼3 ∘C), short-term acute warming (1 year, ∼4 ∘C) and watering field experiments in a desert grassland of northern China. While experimental warming significantly reduced average Rs by 32.5 % and 40.8 % under long-term moderate and short-term acute warming regimes, respectively, watering pulses (fully irrigating the soil to field capacity) stimulated it substantially. This indicates that climatic warming constrains soil carbon release, which is controlled mainly by decreased soil moisture, consequently influencing soil carbon dynamics. Warming did not change the exponential relationship between Rs and soil temperature, whereas the relationship between Rs and soil moisture was better fitted to a sigmoid function. The belowground biomass, soil nutrition, and microbial biomass were not significantly affected by either long-term or short-term warming regimes, respectively. The results of this study highlight the great dependence of soil carbon emission on warming regimes of different durations and the important role of precipitation pulses during the growing season in assessing the terrestrial ecosystem carbon balance and cycle.

Review of drought impacts on carbon cycling in grassland ecosystems

Grasslands play a key role in both carbon and water cycles. In semi-arid and arid grassland areas, the frequency and intensity of droughts are increasing. However, the influence of a drought on grassland carbon cycling is still unclear. In this paper, the relationship between drought and grassland carbon cycling is described from the perspective of drought intensity, frequency, duration, and timing. Based on a large amount of literature, we determined that drought is one of the most prominent threats to grassland carbon cycling, although the impacts of different drought conditions are uncertain. The effects of a drought on grassland carbon cycling are more or less altered by drought-induced disturbances, whether individually or in combination. Additionally, a new conceptual model is proposed to better explain the mechanism of droughts on grassland carbon cycling. At present, evaluations of the effects of droughts on grassland carbon cycling are mainly qualitative. A data fusion model is indispensable for evaluating the fate of carbon cycling in a sustainable grassland system facing global change. In the future, multi-source data and models, based on the development of single and multiple disturbance experiments at the ecosystem level, can be utilized to systematically evaluate drought impacts on grassland carbon cycling at different timescales. Furthermore, more advanced models should be developed to address extreme drought events and their consequences on energy, water, and carbon cycling.

Impacts of low-level liming on soil respiration and forage production in a fertilized upland grassland in Central France

Liming is a common agricultural practice for improving acidic soils, but the addition of liming materials may also promote soil carbon dioxide (CO2) emissions, with adverse effects for climate regulation. In grasslands, current understanding of liming impacts on greenhouse gas emissions is limited by a lack of field data on liming and soil respiration. Here we used a two-year field trial and in situ chamber measurements to evaluate the effects of repeated, low-level liming on soil CO2 emissions from an acidic managed grassland with high soil organic matter content. Soil pH, temperature and moisture were measured during the experiment, as well as microbial and plant biomass, in order to assess possible liming-induced changes to drivers of grassland carbon cycling. Soil CO2 emissions showed significant variation during the two-year study, driven primarily by fluctuations in soil temperature. Soil respiration rates were unaffected by liming treatment, despite significant lime-induced increases in soil pH. Liming was associated with a decrease in biomass produced per gram nitrogen, as well as a decrease in forage C:N in the second year and transient decreases in microbial C:N, but neither plant nor microbial biomass showed significant responses to liming addition. Collectively, our results suggest that positive effects of low-level liming on plants and soil are not offset by increases in soil CO2 emissions in situ, highlighting the potential for sustainable liming practices in fertilized grasslands.

Warm- and cold- season grazing affect soil respiration differently in alpine grasslands

As a traditional practice in grasslands, grazing significantly affects soil respiration (R-s). To improve our understanding of grassland carbon cycling, it is critical to partition the responses of soil respiration to grazing into autotrophic (R-a) and heterotrophic (R-h) respiration. In addition, it remains unclear how grazing patterns, such as warm- and cold-season grazing, influence R-s, and its components in alpine grasslands that are subject to increasing grazing pressure. Here, we conducted a six-year manipulative experiment combining with a meta analysis, to investigate the responses of 12, and its components to moderate grazing in a Tibetan alpine grassland. Grazing patterns included warm-season grazing by sheep during the growing seasons of 2008 to 2010 and simulating cold-season grazing by clipping during the non-growing seasons of 2011 to 2013. Our results showed that warm-season grazing minimally affected R-s, while cold-season grazing significantly increased R-s, by 13.1%. This result was supported by a meta-analysis at seven grassland sites across the Tibetan Plateau. Further, we found that warm-season grazing did not affect R-a or R-h, whereas cold-season grazing enhanced Ra (23.2%) more than R-h (4.9%). Cold-season grazing affected 12, and R-a differently depending on interannual variation in climate conditions. A significant increase of 17.1% and 26.3%, respectively, was recorded in dry and cold years, but no change was recorded in wet and warm years. This study highlights the differential responses of R, components to grazing, and suggests that different grazing patterns should be considered when evaluating future carbon cycles in grazing ecosystems on the Tibetan Plateau.

Solar park microclimate and vegetation management effects on grassland carbon cycling

Increasing energy demands and the drive towards low carbon (C) energy sources has prompted a rapid increase in ground-mounted solar parks across the world. This represents a significant global land use change with implications for the hosting ecosystems that are poorly understood. In order to investigate the effects of a typical solar park on the microclimate and ecosystem processes, we measured soil and air microclimate, vegetation and greenhouse gas emissions for twelve months under photovoltaic (PV) arrays, in gaps between PV arrays and in control areas at a UK solar park sited on species-rich grassland. Our results show that the PV arrays caused seasonal and diurnal variation in air and soil microclimate. Specifically, during the summer we observed cooling, of up to 5.2 °C, and drying under the PV arrays compared with gap and control areas. In contrast, during the winter gap areas were up to 1.7 °C cooler compared with under the PV arrays and control areas. Further, the diurnal variation in both temperature and humidity during the summer was reduced under the PV arrays. We found microclimate and vegetation management explained differences in the above ground plant biomass and species diversity, with both lower under the PV arrays. Photosynthesis and net ecosystem exchange in spring and winter were also lower under the PV arrays, explained by microclimate, soil and vegetation metrics. These data are a starting point to develop understanding of the effects of solar parks in other climates, and provide evidence to support the optimisation of solar park design and management to maximise the delivery of ecosystem services from this growing land use.

Not all droughts are created equal: the impacts of interannual drought pattern and magnitude on grassland carbon cycling.

Climate extremes, such as drought, may have immediate and potentially prolonged effects on carbon cycling. Grasslands store approximately one-third of all terrestrial carbon and may become carbon sources during droughts. However, the magnitude and duration of drought-induced disruptions to the carbon cycle, as well as the mechanisms responsible, remain poorly understood. Over the next century, global climate models predict an increase in two types of drought: chronic but subtle 'press-droughts', and shorter term but extreme 'pulse-droughts'. Much of our current understanding of the ecological impacts of drought comes from experimental rainfall manipulations. These studies have been highly valuable, but are often short term and rarely quantify carbon feedbacks. To address this knowledge gap, we used the Community Land Model 4.0 to examine the individual and interactive effects of pulse- and press-droughts on carbon cycling in a mesic grassland of the US Great Plains. A series of modeling experiments were imposed by varying drought magnitude (precipitation amount) and interannual pattern (press- vs. pulse-droughts) to examine the effects on carbon storage and cycling at annual to century timescales. We present three main findings. First, a single-year pulse-drought had immediate and prolonged effects on carbon storage due to differential sensitivities of ecosystem respiration and gross primary production. Second, short-term pulse-droughts caused greater carbon loss than chronic press-droughts when total precipitation reductions over a 20-year period were equivalent. Third, combining pulse- and press-droughts had intermediate effects on carbon loss compared to the independent drought types, except at high drought levels. Overall, these results suggest that interannual drought pattern may be as important for carbon dynamics as drought magnitude and that extreme droughts may have long-lasting carbon feedbacks in grassland ecosystems.

Litters of photosynthetically divergent grasses exhibit differential metabolic responses to warming and elevated CO2

Climatic stress such as warming would alter physiological pathways in plants leading to changes in tissue chemistry. Elevated CO2 could partly mitigate warming induced moisture stress, and the degree of this mitigation may vary with plant functional types. We studied the composition of structural and non‐structural metabolites in senesced tissues of Bouteloua gracilis (C4) and Pascopyrum smithii (C3) at the Prairie Heating and CO2 Enrichment experiment, Wyoming, USA. We hypothesized that P. smithii and B. gracilis would respond to unfavorable global change factors by producing structural metabolites and osmoregulatory compounds that are necessary to combat stress. However, due to the inherent variation in the tolerance of their photosynthetic pathways to warming and CO2, we hypothesized that these species will exhibit differential response under different combinations of warming and CO2 conditions. Due to a lower thermo‐tolerance of the C4 photosynthesis we expected B. gracilis to exhibit a greater metabolic response under warming with ambient CO2 (cT) and P. smithii to exhibit a similar response under warming combined with elevated CO2 (CT). Our hypothesis was supported by the differential response of structural compounds in these two species, where cT increased the content of lignin and cuticular‐matrix in B. gracilis. In P. smithii a similar response was observed in plants exposed to CT, possibly due to the partial alleviation of moisture stress. With warming, the total cell‐wall bound phenolic acids that cross link polysaccharides to lignins increased in B. gracilis and decreased in P. smithii, indicating a potentially adaptive response of C4 pathway to warming alone. Similarly, in B. gracilis, extractable polar metabolites such as sugars and phenolic acids increased with the main effect of warming. Conversely, in P. smithii, only sugars showed a higher abundance in plants exposed to warming treatments indicating that warming alone might be metabolically too disruptive for the C3 photosynthetic pathway. Here we show for the first time, that along with traditionally probed extractable metabolites, warming and elevated CO2 differentially influence the structural metabolites in litters of photosynthetically divergent grass species. If these unique metabolite responses occur in other species of similar functional types, this could potentially alter carbon cycling in grasslands due to the varying degradability of these litters.

Modeling the grazing effect on dry grassland carbon cycling with Biome-BGC model

Identifying the factors that determine the carbon source/sink strength of ecosystems is important for reducing uncertainty in the global carbon cycle. Using the modified Biome-BGC grazing model, we modeled carbon dynamics in Xinjiang, Northwest China for grasslands that varied in grazing intensity. In general, this regional simulation estimated that the grassland ecosystems in Xinjiang acted as a net carbon source, with a value of 0.38PgC over the period 1979–2007. There were significant effects of grazing on carbon dynamics. An over-compensatory effect in net primary productivity (NPP) and vegetation carbon (C) stock was observed when grazing intensity was lower than 0.40head/ha. Grazing resulted in a net carbon source of 23.45gCm−2yr−1, which equaled 0.37Pg in Xinjiang in the last 29 years. In general, grazing decreased vegetation C stock, while an increasing trend was observed with low grazing intensity. The soil C increased significantly (17%) with long-term grazing, while the soil C stock exhibited a steady trend without grazing. These findings have implications for grassland ecosystem management as it relates to carbon sequestration and climate change mitigation, e.g., removal of grazing should be considered in strategies that aim to increase terrestrial carbon sequestrations at local and regional scales. One of the greatest limitations in quantifying the effects of herbivores on carbon cycling is identifying the grazing systems and intensities within a given region. We hope our study emphasizes the need for large-scale assessments of how grazing impacts carbon cycling.

Grazing exclusion alters soil microbial respiration, root respiration and the soil carbon balance in grasslands of the Loess Plateau, northern China

Grassland ecosystems are a significant component of the global carbon cycle. To better understand how grazing affects the carbon cycle of grasslands, soil microbial respiration (Rm) and root respiration (Rr), which are the main soil respiration components, we investigated with a trenching method in grazed grasslands (GG) and fenced (FG) grasslands on the Loess Plateau, northern China in 2008. The annual carbon balance in the two grasslands were also assessed and compared. After exclusion of grazing for about 3 years, soil organic carbon (SOC) and microbial biomass carbon (MBC) in the surface soil increased significantly (P < 0.05), resulting in the increase of Rm in most seasons. Exclusion of grazing did not change the diurnal variations of Rm, Rr and total soil respiration (Rt). Grazing decreased the temperature dependence of Rm. The annual accumulations of Rm were 165.9 g C m−2 in FG and 116.1 g C m−2 in GG. On most dates, Rr in FG was higher than in GG, but significant differences were only found in some seasons. The seasonal average value of Rr was 0.374 µmol carbon dioxide (CO2) m−2 s−1 in FG, 21.0% higher than that in GG (0.309 µmol CO2 m−2 s−1). Net primary production (NPP) in FG and GG were 243.6 and 205.8 g C m−2, respectively. The annual C balance resulted in net C sequestrations of 77.7 and 89.7 g C m−2 in FG and GG, respectively, suggesting that the grassland in this region may act as a C sink both under grazing and fencing.