Climate change reduces the net sink of CH\u3csub\u3e4\u3c/sub\u3e and N\u3csub\u3e2\u3c/sub\u3eO in a semiarid grassland

Atmospheric concentrations of methane (CH4) and nitrous oxide (N2O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2O fluxes in a well-drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH4 and N2O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell-shaped relationship was observed with soil moisture with highest CH4 uptake at intermediate soil moisture. Both N2O emission and uptake occurred at our site with some years showing cumulative N2O emission and other years showing cumulative N2O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2O expressed in CO2 equivalents (across 5 years by 7% and 11% for elevated CO2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2O expressed in CO2-equivalents

Microclimatic performance of a free-air warming and CO2 enrichment experiment in windy Wyoming, USA

In order to plan for global changing climate experiments are being conducted in many countries, but few have monitored the effects of the climate change treatments (warming, elevated CO2) on the experimental plot microclimate. During three years of an eight year study with year-round feedback-controlled infra-red heater warming (1.5/3.0°C day/night) and growing season free-air CO2 enrichment (600 ppm) in the mixed-grass prairie of Wyoming, USA, we monitored soil, leaf, canopy-air, above-canopy-air temperatures and relative humidity of control and treated experimental plots and evaluated ecologically important temperature differentials. Leaves were warmed somewhat less than the target settings (1.1 & 1.5°C day/night) but soil was warmed more creating an average that matched the target settings extremely well both during the day and night plus the summer and winter. The site typically has about 50% bare or litter covered soil, therefore soil heat transfer is more critical than in dense canopy ecosystems. The Wyoming site commonly has strong winds (5 ms-1 average) and significant daily and seasonal temperature fluctuations (as much as 30°C daily) but the warming system was nearly always able to maintain the set temperatures regardless of abiotic variation. The within canopy-air was only slightly warmed and above canopy- air was not warmed by the system, therefore convective warming was minor. Elevated CO2 had no direct effect nor interaction with the warming treatment on microclimate. Relative humidity within the plant canopy was only slightly reduced by warming. Soil water content was reduced by warming but increased by elevated CO2. This study demonstrates the importance of monitoring the microclimate in manipulative field global change experiments so that critical physiological and ecological conclusions can be determined. Highly variable energy demand fluctuations showed that passive IR heater warming systems will not maintain desired warming for much of the time

Long-term exposure to elevated CO\u3csub\u3e2\u3c/sub\u3e enhances plant community stability by suppressing dominant plant species in a mixed-grass prairie

Climate controls vegetation distribution across the globe, and some vegetation types are more vulnerable to climate change, whereas others are more resistant. Because resistance and resilience can influence ecosystem stability and determine how communities and ecosystems respond to climate change, we need to evaluate the potential for resistance as we predict future ecosystem function. In a mixed-grass prairie in the northern Great Plains, we used a large field experiment to test the effects of elevated CO2, warming, and summer irrigation on plant community structure and productivity, linking changes in both to stability in plant community composition and biomass production. We show that the independent effects of CO2 and warming on community composition and productivity depend on interannual variation in precipitation and that the effects of elevated CO2 are not limited to water saving because they differ from those of irrigation. We also show that production in this mixed-grass prairie ecosystem is not only relatively resistant to interannual variation in precipitation, but also rendered more stable under elevated CO2 conditions. This increase in production stability is the result of altered community dominance patterns: Community evenness increases as dominant species decrease in biomass under elevated CO2. In many grasslands that serve as rangelands, the economic value of the ecosystem is largely dependent on plant community composition and the relative abundance of key forage species. Thus, our results have implications for how we manage native grasslands in the face of changing climate

Climate change reduces the net sink of CH\u3csub\u3e4\u3c/sub\u3e and N\u3csub\u3e2\u3c/sub\u3eO in a semiarid grassland

Atmospheric concentrations of methane (CH4) and nitrous oxide (N2O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2O fluxes in a well-drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH4 and N2O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell-shaped relationship was observed with soil moisture with highest CH4 uptake at intermediate soil moisture. Both N2O emission and uptake occurred at our site with some years showing cumulative N2O emission and other years showing cumulative N2O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2O expressed in CO2 equivalents (across 5 years by 7% and 11% for elevated CO2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2O expressed in CO2-equivalents

Elevated CO\u3csub\u3e2\u3c/sub\u3e effects on semi-arid grassland plants in relation to water availability and competition

1. It has been suggested that much of the elevated CO2 effect on plant productivity and N cycling in semi-arid grasslands is related to a CO2-induced increase in soil moisture, but the relative importance of moisture-mediated and direct effects of CO2 remain unclear. 2. We grew five grassland species common to the semi-arid grasslands of northern Colorado, USA, as monocultures and as mixtures of all five species in pots. We examined the effects of atmospheric CO2 concentration (ambient vs. 780 p.p.m.) and soil moisture (15 vs. 20% m⁄m) on plant biomass and plant N uptake. Our objective was to separate CO2 effects not related to water from water-mediated CO2 effects by frequently watering the pots, thereby eliminating most of the elevated CO2 effects on soil moisture, and including a water treatment similar in magnitude to the water-savings effect of CO2. 3. Biomass of the C3 grasses Hesperostipa comata and Pascopyrum smithii increased under elevated CO2, biomass of the C4 grass Bouteloua gracilis increased with increased soil moisture, while biomass of the forbs Artemisia frigida and Linaria dalmatica had no or mixed responses. Increased plant N uptake contributed to the increase in plant biomass with increased soil moisture while the increase in plant biomass with CO2 enrichment was mostly a result of increased N use efficiency (NUE). Species-specific responses to elevated CO2 and increased soil moisture differed between monocultures and mixtures. Both under elevated CO2 and with increased soil moisture, certain species gained N in mixtures at the expense of species that lost N, but elevated CO2 led to a different set of winners and losers than did increased water. 4. Elevated CO2 can directly increase plant productivity of semi-arid grasslands through increased NUE, while a CO2-induced increase in soil moisture stimulating net N mineralization could further enhance plant productivity through increased N uptake. Our results further indicate that the largest positive and negative effects of elevated CO2 and increased soil moisture on plant productivity occur with interspecific competition. Responses of this grassland community to elevated CO2 and water may be both contingent upon and accentuated by competition

Mediation of soil C decomposition by arbuscular mycorrizhal fungi in grass rhizospheres under elevated CO2

Arbuscular mycorrhizal (AMF) function has mostly been studied from the plant perspective, but there is a shortage of empirical assessments of their ecosystem level impacts on soil carbon (C). Our understanding of the role of AMF on C processing belowground has been restricted mostly to fresh plant residues, not stabilized soil organic matter. The mechanisms by which elevated CO2 (eCO2) alter soil C remain an open question but AMF likely play a role via C and nutrients, which could in turn, be plant species dependent. We assessed AMF as mediators of C processing in the rhizosphere of two grasses under eCO2. We exposed a C4 and a C3 grass to a combination of ambient and eCO2 with and without modification of the AMF communities and using stable isotopes quantified the respiration of native soil C(as rhizosphere priming), its contribution to dissolved and microbial C and the final remaining C pool. The AMF treatment impacted soil C respiration under the C3-plant and only under eCO2. eCO2 suppressed decomposition (negative priming) but this effect disappeared when the AMF community was reduced. In contrast to studies of fresh plant residues suggesting that AMF can enhance C loss, our observations indicate that AMF may promote C storage in the soil organic matter pool. Results support that AMF can mediate the effect of eCO2 on soil C in the rhizosphere of some plant species, a potential mechanism explaining variation in impacts of eCO2 on soil C storage and C balances across species and ecosystems

CO2 enhances productivity, alters species composition, and reduces digestibility of Shortgrass Steppe vegetation

Includes bibliographical references (pages 218-219).The impact of increasing atmospheric CO2 concentrations has been studied in a number of field experiments, but little information exists on the response of semiarid rangelands to CO2, or on the consequences for forage quality. This study was initiated to study the CO2 response of the shortgrass steppe, an important semiarid grassland on the western edge of the North American Great Plains, used extensively for livestock grazing. The experiment was conducted for five years on native vegetation at the USDA-ARS Central Plains Experimental Range in northeastern Colorado, USA. Three perennial grasses dominate the study site, Bouteloua gracilis, a C4 grass, and two C3 grasses, Pascopyrum smithii and Stipa comata. The three species comprise 88% of the aboveground phytomass. To evaluate responses to rising atmospheric CO2, we utilized six open-top chambers, three with ambient air and three with air CO2 enriched to 720 mmol/mol, as well as three unchambered controls. We found that elevated CO2 enhanced production of the shortgrass steppe throughout the study, with 41% greater aboveground phytomass harvested annually in elevated compared to ambient plots. The CO2-induced production response was driven by a single species, S. comata, and was due in part to greater seedling recruitment. The result was species movement toward a composition more typical of the mixed-grass prairie. Growth under elevated CO2 reduced the digestibility of all three dominant grass species. Digestibility was also lowest in the only species to exhibit a CO2-induced production enhancement, S. comata. The results suggest that rising atmospheric CO2 may enhance production of lower quality forage and a species composition shift toward a greater C3 component

Elevated CO2 and warming cause interactive effects on soil carbon and shifts in carbon use by bacteria

Accurate predictions of soil C feedbacks to climate change depend on an improved understanding of responses of soil C pools and C use by soil microbial groups. We assessed soil and microbial C in a 7-year manipulation of CO2 and warming in a semi-arid grassland. Continuous field isotopic labelling under elevated CO2 further allowed us to study the dynamics of the existing C (Old C) in soil and microbes as affected by warming. Warming reduced soil C under elevated CO2 but had no impact under ambient CO2. Loss of soil C under warming and elevated CO2 was attributed to increased proportional loss of Old C. Warming also reduced the proportion of Old C in microbes, specifically the bacteria, but not the fungi. These findings highlight that warming impacts are C pool and microbial taxa dependent and demonstrate interactive effects of warming and atmospheric CO2 on soil C

Plant rhizosphere influence on microbial C metabolism : the role of elevated CO2, N availability and root stoichiometry

Microbial decomposer C metabolism is considered a factor controlling soil C stability, a key regulator of global climate. The plant rhizosphere is now recognized as a crucial driver of soil C dynamics but specific mechanisms by which it can affect C processing are unclear. Climate change could affect microbial C metabolism via impacts on the plant rhizosphere. Using continuous 13C labelling under controlled conditions that allowed us to quantify SOM derived-C in all pools and fluxes, we evaluated the microbial metabolism of soil C in the rhizosphere of a C4 native grass exposed to elevated CO2 and under variation in N concentrations in soil and in plant root C:N stoichiometry. Our results demonstrated that this plant can influence soil C metabolism and further, that elevated CO2 conditions can alter this role by increasing microbial C efficiency as indicated by a reduction in soil-derived C respiration per unit of soil C-derived microbial biomass. Moreover, under elevated CO2 increases in soil N, and notably, root tissue N concentration increased C efficiency, suggesting elevated CO2 shifted the stoichiometric balance so N availability was a more critical factor regulating efficiency than under ambient conditions. The root C:N stoichiometry effect indicates that plant chemical traits such as root N concentration are able to influence the metabolism of soil C and that elevated CO2 conditions can modulate this role. Increased efficiency in soil C use was associated with negative rhizosphere priming and we hypothesize that the widely observed phenomenon of rhizosphere priming may result, at least in part, from changes in the metabolic efficiency of microbial populations. Observed changes in the microbial community support that shifting microbial populations were a contributing factor to the observed metabolic responses. Our case study points at greater efficiency of the SOM-degrading populations in a high CO2, high N world, potentially leading to greater C storage of microbially assimilated C in soil

Data from: Elevated CO2 and water addition enhance nitrogen turnover in grassland plants with implications for temporal stability

Temporal variation in soil nitrogen (N) availability affects growth of grassland communities that differ in their use and reuse of N. In a seven-year-long climate change experiment in a semiarid grassland, the temporal stability of plant biomass production varied with plant N turnover (reliance on externally acquired N relative to internally recycled N). Species with high N turnover were less stable in time compared to species with low N turnover. In contrast, N turnover at the community level was positively associated with asynchrony in biomass production, which in turn increased community temporal stability. Elevated CO2 and summer irrigation, but not warming, enhanced community N turnover and stability, possibly because treatments promoted greater abundance of species with high N turnover. Our study highlights the importance of plant N turnover for determining the temporal stability of individual species and plant communities affected by climate change