Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2

We measured soil bacteria, fungi, protozoa, nematodes, and biological activity in serpentine and sandstone annual grasslands after 4 years of exposure to elevated atmospheric CO₂. Measurements were made during the early part of the season, when plants were in vegetative growth, and later in the season, when plants were approaching their maximum biomass. In general, under ambient CO₂, bacterial biomass, total protozoan numbers, and numbers of bactivorous nematodes were similar in the two grasslands. Active and total fungal biomasses were higher on the more productive sandstone grassland compared to the serpentine. However, serpentine soils contained nearly twice the number of fungivorous nematodes compared to the sandstone, perhaps explaining the lower standing crop of fungal biomass in the serpentine and suggesting higher rates of energy flow through the fungal-based soil food web. Furthermore, root biomass in the surface soils of these grasslands is comparable, but the serpentine contains 6 times more phytophagous nematodes compared to the sandstone, indicating greater below-ground grazing pressure on plants in stressful serpentine soils. Elevated CO₂ increased the biomass of active fungi and the numbers of flagellates in both grasslands during the early part of the season and increased the number of phytophagous nematodes in the serpentine. Elevated CO₂ had no effect on the total numbers of bactivorous or fungivorous nematodes, but decreased the diversity of the nematode assemblage in the serpentine at both sampling dates. Excepting this reduction in nematode diversity, the effects of elevated CO₂ disappeared later in the season as plants approached their maximum biomass. Elevated CO₂ had no effect on total and active bacterial biomass, total fungal biomass, or the total numbers of amoebae and ciliates in either grassland during either sampling period. However, soil metabolic activity was higher in the sandstone grassland in the early season under elevated CO₂, and elevated CO₂ altered the patterns of use of individual carbon substrates in both grasslands at this time. Rates of substrate use were also significantly higher in the sandstone, indicating increased bacterial metabolic activity. These changes in soil microbiota are likely due to an increase in the flux of carbon from roots to soil in elevated CO₂, as has been previously reported for these grasslands. Results presented here suggest that some of the carbon distributed below ground in response to elevated CO₂ affects the soil microbial food web, but that these effects may be more pronounced during the early part of the growing season.     

Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands

Root, arbuscular-mycorrhizal (AM), soil faunal (protozoa and microarthropods), and microbial responses to field exposure to CO₂ for six growing seasons were measured in spring 1997 in two adjacent grassland communities. The grasslands showed contrasting root responses to CO₂ enrichment: whereas root length was not affected in the sandstone grassland, it was greater in the serpentine grassland, as was specific root length. AM fungal hyphal lengths were greater in the sandstone, but were unaffected in the serpentine community. This lent support to the hypothesis that there may be a tradeoff in resource allocation to more fine roots or greater mycorrhizal extraradical hyphal length. AM root infection was greater in both communities at elevated CO₂, as was the proportion of roots containing arbuscules. Our data on total hyphal lengths, culturable and active fungi, bacteria, and protozoa supported the hypothesis that the fungal food chain was more strongly stimulated than the bacterial chain. This study is one of the first to test these hypotheses in natural multi-species communities in the field. Received: 23 October 1998 / Accepted: 19 February 1999     

Elevated CO2 increases belowground respiration in California grasslands

This study was designed to identify potential effects of elevated CO₂ on belowground respiration (the sum of root and heterotrophic respiration) in field and microcosm ecosystems and on the annual carbon budget. We made three sets of respiration measurements in two CO₂ treatments, i.e., (1) monthly in the sandstone grassland and in microcosms from November 1993 to June 1994; (2) at the annual peak of live biomass (March and April) in the serpentine and sandstone grasslands in 1993 and 1994; and (3) at peak biomass in the microcosms with monocultures of seven species in 1993. To help understand ecosystem carbon cycling, we also made supplementary measurements of belowground respiration monthly in sandstone and serpentine grasslands located within 500 m of the CO₂ experiment site. The seasonal average respiration rate in the sandstone grassland was 2.12 mol m^-2 s^-1 in elevated CO₂, which was 42% higher than the 1.49 mol m^-2 s^-1 measured in ambient CO₂ (P=0.007). Studies of seven individual species in the microcosms indicated that respiration was positively correlated with plant biomass and increased, on average, by 70% with CO₂. Monthly measurements revealed a strong seasonality in belowground respiration, being low (0–0.5 mol CO₂ m^-2 s^-1 in the two grasslands adjacent to the CO₂ site) in the summer dry season and high (2–4 mol CO₂ m^-2 s^-1 in the sandstone grassland and 2–7 mol CO₂ m^-2 s^-1 in the microcosms) during the growing season from the onset of fall rains in November to early spring in April and May. Estimated annual carbon effluxes from the soil were 323 and 440 g C m^-2 year^-1 for the sandstone grasslands in ambient and elevated CO₂. That CO₂-stimulated increase in annual soil carbon efflux is more than twice as big as the increase in aboveground net primary productivity (NPP_a) and approximately 60% of NPP_a in this grassland in the current CO₂ environment. The results of this study suggest that below-ground respiration can dissipate most of the increase in photosynthesis stimulated by elevated CO₂.CIWDPB Publication # 1271     

Effects of increased soil water availability on grassland ecosystem carbon dioxide fluxes

There is considerable interest in how ecosystems will respond to changes in precipitation. Alterations in rain and snowfall are expected to influence the spatio-temporal patterns of plant and soil processes that are controlled by soil moisture, and potentially, the amount of carbon (C) exchanged between the atmosphere and ecosystems. Because grasslands cover over one third of the terrestrial landscape, understanding controls on grassland C processes will be important to forecast how changes in precipitation regimes will influence the global C cycle. In this study we examined how irrigation affects carbon dioxide (CO₂) fluxes in five widely variable grasslands of Yellowstone National Park during a year of approximately average growing season precipitation. We irrigated plots every 2 weeks with 25% of the monthly 30-year average of precipitation resulting in plots receiving approximately 150% of the usual growing season water in the form of rain and supplemented irrigation. Ecosystem CO₂ fluxes were measured with a closed chamber-system once a month from May-September on irrigated and unirrigated plots in each grassland. Soil moisture was closely associated with CO₂ fluxes and shoot biomass, and was between 1.6% and 11.5% higher at the irrigated plots (values from wettest to driest grassland) during times of measurements. When examining the effect of irrigation throughout the growing season (May–September) across sites, we found that water additions increased ecosystem CO₂ fluxes at the two driest and the wettest sites, suggesting that these sites were water-limited during the climatically average precipitation conditions of the 2005 growing season. In contrast, no consistent responses to irrigation were detected at the two sites with intermediate soil moisture. Thus, the ecosystem CO₂ fluxes at those sites were not water-limited, when considering their responses to supplemental water throughout the whole season. In contrast, when we explored how the effect of irrigation varied temporally, we found that irrigation increased ecosystem CO₂ fluxes at all the sites late in the growing season (September). The spatial differences in the response of ecosystem CO₂ fluxes to irrigation likely can be explained by site specific differences in soil and vegetation properties. The temporal effects likely were due to delayed plant senescence that promoted plant and soil activity later into the year. Our results suggest that in Yellowstone National Park, above-normal amounts of soil moisture will only stimulate CO₂ fluxes across a portion of the ecosystem. Thus, depending on the topographic location, grassland CO₂ fluxes can be water-limited or not. Such information is important to accurately predict how changes in precipitation/soil moisture will affect CO₂ dynamics and how they may feed back to the global C cycle.     

Comparison of the effect of elevated CO2 on an invasive species (Centaurea solstitialis) in monoculture and community settings

The ongoing increase in atmospheric CO₂ concentration ([CO₂]) is likely to change the species composition of plant communities. To investigate whether growth of a highly invasive plant species, Centaurea solstitialis (yellow starthistle), was affected by elevated [CO₂], and whether the success of this species would increase under CO₂ enrichment, I grew the species in serpentine soil microcosms, both as a monoculture and as a component of a grassland community. Centaurea grown in monoculture responded strongly to [CO₂] enrichment of 350 mol mol^–1, increasing aboveground biomass production by 70%, inflorescence production by 74%, and midday photosynthesis by an average of 132%. When grown in competition with common serpentine grassland species, Centaurea responded to CO₂ enrichment with similar but nonsignificant increases (+69% aboveground biomass, +71% inflorescence production), while total aboveground biomass of the polyculture increased by 28%. Centaurea's positive CO₂ response in monoculture and parallel (but non-significant) response in polyculture provoke questions about possible consequences of increasing CO₂ for more typical California grasslands, where the invader already causes major problems.     

Shrub establishment under experimental global changes in a California grassland

Accelerating invasion of grasslands by woody species is a widespread global phenomenon. The native shrub Baccharis pilularis has recently increased in abundance in some California grasslands, with large local community and ecosystem effects. I investigated potential contributions of (1) future global climate and atmospheric changes and (2) variation in moisture and nutrient availability to increased Baccharis germination and early establishment rates. I examined responses of Baccharis seeds and seedlings to simulated warming (+ 1−2 °C) and elevated CO₂ (+ 300 ppm) in a 2-year field experiment. Warming and CO₂ treatments were applied at ambient and increased water and nitrogen levels chosen to simulate future increases in precipitation (+ 50%) and N deposition (+ 7 gN m⁻² y⁻¹). Elevated CO₂ and water addition each increased or accelerated germination. Herbivory strongly reduced seedling populations during the winter wet season; drought further reduced seedling survival in the spring. Overall Baccharis survivorship was extremely low (<0.1%) across all treatments, complicating the interpretation of global change effects.     

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO₂ exchange and the emission or uptake of non-CO₂ GHGs (CH₄ and N₂O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH₄ and N₂O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (R_H) in both forests and grasslands, while a significant N-induced increase in soil CO₂ fluxes (R_S, soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N₂O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH₄, suggesting a declined sink capacity of forests and grasslands for atmospheric CH₄ under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO₂-eq year⁻¹ vs. grassland, 0.58 Pg CO₂-eq year⁻¹) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO₂-eq year⁻¹ vs. grassland, 0.18 Pg CO₂-eq year⁻¹) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%–4.8%) by the combined effects of its stimulation of N₂O emissions together with the reduced soil uptake of CH₄.     

The Effect of Increased N Deposition on Nitrous oxide, Methane and Carbon dioxide Fluxes from Unmanaged Forest and Grassland Communities in Michigan

Atmospheric nitrogen deposition is anticipated to increase over the next decades with possible implications for future forest-atmosphere interactions. Increased soil N₂O emissions, depressed CH₄ uptake and depressed soil respiration CO₂ loss is considered a likely response to increased N deposition. This study examined fluxes of N₂O, CH₄ and CO₂ over two growing seasons from soils in unmanaged forest and grassland communities on abandoned agricultural areas in Michigan. All sites were subject to simulated increased N-deposition in the range of 1–3 g N m⁻² annually. Nitrous oxide fluxes and soil N concentrations in coniferous and grassland sites were on the whole unaffected by the increased N-inputs. It is noteworthy though that N₂O emissions increased three-fold in the coniferous sites in the first growing season in response to the low N treatment, although the response was barely significant (p<0.06). In deciduous forests, we observed increased levels of soil mineral N during the second year of N fertilization, however N₂O fluxes did not increase. Rates of methane oxidation were similar in all sites with no affect of field N application. Likewise, we did not observe any changes in soil CO₂ efflux in response to N additions. The combination of tillage history and vegetation type was important for the trace gas fluxes, i.e. soil CO₂ efflux was greater in successional grassland sites compared with the forested sites and CH₄ uptake was reduced in post-tillage coniferous- and successional sites compared with the old-growth deciduous site. Our results indicate that short-term increased N availability influenced individual processes linked to trace gas turnover in the soil independently from the ecosystem N status. However, changes in whole system fluxes were not evident and were very likely mediated by competitive N uptake processes.     

Species-specific responses of plant communities to altered carbon and nutrient availability

In a field microcosm experiment, species‐specific responses of aboveground biomass of two California annual grassland communities to elevated CO₂ and nutrient availability were investigated. One community grows on shallow, nutrient‐poor serpentine‐derived soil whereas the other occurs on deeper, modestly fertile sandstone/greenstone‐derived substrate. In most species, CO₂ effects did not appear until late in the growing season, probably because the elevated CO₂ increased water‐use‐efficiency easing, the onset of the summer drought. Responses of aboveground biomass to elevated CO₂ differed depending on nutrient availability. Similarly, biomass responses to nutrient treatments differed depending on the CO₂ status. For the majority of the species, production increased most under elevated CO₂ with added nutrients (N,P,K, and micro nutrients). Some species were losers under conditions that increased overall community production, including Bromus hordeaceus in the serpentine community (negative biomass response under elevated CO₂) and Lotus wrangelianus in both communities (negative biomass response with added nitrogen). Treatment and competitive effects on species‐specific biomass varied in both magnitude and direction, especially in the serpentine community, significantly affecting community structure. Individual resource environments are likely to be affected by neighbouring plants, and these competitive interactions complicate predictions of species' responses to elevated CO₂.     

Root dynamics in a semi-natural grassland in relation to atmospheric carbon dioxide enrichment,soil water and shoot biomass

Plant responses to increasing atmospheric CO₂ concentrations have been studied intensively. However, the effects of elevated CO₂ on root dynamics, which is important for global carbon budgets as well as for nutrient cycling in ecosystems, has received much less attention. We used minirhizotrons inside open-top chambers to study the effects of elevated atmospheric carbon dioxide concentration on root dynamics in a nutrient-poor semi-natural grassland in central Sweden. We conducted our investigation over three consecutive growing seasons during which three treatments were applied at the site: Elevated (≈ 700 μmol mol^-1) and ambient (≈ 360 μmol mol^-1) chamber levels of CO₂ and a control, without a chamber. During 1997, a summer with two dry periods, the elevated treatment compared with ambient had 25% greater mean root counts, 65% greater above-ground biomass and 15% greater soil moisture. The chambers seemed responsible for changes in root dynamics, whereas the elevated CO₂ treatment in general increased the absolute sum of root counts compared with the ambient chamber. In 1998, a wet growing season, there were no significant differences in shoot biomass or root dynamics and both chamber treatments had lower soil moisture than the control. We found that as seasonal dryness increased, the ratio of elevated – ambient shoot biomass production increased while the root to shoot ratio decreased. We conclude that this grasslands response to elevated CO₂ is dependent on seasonal weather conditions and that CO₂ enrichment will most significantly increase production in such a grassland when under water stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil C and N responses to woody plant expansion in a mesic grassland

Changes in land management and reductions in fire frequency have contributed to increased cover of woody species in grasslands worldwide. These shifts in plant community composition have the potential to alter ecosystem function, particularly through changes in soil processes and properties. In semi-arid grasslands, the invasion of shrubs and trees is often accompanied by increases in soil resources and more rapid N and C cycling. We assessed the effects of shrub encroachment in a mesic grassland in Kansas (USA) on soil CO₂ flux, extractable inorganic N, and N mineralization beneath shrub communities (Cornus drummondii) and surrounding undisturbed grassland sites. In this study, a shift in plant community composition from grassland to shrubland resulted in a 16% decrease in annual soil CO₂ flux(4.78 kg CO₂ m^–2 year^–1 for shrub dominated sites versus 5.84 kg CO₂ m^–2 year^–1 for grassland sites) with no differences in total soil C or N or inorganic N. There was considerable variability in N mineralization rates within sites, which resulted in no overall difference in cumulative N mineralized during this study (4.09 g N m^–2 for grassland sites and 3.03 g N m^–2 for shrub islands). These results indicate that shrub encroachment into mesic grasslands does not significantly alter N availability (at least initially), but does alter C cycling by decreasing soil CO₂ flux.     

Large Greenhouse Gas Emissions from a Temperate Peatland Pasture

Agricultural drainage is thought to alter greenhouse gas emissions from temperate peatlands, with CH₄ emissions reduced in favor of greater CO₂ losses. Attention has largely focussed on C trace gases, and less is known about the impacts of agricultural conversion on N₂O or global warming potential. We report greenhouse gas fluxes (CH₄, CO₂, N₂O) from a drained peatland in the Sacramento-San Joaquin River Delta, California, USA currently managed as a rangeland (that is, pasture). This ecosystem was a net source of CH₄ (25.8 ± 1.4 mg CH₄-C m⁻² d⁻¹) and N₂O (6.4 ± 0.4 mg N₂O-N m⁻² d⁻¹). Methane fluxes were comparable to those of other managed temperate peatlands, whereas N₂O fluxes were very high; equivalent to fluxes from heavily fertilized agroecosystems and tropical forests. Ecosystem scale CH₄ fluxes were driven by “hotspots” (drainage ditches) that accounted for less than 5% of the land area but more than 84% of emissions. Methane fluxes were unresponsive to seasonal fluctuations in climate and showed minimal temporal variability. Nitrous oxide fluxes were more homogeneously distributed throughout the landscape and responded to fluctuations in environmental variables, especially soil moisture. Elevated CH₄ and N₂O fluxes contributed to a high overall ecosystem global warming potential (531 g CO₂-C equivalents m⁻² y⁻¹), with non-CO₂ trace gas fluxes offsetting the atmospheric “cooling” effects of photoassimilation. These data suggest that managed Delta peatlands are potentially large regional sources of greenhouse gases, with spatial heterogeneity in soil moisture modulating the relative importance of each gas for ecosystem global warming potential.     

Effects of Climate Change Drivers on Nitrous Oxide Fluxes in an Upland Temperate Grassland

Despite increasing interest in the patterns of trace gas emissions in terrestrial ecosystems, little is known about the impacts of climate change on nitrous oxide (N₂O) fluxes. The aim of this study was to determine the importance of the three main drivers of climate change (warming, summer drought, and elevated CO₂ concentrations) on N₂O fluxes from an extensively managed, upland grassland. Over a 2-year period, we monitored N₂O fluxes in an in situ ecosystem manipulation experiment simulating the climate predicted for the study area in 2080 (3.5°C temperature increase, 20% reduction in summer rainfall and atmospheric CO₂ levels of 600 ppm). N₂O fluxes showed significant seasonal and interannual variation irrespective of climate treatment, and were higher in summer and autumn compared with winter and spring. Overall, N₂O emissions showed a positive correlation with soil temperature and rainfall. Elevated temperature had a positive impact on mean annual N₂O fluxes but effects were only significant in 2007. Contrary to expectations, neither combined summer drought and warming nor the simultaneous application of elevated atmospheric CO₂ concentrations, summer drought and warming had any significant effect on annual N₂O fluxes. However, the maximum N₂O flux rates observed during the study occurred when elevated CO₂ was combined with warming and drought, suggesting the potential for important, short-term N₂O–N losses in enriched CO₂ environments. Taken together, our results suggest that the N₂O responses of temperate, extensively managed grasslands to future climate change scenarios may be primarily driven by temperature effects.     

Greenhouse gas fluxes over managed grasslands in Central Europe

Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site‐specific management strategies strongly affect the biosphere–atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO₂ exchange, while long‐term N₂O and CH₄ flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO₂, N₂O, and CH₄ fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO₂ sink (−1,783 to −91 g CO₂ m⁻² year⁻¹), but a N₂O source (18–638 g CO₂‐eq. m⁻² year⁻¹), and either a CH₄ sink or source (−9 to 488 g CO₂‐eq. m⁻² year⁻¹). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between −2,761 and −58 g CO₂‐eq. m⁻² year⁻¹, with N₂O and CH₄ emissions offsetting concurrent CO₂ uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N₂O and CH₄ fluxes was generally low and varied considerably within years. However, after site‐specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity (“sweet spots”) and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N₂O and CH₄ emissions. The N₂O‐N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%–8.6%). Although grassland management led to increased N₂O and CH₄ emissions, the CO₂ sink strength was generally the most dominant component of the annual GHG budget.     

Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2

Atmospheric CO₂ enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO₂ enrichment, with an emphasis on water relations. Increasing CO₂ led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO₂ enrichment were relatively greater in dry years. In contrast, CO₂-induced aboveground biomass increases in the Texas C₃/C₄ grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO₂ in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO₂-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO₂ were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO₂ on plant and soil water relations may contribute substantially to experimentally induced CO₂-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO₂ effects on photosynthesis and those indirect CO₂ effects via soil moisture as documented here.     

Water- and Plant-Mediated Responses of Ecosystem Carbon Fluxes to Warming and Nitrogen Addition on the Songnen Grassland in Northeast China

Background

Understanding how grasslands are affected by a long-term increase in temperature is crucial to predict the future impact of global climate change on terrestrial ecosystems. Additionally, it is not clear how the effects of global warming on grassland productivity are going to be altered by increased N deposition and N addition.

Methodology/Principal Findings

In-situ canopy CO₂ exchange rates were measured in a meadow steppe subjected to 4-year warming and nitrogen addition treatments. Warming treatment reduced net ecosystem CO₂ exchange (NEE) and increased ecosystem respiration (ER); but had no significant impacts on gross ecosystem productivity (GEP). N addition increased NEE, ER and GEP. However, there were no significant interactions between N addition and warming. The variation of NEE during the four experimental years was correlated with soil water content, particularly during early spring, suggesting that water availability is a primary driver of carbon fluxes in the studied semi-arid grassland.

Conclusion/Significance

Ecosystem carbon fluxes in grassland ecosystems are sensitive to warming and N addition. In the studied water-limited grassland, both warming and N addition influence ecosystem carbon fluxes by affecting water availability, which is the primary driver in many arid and semiarid ecosystems. It remains unknown to what extent the long-term N addition would affect the turn-over of soil organic matter and the C sink size of this grassland.     

Effects of an increase in summer precipitation on leaf, soil, and ecosystem fluxes of CO2 and H2O in a sotol grassland in Big Bend National Park, Texas

Global climate models predict that in the next century precipitation in desert regions of the USA will increase, which is anticipated to affect biosphere/atmosphere exchanges of both CO₂ and H₂O. In a sotol grassland ecosystem in the Chihuahuan Desert at Big Bend National Park, we measured the response of leaf-level fluxes of CO₂ and H₂O 1 day before and up to 7 days after three supplemental precipitation pulses in the summer (June, July, and August 2004). In addition, the responses of leaf, soil, and ecosystem fluxes of CO₂ and H₂O to these precipitation pulses were also evaluated in September, 1 month after the final seasonal supplemental watering event. We found that plant carbon fixation responded positively to supplemental precipitation throughout the summer. Both shrubs and grasses in watered plots had increased rates of photosynthesis following pulses in June and July. In September, only grasses in watered plots had higher rates of photosynthesis than plants in the control plots. Soil respiration decreased in supplementally watered plots at the end of the summer. Due to these increased rates of photosynthesis in grasses and decreased rates of daytime soil respiration, watered ecosystems were a sink for carbon in September, assimilating on average 31 mmol CO₂ m⁻² s⁻¹ ground area day⁻¹. As a result of a 25% increase in summer precipitation, watered plots fixed eightfold more CO₂ during a 24-h period than control plots. In June and July, there were greater rates of transpiration for both grasses and shrubs in the watered plots. In September, similar rates of transpiration and soil water evaporation led to no observed treatment differences in ecosystem evapotranspiration, even though grasses transpired significantly more than shrubs. In summary, greater amounts of summer precipitation may lead to short-term increased carbon uptake by this sotol grassland ecosystem.     

Simulated effects of low atmospheric CO2 on structure and composition of North American vegetation at the Last Glacial Maximum

1. Physiological experiments have indicated that the lower CO₂ levels of the last glaciation (200 μmol mol^?1) probably reduced plant water-use efficiency (WUE) and that they combined with increased aridity and colder temperatures to alter vegetation structure and composition at the Last Glacial Maximum (LGM). 2. The effects of low CO₂ on vegetation structure were investigated using BIOME3 simulations of leaf area index (LAI), and a two-by-two factorial experimental design (modern/LGM CO₂, modern/LGM climate).3. Using BIOME3, and a combination of lowered CO₂ and simulated LGM climate (from the NCAR-CCM1 model), results in the introduction of additional xeric vegetation types between open woodland and closed-canopy forest along a latitudinal gradient in eastern North America.4. The simulated LAI of LGM vegetation was 25–60% lower in many regions of central and eastern United States relative to modern climate, indicating that glacial vegetation was much more open than today.5. Comparison of factorial simulations show that low atmospheric CO₂ has the potential to alter vegetation structure (LAI) to a greater extent than LGM climate.6. If the magnitude of LAI reductions simulated for glacial North America were global, then low atmospheric CO₂ may have promoted atmospheric warming and increased aridity, through alteration of rates of water and heat exchange with the atmosphere.     

Differential influence of elevated CO<Subscript>2</Subscript> on gas exchange and water use efficiency of four indigenous shrub species distributed in different sandy environments in central Inner Mongolia

In view of the increase in global warming and carbon dioxide (CO₂) concentrations, it is essential to investigate the influences of climate change on plant growth and water use in arid and semi-arid grassland species. Experiments were conducted to understand the ecophysiological response of four indigenous species to elevated CO₂ in the semi-arid sandy grassland of central Inner Mongolia. Seedlings of the four species were grown for 8 weeks at four different consistently elevated CO₂ concentrations in the environment-controlled growth chambers. All four elevated CO₂ concentrations (400, 800, 1200, 1600 ppm) were found to result in decreased stomatal conductance (26–86%), decreased transpiration rate (21–80%), increased shoot water potential (1–42%) and increased water use efficiency (WUE) (10–412%) for two Artemisia species and Caragana korshinskii. Under our experimental conditions, the two Artemisia species and C. korshinskii would benefit more than Hedysarum laeve from exposure to elevated CO₂ in terms of higher shoot water potential and WUE combined with lower stomatal conductance and transpiration rate. The results indicate that in a warmer, CO₂-enriched future atmospheric environment, WUE in semi-arid grasslands may be higher than previously expected. Our findings will provide information for screening appropriate species for restoration of the degraded sandy grasslands in semi-arid areas under future climate change scenarios.     

Soil Respiration in European Grasslands in Relation to Climate and Assimilate Supply

Soil respiration constitutes the second largest flux of carbon (C) between terrestrial ecosystems and the atmosphere. This study provides a synthesis of soil respiration (R _s) in 20 European grasslands across a climatic transect, including ten meadows, eight pastures and two unmanaged grasslands. Maximum rates of R _s ( ), R _s at a reference soil temperature (10°C; ) and annual R _s (estimated for 13 sites) ranged from 1.9 to 15.9 μmol CO₂ m⁻² s⁻¹, 0.3 to 5.5 μmol CO₂ m⁻² s⁻¹ and 58 to 1988 g C m⁻² y⁻¹, respectively. Values obtained for Central European mountain meadows are amongst the highest so far reported for any type of ecosystem. Across all sites was closely related to . Assimilate supply affected R _s at timescales from daily (but not necessarily diurnal) to annual. Reductions of assimilate supply by removal of aboveground biomass through grazing and cutting resulted in a rapid and a significant decrease of R _s. Temperature-independent seasonal fluctuations of R _s of an intensively managed pasture were closely related to changes in leaf area index (LAI). Across sites increased with mean annual soil temperature (MAT), LAI and gross primary productivity (GPP), indicating that assimilate supply overrides potential acclimation to prevailing temperatures. Also annual R _s was closely related to LAI and GPP. Because the latter two parameters were coupled to MAT, temperature was a suitable surrogate for deriving estimates of annual R _s across the grasslands studied. These findings contribute to our understanding of regional patterns of soil C fluxes and highlight the importance of assimilate supply for soil CO₂ emissions at various timescales.