Global change could amplify fire effects on soil greenhouse gas emissions

Background

Little is known about the combined impacts of global environmental changes and ecological disturbances on ecosystem functioning, even though such combined impacts might play critical roles in shaping ecosystem processes that can in turn feed back to climate change, such as soil emissions of greenhouse gases.

Methodology/Principal Findings

We took advantage of an accidental, low-severity wildfire that burned part of a long-term global change experiment to investigate the interactive effects of a fire disturbance and increases in CO₂ concentration, precipitation and nitrogen supply on soil nitrous oxide (N₂O) emissions in a grassland ecosystem. We examined the responses of soil N₂O emissions, as well as the responses of the two main microbial processes contributing to soil N₂O production – nitrification and denitrification – and of their main drivers. We show that the fire disturbance greatly increased soil N₂O emissions over a three-year period, and that elevated CO₂ and enhanced nitrogen supply amplified fire effects on soil N₂O emissions: emissions increased by a factor of two with fire alone and by a factor of six under the combined influence of fire, elevated CO₂ and nitrogen. We also provide evidence that this response was caused by increased microbial denitrification, resulting from increased soil moisture and soil carbon and nitrogen availability in the burned and fertilized plots.

Conclusions/Significance

Our results indicate that the combined effects of fire and global environmental changes can exceed their effects in isolation, thereby creating unexpected feedbacks to soil greenhouse gas emissions. These findings highlight the need to further explore the impacts of ecological disturbances on ecosystem functioning in the context of global change if we wish to be able to model future soil greenhouse gas emissions with greater confidence.     

Changes in soil greenhouse gas fluxes by land use change from primary forest

Primary forest conversion is a worldwide serious problem associated with human disturbance and climate change. Land use change from primary forest to plantation, grassland or agricultural land may lead to profound alteration in the emission of soil greenhouse gases (GHG). Here, we conducted a global meta‐analysis concerning the effects of primary forest conversion on soil GHG emissions and explored the potential mechanisms from 101 studies. Our results showed that conversion of primary forest significantly decreased soil CO₂ efflux and increased soil CH₄ efflux, but had no effect on soil N₂O efflux. However, the effect of primary forest conversion on soil GHG emissions was not consistent across different types of land use change. For example, soil CO₂ efflux did not respond to the conversion from primary forest to grassland. Soil N₂O efflux showed a prominent increase within the initial stage after conversion of primary forest and then decreased over time while the responses of soil CO₂ and CH₄ effluxes were consistently negative or positive across different elapsed time intervals. Moreover, either within or across all types of primary forest conversion, the response of soil CO₂ efflux was mainly moderated by changes in soil microbial biomass carbon and root biomass while the responses of soil N₂O and CH₄ effluxes were related to the changes in soil nitrate and soil aeration‐related factors (soil water content and bulk density), respectively. Collectively, our findings highlight the significant effects of primary forest conversion on soil GHG emissions, enhance our knowledge on the potential mechanisms driving these effects and improve future models of soil GHG emissions after land use change from primary forest.     

Terrestrial N2O emissions and related functional genes under climate change: A global meta‐analysis

Nitrous oxide (N₂O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N₂O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N₂O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N₂O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N₂O emissions (most effectively induced by open‐top chambers). Whole‐day or whole‐year warming treatment significantly enhanced N₂O emissions, but daytime, nighttime or short‐season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N₂O emission by an average of 55%, while decreased precipitation suppressed N₂O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U‐shape relationship with soil moisture; further insight into biotic mechanisms underlying N₂O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N₂O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.     

Interactive effects of elevated CO2, warming,reduced rainfall,and nitrogen on leaf gas exchange in five perennial grassland species

Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi-factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open-air experiment in Minnesota, USA, including elevated CO₂ (eCO₂), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context-dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO₂ depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C₄ grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.     

Do “hot moments” become hotter under climate change? Soil nitrogen dynamics from a climate manipulation experiment in a post-harvest forest

Whole-tree forest harvest can increase soil nitrous oxide (N₂O) effluxes and leaching of nitrogen (N) from soils. These altered N dynamics are often linked to harvesting effects on microclimate, suggesting that this “hot moment” for N cycling may become hotter with climate change. We hypothesized that increases in temperature and precipitation during this post-harvest period would increase availability of soil mineral N and soil-atmosphere N₂O efflux. To test this hypothesis we implemented a climate manipulation experiment after a forest harvest, and measured soil N₂O fluxes and inorganic N accumulating on ion exchange resins. Climate treatments were: control (A, ambient), heated (H, +2.5 °C), wetted (W, +23 % precipitation), and a two-factor treatment (H+W). For all treatments, the first year after harvest had highest N₂O efflux and resin N. Wetting significantly increased cumulative soil N₂O fluxes, but only when soils were not heated too. The cumulative soil-to-atmosphere N₂O efflux from W (5.8 mg N₂O–N m^?2) was significantly higher than A (?1.9 mg N₂O–N m^?2), but H+W (~0 mg N₂O–N m^?2) was similar to A. Regardless of wetting, heating increased resin N, but only on certain dates. Cumulative resin N was on average 125 % greater in the H plots than non-heated plots. Thus, changes in temperature and precipitation each impart distinct changes to the soil N cycle. Heating increased resin N regardless of water inputs, while wetting increasing N₂O but not when combined with heating. Our results suggest that climate change may exacerbate soil N losses from whole-tree harvest in the future, but the form and quantity of N loss will depend on how the future climate changes.     

Nitrification,denitrification, and related functional genes under elevated CO2: A meta-analysis in terrestrial ecosystems

Global change may have profound effects on soil nitrogen (N) cycling that can induce positive feedback to climate change through increased nitrous oxide (N₂O) emissions mediated by nitrification and denitrification. We conducted a meta-analysis of the effects of elevated CO₂ on nitrification and denitrification based on 879 observations from 58 publications and 46 independent elevated CO₂ experiments in terrestrial ecosystems. We investigated the effects of elevated CO₂ alone or combined with elevated temperature, increased precipitation, drought, and N addition. We assessed the response to elevated CO₂ of gross and potential nitrification, potential denitrification, and abundances of related functional genes (archaeal amoA, bacterial amoA, nirK, nirS, and nosZ). Elevated CO₂ increased potential nitrification (+28%) and the abundance of bacterial amoA functional gene (+62%) in cropland ecosystems. Elevated CO₂ increased potential denitrification when combined with N addition and higher precipitation (+116%). Elevated CO₂ also increased the abundance of nirK (+25%) and nirS (+27%) functional genes in terrestrial ecosystems and of nosZ (+32%) functional gene in cropland ecosystems. The increase in the abundance of nosZ under elevated CO₂ was larger at elevated temperature and high N (+62%). Four out of 14 two-way interactions tested between elevated CO₂ and elevated temperature, elevated CO₂ and increased precipitation, and elevated CO₂ and N addition were marginally significant and mostly synergistic. The effects of elevated CO₂ on potential nitrification and abundances of bacterial amoA and nirS functional genes increased with mean annual temperature and mean annual precipitation. Our meta-analysis thus suggests that warming and increased precipitation in large areas of the world could reinforce positive responses of nitrification and denitrification to elevated CO₂ and urges the need for more investigations in the tropical zone and on interactive effects among multiple global change factors, as we may largely underestimate the effects of global change on soil N₂O emissions.     

Nitrogen addition,rather than altered precipitation,stimulates nitrous oxide emissions in an alpine steppe

Anthropogenic‐driven global change, including changes in atmospheric nitrogen (N) deposition and precipitation patterns, is dramatically altering N cycling in soil. How long‐term N deposition, precipitation changes, and their interaction influence nitrous oxide (N₂O) emissions remains unknown, especially in the alpine steppes of the Qinghai–Tibetan Plateau (QTP). To fill this knowledge gap, a platform of N addition (10 g m⁻² year⁻¹) and altered precipitation (±50% precipitation) experiments was established in an alpine steppe of the QTP in 2013. Long‐term N addition significantly increased N₂O emissions. However, neither long‐term alterations in precipitation nor the co‐occurrence of N addition and altered precipitation significantly affected N₂O emissions. These unexpected findings indicate that N₂O emissions are particularly susceptible to N deposition in the alpine steppes. Our results further indicated that both biotic and abiotic properties had significant effects on N₂O emissions. N₂O emissions occurred mainly due to nitrification, which was dominated by ammonia‐oxidizing bacteria, rather than ammonia‐oxidizing archaea. Furthermore, the alterations in belowground biomass and soil temperature induced by N addition modulated N₂O emissions. Overall, this study provides pivotal insights to aid the prediction of future responses of N₂O emissions to long‐term N deposition and precipitation changes in alpine ecosystems. The underlying microbial pathway and key predictors of N₂O emissions identified in this study may also be used for future global‐scale model studies.     

Climate warming interacts with other global change drivers to influence plant phenology: A meta-analysis of experimental studies

Shifts in plant phenology influence ecosystem structures and functions, yet how multiple global change drivers interact to affect phenology remains elusive. We conducted a meta-analysis of 242 published articles to assess interactions between warming (W) and other global change drivers including nitrogen addition (N), increased precipitation (IP), decreased precipitation (DP) and elevated CO₂ (eCO₂) on multiple phenophases in experimental studies. We show that leaf out and first flowering were most strongly affected by warming, while warming and decreased precipitation were the most pronounced drivers for leaf colouring. Moreover, interactions between warming and other global change drivers were common and both synergistic and antagonistic interactions were observed: interactions W + IP and W + eCO₂ were frequently synergistic, whereas interactions W + N and W + DP were mostly antagonistic. These findings demonstrate that global change drivers often affect plant phenology interactively. Incorporating the multitude of interactions into models is crucial for accurately predicting plant responses to global changes.     

Environmental drivers interactively affect individual tree growth across temperate European forests

Forecasting the growth of tree species to future environmental changes requires a better understanding of its determinants. Tree growth is known to respond to global‐change drivers such as climate change or atmospheric deposition, as well as to local land‐use drivers such as forest management. Yet, large geographical scale studies examining interactive growth responses to multiple global‐change drivers are relatively scarce and rarely consider management effects. Here, we assessed the interactive effects of three global‐change drivers (temperature, precipitation and nitrogen deposition) on individual tree growth of three study species (Quercus robur/petraea, Fagus sylvatica and Fraxinus excelsior). We sampled trees along spatial environmental gradients across Europe and accounted for the effects of management for Quercus. We collected increment cores from 267 trees distributed over 151 plots in 19 forest regions and characterized their neighbouring environment to take into account potentially confounding factors such as tree size, competition, soil conditions and elevation. We demonstrate that growth responds interactively to global‐change drivers, with species‐specific sensitivities to the combined factors. Simultaneously high levels of precipitation and deposition benefited Fraxinus, but negatively affected Quercus’ growth, highlighting species‐specific interactive tree growth responses to combined drivers. For Fagus, a stronger growth response to higher temperatures was found when precipitation was also higher, illustrating the potential negative effects of drought stress under warming for this species. Furthermore, we show that past forest management can modulate the effects of changing temperatures on Quercus’ growth; individuals in plots with a coppicing history showed stronger growth responses to higher temperatures. Overall, our findings highlight how tree growth can be interactively determined by global‐change drivers, and how these growth responses might be modulated by past forest management. By showing future growth changes for scenarios of environmental change, we stress the importance of considering multiple drivers, including past management and their interactions, when predicting tree growth.     

Throughfall exclusion and leaf litter addition drive higher rates of soil nitrous oxide emissions from a lowland wet tropical forest

Tropical forests are a significant global source of the greenhouse gas nitrous oxide (N₂O). Predicted environmental changes for this biome highlight the need to understand how simultaneous changes in precipitation and labile carbon (C) availability may affect soil N₂O production. We conducted a small‐scale throughfall and leaf litter manipulation in a lowland tropical forest in southwestern Costa Rica to test how potential changes in both water and litter derived labile C inputs to soils may alter N₂O emissions. Experimentally reducing throughfall in this wet tropical forest significantly increased soil emissions of N₂O, and our data suggest that at least part of this response was driven by an increase in the concentration of dissolved organic carbon [DOC] inputs delivered from litter to soil under the drier conditions. Furthermore, [DOC] was significantly correlated with N₂O emissions across both throughfall and litterfall manipulation plots, despite the fact that native NO₃^? pools in this site were generally small. Our results highlight the importance of understanding not only the potential direct effects of changing precipitation on soil biogeochemistry, but also the indirect effects resulting from interactions between the hydrologic, C and N cycles. Finally, over all sampling events we observed lower mean N₂O emissions (<1 ng N₂O‐N cm^?2 h^?1) than reported for many other lowland tropical forests, perhaps reflecting a more general pattern of increasing relative N constraints to biological activity as one moves from drier to wetter portions of the lowland tropical forest biome.     

Soil plus root respiration and microbial biomass following water, nitrogen, and phosphorus application at a high arctic semi desert

In order to investigate the effects of anticipated increased precipitation and changing soil nutrient levels on soil CO₂ efflux from high arctic semi desert, a field experiment was carried out in Northeast Greenland. Water, phosphorus, and nitrogen were added to plots in a fully factorial design. Soil microbial biomass carbon was analysed after one year, and respiration from soil plus roots was measured in situ throughout the third growing season after initiation of the experiment. Soil plus root respiration was enhanced by up to 47%, and the microbial biomass by 24%, by the weekly water additions, but not by nutrient additions. The direct effect of increased soil moisture on CO₂ efflux suggests that future changes of precipitation levels and patterns may strongly affect below-ground respiration in arctic semi deserts, with direction of responses depending upon amounts and frequencies of precipitation events. Morover, low CO₂ emission at low light intensities regardless of treatment suggests that the major part of the below-ground respiration originated from turnover of recently fixed C. Hence, the more recalcitrant soil organic matter C pool may not change in proportion to changes in below-ground respiration rate.     

Moderate precipitation reduction enhances nitrogen cycling and soil nitrous oxide emissions in a semi-arid grassland

The ongoing climate change is predicted to induce more weather extremes such as frequent drought and high-intensity precipitation events, causing more severe drying-rewetting cycles in soil. However, it remains largely unknown how these changes will affect soil nitrogen (N)-cycling microbes and the emissions of potent greenhouse gas nitrous oxide (N₂O). Utilizing a field precipitation manipulation in a semi-arid grassland on the Loess Plateau, we examined how precipitation reduction (ca. −30%) influenced soil N₂O and carbon dioxide (CO₂) emissions in field, and in a complementary lab-incubation with simulated drying-rewetting cycles. Results obtained showed that precipitation reduction stimulated plant root turnover and N-cycling processes, enhancing soil N₂O and CO₂ emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N₂O emissions primarily originated from nitrification process. The incubation experiment further showed that in field soils under precipitation reduction, drying-rewetting stimulated N mineralization and ammonia-oxidizing bacteria in favor of genera Nitrosospira and Nitrosovibrio, increasing nitrification and N₂O emissions. These findings suggest that moderate precipitation reduction, accompanied with changes in drying-rewetting cycles under future precipitation scenarios, may enhance N cycling processes and soil N₂O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.     

Heat‐wave effects on greenhouse gas emissions from shallow lake mesocosms

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Responses of CO2, N2O and CH4 fluxes between atmosphere and forest soil to changes in multiple environmental conditions

To investigate the effects of multiple environmental conditions on greenhouse gas (CO₂, N₂O, CH₄) fluxes, we transferred three soil monoliths from Masson pine forest (PF) or coniferous and broadleaved mixed forest (MF) at Jigongshan to corresponding forest type at Dinghushan. Greenhouse gas fluxes at the in situ (Jigongshan), transported and ambient (Dinghushan) soil monoliths were measured using static chambers. When the transported soil monoliths experienced the external environmental factors (temperature, precipitation and nitrogen deposition) at Dinghushan, its annual soil CO₂ emissions were 54% in PF and 60% in MF higher than those from the respective in situ treatment. Annual soil N₂O emissions were 45% in PF and 44% in MF higher than those from the respective in situ treatment. There were no significant differences in annual soil CO₂ or N₂O emissions between the transported and ambient treatments. However, annual CH₄ uptake by the transported soil monoliths in PF or MF was not significantly different from that at the respective in situ treatment, and was significantly lower than that at the respective ambient treatment. Therefore, external environmental factors were the major drivers of soil CO₂ and N₂O emissions, while soil was the dominant controller of soil CH₄ uptake. We further tested the results by developing simple empirical models using the observed fluxes of CO₂ and N₂O from the in situ treatment and found that the empirical models can explain about 90% for CO₂ and 40% for N₂O of the observed variations at the transported treatment. Results from this study suggest that the different responses of soil CO₂, N₂O, CH₄ fluxes to changes in multiple environmental conditions need to be considered in global change study.     

Co-Regulations of Spartina alterniflora Invasion and Exogenous Nitrogen Loading on Soil N2O Efflux in Subtropical Mangrove Mesocosms

Both plant invasion and nitrogen (N) enrichment should have significant impact on mangrove ecosystems in coastal regions around the world. However, how N₂O efflux in mangrove wetlands responds to these environmental changes has not been well studied. Here, we conducted a mesocosm experiment with native mangrove species Kandelia obovata, invasive salt marsh species Spartina alterniflora, and their mixture in a simulated tide rotation system with or without nitrogen addition. In the treatments without N addition, the N₂O effluxes were relatively low and there were no significant variations among the three vegetation types. A pulse loading of exogenous ammonium nitrogen increased N₂O effluxes from soils but the stimulatory effect gradually diminished over time, suggesting that frequent measurements are necessary to accurately understand the behavior of N-induced response of N₂O emissions. With the N addition, the N₂O effluxes from the invasive S. alterniflora were lower than that from native K. obovata mesocosms. This result may be attributed to higher growth of S. alterniflora consuming most of the available nitrogen in soils, and thus inhibiting N₂O production. We concluded that N loading significantly increased N₂O effluxes, while the invasion of S. alterniflora reduced N₂O effluxes response to N loading in this simulated mangrove ecosystem. Thus, both plant invasion and excessive N loading can co-regulate soil N₂O emissions from mangrove wetlands, which should be considered when projecting future N₂O effluxes from this type of coastal wetland.     

Microbes drive global soil nitrogen mineralization and availability

Soil net nitrogen mineralization rate (N_min), which is critical for soil nitrogen availability and plant growth, is thought to be primarily controlled by climate and soil physical and/or chemical properties. However, the role of microbes on regulating soil N_min has not been evaluated on the global scale. By compiling 1565 observational data points of potential net N_min from 198 published studies across terrestrial ecosystems, we found that N_min significantly increased with soil microbial biomass, total nitrogen, and mean annual precipitation, but decreased with soil pH. The variation of N_min was ascribed predominantly to soil microbial biomass on global and biome scales. Mean annual precipitation, soil pH, and total soil nitrogen significantly influenced N_min through soil microbes. The structural equation models (SEM) showed that soil substrates were the main factors controlling N_min when microbial biomass was excluded. Microbe became the primary driver when it was included in SEM analysis. SEM with soil microbial biomass improved the N_min prediction by 19% in comparison with that devoid of soil microbial biomass. The changes in N_min contributed the most to global soil NH₄⁺‐N variations in contrast to climate and soil properties. This study reveals the complex interactions of climate, soil properties, and microbes on N_min and highlights the importance of soil microbial biomass in determining N_min and nitrogen availability across the globe. The findings necessitate accurate representation of microbes in Earth system models to better predict nitrogen cycle under global change.     

Effects of Plant Species Diversity and Composition on Nitrogen Cycling and the Trace Gas Balance of Soils

Experiments addressing the role of plant species diversity for ecosystem functioning have recently proliferated. Most studies have focused on plant biomass responses. However, microbial processes involved in the production of N₂O and the oxidation of atmospheric CH₄ could potentially be affected via effects on N cycling, on soil diffusive properties (due to changes in water relations and root architecture) and by more direct interactions of plants with soil microbes. We studied ecosystem-level CH₄ and N₂O fluxes in experimental communities assembled from two pasture soils and from combinations of 1, 3, 6, 8 or 9 species typical for these pastures. The soils contrasted with respect to texture and fertility. N₂O emissions decreased with diversity and increased in the presence of legumes. Soils were sinks for CH₄ at all times; legume monocultures were a smaller sink for atmospheric CH₄ than non-legume monocultures, but no effect of species richness per se was detected. However, both the exchange of CH₄ and N₂O strongly depended on plant community composition, and on the interaction of composition with soil type, indicating that the functional role of species and their interactions differed between soils. N₂O fluxes were mainly driven by effects on soil nitrate and on nitrification while soil moisture had less of an effect. Soil microbial C and N and N mineralisation rates were not altered. The driver of the interactive soil type×plant community composition-effects was less clear. Because soil methanotrophs may take longer to respond to alterations of N cycling than the 1/2 year treatment in this study, we also tested species richness-effects in two separate 5-year field studies, but results were ambiguous, indicating complex interactions with soil disturbance. In conclusion, our study demonstrates that plant community composition can affect the soil trace gas balance, whereas plant species richness per se was less important; it also indicates a potential link between the botanical composition of plant communities and global warming.     

Effects of Urbanization-Induced Environmental Changes on Ecosystem Functioning in the Phoenix Metropolitan Region,USA

Abstract Urban ecosystems are profoundly modified by human activities and thereby provide a unique “natural laboratory” to study potential ecosystem responses to anthropogenic environmental changes. Because urban environments are now affected by urban heat islands, carbon dioxide domes, and high-level nitrogen deposition, to some extent they portend the future of the global ecosystem. Urbanization in the metropolitan region of Phoenix, Arizona (USA) has resulted in pronounced changes in air temperature (T _air), atmospheric CO₂ concentration, and nitrogen deposition (N_dep). In this study, we used a process-based ecosystem model to explore how the Larrea tridentata dominated Sonoran Desert ecosystem may respond to these urbanization-induced environmental changes. We found that water availability controls the magnitude and pattern of responses of the desert ecosystem to elevated CO₂, air temperature, N deposition and their combinations. Urbanization effects were much stronger in wet years than normal and dry years. At the ecosystem level, aboveground net primary productivity (ANPP) and soil organic matter (SOM) both increased with increasing CO₂ and N_dep individually and in combinations with changes in T _air. Soil N (N_soil) responded positively to increased N deposition and air temperature, but negatively to elevated CO₂. Correspondingly, ANPP and SOM of the Larrea ecosystem decreased along the urban–suburban–wildland gradient, whereas N_soil peaked in the suburban area. At the plant functional type (FT) level, ANPP generally responded positively to elevated CO₂ and N_dep, but negatively to increased T _air. C₃ winter annuals showed a greater ANPP response to higher CO₂ levels (>420 ppm) than shrubs, which could lead over the long term to changes in species composition, because competition among functional groups is strong for resources such as soil water and nutrients. Overall, the combined effects of the three environmental factors depended on rainfall variability and nonlinear interactions within and between plant functional types and environmental factors. We intend to use these simulation results as working hypotheses to guide our field experiments and observations. Experimental testing of these hypotheses through this process should improve our understanding of urban ecosystems under increasing environmental stresses.     

Responses of grassland production to single and multiple global environmental changes

In this century, increasing concentrations of carbon dioxide (CO₂) and other greenhouse gases in the Earth's atmosphere are expected to cause warmer surface temperatures and changes in precipitation patterns. At the same time, reactive nitrogen is entering natural systems at unprecedented rates. These global environmental changes have consequences for the functioning of natural ecosystems, and responses of these systems may feed back to affect climate and atmospheric composition. Here, we report plant growth responses of an ecosystem exposed to factorial combinations of four expected global environmental changes. We exposed California grassland to elevated CO₂, temperature, precipitation, and nitrogen deposition for five years. Root and shoot production did not respond to elevated CO₂ or modest warming. Supplemental precipitation led to increases in shoot production and offsetting decreases in root production. Supplemental nitrate deposition increased total production by an average of 26%, primarily by stimulating shoot growth. Interactions among the main treatments were rare. Together, these results suggest that production in this grassland will respond minimally to changes in CO₂ and winter precipitation, and to small amounts of warming. Increased nitrate deposition would have stronger effects on the grassland. Aside from this nitrate response, expectations that a changing atmosphere and climate would promote carbon storage by increasing plant growth appear unlikely to be realized in this system.     

微生物介导的碳氮循环过程对全球气候变化的响应

土壤是地球表层最为重要的碳库也是温室气体的源或汇。自工业革命以来,对土壤温室气体的容量、收支平衡和通量等已有较多研究和估算,但对关键过程及其源/汇的研究却十分有限。微生物是土壤碳氮转化的主要驱动者, 在生态系统碳氮循环过程中扮演重要的角色,对全球气候变化有着响应的响应、适应及反馈,然而其个体数量,群落结构和多样性如何与气候扰动相互关联、进而怎样影响生态系统过程的问题仍有待进一步探索。从微生物介导的碳氮循环过程入手,重点讨论微生物对气候变化包括温室气体(CO₂,CH₄,N₂O)增加、全球变暖、大气氮沉降等的响应和反馈,并由此提出削减温室气体排放的可能途径和今后发展的方向。