Future Global Change Scenarios Research Articles

Soil Methanotrophy Model (MeMo v1.0): a process-based model to quantify global uptake of atmospheric methane by soil

Abstract. Soil bacteria known as methanotrophs are the sole biological sink for atmospheric methane (CH4), a potent greenhouse gas that is responsible for ∼ 20 % of the human-driven increase in radiative forcing since pre-industrial times. Soil methanotrophy is controlled by a plethora of factors, including temperature, soil texture, moisture and nitrogen content, resulting in spatially and temporally heterogeneous rates of soil methanotrophy. As a consequence, the exact magnitude of the global soil sink, as well as its temporal and spatial variability, remains poorly constrained. We developed a process-based model (Methanotrophy Model; MeMo v1.0) to simulate and quantify the uptake of atmospheric CH4 by soils at the global scale. MeMo builds on previous models by Ridgwell et al. (1999) and Curry (2007) by introducing several advances, including (1) a general analytical solution of the one-dimensional diffusion–reaction equation in porous media, (2) a refined representation of nitrogen inhibition on soil methanotrophy, (3) updated factors governing the influence of soil moisture and temperature on CH4 oxidation rates and (4) the ability to evaluate the impact of autochthonous soil CH4 sources on uptake of atmospheric CH4. We show that the improved structural and parametric representation of key drivers of soil methanotrophy in MeMo results in a better fit to observational data. A global simulation of soil methanotrophy for the period 1990–2009 using MeMo yielded an average annual sink of 33.5 ± 0.6 Tg CH4 yr−1. Warm and semi-arid regions (tropical deciduous forest and open shrubland) had the highest CH4 uptake rates of 602 and 518 mg CH4 m−2 yr−1, respectively. In these regions, favourable annual soil moisture content (∼ 20 % saturation) and low seasonal temperature variations (variations < ∼ 6 ∘C) provided optimal conditions for soil methanotrophy and soil–atmosphere gas exchange. In contrast to previous model analyses, but in agreement with recent observational data, MeMo predicted low fluxes in wet tropical regions because of refinements in formulation of the influence of excess soil moisture on methanotrophy. Tundra and mixed forest had the lowest simulated CH4 uptake rates of 176 and 182 mg CH4 m−2 yr−1, respectively, due to their marked seasonality driven by temperature. Global soil uptake of atmospheric CH4 was decreased by 4 % by the effect of nitrogen inputs to the system; however, the direct addition of fertilizers attenuated the flux by 72 % in regions with high agricultural intensity (i.e. China, India and Europe) and by 4–10 % in agriculture areas receiving low rates of N input (e.g. South America). Globally, nitrogen inputs reduced soil uptake of atmospheric CH4 by 1.38 Tg yr−1, which is 2–5 times smaller than reported previously. In addition to improved characterization of the contemporary soil sink for atmospheric CH4, MeMo provides an opportunity to quantify more accurately the relative importance of soil methanotrophy in the global CH4 cycle in the past and its capacity to contribute to reduction of atmospheric CH4 levels under future global change scenarios.

Integrated assessment of future potential global change scenarios and their hydrological impacts in coastal aquifers – a new tool to analyse management alternatives in the Plana Oropesa-Torreblanca aquifer

Abstract. Any change in the components of the water balance in a coastal aquifer, whether natural or anthropogenic, can alter the freshwater–salt water equilibrium. In this sense climate change (CC) and land use and land cover (LULC) change might significantly influence the availability of groundwater resources in the future. These coastal systems demand an integrated analysis of quantity and quality issues to obtain an appropriate assessment of hydrological impacts using density-dependent flow solutions. The aim of this work is to perform an integrated analysis of future potential global change (GC) scenarios and their hydrological impacts in a coastal aquifer, the Plana Oropesa-Torreblanca aquifer. It is a Mediterranean aquifer that extends over 75 km2 in which important historical LULC changes have been produced and are planned for the future. Future CC scenarios will be defined by using an equi-feasible and non-feasible ensemble of projections based on the results of a multi-criteria analysis of the series generated from several regional climatic models with different downscaling approaches. The hydrological impacts of these CC scenarios combined with future LULC scenarios will be assessed with a chain of models defined by a sequential coupling of rainfall-recharge models, crop irrigation requirements and irrigation return models (for the aquifer and its neighbours that feed it), and a density-dependent aquifer approach. This chain of models, calibrated using the available historical data, allow testing of the conceptual approximation of the aquifer behaviour. They are also fed with series representatives of potential global change scenarios in order to perform a sensitivity analysis regarding future scenarios of rainfall recharge, lateral flows coming from the hydraulically connected neighbouring aquifer, agricultural recharge (taking into account expected future LULC changes) and sea level rise (SLR). The proposed analysis is valuable for improving our knowledge about the aquifer, and so comprises a tool to design sustainable adaptation management strategies taking into account the uncertainty in future GC conditions and their impacts. The results show that GC scenarios produce significant increases in the variability of flow budget components and in the salinity.

Effects of warming and nitrogen addition on nutrient resorption efficiency in an alpine meadow on the northern Tibetan Plateau

ABSTRACTThe increase in two controversial global environmental issues, climate warming and nitrogen (N) deposition, may have distinct effects on the processes and functioning of terrestrial ecosystems. Nutrient resorption is an important determinant of plant community nutrient dynamics, especially in nutrient-limited ecosystems, but information about N and phosphorus (P) resorption in alpine ecosystems is still lacking. A long-term simulated warming and exogenous N addition experiment initiated in July 2010 was conducted in an alpine meadow in Damxung County in northern Tibet. The experiment consisted of conditions of warming and no warming crossed with three N addition levels: 0 (CK), 20 (N20), and 40 (N40) kg N · ha−1 · year−1. With increasing N addition levels, the N content and the N/P ratio in plant leaves gradually increased, while the P limitation of plant growth was aggravated by N addition. The moderate N addition level (N20) increased plant N resorption efficiency (NRE), while the high N addition level (N40) had no effect on the NRE of Kobresia pygmaea or Anaphalis xylorhiza. N addition significantly increased the P resorption efficiency (PRE) in Stipa capillacea leaves. However, N addition did not change the community NRE or the community PRE. The soil N content decreased under the warming treatment. At the community level, warming significantly increased the NRE by 12% and 16%, and the PRE by 26% and 24% under the CK and N40 treatments, respectively. The NRE and PRE were higher in S. capillacea than in K. pygmaea and A. xylorhiza, especially at the high N addition level (38% and 45% higher NRE and 36% and 15% higher PRE compared to K. pygmaea and A. xylorhiza, respectively). Correlation analysis showed that the NRE and PRE in plant leaves were mainly mediated by soil inorganic N availability, and tended to decrease with increase of soil N availability, suggesting that N loss due to warming could induce changes in nutrient resorption in alpine ecosystems. The species-specific responses to N addition and the stronger competitive advantage of S. capillacea may change the community structure and subsequently affect the decomposition process in this alpine meadow under future global climate change scenarios.

Temperature-induced variability in metabolic activity of ecologically important estuarine macrobenthos

Temperature is one of the more important factors affecting metabolism and fitness of aquatic ectothermic animals. The present study aimed, therefore, at quantifying and comparing the oxygen consumption rates of several subtropical macrobenthos organisms living in two South African estuarine systems. Oxygen fluxes were measured in the laboratory at an individual level for temperatures ranging between 16 and 30 °C. The metabolic response differed significantly among taxa, developmental stage, and microhabitat, with higher activation energy for the animals from the permanently open estuary compared to the one from the temporarily open/closed system. The invasive snails showed an exceptional higher metabolic performance at higher temperatures compared to the other organisms collected within the temporarily open/closed system. The results suggest that estuaries experiencing prolonged closure with stable environmental conditions and lower biodiversity host the least performing native species. Setting this pattern in a future global change scenario, this study highlights that longer closures of the estuarine mouth caused by changing rainfall regimes and increased freshwater abstractions from catchments are likely to create the ideal conditions for the proliferation of more adaptable invasive species in temporally open/closed subtropical estuaries.

Interactive effects of increased temperature, pCO2 and the synthetic progestin levonorgestrel on the fitness and breeding of the amphipod Gammarus locusta.

Given the lack of knowledge regarding climate change-chemical exposure interactions, it is vital to evaluate how these two drivers jointly impact aquatic species. Thus, for the first time, we aimed at investigating the combined effects of increased temperature, pCO2 and the synthetic progestin levonorgestrel on survival, growth, consumption rate and reproduction of the amphipod Gammarus locusta. For that, a full factorial design manipulating temperature [ambient temperature and warming (+4°C)], pCO2 [normocapnia and hypercapnia (Δ pH 0.5 units)] and the progestin levonorgestrel (LNG: L1 - 10ngLL-1 and L2 - 1000ngLL-1, control - no progestin and solvent control - vehicle ethanol (0.01%)) was implemented for 21 days. G.locusta was strongly negatively affected by warming, experiencing higher mortality rates (50-80%) than in any other treatments. Instead, growth rates were significantly affected by interactions of LNG with temperature and pCO2. It was observed, in the short-term (7d) that under ambient temperature (18°C) and hypercapnic conditions (pH 7.6), the LNG presence promoted the amphipod's growth, while in the medium-term (21d) this response was not observed. Relative consumption rates (RCRs), during the first week were higher than in the third week. Furthermore, in the first week, RCRs were negatively affected by higher temperature while in the third week, RCRs were negatively affected by acidification. Furthermore, it was observed a negative effect of higher temperature and acidification on G.locusta fecundity, contrarily to LNG. Concluding, the impact of increased temperature and pCO2 was clearly more adverse for the species than exposure to the synthetic progestin, however, some interactions between the progestin and the climate factors were observed. Thus, in a future scenario of global change, the presence of LNG (and other progestins alike) may modulate to a certain level the effects of climate drivers (and vice-versa) on the gammarids fitness and reproduction.

Combined, short-term exposure to reduced seawater pH and elevated temperature induces community shifts in an intertidal meiobenthic assemblage

In future global change scenarios the surface ocean will experience continuous acidification and rising temperatures. While effects of both stressors on marine, benthic communities are fairly well studied, consequences of the interaction of both factors remain largely unknown. We performed a short-term microcosm experiment exposing a soft-bottom community from an intertidal flat in the Westerscheldt estuary to two levels of seawater pH (ambient pHT = 7.9, reduced pHT = 7.5) and temperature (10 °C ambient and 13 °C elevated temperature) in a crossed design. After 8 weeks, meiobenthic community structure and nematode staining ratios, as a proxy for mortality, were compared between treatments and structural changes were related to the prevailing abiotic conditions in the respective treatments (pore water pHT, sediment grain size, total organic matter content, total organic carbon and nitrogen content, phytopigment concentrations and carbonate concentration). Pore water pHT profiles were significantly altered by pH and temperature manipulations and the combination of elevated temperature and reduced pH intensified the already more acidic porewater below the oxic zone. Meiofauna community composition was significantly affected by the combination of reduced pH and elevated temperature resulting in increased densities of predatory Platyhelminthes, reduced densities of Copepoda and Nauplii and complete absence of Gastrotricha compared to the experimental control. Furthermore, nematode staining ratio was elevated when seawater pH was reduced pointing towards reduced degradation rates of dead nematode bodies. The observed synergistic interactions of pH and temperature on meiobenthic communities and abiotic sediment characteristics underline the importance of multistressor experiments when addressing impacts of global change on the marine environment.

Changes in flavonoid biosynthesis and in the berry whole transcriptome of ‘Sangiovese’ under moderate air temperature increase

Among the environmental factors, temperature is the one that poses serious threats to viticulture in the present and future scenarios of global climate change. In this work, we evaluated the effects on berry ripening of two thermal regimes, imposed from veraison to harvest. Potted vines were grown in two air-conditioned greenhouses with high temperature (HT) and low temperature (LT) regimes characterized respectively by 26 and 21°C as average and 41.7 and 35°C as maximum air daily temperature. We carried out analyses of the main berry compositional parameters and of berry skin phenolics on HT and LT berries sampled at six phases during ripening. In the same sampling dates, the berry skin transcriptome under the two temperature regimes was analyzed. The two thermal conditions strongly differentiated HT and LT berries. The HT regime did not increase sugar accumulation at harvest but contributed to a total acidity reduction and to a pH increase. Conversely, changes in growing temperatures greatly impacted on grape phenolic composition as the anthocyanin and flavonol concentrations resulted strongly reduced. Whole transcriptome analysis identified genes with a different expression profile when berries were ripened under HT or LT. Focusing on phenylpropanoid-related genes, we found many differential transcripts involved in stilbene biosynthesis. Nevertheless, the effect observed on genes involved in flavonoid biosynthesis was only marginally in line with the berry biochemical parameters, suggesting a possible post-transcriptional response of this pathway to moderate increments of temperature in 'Sangiovese'.

Review: Host-pathogen dynamics of seagrass diseases under future global change

Human-induced global change is expected to amplify the disease risk for marine biota. However, the role of disease in the rapid global decline of seagrass is largely unknown. Global change may enhance seagrass susceptibility to disease through enhanced physiological stress, while simultaneously promoting pathogen development. This review outlines the characteristics of disease-forming organisms and potential impacts of global change on three groups of known seagrass pathogens: labyrinthulids, oomycetes and Phytomyxea. We propose that hypersalinity, climate warming and eutrophication pose the greatest risk for increasing frequency of disease outbreaks in seagrasses by increasing seagrass stress and lowering seagrass resilience. In some instances, global change may also promote pathogen development. However, there is currently a paucity of information on these seagrass pathosystems. We emphasise the need to expand current research to better understand the seagrass-pathogen relationships, serving to inform predicative modelling and management of seagrass disease under future global change scenarios.

Phenolic contents and genome-wide expression profiling of grapevine berries (Vitis viniferaL. ‘Sangiovese’) ripened under two different temperature regimes

High summer temperatures are among the environmental factors that pose serious threats to viticulture in the present and future scenarios of global climate change. Such extremes are expected to affect different aspects of grapevine biology and metabolism. In this work, we studied the effects of two different temperature regimes, on the evolution of main berry compositional parameters and on grapevine genome-wide expression during ripening. Vitis vinifera 'Sangiovese' potted vines were grown, from veraison to harvest, in two air-conditioned greenhouses with high temperature (HT) and low temperature (LT) regimes characterized by 26 and 21°C as average air daily temperature. We determined berry soluble solids, pH, titratable acidity and phenolic compounds as anthocyanins and flavonols during the ripening process. The results indicated that HT regime did not increase sugar accumulation at harvest but contributed to a total acidity reduction. Conversely, changes in growing temperatures greatly impact on grape phenolic composition as the anthocyanin and flavonol contents resulted strongly reduced in HT compared to LT regime. We applied a clustering analysis approach to the obtained genome-wide microarray data to identify those transcripts with a different transcriptional profile when berries were ripened under HT or LT conditions. We found many differential transcripts involved in heat stress response, many stilbene synthase gene family members, and some other transcripts that corroborate our biochemical results.

Variability of ant community composition in cork oak woodlands across the Mediterranean region: implications for forest management

Abstract: We evaluated the potential use of ants as a powerful tool for environmental monitoring, together with the applicability of the functional group approach as an alternative method for studying ant communities in cork oak woodlands. Variations in ant community composition, diversity and functional groups were studied in two cork oak forested sites across the Mediterranean region. Ants were sampled using pitfall traps placed along linear transects at 12 sites located in the main cork districts of Italy and Morocco (Gallura in Sardinia, and Maâmora, east of Rabat). A total of 13.501 specimens were collected, belonging to 38 species (five shared species). A distinct separation in the NMDS plots between Gallura and Maâmora ant assemblages was clearly visible. Ant species composition was widely different between the two districts and significant differences were detected within the Gallura district at the species level. Opportunist species were well represented in Gallura (about 27% of average Bray-Curtis similarity) as well as cryptic species (over 23%). In the Maâmora forest, generalized Myrmicinae, hot climate specialists and opportunists contributed equally to the average similarity (together about 53%). Multi-scale ant diversity showed that the true turnover was higher in Gallura than in Maâmora. These findings support the idea that the functional group approach, rather than species diversity per se, could be considered as a valuable tool to detect the response of the ant community to environmental changes in Mediterranean cork oak woodlands. Using ants as bioindicators could help not only in detecting early warning signs of habitat disturbance, but also in defining a useful management strategy to increase the resilience of agroforestry systems under future global change scenarios.

Global change effects on plankton community structure and trophic interactions in a Patagonian freshwater eutrophic system

The short- and mid-term effects of a simulated global change scenario (i.e., Future) of increased nutrients, acidification, and solar radiation, in the presence or absence of grazers, were evaluated on a freshwater plankton community of Patagonia, Argentina. We used a cluster experimental design with microcosms incubated outdoors simulating the in situ (i.e., Present) and the Future conditions. Short-term changes in net productivity and respiration, together with mid-term changes in the community (abundance, biomass, and phytoplankton cell size) were measured. Phytoplankton had lower net productivity and higher respiration and zooplankton had, in general, higher respiration under the Future than that under the Present condition when organisms were exposed to UVR. The mid-term impacts of the Future condition were neither significant on zooplankton abundances, nor in phytoplankton abundances, biomass, and cell size. Nevertheless, the zooplankton–phytoplankton interaction strength was greater under the Future condition. Zooplankton exerted a strong top-down pressure, regardless of the experimental scenarios, grazing preferentially on small phytoplankton cells, thus decreasing their abundances and biomass. Overall, there were significant short-term impact of our Future global change scenario; however, its effects on mid-term time scales were not significant, and indeed, the zooplankton top-down pressure was the main driver that shaped the phytoplankton community.

Nitrogen deposition does not reduce water deficit in Ailanthus altissima seedlings

Water deficit and nitrogen (N) deposition are two important factors affecting plant growth in the context of global change. Here we selected Ailanthus altissima, a locally important tree species in North China, to study the combined effects of soil water status and N addition on plant growth. Ailanthus altissima seedlings were exposed to two soil moisture conditions (35% and 75% of the saturated soil moisture content) and three N treatments (0, 5, and 15gNm−2year−1) in a greenhouse experiment. After 50 d of treatment, plant growth, biomass production and allocation, and leaf physiological parameters were determined. Water deficit significantly restricted some parameters of growth and biomass accumulation, like height and basal diameter, total biomass, lateral root biomass, leaf biomass, and stem biomass. Some physiological parameters were also affected significantly by water deficit, leaf N content and qP decreased as soil moisture decreased and WUE was highest in water deficit condition. However, the high N load increased crown area and total biomass, lateral root biomass, and root biomass. With respect to physiological parameters, net photosynthetic rate, leaf N content, ΦPSII and ETR were clearly higher in the high N addition group than in the control group. Overall, an interaction between water deficit and N load had no significant effects on plant growth. Based on the present study with A. altissima seedlings, considering their rapid growth rate, we conclude that this species is better adapted than other deciduous trees to water deficit and high N additions under future global change scenarios, making it suited for vegetation restoration and reconstruction projects in the warm temperate zone of China.

Whole Plant Temperature Manipulation Affects Flavonoid Metabolism and the Transcriptome of Grapevine Berries.

Among environmental factors, temperature is the one that poses serious threats to viticulture in the present and future scenarios of global climate change. In this work, we evaluated the effects on berry ripening of two thermal regimes, imposed from veraison to harvest. Potted vines were grown in two air-conditioned greenhouses with High Temperature (HT) and Low Temperature (LT) regimes characterized by 26 and 21°C as average and 42 and 35°C as maximum air daily temperature, respectively. We conducted analyses of the main berry compositional parameters, berry skin flavonoids and berry skin transcriptome on HT and LT berries sampled during ripening. The two thermal conditions strongly differentiated the berries. HT regime increased sugar accumulation at the beginning of ripening, but not at harvest, when HT treatment contributed to a slight total acidity reduction and pH increase. Conversely, growing temperatures greatly impacted on anthocyanin and flavonol concentrations, which resulted as strongly reduced, while no effects were found on skin tannins accumulation. Berry transcriptome was analyzed with several approaches in order to identify genes with different expression profile in berries ripened under HT or LT conditions. The analysis of whole transcriptome showed that the main differences emerging from this approach appeared to be more due to a shift in the ripening process, rather than to a strong rearrangement at transcriptional level, revealing that the LT temperature regime could delay berry ripening, at least in the early stages. Moreover, the results of the in-depth screening of genes differentially expressed in HT and LT did not highlight differences in the expression of transcripts involved in the biosynthesis of flavonoids (with the exception of PAL and STS) despite the enzymatic activities of PALs and UFGT being significantly higher in LT than HT. This suggests only a partial correlation between molecular and biochemical data in our conditions and the putative existence of post-transcriptional and post-translational mechanisms playing significant roles in the regulation of flavonoid metabolic pathways and in particular of anthocyanins.

Biodiversity in perennial and intermittent rivers: a meta‐analysis

Comprehensive knowledge of the effects of disturbances on biodiversity is crucial for conservation and management, not least because ecosystems with low biodiversity may be the most vulnerable. In rivers, the role of disturbance in shaping aquatic biodiversity has mainly focused on floods. Perennial rivers (PRs) often flood, whereas intermittent rivers (IRs) flood, stop flowing and dry. Despite the recent and significant increase in research on IRs, controversy remains about whether they are more or less biodiverse than PRs. Our aim was to determine (Q1) if PRs and IRs differ in biodiversity and (Q2) if the direction and magnitude of the differences (effect sizes) are related to environmental (climate, season, habitat, longitudinal zonation and anthropogenic disturbance) and/or biological factors (taxonomic group). We conducted a meta‐analysis on 44 published studies of PR and IR biodiversity that had replicated data. We applied random effects models to the data to obtain weighted mean effect sizes for differences between PRs and IRs, and their confidence intervals, by first considering all studies and then by splitting studies into groups on the basis of the above factors. We found that biodiversity was significantly higher in PRs than in IRs (Q1). We also detected significant differences (PRs> IRs) in studies of macroinvertebrates, in those conducted within arid and temperate climates, dry and wet sampling seasons, headwaters, and regions subject to different levels of anthropogenic disturbance (Q2). Our meta‐analysis suggests that the expected increase in the prevalence of IRs in certain regions of the world due to global change could result in a decrease in freshwater biodiversity. To better manage and preserve aquatic biodiversity under future global change scenarios and to avoid potential ecosystem consequences of biodiversity loss, conservation efforts should be targeted towards those environmental conditions or taxonomic groups with significant differences (PRs > IRs).

Predicting River Macroinvertebrate Communities Distributional Shifts under Future Global Change Scenarios in the Spanish Mediterranean Area

Several studies on global change over the next century predict increases in mean air temperatures of between 1°C to 5°C that would affect not only water temperature but also river flow. Climate is the predominant environmental driver of thermal and flow regimes of freshwater ecosystems, determining survival, growth, metabolism, phenology and behaviour as well as biotic interactions of aquatic fauna. Thus, these changes would also have consequences for species phenology, their distribution range, and the composition and dynamics of communities. These effects are expected to be especially severe in the Mediterranean basin due its particular climate conditions, seriously threatening Southern European ecosystems. In addition, species with restricted distributions and narrow ecological requirements, such as those living in the headwaters of rivers, will be severely affected. The study area corresponds to the Spanish Mediterranean and Balearic Islands, delimited by the Köppen climate boundary. With the application of the MEDPACS (MEDiterranean Prediction And Classification System) predictive approach, the macroinvertebrate community was predicted for current conditions and compared with three posible scenarios of watertemperature increase and its associated water flow reductions. The results indicate that the aquatic macroinvertebrate communities will undergo a drastic impact, with reductions in taxa richness for each scenario in relation to simulated current conditions, accompanied by changes in the taxa distribution pattern. Accordingly, the distribution area of most of the taxa (65.96%) inhabiting the mid-high elevations would contract and rise in altitude. Thus, families containing a great number of generalist species will move upstream to colonize new zones with lower water temperatures. By contrast, more vulnerable taxa will undergo reductions in their distribution area.

Differential impacts of global change variables on coastal South Atlantic phytoplankton: Role of seasonal variations

Global change is associated to the increase in temperature (T), nutrient inputs (Nut) and solar radiation in the water column. To address their joint impact on the net community production [NCP], respiration [CR] and PSII performance (ΦPSII) of coastal phytoplankton communities from the South Atlantic Ocean over a seasonal succession, we performed a factorial design. For this, we used a 2 × 2 × 2 matrix set-up, with and without UVR, ambient and enriched nutrients, and in situ T and in situ T + 3 °C. The future scenario of global change exerted a dual impact, from an enhancement of NCP and ΦPSII during the pre-bloom to an inhibition of both processes towards the bloom period, when the in situ T and irradiances were lower and the community was dominated by diatoms. The increased inhibition of NCP and ΦPSII during the most productive stage of the annual succession could produce significant alterations of the CO2-sink capacity of coastal areas in the future.

The impacts of precipitation increase and nitrogen addition on soil respiration in a semiarid temperate steppe

AbstractSoil respiration, Rs, is strongly controlled by water availability in semiarid grasslands. However, how Rs is affected by precipitation change (either as rainfall or as snowfall) especially under increasing nitrogen (N) deposition has been uncertain. A manipulative experiment to investigate the responses of growing season Rs to changes in spring snowfall or summer rainfall with or without N addition was conducted in the semiarid temperate steppe of China during three hydrologically contrasting years. Our results showed that both spring snow addition and summer water addition significantly increased Rs by increasing soil moisture. The effect of spring snow addition only occurred in years with both relatively lower natural snowfall and later snowmelt time. Summer water addition showed a much stronger effect on Rs by increasing plant root growth and microbial activities, but the magnitude also largely depended on the possible legacy effect of previous year precipitation. Our results indicated that precipitation increase in the form of snowfall had weaker effects than that in the form of rainfall as the former only accounted for less than 30% of total precipitation. Compared with other ecosystem processes, Rs was less responsible for increase in N deposition as it did not increase root productivity and microbial activities in the soils. Our results provided field data constraints for modeling the ecosystem carbon balance under the future global change scenarios in semiarid grasslands.

Sensitivities to nitrogen and water addition vary among microbial groups within soil aggregates in a semiarid grassland

We investigated whether enhanced nitrogen (N) and water inputs would redistribute the microbial community within different soil aggregate size classes in a field manipulation experiment initiated in 2005. Distribution of microbial groups was monitored in large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), and microaggregates (<250 μm) in a semiarid grassland. Both arbuscular mycorrhizal (AM) fungi and saprophytic fungi were the most abundant in soil macroaggregates. The gram-negative bacteria were more abundant in soil microaggregates. Total phospholipid fatty acid (PLFA) concentration in general and actinomycetes in particular decreased with N addition under ambient precipitation but was unaffected by combined additions of N and water within the three soil aggregate fractions as compared to control plots. In contrast, the abundance of saprophytic fungi decreased with combined N and water addition, but it was not affected by N addition under ambient precipitation. The abundance of gram-positive bacteria increased with N addition under both ambient and elevated water conditions for all soil aggregate fractions. In summary, the higher short-term nutrient and water availabilities provoked a shift in soil microbial community composition and increased total PLFA abundance irrespectively of the level of soil aggregation. In the long term, this could destabilize soil carbon pools and influence the nutrient limitation of soil biota within different soil aggregate size classes under future global change scenarios.

Climate and land use changes effects on soil organic carbon stocks in a Mediterranean semi-natural area.

A thorough knowledge of the effects of climate and land use changes on the soil carbon pool is critical to planning effective strategies for adaptation and mitigation in future scenarios of global climate and land use change. In this study, we used CarboSOIL model to predict changes in soil organic carbon stocks in a semi-natural area of Southern Spain in three different time horizons (2040, 2070, 2100), considering two general circulation models (BCM2 and ECHAM5) and three IPCC scenarios (A1b, A2, B2). The effects of potential land use changes from natural vegetation (Mediterranean evergreen oak woodland) to agricultural land (olive grove and cereal) on soil organic carbon stocks were also evaluated. Predicted values of SOC contents correlated well those measured (R2 ranging from 0.71 at 0-25cm to 0.97 at 50-75cm) showing the efficiency of the model. Results showed substantial differences among time horizons, climate and land use scenarios and soil depth with larger decreases of soil organic carbon stocks in the long term (2100 time horizon) and particularly in olive groves. The combination of climate and land use scenarios (in particular conversion from current 'dehesa' to olive groves) resulted in yet higher losses of soil organic carbon stocks, e.g. -30, -15 and -33% in the 0-25, 25-50 and 50-75cm sections respectively. This study shows the importance of soil organic carbon stocks assessment under both climate and land use scenarios at different soil sections and point towards possible directions for appropriate land use management in Mediterranean semi natural areas.

Responses of ecosystem respiration to nitrogen enrichment and clipping mediated by soil acidification in an alpine meadow

Clipping is one of the most important land-use practices in grassland ecosystems, yet the extent of clipping effect on ecosystem carbon (C) cycles depends on its interaction with nitrogen (N) deposition in future global climate change scenario, especially in the N-limited alpine meadow ecosystem. We conducted a field N addition (40kgNha−1 yr−1) and clipping experiment in an alpine meadow on the northern Tibet Plateau to investigate the effects of exogenous N input and clipping on ecosystem (Re) and soil respiration (Rs). Generally, N enrichment enhanced Re, Rs and aboveground biomass (AGB) in the second year of the experiment, but the extent of the enhancements decreased from the third year of the experiment. Clipping didn’t affect Re, Rs and AGB relative to the control, while clipping and N addition significantly increased belowground biomass. Soil inorganic N increased, while soil pH decreased with the year of N additions. Significant positive correlations of aboveground biomass with Re and Rs suggest the increase of plant photosynthetic C fixation under N addition is a key force in driving thus ecosystem CO2 efflux. However, the positive correlation between soil pH and aboveground biomass indicated soil acidification could be one of important factors decelerating the positive responses of plant growth to soil N availability increase. All the results suggest soil acidification due to N addition influenced the effects of N enrichment and clipping on ecosystem CO2 efflux through modulating plant growth and photosynthetic C allocation.