• Energy Research
• 6. Clean water close

AbstractMicroorganisms dominate the decomposition of organic matter and their activities are strongly influenced by temperature. As the carbon (C) flux from soil to the atmosphere due to microbial activity is substantial, understanding temperature relationships of microbial processes is critical. It has been shown that microbial temperature relationships in soil correlate with the climate, and microorganisms in field experiments become more warm‐tolerant in response to chronic warming. It is also known that microbial temperature relationships reflect the seasons in aquatic ecosystems, but to date this has not been investigated in soil. Although climate change predictions suggest that temperatures will be mostly affected during winter in temperate ecosystems, no assessments exist of the responses of microbial temperature relationships to winter warming. We investigated the responses of the temperature relationships of bacterial growth, fungal growth, and respiration in a temperate grassland to seasonal change, and to 2 years’ winter warming. The warming treatments increased winter soil temperatures by 5–6°C, corresponding to 3°C warming of the mean annual temperature. Microbial temperature relationships and temperature sensitivities (Q10) could be accurately established, but did not respond to winter warming or to seasonal temperature change, despite significant shifts in the microbial community structure. The lack of response to winter warming that we demonstrate, and the strong response to chronic warming treatments previously shown, together suggest that it is the peak annual soil temperature that influences the microbial temperature relationships, and that temperatures during colder seasons will have little impact. Thus, mean annual temperatures are poor predictors for microbial temperature relationships. Instead, the intensity of summer heat‐spells in temperate systems is likely to shape the microbial temperature relationships that govern the soil‐atmosphere C exchange.

Global biogeochemical cycles of carbon and other nutrients are increasingly affected by human activities (Griggs et al., 2013). So far, modeling has been central for our understanding of how this will affect ecosystem functioning and the biogeochemical cycling of elements (Treseder et al., 2012). These models adopt a reductive approach built on the flow of elements between pools that are difficult or even impossible to verify with empirical evidence. Furthermore, while some of these models include the response in physiology, ecology and biogeography of primary producers to environmental change, the microbial part of the ecosystem is generally poorly represented or lacking altogether (Stein and Nicol, 2011; Treseder et al., 2012). The principal pool of carbon and other nutrients in soil is the organic matter (Schimel, 1995). The turnover time of this reservoir is governed by the rate at which microorganisms consume it. The rate of organic matter degradation in a soil is determined by both the indigenous microbial community and the environmental conditions (e.g., temperature, pH, soil water capacity, etc.), which govern the biogeochemical activities of the microorganisms (Waksman and Gerretsen, 1931; Schmidt et al., 2011). The dependences of these biogeochemical activity rates on environmental conditions such as pH, moisture and temperature have been frequently studied (Conant et al., 2011; Schmidt et al., 2011). However, while various microorganisms involved in carrying out biogeochemical processes have been identified, biogeochemical process rates are only rarely measured together with microbial growth, and one of the biggest challenges for advancing our understanding of biogeochemical processes is to systematically link biogeochemistry to the rate of specific metabolic processes (Rousk and Baath, 2011; Stein and Nicol, 2011). We also need to identify the factors governing these activities and if it results in feedback mechanisms that alter the growth, activity and interaction between primary producers and microorganisms (Treseder et al., 2012). By determining how different groups of microorganisms respond to individual environmental conditions by allocating e.g. carbon to production of biomass, CO2 and other products, a mechanistic as well as quantitative understanding of formation and decomposition of organic matter, and the production and consumption of greenhouse gases, can be achieved. In this Research Topic, supported by the Swedish research councils' program “Biodiversity and Ecosystem Services in a Changing Landscape” (BECC), we intend to promote an alternative framework to address how cycling of carbon and other nutrients will be altered in a changing environment from the first-principle mechanisms that drive them—namely the ecology, physiology and biogeography of microorganisms. In order to improve the predictive power of current models, the alternative framework supports the development of new models of biogeochemical cycles that factor in microbial physiology, ecology, and biogeochemistry. Our ambition has been richly rewarded by an extensive list of submissions. We are pleased to present contributions including primary research targeting the microbial control of biogeochemistry, comprehensive reviews of how microbial processes and communities relate to biogeochemical cycles, identification of critical challenges that remain, and new perspectives and ideas of how to optimize progress in our understanding of the microbial regulation of biogeochemistry. Our Research Topic presents new findings about the importance of the microbial community composition, their metabolic state, and the activity of enzymes for the fate and degradation of specific substrates such as chitin (Beier and Bertilsson, 2013), the degradation of more complex compounds such as those constituting plant litter (Moorhead et al., 2013; Rinkes et al., 2013), and the metabolism and biogeochemical cycling of one-carbon compounds (Aronson et al., 2013; Basiliko et al., 2013; Kappler and Nouwens, 2013). The environmental control and land-use perturbation of microbial communities and methane production were assessed in a comprehensive review (Aronson et al., 2013) as well as a case study (Basiliko et al., 2013) and a meta-analysis (Holden and Treseder, 2013). Other contributions have focused on how environmental variables that are affected by climate change can modulate microbial activities by e.g. their influence on the production and activity of enzymes (Steinweg et al., 2013), while Bradford (2013) has provided a comprehensive review of how microbial processes respond to warmer temperatures. These reviews are accompanied by a new suggestion for how we can achieve better predictions for microbial responses (and feedbacks) to climate change (de Vries and Shade, 2013), while Moorhead et al. (2013) identify knowledge gaps and provide important insights about how data on microbial communities, environmental conditions, and enzyme activities can be used to better inform enzyme-based models. Several submissions have highlighted the importance for plant-microbial feedbacks for the regulation of organic matter decomposition and formation (Moorhead et al., 2013; Thomson et al., 2013; Churchland and Grayston, under review), the production of biogenic volatile organic compounds (Rinnan et al., 2013), and the community composition of methanogens and sulfate reducing bacteria (Zeleke et al., 2013). A very active research area in soil microbial ecology is presently how small amounts of labile carbon sources can trigger, or “prime,” the decomposition of soil organic matter. A route toward a more general understanding of the regulation of plant-soil interaction for biogeochemistry, that may well facilitate our understanding of “priming effects,” could be the incorporation of stoichiometric concepts (Dijkstra et al., 2013; Mooshammer et al., 2014). Stoichiometric variations in the concentration of nutrients, combined with variations in carbon and nutrient demands of different decomposer groups, also seems to be reflected in the degradation rate of plant litter (Rinkes et al., 2013). A comprehensive review of biogenic fixation of nitrogen demonstrates the importance of interactions between different biogeochemical cycles for nitrogen fixation in ecosystems with nitrogen-limited plant productivity (Rousk et al., 2013). These contributions emphasize that stoichiometric variations in nutrient concentrations are of importance for both factors that could determine the propensity for organic matter to accumulate in an ecosystem, and thus for carbon to be sequestered. Some contributions to this Research Topic have also highlighted methodological challenges that urgently need attention. For instance, the ability of contemporary isotopic tracer methods to estimate microbial contributions to biogeochemical processes could be systematically overestimated (Hobbie and Hobbie, 2013), suggesting that estimates of the turnover of low molecular weight organic compounds, and possibly also for estimations of nitrogen transformation rates, need to be revised. Additionally, there is a need to move from laboratory-based estimations of the microbial role in ecosystem level processes, often omitting crucial components such as the presence of plants, to field-based assessments in intact systems (Rinkes et al., 2013). The contributions to our Research Topic have opened up new horizons and stimulated conceptual developments in our basic understanding of the regulating factors of global biogeochemical cycles. Within this forum, we have begun to bridge Microbial Ecology and Biogeochemistry, connecting microbial activities at the microcosm scale to carbon fluxes at the ecosystem-scale, and linking above- and belowground ecosystem functioning. We are hopeful that we have initiated conceptual developments that can reach far beyond this Research Topic. It is a mere first step, but we are confident it is directed toward a predictive understanding of the microbial regulation of global biogeochemical cycles.

AbstractWe investigated how the legacy of warming and summer drought affected microbial communities in five different replicated long‐term (>10 years) field experiments across Europe (EU‐FP7 INCREASE infrastructure). To focus explicitly on legacy effects (i.e., indirect rather than direct effects of the environmental factors), we measured microbial variables under the same moisture and temperature in a brief screening, and following a pre‐incubation at stable conditions. Specifically, we investigated the size and composition of the soil microbial community (PLFA) alongside measurements of bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth rates, previously shown to be highly responsive to changes in environmental factors, and microbial respiration. We found no legacy effects on the microbial community size, composition, growth rates, or basal respiration rates at the effect sizes used in our experimental setup (0.6 °C, about 30% precipitation reduction). Our findings support previous reports from single short‐term ecosystem studies thereby providing a clear evidence base to allow long‐term, broad‐scale generalizations to be made. The implication of our study is that warming and summer drought will not result in legacy effects on the microbial community and their processes within the effect sizes here studied. While legacy effects on microbial processes during perturbation cycles, such as drying–rewetting, and on tolerance to drought and warming remain to be studied, our results suggest that any effects on overall ecosystem processes will be rather limited. Thus, the legacies of warming and drought should not be prioritized factors to consider when modeling contemporary rates of biogeochemical processes in soil.

AbstractClimate change will alter precipitation patterns with consequences for soil C cycling. An understanding of how fluctuating soil moisture affects microbial processes is therefore critical to predict responses to future global change. We investigated how long‐term experimental field drought influences microbial tolerance to lower moisture levels (“resistance”) and ability to recover when rewetted after drought (“resilience”), using soils from a heathland which had been subjected to experimental precipitation reduction during the summer for 18 years. We tested whether drought could induce increased resistance, resilience, and changes in the balance between respiration and bacterial growth during perturbation events, by following a two‐tiered approach. We first evaluated the effects of the long‐term summer drought on microbial community functioning to drought and drying–rewetting (D/RW), and second tested the ability to alter resistance and resilience through additional perturbation cycles. A history of summer drought in the field selected for increased resilience but not resistance, suggesting that rewetting after drought, rather than low moisture levels during drought, was the selective pressure shaping the microbial community functions. Laboratory D/RW cycles also selected for communities with a higher resilience rather than increased resistance. The ratio of respiration to bacterial growth during D/RW perturbation was lower for the field drought‐exposed communities and decreased for both field treatments during the D/RW cycles. This suggests that cycles of D/RW also structure microbial communities to respond quickly and efficiently to rewetting after drought. Our findings imply that microbial communities can adapt to changing climatic conditions and that this might slow the rate of soil C loss predicted to be induced by future cyclic drought.

AbstractClimate change is exposing high‐latitude systems to warming and a shift towards more shrub‐dominated plant communities, resulting in increased leaf‐litter inputs at the soil surface, and more labile root‐derived organic matter (OM) input in the soil profile. Labile OM can stimulate the mineralization of soil organic matter (SOM); a phenomenon termed “priming.” In N‐poor subarctic soils, it is hypothesized that microorganisms may “prime” SOM in order to acquire N (microbial N‐mining). Increased leaf‐litter inputs with a high C/N ratio might further exacerbate microbial N demand, and increase the susceptibility of N‐poor soils to N‐mining. We investigated the N‐control of SOM mineralization by amending soils from climate change–simulation treatments in the subarctic (+1.1°C warming, birch litter addition, willow litter addition, and fungal sporocarp addition) with labile OM either in the form of glucose (labile C; equivalent to 400 µg C/g fresh [fwt] soil) or alanine (labile C + N; equivalent to 400 µg C and 157 µg N/g fwt soil), to simulate rhizosphere inputs. Surprisingly, we found that despite 5 yr of simulated climate change treatments, there were no significant effects of the field‐treatments on microbial process rates, community structure or responses to labile OM. Glucose primed the mineralization of both C and N from SOM, but gross mineralization of N was stimulated more than that of C, suggesting that microbial SOM use increased in magnitude and shifted to components richer in N (i.e., selective microbial N‐mining). The addition of alanine also resulted in priming of both C and N mineralization, but the N mineralization stimulated by alanine was greater than that stimulated by glucose, indicating strong N‐mining even when a source of labile OM including N was supplied. Microbial carbon use efficiency was reduced in response to both labile OM inputs. Overall, these findings suggest that shrub expansion could fundamentally alter biogeochemical cycling in the subarctic, yielding more N available for plant uptake in these N‐limited soils, thus driving positive plant–soil feedbacks.