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Climate change models predict shifts in the frequency and magnitude of rain events (precipitation patterns). We studied how precipitation history shapes microbial community responses to rewetting and how these effects depend on N status. Twelve weeks of contrasting precipitation and N input left a legacy effect by shaping present (DNA-based) and potentially active (rRNA-based) bacterial and fungal communities. This legacy effect determined the microbial response to rewetting, as demonstrated by differences in the post-wet potentially active bacterial and fungal communities as well as the flux of soil-emitted CO 2 over a 29-h period. Despite contrasting effects of precipitation and N input history on fungal:bacterial ratio and microbial community composition, the timing of the potentially active bacterial and fungal response to rewetting was not altered. Thus, regardless of precipitation or N input history, potentially active bacteria responded with a small shift in community composition within 1 h of rewetting but did not change further for the remaining 28 h analyzed. The potentially active fungi did not respond to rewetting within 29 h. Even though more extreme fluctuations in soil moisture changed soil microbial community composition, the short-term response of microbial communities to rewetting was conserved. Soil CO 2 efflux upon rewetting was higher from systems with a history of frequent precipitation and underline the role of plants in drying-rewetting processes. We suggest that shifts in the fungal:bacterial ratio, as well as N-cycling potentials, may have consequences on food web stability and soil biogeochemical cycling. • Precipitation and N input legacy shaped soil bacterial and fungal communities. • Soils with a frequent precipitation legacy emitted more CO 2 upon rewetting. • The potentially active bacterial community shifted regardless of legacy. • Fungal communities did not respond to rewetting (assessed over 29-h). [ABSTRACT FROM AUTHOR]

Tundra shrubs are slow-growing species limited by low air temperature and scarce nutrient availability. However, shrub expansion has been widely observed in the Arctic during the last decades and attributed to climate warming. Shift in shrub growth, wood structure and abundance affects the surface albedo and permafrost thawing and these changes may feedback to climate. Despite the importance of shrub–climate feedbacks, uncertainties about shrub growth sensitivity to climate remain. Here, we explored the indirect effects of climate warming on shrub growth (vertical and radial), bark thickness, and bark investment in four arctic shrub species. We combined a field experiment addressing two suggested growth drivers – thawing depth and nutrient availability – with dendroecology in a Siberian tundra ecosystem. We used heating cables to increase the thawing depth. To enhance the nutrient availability, we fertilized the surface soil layers. We found that shrub growth was mainly limited by nutrient availability, as indicated by the fertilization treatment effects on shrub growth ring widths. We also found a bark thickness decrease with the combined soil heating and nutrient addition treatment and a negative correlation between bark investment and growth rate for two of the species. These findings suggest that tundra shrubs, especially deciduous species, will grow faster and taller driven by an increasing nutrient availability in the surface soil layers. However, shrubs might become more vulnerable to pests, herbivory, and climate extremes, such as frost or drought events, due to thinner bark and lower bark investment. Using dendroecological approaches in field experiments simulating projected climate scenarios for the Arctic, and an increasing number of study species and locations will reduce uncertainties related to shrub growth sensitivity to climate and other processes driving shrub dynamics. [ABSTRACT FROM AUTHOR]

Soil ecosystems actively regulate climate by controlling methane and nitrous oxide fluxes into the atmosphere. Soils have been, however, drastically altered by agricultural practices, such as nitrogen amendment which increases nitrous oxide emission while it reduces methane uptakes in well-aerated soils by affecting methane-oxidizing bacteria. New nitrification inhibitors, such as 3,4-dimethylpyrazole phosphate (DMPP), are often applied in combination with nitrogen-based fertilizer to increase plant productivity by increasing available ammonium and inhibiting denitrification processes reducing in turn nitrous oxide emissions. However, the increase in ammonium due to nitrification inhibition might also affect methane oxidizing bacteria. We therefore investigated the effects of nitrogen-based fertilizer and DMPP on methane and nitrous oxide fluxes in an extensively managed grassland. We also determined the spatial distribution of active methane oxidizing bacteria by radiolabeling. Short-term reduction in methane uptake and methanotrophic activity occurred after application of 600 kg N ha −1 while DMPP did not alter methane uptake but reduced nitrous oxide emission. The combination of both radio-labeling and field measurement revealed that methane uptake collapsed in the field when methanotrophic activity was inhibited not only in the surface but also in deeper soil. Finally, both methane uptake and methanotrophic activity recovered with time. [ABSTRACT FROM AUTHOR]

Highlights: [•] A long-term nitrogen-addition experiment was conducted in a Norway spruce forest. [•] Net soil N2O and CH4 emissions increased due to this N addition. [•] Trees (40% of the basal area) were then girdled and, one year later, felled. [•] This reduced net emissions of CH4 but increased those of N2O. [•] Soil respiration was not significantly affected (but tended to be reduced by N). [ABSTRACT FROM AUTHOR]

Elevated concentration of atmospheric carbon dioxide will affect carbon cycling in terrestrial ecosystems. Possible effects include increased carbon input into the soil through the rhizosphere, altered nutrient concentrations of plant litter and altered soil moisture. Consequently, the ongoing rise in atmospheric carbon dioxide might indirectly influence soil biota, decomposition and nutrient transformations.N-mineralisation and activities of the enzymes invertase, xylanase, urease, protease, arylsulfatase, and alkaline phosphatase were investigated in spring and summer in calcareous grassland, which had been exposed to ambient and elevated CO2 concentrations (365 and 600 μl l−1) for six growing seasons.In spring, N-mineralisation increased significantly by 30% at elevated CO2, while there was no significant difference between treatments in summer (+3%). The response of soil enzymes to CO2 enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly by 32 and 21%. In summer, differences of activities between CO2 treatments were greatest in the case of urease and protease (+21 and +17% at elevated CO2).The stimulation of N-mineralisation and enzyme activities at elevated CO2 was probably caused by higher soil moisture and/or increased root biomass. We conclude that elevated CO2 will enhance below-ground C- and N-cycling in grasslands. [Copyright &y& Elsevier]

Alpine soils are warming strongly, leading to profound alterations in carbon cycling and greenhouse gas budgets, mediated via the soil microbiome. To explore microbial responses to global warming, we incubated eight alpine soils between 4 and 35 °C and linked the temperature dependency of bacterial growth with alterations in community structures and the identification of temperature sensitive taxa. The temperature optimum for bacterial growth was between 27 and 30 °C and was higher in soils from warmer environments. This temperature framing the upper limit of naturally occurring temperatures was a tipping point above which the temperature range for growth shifted towards higher temperatures together with pronounced changes in community structures and diversity based on both 16S rRNA gene and transcript sequencing. For instance, at the highest temperature, we observed a strong increase in OTUs affiliated with Burkholderia-Paraburkholderia, Phenylobacterium, Pseudolabrys, Edaphobacter and Sphingomonas. Dominance at high temperature was explained by a priori adaptation to high temperature, high growth potential as well as stress resistance. At the highest temperature, we moreover observed an overall increase in copiotrophic properties in the community along with high growth rates. Further, temperature effects on community structures depended on the long-term climatic legacy of the soils. These findings contribute to extrapolating from single to multiple sites across a large range of conditions. • Temperature impacts on alpine soil microbiome depend on climatic legacy of the soil. • Strong temperature effects above community optimum for growth, minor effects below. • High bacterial growth rates and copiotrophic features at high temperature. • High temperature promotes heat-adapted, stress resistant and fast growing bacteria. • Burkholderia, Phenylobacterium, Pseudolabrys and Edaphobacter favored by heat. [ABSTRACT FROM AUTHOR]

During leaf litter decomposition, nutrients are released, can be transferred among different litter species, are metabolized by soil organisms and are taken up by plants again. However, it remains unclear to which extent leaf litter species richness affects these processes of nutrient cycling, and whether effects on one of those processes propagate to the subsequent one. We established a common garden decomposition experiment in a Chinese subtropical secondary forest, to trace two essential nutrients during decomposition and their uptake by plants along a litter species diversity gradient. Unlabelled, and 15 N and Li (as surrogate for K) labelled leaves of three native tree species were used to create replicated 1-, 2- and 3-species mixtures, each with one species labelled per mixture. Litter mixtures were placed in mesocosms with one growing herbaceous phytometer plant. Over six months, litter and phytometer plants of each mixture were sampled at four points in time and the different process steps of nutrient dynamics were determined. Our results showed species and nutrient specific decomposition dynamics, which propagated through the processes of mass loss, nutrient release and transfer among species, and nutrient uptake dynamics of phytometer plants. However, we found no litter species diversity effects along the different litter decomposition processes. Rather specific diversity effects occurred in few cases at different points in time for mass loss, Li release and transfer dynamics. These effects were not caused by nutrient transfer from labelled to unlabelled litter, suggesting that species identity effects on decomposer dynamics may outweigh effects of nutrient transfer among litter species in mixtures. Further, the observed litter species diversity effects did not affect the 15 N uptake of phytometer plants. Hence, the influence of species diversity on nutrient cycling and plant available nutrient stocks is mainly determined by the amount and variety of chemical compounds that different species exhibit and release to the soil. [ABSTRACT FROM AUTHOR]

Leaf litter decomposition is closely linked to soil nutrient cycling. Both vary with environmental conditions, leaf litter diversity, faunal decomposer community and leaf litter chemistry. Polyphenols, i.e. phenolics and tannins, are important secondary metabolites in leaf litter and are considered a major impediment to whole-leaf decomposition. While the function of polyphenols is well studied, the mechanisms and drivers of their decomposition are largely unknown. We reasoned that polyphenol decomposition is driven by the same factors as whole-leaf decomposition. We hypothesized that polyphenol decomposition rates increase with leaf litter richness, decrease with macrofauna exclusion and are related to traits characterizing leaf litter quality. We measured decomposition rates of polyphenols in leaf litter of seven subtropical Chinese tree species, sampled at five dates and in a fully factorial design that manipulated litter richness and macrofauna access. We further estimated leaf carbon and nitrogen contents and leaf toughness using near-infrared spectroscopy. We showed that 1) phenolics and tannin decomposition rates did not depend on leaf litter species richness, 2) the decomposition rates of phenolics and tannins were up to one magnitude higher than whole-leaf decomposition rates, 3) the exclusion of macrofauna increased phenolics and tannin decomposition rates, 4) the leaf nitrogen content positively affected the phenolics decomposition rates and 5) the tannin-to-nitrogen ratio was the best predictor of whole-leaf decomposition. We conclude that the fast decomposition of phenolics and tannins in the early stages of whole-leaf litter decomposition is an essential ecological process. Low molecular weight phenolics that enter the soil can accelerate microbial growth, while potentially toxic tannins leave the leaf tissue, and thus, enable the consecutive whole-leaf decomposition. Our study is the first to show that macrofauna occurrence negatively affects the decomposition of ecological relevant secondary plant metabolites. This points to the importance of considering biotic interactions between different trophic levels to fully understand the mechanisms of leaf litter decomposition. • Litter species identity rather than litter diversity drives polyphenol decomposition. • Soil macrofauna negatively affects the decomposition of secondary plant metabolites. • Leaf nitrogen content positively affects the phenolics decomposition rates. • Tannin-to-nitrogen ratio is best predictor of whole-leaf decomposition. [ABSTRACT FROM AUTHOR]

Positive relationships between biodiversity functioning have been found in communities of plants but also of soil microbes. The beneficial effects of diversity are thought to be driven by niche partitioning among community members, which leads to more complete or more efficient community-level resource use through various mechanisms. An intriguing related question is whether environmentally more heterogeneous habitats provide a larger total niche space and support stronger diversity—functioning relationships because they harbor more species or allow species to partition the available niche space more efficiently. Here, we tested this hypothesis by assembling communities of 1, 2 or 4 methanotrophic isolates and exposing them to temporally (constant or diurnal temperature cycling) and structurally (one or two aggregate size classes) more heterogeneous conditions. In total, we incubated 396 microcosms for 41 days and found that more biodiverse communities consumed more methane (CH 4) and tended to have a larger community size (higher pmoA copy numbers). Diurnal temperature cycling strongly reduced CH 4 oxidation and growth, whereas soil aggregate composition and diversity had no detectable effect. Biodiversity effects varied greatly with the identity of the community members that were combined. With respect to community level CH 4 consumption, strain interactions were positive or neutral but never negative, and could neither be explained by 14 structural and function traits we collected or by the observed competitive hierarchy among the strains. Overall, our results indicate that methanotrophic diversity promotes methanotrophic community functioning. The strains that performed best varied with environmental conditions, suggesting that a high biodiversity is important for maintaining methanotrophic functioning as environmental conditions fluctuate over time. • Mechanistic diallels were used to analyze interactions among methanotrophs. • CH4 consumption increased with methanotroph diversity. • Diversity-functioning relationships were not promoted by environmental heterogeneity. • Interactive effects of strains on CH4 consumption all were positive or neutral. • Contributions of strains to CH4 consumption did not follow competitive hierarchies. [ABSTRACT FROM AUTHOR]

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added 14C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the 14C label in mineralized and leached DOM and quantified the spatial distribution of DO14C retained in soils using a new autoradiographic technique. Leaching of DO14C through the 10 cm-long soil columns amounted up to 28% of the added DO14C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO14C as only 2-9% were mineralized, with the highest rates in mature soils. In line with the mass balance of 14C fluxes, measured 14C activities in soils indicated that the major part of litter DO14C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO14C in the upper 3 cm of the soil columns. In the deeper soil, the 14C label was concentrated along soil pores and textural discontinuities with similarly high 14C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO14C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO14C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization. [ABSTRACT FROM AUTHOR]

Abstract: The oxidation of atmospheric methane (CH4) by soil methanotrophic bacteria constitutes the only biological sink for this greenhouse gas. However, anthropogenic activities, in particular N fertilization, often (but not always) reduce this sink, by mechanisms only partly understood. We argue that the difficulty in developing a process-based understanding of the mechanisms involved is, in part, due to complex interactions with environmental conditions and N cycling, in combination with a lack of information on how the involved processes and organisms are organized within soil structure. We have developed a novel method which permits mapping of the spatial distribution of the active soil methanotrophs at a resolution well below 100 μm. In the present study, we applied this technique to a selection of plots from the Park Grass experiment at Rothamsted, UK, to which either no fertilizer, or (NH4)2SO4, NaNO3, or manure were applied for over 150 years. We measured the spatial distribution of CH4 assimilation four times throughout the 2008 growing season, together with field-based measurements of the soil CH4 sink. In general, methanotrophic activity was most pronounced within the top 10 cm of soil, and along the surface of aggregates and pore channels. Soil CH4 oxidation was controlled by soil moisture, with no differences between the plots after correcting for differences in soil moisture within the field site. Exceptions were on the (NH4)2SO4-treated plots in which acidification had occurred due to no or little liming. In these plots, methanotrophic activity was restricted to spots in deeper soil layers, which contributed only little to the sink for atmospheric CH4 due to diffusive limitation of the top soil layers. Our results suggest that spatial distribution of CH4 assimilation is controlled by local concentrations of NH4 + and/or pH within the soil structure. The effect of pH may be direct, or indirect through a reduction in nitrification rates and therefore increased NH4 + concentrations, or indirect through a mobilization of Al3+ which also might reduce methanotrophic activity. The concentration of ammonium ions, and their persistence in soil, will depend on the quantity of N applied, its rate of release through mineralization, and its rate of removal by either plant or microbial assimilation or nitrification. Our findings underline the importance of developing a detailed understanding of the spatial organisation of these processes since this will determine the nature and strength of their interactions. The technique we have shown here provides a powerful tool to achieve this goal. [Copyright &y& Elsevier]