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Biodiversity enhances ecosystem functions such as biomass production and nutrient cycling. Although the majority of the terrestrial biodiversity is hidden in soils, very little is known about the importance of the diversity of microbial communities for soil functioning. Here, we tested effects of biodiversity on the functioning of methanotrophs, a specialized group of soil bacteria that plays a key role in mediating greenhouse gas emissions from soils. Using pure strains of methanotrophic bacteria, we assembled artificial communities of different diversity levels, with which we inoculated sterile soil microcosms. To assess the functioning of these communities, we measured methane oxidation by gas chromatography throughout the experiment and determined changes in community composition and community size at several time points by quantitative PCR and sequencing. We demonstrate that microbial diversity had a positive overyielding effect on methane oxidation, in particular at the beginning of the experiment. This higher assimilation of CH 4 at high diversity translated into increased growth and significantly larger communities towards the end of the study. The overyielding of mixtures with respect to CH 4 consumption and community size were positively correlated. The temporal CH 4 consumption profiles of strain monocultures differed, raising the possibility that temporal complementarity of component strains drove the observed community-level strain richness effects; however, the community niche metric we derived from the temporal activity profiles did not explain the observed strain richness effect. The strain richness effect also was unrelated to both the phylogenetic and functional trait diversity of mixed communities. Overall, our results suggest that positive biodiversity-ecosystem-function relationships show similar patterns across different scales and may be widespread in nature. Additionally, biodiversity is probably also important in natural methanotrophic communities for the ecosystem function methane oxidation. Therefore, maintaining soil conditions that support a high diversity of methanotrophs may help to reduce the emission of the greenhouse gas methane., (© 2018 by the Ecological Society of America.)

Soil microbial biomass is a key determinant of carbon dynamics in the soil. Several studies have shown that soil microbial biomass significantly increases with plant species diversity, but it remains unclear whether plant species diversity can also stabilize soil microbial biomass in a changing environment. This question is particularly relevant as many global environmental change (GEC) factors, such as drought and nutrient enrichment, have been shown to reduce soil microbial biomass. Experiments with orthogonal manipulations of plant diversity and GEC factors can provide insights whether plant diversity can attenuate such detrimental effects on soil microbial biomass. Here, we present the analysis of 12 different studies with 14 unique orthogonal plant diversity × GEC manipulations in grasslands, where plant diversity and at least one GEC factor (elevated CO2 , nutrient enrichment, drought, earthworm presence, or warming) were manipulated. Our results show that higher plant diversity significantly enhances soil microbial biomass with the strongest effects in long-term field experiments. In contrast, GEC factors had inconsistent effects with only drought having a significant negative effect. Importantly, we report consistent non-significant effects for all 14 interactions between plant diversity and GEC factors, which indicates a limited potential of plant diversity to attenuate the effects of GEC factors on soil microbial biomass. We highlight that plant diversity is a major determinant of soil microbial biomass in experimental grasslands that can influence soil carbon dynamics irrespective of GEC., (© 2015 John Wiley & Sons Ltd.)

The restoration of peatlands is an important strategy to counteract subsidence and loss of biodiversity. However, responses of important microbial soil processes are poorly understood. We assessed functioning, diversity and spatial organization of methanotrophic communities in drained and rewetted peat meadows with different water table management and agricultural practice. Results show that the methanotrophic diversity was similar between drained and rewetted sites with a remarkable dominance of the genus Methylocystis. Enzyme kinetics depicted no major differences, indicating flexibility in the methane (CH4) concentrations that can be used by the methanotrophic community. Short-term flooding led to temporary elevated CH4 emission but to neither major changes in abundances of methane-oxidizing bacteria (MOB) nor major changes in CH4 consumption kinetics in drained agriculturally used peat meadows. Radiolabeling and autoradiographic imaging of intact soil cores revealed a markedly different spatial arrangement of the CH4 consuming zone in cores exposed to near-atmospheric and elevated CH4. The observed spatial patterns of CH4 consumption in drained peat meadows with and without short-term flooding highlighted the spatial complexity and responsiveness of the CH4 consuming zone upon environmental change. The methanotrophic microbial community is not generally altered and harbors MOB that can cover a large range of CH4 concentrations offered due to water-table fluctuations, effectively mitigating CH4 emissions., (© FEMS 2015. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Increased warming in spring and prolonged summer drought may alter soil microbial denitrification. We measured potential denitrification activity and denitrifier marker gene abundances (nirK, nirS, nosZ) in grasslands soils in three geographic regions characterized by site-specific land-use indices (LUI) after warming in spring, at an intermediate sampling and after summer drought. Potential denitrification was significantly increased by warming, but did not persist over the intermediate sampling. At the intermediate sampling, the relevance of grassland land-use intensity was reflected by increased potential N2O production at sites with higher LUI. Abundances of total bacteria did not respond to experimental warming or drought treatments, displaying resilience to minor and short-term effects of climate change. In contrast, nirS- and nirK-type denitrifiers were more influenced by drought in combination with LUI and pH, while the nosZ abundance responded to the summer drought manipulation. Land-use was a strong driver for potential denitrification as grasslands with higher LUI also had greater potentials for N2O emissions. We conclude that both warming and drought affected the denitrifying communities and the potential denitrification in grassland soils. However, these effects are overruled by regional and site-specific differences in soil chemical and physical properties which are also related to grassland land-use intensity., (© FEMS 2015. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Loss of biodiversity and increased nutrient inputs are two of the most crucial anthropogenic factors driving ecosystem change. Although both received considerable attention in previous studies, information on their interactive effects on ecosystem functioning is scarce. In particular, little is known on how soil biota and their functions are affected by combined changes in plant diversity and fertilization., Methodology/principal Findings: We investigated the effects of plant diversity, functional community composition, and fertilization on the biomass and respiration of soil microbial communities in a long-term biodiversity experiment in semi-natural grassland (Jena Experiment). Plant species richness enhanced microbial basal respiration and microbial biomass, but did not significantly affect microbial specific respiration. In contrast, the presence of legumes and fertilization significantly decreased microbial specific respiration, without altering microbial biomass. The effect of legumes was superimposed by fertilization as indicated by a significant interaction between the presence of legumes and fertilization. Further, changes in microbial stoichiometry (C-to-N ratio) and specific respiration suggest the presence of legumes to reduce N limitation of soil microorganisms and to modify microbial C use efficiency., Conclusions/significance: Our study highlights the role of plant species and functional group diversity as well as interactions between plant community composition and fertilizer application for soil microbial functions. Our results suggest soil microbial stoichiometry to be a powerful indicator of microbial functioning under N limited conditions. Although our results support the notion that plant diversity and fertilizer application independently affect microbial functioning, legume effects on microbial N limitation were superimposed by fertilization, indicating significant interactions between the functional composition of plant communities and nutrient inputs for soil processes.

Soil compaction is a major disturbance associated with logging, but we lack a fundamental understanding of how this affects the soil microbiome. We assessed the structural resistance and resilience of the microbiome using a high-throughput pyrosequencing approach in differently compacted soils at two forest sites and correlated these findings with changes in soil physical properties and functions. Alterations in soil porosity after compaction strongly limited the air and water conductivity. Compaction significantly reduced abundance, increased diversity, and persistently altered the structure of the microbiota. Fungi were less resistant and resilient than bacteria; clayey soils were less resistant and resilient than sandy soils. The strongest effects were observed in soils with unfavorable moisture conditions, where air and water conductivities dropped well below 10% of their initial value. Maximum impact was observed around 6-12 months after compaction, and microbial communities showed resilience in lightly but not in severely compacted soils 4 years post disturbance. Bacteria capable of anaerobic respiration, including sulfate, sulfur, and metal reducers of the Proteobacteria and Firmicutes, were significantly associated with compacted soils. Compaction detrimentally affected ectomycorrhizal species, whereas saprobic and parasitic fungi proportionally increased in compacted soils. Structural shifts in the microbiota were accompanied by significant changes in soil processes, resulting in reduced carbon dioxide, and increased methane and nitrous oxide emissions from compacted soils. This study demonstrates that physical soil disturbance during logging induces profound and long-lasting changes in the soil microbiome and associated soil functions, raising awareness regarding sustainable management of economically driven logging operations.

Random reductions in plant diversity can affect ecosystem functioning, but it is still unclear which components of plant diversity (species number - namely richness, presence of particular plant functional groups, or particular combinations of these) and associated biotic and abiotic drivers explain the observed relationships, particularly for soil processes. We assembled grassland communities including 1 to 16 plant species with a factorial separation of the effects of richness and functional group composition to analyze how plant diversity components influence soil nitrifying and denitrifying enzyme activities (NEA and DEA, respectively), the abundance of nitrifiers (bacterial and archaeal amoA gene number) and denitrifiers (nirK, nirS and nosZ gene number), and key soil environmental conditions. Plant diversity effects were largely due to differences in functional group composition between communities of identical richness (number of sown species), though richness also had an effect per se. NEA was positively related to the percentage of legumes in terms of sown species number, the additional effect of richness at any given legume percentage being negative. DEA was higher in plots with legumes, decreased with increasing percentage of grasses, and increased with richness. No correlation was observed between DEA and denitrifier abundance. NEA increased with the abundance of ammonia oxidizing bacteria. The effect of richness on NEA was entirely due to the build-up of nitrifying organisms, while legume effect was partly linked to modified ammonium availability and nitrifier abundance. Richness effect on DEA was entirely due to changes in soil moisture, while the effects of legumes and grasses were partly due to modified nitrate availability, which influenced the specific activity of denitrifiers. These results suggest that plant diversity-induced changes in microbial specific activity are important for facultative activities such as denitrification, whereas changes in microbial abundance play a major role for non-facultative activities such as nitrification.

Changes in frequency and intensity of drought events are anticipated in many areas of the world. In pasture, drought effects on soil nitrogen (N) cycling are spatially and temporally heterogeneous due to N redistribution by grazers. We studied soil N cycling responses to simulated summer drought and N deposition by grazers in a 3-year field experiment replicated in two grasslands differing in climate and management. Cattle urine and NH4NO3 application increased soil NH4(+) and NO3(-) concentrations, and more so under drought due to reduced plant uptake and reduced nitrification and denitrification. Drought effects were, however, reflected to a minor extent only in potential nitrification, denitrifying enzyme activity (DEA), and the abundance of functional genes characteristic of nitrifying (bacterial and archaeal amoA) and denitrifying (narG, nirS, nirK, nosZ) micro-organisms. N2O emissions, however, were much reduced under drought, suggesting that this effect was driven by environmental limitations rather than by changes in the activity potential or the size of the respective microbial communities. Cattle urine stimulated nitrification and, to a lesser extent, also DEA, but more so in the absence of drought. In contrast, NH4NO3 reduced the activity of nitrifiers and denitrifiers due to top-soil acidification. In summary, our data demonstrate that complex interactions between drought, mineral N availability, soil acidification, and plant nutrient uptake control soil N cycling and associated N2O emissions. These interactive effects differed between processes of the soil N cycle, suggesting that the spatial heterogeneity in pastures needs to be taken into account when predicting changes in N cycling and associated N2O emissions in a changing climate.

Temperate forest soils are usually efficient sinks for the greenhouse gas methane, at least in the absence of significant amounts of methanogens. We demonstrate here that trafficking with heavy harvesting machines caused a large reduction in CH(4) consumption and even turned well-aerated forest soils into net methane sources. In addition to studying methane fluxes, we investigated the responses of methanogens after trafficking in two different forest sites. Trafficking generated wheel tracks with different impact (low, moderate, severe, and unaffected). We found that machine passes decreased the soils' macropore space and lowered hydraulic conductivities in wheel tracks. Severely compacted soils yielded high methanogenic abundance, as demonstrated by quantitative PCR analyses of methyl coenzyme M reductase (mcrA) genes, whereas these sequences were undetectable in unaffected soils. Even after a year after traffic compression, methanogen abundance in compacted soils did not decline, indicating a stability of methanogens here over time. Compacted wheel tracks exhibited a relatively constant community structure, since we found several persisting mcrA sequence types continuously present at all sampling times. Phylogenetic analysis revealed a rather large methanogen diversity in the compacted soil, and most mcrA gene sequences were mostly similar to known sequences from wetlands. The majority of mcrA gene sequences belonged either to the order Methanosarcinales or Methanomicrobiales, whereas both sites were dominated by members of the families Methanomicrobiaceae Fencluster, with similar sequences obtained from peatland environments. The results show that compacting wet forest soils by heavy machinery causes increases in methane production and release.

Human activities affect both tree species composition and diversity in forested ecosystems. This in turn alters the species diversity of plant litter and litter quality, which may have cascading effects on soil microbial communities and their functions for decomposition and nutrient cycling. We tested microbial responses to litter species diversity in a leaf litter decomposition experiment including monocultures, 2-, and 4-species mixtures in the subtropical climate zone of southeastern China. Soil microbial community composition was assessed by lipid analysis, and microbial functions were measured using extracellular enzyme activity and gross rates of nitrogen mineralization. We observed a positive relationship between litter species diversity and abundances of mycorrhizal fungi and actinomycetes. Alternatively, enzyme activities involved in carbon and phosphorus acquisition, and enzyme indices of relative carbon limitation, were higher only in the 4-species mixtures. This suggests that the minimum basal substrate level for enzyme production was reached, or that limitation was higher, at the highest diversity level only. Responses to litter diversity also changed over time, where phosphatase responses to litter diversity were strongest early in decomposition and the indices of carbon limitation relative to other nutrients showed stronger responses later in decomposition. Enzyme activities were related to lipid biomarker data and the mass of litter remaining at the third time point, but relationships between enzyme activity and the mass of litter remaining were not consistent across other time points. We conclude that litter species richness will likely only reduce microbial functions at key intervals of diversity loss while microbial growth is more sensitive to incremental diversity loss, with no clear relationships between them or to ecosystem functions. The observed litter diversity effects on soil microbial biomass and enzyme activity indicate interactions of aboveground and belowground communities, and together with environmental conditions they are important for maintaining ecosystem functions. [ABSTRACT FROM AUTHOR]

Background: Effects of elevated atmospheric CO2 concentrations on plant growth and associated C cycling have intensively been studied, but less is known about effects on the fluxes of radiatively active trace gases other than CO2. Net soil-atmosphere CH4 fluxes are determined by the balance of soil microbially-driven methane (CH4) oxidation and methanogenesis, and both might change under elevated CO2. Methods and Results: Here, we studied CH4 dynamics in a permanent grassland exposed to elevated CO2 for 14 years. Soil-atmosphere fluxes of CH4 were measured using large static chambers, over a period of four years. The ecosystem was a net sink for atmospheric CH4 for most of the time except summer to fall when net CH4 emissions occurred. We did not detect any elevated CO2 effects on CH4 fluxes, but emissions were difficult to quantify due to their discontinuous nature, most likely because of ebullition from the saturated zone. Potential methanotrophic activity, determined by incubation of fresh sieved soil under standardized conditions, also did not reveal any effect of the CO2 treatment. Finally, we determined the spatial micro-distribution of methanotrophic activity at less than 5× atmospheric (10 ppm) and elevated (10000 ppm) CH4 concentrations, using a novel auto-radiographic technique. These analyses indicated that domains of net CH4 assimilation were distributed throughout the analyzed top 15 cm of soils, with no dependence on CH4 concentration or CO2 treatment. Conclusions: Our investigations suggest that elevated CO2 exerts no or only minor effects on CH4 fluxes in the type of ecosystem we studied, at least as long as soil moisture differences are small or absent as was the case here. The autoradiographic analyses further indicate that the spatial niche of CH4 oxidation does not shift in response to CO2 enrichment or CH4 concentration, and that the same type of methanotrophs may oxidize CH4 from atmospheric and soil-internal sources. [ABSTRACT FROM AUTHOR]

Soil methanotrophic bacteria constitute the only globally relevant biological sink for atmospheric methane (CH4). Nitrogen (N) fertilizers as well as soil moisture regime affect the activity of these organisms, but the mechanisms involved are not well understood to date. In particular, virtually nothing is known about the spatial distribution of soil methanotrophs within soil structure and how this regulates CH4 fluxes at the ecosystem scale. We studied the spatial distribution of CH4 assimilation and its response to a factorial drought×N fertilizer treatment in a 3-year experiment replicated in two grasslands differing in management intensity. Intact soil cores were labelled with 14CH4 and methanotrophic activity mapped at a resolution of ∼ 100 μm using an autoradiographic technique. Under drought, the main zone of CH4 assimilation shifted down the soil profile. Ammonium nitrate (NH4NO3) and cattle urine reduced CH4 assimilation in the top soil, but only when applied under drought, presumably because NH4 1 from fertilizers was not removed by plant uptake and nitrification under these conditions. Ecosystem-level CH4 fluxes measured in the field did show no or only very small inhibitory effects, suggesting that deeper soil layers fully compensated for the reduction in top soil CH4 assimilation. Our results indicate that the ecosystem-level CH4 sink cannot be inferred from measurements of soil samples that do not reflect the spatial organization of soils (e.g. stratification of organisms, processes, and mechanisms). The autoradiographic technique we have developed is suited to study methanotrophic activity in a relevant spatial context and does not rely on the genetic identity of the soil bacterial communities involved, thus ideally complementing DNA-based approaches. [ABSTRACT FROM AUTHOR]

Elevated CO2 generally increases plant productivity, and has been found to alter plant community composition in many ecosystems. Because soil microbes depend on plant-derived C and are often associated with specific plant species, elevated CO2 has the potential to alter structure and functioning of soil microbial communities. We investigated soil microbial community structure of a species-rich semi-natural calcareous grassland that had been exposed to elevated CO2 (600 μL L-1) for 6 growing seasons. We analysed microbial community structure using phospholipid fatty acid (PLFA) profiles and DNA fingerprints obtained by Denaturing Gradient Gel Electrophoresis (DGGE) of 16S rDNA fragments amplified by the Polymerase Chain Reaction (PCR). PLFA profiles were not affected by CO2 enrichment and the ratio of fungal and bacterial PLFA did not change. Ordination analysis of DNA fingerprints revealed a significant relation between CO2 enrichment and variation in DNA fingerprints in summer (P=0.01), but not in spring. This variation was due to changes in low-intensity bands, while dominant bands did not differ between CO2 treatments. Diversity of the bacterial community, as assessed by number of bands in DNA fingerprints and calculation of Shannon diversity indices, was not affected by elevated CO2. Overall, only minor effects on microbial community structure were detected, corroborating earlier findings that soil carbon inputs did probably change much less than suggested by plant photosynthetic responses. [ABSTRACT FROM AUTHOR]

Microbial responses to three years of CO2 enrichment (600 μL L–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO2. Microbial C:N ratios significantly decreased under elevated CO2 (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO2 (qCO2 = basal respiration/microbial C) did not exhibit any significant differences between CO2 treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO2 and phosphorus (P) fertilizer (1 g P m–2 y–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO2. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO2 on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO2 enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is... [ABSTRACT FROM AUTHOR]

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]

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]