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Soil microorganisms carry out important processes, including support of plant growth and cycling of carbon and other nutrients. However, the majority of soil microbes have not yet been isolated and their functions are largely unknown. Although metagenomic sequencing reveals microbial identities and functional gene information, it includes DNA from microbes with vastly varying physiological states. Therefore, metagenomics is only predictive of community functional potential. We posit that the next frontier lies in understanding the metaphenome, the product of the combined genetic potential of the microbiome and available resources. Here we describe examples of opportunities towards gaining understanding of the soil metaphenome. [ABSTRACT FROM AUTHOR]

Diversifying biologically simple cropping systems often entails altering other management practices, such as tillage regime or nitrogen (N) source. We hypothesized that the interaction of crop rotation, N source, and tillage in diversified cropping systems would promote microbially-mediated soil C and N cycling while attenuating inorganic N pools. We studied a cropping systems trial in its 10th year in Iowa, USA, which tested a 2-yr cropping system of corn ( Zea mays L.)/soybean [ Glycine max (L.) Merr.] managed with conventional fertilizer N inputs and conservation tillage, a 3-yr cropping system of corn/soybean/small grain + red clover ( Trifolium pratense L.), and a 4-yr cropping system of corn/soybean/small grain + alfalfa ( Medicago sativa L.)/alfalfa. Three year and 4-yr cropping systems were managed with composted manure, reduced N fertilizer inputs, and periodic moldboard ploughing. We assayed soil microbial biomass carbon (MBC) and N (MBN), soil extractable NH 4 and NO 3 , gross proteolytic activity of native soil, and potential activity of six hydrolytic enzymes eight times during the growing season. At the 0–20 cm depth, native protease activity in the 4-yr cropping system was greater than in the 2-yr cropping system by a factor of 7.9, whereas dissolved inorganic N pools did not differ between cropping systems (P = 0.292). At the 0–20 cm depth, MBC and MBN the 4-yr cropping system exceeded those in the 2-yr cropping system by factors of 1.51 and 1.57. Our findings suggest that diversified crop cropping systems, even when periodically moldboard ploughed, support higher levels of microbial biomass, greater production of bioavailable N from SOM, and a deeper microbially active layer than less diverse cropping systems. [ABSTRACT FROM AUTHOR]

Given that analytical precision affects the number of biological replicates required to achieve adequate statistical power, and analytical variation is often high for soil assays, we investigated ways to reduce variation of soil hydrolase assays. For two mineral soils (sandy and loamy) and one organic soil (peat), we compared variation of fluorescence for acid phosphatase, β-glucosidase, and N-acetyl-glucosaminidase using incubation methods that differed in assay volume and the ability of reagents to mix. For fluorescence of β-glucosidase and N-acetyl-glucosaminidase from the mineral soils, which had coefficients of variation that exceeded 10% on average, well-mixed, larger volumes significantly reduced variation. This reduction was not observed for fluorescence of acid phosphatase from peat, which had consistently low variation. For all soils tested, thorough mixing of NaOH also reduced variation. Because the goal of enzyme assays is to estimate the total enzyme pool in a sample of soil, modifications to reduce analytical variation, such as those proposed here, should be considered. These modifications can potentially increase our ability to detect treatment differences within the heterogeneous soil matrix. [ABSTRACT FROM AUTHOR]

Abstract: We used natural abundance 13C:12C (δ 13C) and 8 yr of labeling with 13C-depleted CO2 in a Pinus taeda Free Air CO2 Enrichment (FACE) experiment to investigate carbon sources of saprotrophic fungi, ectomycorrhizal fungi, and fungi of uncertain life history. Sporocarp δ 13C identified Sistotrema confluens as ectomycorrhizal, as suspected previously from morphological characteristics. Saprotrophic δ 13C declined by 2 ‰–13 ‰ between ambient to elevated CO2 treatments and corresponded to different carbon sources, including surface litter (Rhodocollybia, Mycena), pine cones (Baeospora), wood (Gymnopilus, Pholiota), and soil (Ramariopsis). Ectomycorrhizal fungi, foliage, and surficial litter declined 12 ‰ in δ 13C between ambient and elevated treatments, confirming that these fungi depend on recent photosynthate. The δ 13C of ectomycorrhizal genera correlated between treatments with a slope (4.3 ± 1.2) greater than the expected value of one. This suggested that Inocybe and Cortinarius incorporated some pre-treatment, soil-derived carbon (presumably from amino acids) whereas Lactarius and Russula only incorporated current-year photosynthate or recent, litter-derived carbon. Combining natural abundance and tracer 13C measurements proved a powerful technique to examine carbon sources of different fungi. [Copyright &y& Elsevier]

Within soil aggregates, binding of organic matter is known to occlude it from microbial attack. Within aggregate fractions of different sizes, microbial communities and activities have also been shown to differ. As a result the soil physical structure, organic inputs and microbial activity together impact the rate at which organic matter is decomposed and stored within soil. However, methods developed for isolating soil aggregates may affect subsequent biological assays. In this study, we sought to understand how enzyme activity within soil aggregates is influenced by aggregate isolation methodology, including wet, dry, and 'optimal moisture' sieving procedures within two contrasting ecosystems (a corn agroecosystem and a 2-yr old, diverse planted tallgrass prairie). Mass distribution of aggregates from wet-sieving was skewed toward small macroaggregates (250-1000 μm) and microaggregates (<250 μm), but the distribution of dry and optimal moisture aggregates was highly skewed toward large macroaggregates (>2000 μm). Wet-sieved macroaggregates (>1000 μm) had greater aggregate potential enzyme activity (nmol substrate h−1 g−1 dry aggregate) than smaller aggregate fractions and whole soil, particularly for C-cycling enzymes cellobiohydrolase and β-glucosidase. Also, wet-sieved aggregates from corn systems had higher potential cellobiohydrolase and β-glucosidase activity than aggregates isolated from prairie. Neither of these relationships was observed in dry and optimal moisture aggregates, suggesting that elevated activities are characteristic of water-stable aggregates and possibly stimulated by soil rewetting. The proportional contribution to total enzyme activity observed in water-stable microaggregates accounted for 46-62% of whole soil activity; although water-stable large macroaggregates (>2000 μm) had greater aggregate enzyme activity, they contributed a minority of overall soil activity. In contrast, the proportional contribution of large macroaggregates comprised 70-78% of whole soil activity when dry sieved and 38-66% under optimal moisture sieving. Wet-sieving soil aggregates is most useful to examine long-term changes in soil organic matter and microbial activity between soil types. Optimal moisture and dry sieved aggregates may be useful alternatives to more closely capture short-term in situ measures of seasonal and intra-annual soil microbial activity. [ABSTRACT FROM AUTHOR]

Abstract: The impact of rising atmospheric carbon dioxide (CO2) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O3) or vary based on species-specific growth responses to elevated CO2. To understand how projected increases in atmospheric CO2 and O3 alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO2–O3 Enrichment (FACE) experiment. Tracer amounts of 15NH4 + were applied to the forest floor of Populus tremuloides, P. tremuloides–Betula papyrifera and P. tremuloides–Acer saccharum communities exposed to factorial CO2 and O3 treatments. The 15N tracer and strongly depleted 13C–CO2 were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO2 relative to ambient CO2. As main effects, neither CO2 nor O3 significantly altered 15N recovery in SOM. Elevated CO2 significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO2 has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O3 had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO2 can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response. [Copyright &y& Elsevier]

The ongoing increase of atmospheric temperature may induce soil organic carbon (SOC) loss and exacerbate the greenhouse effect. As a result, there is a great effort to understand the relationship between temperature and the heterotrophic soil respiration rate (R SOIL) as it has significant implications for anticipated change of the Earth system. Soil respiration depends on the size of respiring microbial biomass (MBC) and when R SOIL is measured without concurrent measurement of MBC, the apparent temperature sensitivity of R SOIL could be misinterpreted since MBC can change with temperature within days or weeks of warming. The effect of temperature driven changes in MBC on the apparent temperature sensitivity of R SOIL was evaluated using a meta-analysis of 27 laboratory and field experiments conducted at different temporal scales (1–730 d) and under a wide range of temperatures (2–50 °C) and soil conditions. Across all studies, the apparent temperature sensitivity decreased when MBC decreased with increasing temperature and vice versa. We observed a steep decrease of MBC above optimal temperature (27.1 ± 1.0 °C), which attenuated the apparent temperature sensitivity of R SOIL , an aspect previously explained by the existence of reaction rate temperature optima. The temperature response of the MBC specific respiration rate was, however, highly non-linear and soil specific. Including MBC in soil biogeochemical models requires careful consideration of the variability of temperature-associated physiological changes of soil microorganisms. Without it, microbially explicit models cannot predict temperature induced SOC loss better than older, empirical models based on first order reaction kinetics. Image 1 • Temperature sensitivity of heterotrophic soil respiration is affected by temperature driven changes in microbial biomass. • Attenuation of the temperature sensitivity above ∼25 °C is caused by increased death rate of microbial biomass. • The relationship between temperature and microbial biomass specific respiration is highly non-linear and soil specific. [ABSTRACT FROM AUTHOR]

Abstract Soil microbial communities are highly spatially organized, shaped in part by the structure of soil itself. Understanding how spatially discrete microbial communities change across years and seasons in response to environmental factors, plant phenology and aggregate turnover, is key to understanding how varying management practices impact the ecology of soil microbial communities. We investigated both seasonal (within year) and annual (across sampling years) changes of discrete microbial communities in soil aggregate fractions, large macroaggregates (LM) and microaggregates (MICRO) in three different bioenergy management systems. We hypothesized that 1) seasonal changes due to plant phenology and aggregate turnover will be most pronounced within the MICRO aggregate soil microbial community; 2) inter-annual variability will lead to changes in microbial diversity across aggregate sizes and the magnitude of change will be mediated by management regime. We found that LM and MICRO aggregates have unique microbial communities within soil. MICRO aggregate microbial communities are more diverse and change more dynamically across the sampling season, peaking in diversity at peak plant growth and maximum biomass. The number of families indicative of specific MICRO aggregate habitats increases over the growing season for both bacteria (from 3 to 51) and fungi (from 8 to 14). The LM aggregates harbored less diverse, yet more stable, communities within a growing season. By contrast, between years the LM aggregates were the most responsive to inter-annual variability. Our study demonstrates the importance of including the spatio-temporal dynamics of soil microbes. We identified "hot spots" of microbial diversity within soil, with a greater diversity of microbes found under prairies, within the MICRO aggregates, and seasonally during peak plant biomass. Targeted analysis of the MICRO aggregates can contribute to deeper understanding of potential diversity and functioning of soil microbial communities for ecosystem maintenance as well as the response to climatic events and environmental change. Graphical abstract Image 1 Highlights • Microbial communities are distinct in soil aggregate fractions. • Microaggregates house the greatest microbial diversity. • Microaggregate communities fluctuate within a growing season. • Greatest microbial diversity is found in microaggregates during peak plant growth. • Diversity begets diversity, higher plant diversity increased microbial diversity. [ABSTRACT FROM AUTHOR]

Highlights • Planted prairie bioenergy systems with high plant diversity supported greater microbial diversity than corn systems. • Microbial activity increased with increasing plant root inputs and greater precipitation. • Prairies with increased microbial diversity exhibited increased functional resiliency, as measured by cellulose-degrading enzyme activity, than corn systems. Abstract Agricultural land use is a major threat to biodiversity and ecosystem functions in tallgrass prairies. However, there are proposed bioenergy systems that can use biomass harvested from restored tallgrass prairie, creating a potential free market incentive for landowners to restore prairies. These alternative management practices may alter associated soil microbial communities and their ecosystem services. We examined changes in soil microbial community structure, function, and resiliency to drought following two prairie restorations from row-crop agriculture and through subsequent succession in a fertilized and unfertilized tallgrass prairie. The soil microbial community structure was assessed through amplicon (16S and ITS) sequencing, function through potential extracellular enzyme activity, and resiliency indices were calculated for both microbial diversity measures and extracellular enzyme activity. We hypothesized that 1) distinct soil microbial communities in each management system will continue to develop over time reflecting the extent of divergence between the plant communities, due to the strong selective forces plant communities have on the soil microbiome. 2) Microbial extracellular enzymatic function will continue to diverge between the management systems across sampling years. 3) We will see increased resiliency to drought in the prairies potentially due to the greater diversity in this management system for the microbial and plant community, creating a possible enhancement in functional redundancy. Our experiment demonstrates that soil microbial communities continue to diverge from row-crop agriculture as prairie restoration progresses. Planted prairie bioenergy systems with higher plant diversity supported greater microbial diversity than corn systems. Corn monocultures were less resistant to drought stress, as evidenced by decreased microbial activity and richness. Prairies with increased microbial diversity exhibited increased functional resiliency than corn systems, as measured by cellulose-degrading enzyme activity. Prairies that received nitrogen fertilization maintained high microbial diversity and activity, even under drought. Our study demonstrates that diverse cropping systems may benefit from nitrogen fertilization to confer resiliency to disturbance events. Increasing resiliency, while maintaining productivity, is key to managing alternative crops that are sustainable systems for biofuel uses. Our multi-year study reveals the benefits of long-term experiments for capturing the dynamic range of microbial mediation of soil carbon and nutrients and the importance of resiliency in both developing sustainable management systems and modeling predictive biogeochemical models. [ABSTRACT FROM AUTHOR]

Abstract: Quantitative PCR (qPCR) is a popular technique used to quantify target sequences from DNA isolated from soil, but PCR inhibition makes it difficult to estimate gene copy number. Here, we evaluated the extent to which inhibition associated with reaction conditions and sample-specific properties influence the linear range of amplification, and the efficiency and sensitivity of qPCR assays of three bacterial gene targets. We adopted a sample pool approach and exploited the mathematical basis of qPCR to correct for sample-specific effects on amplification. Results revealed that qPCR efficiency and sensitivity were dependent on all conditions tested. In addition, the effect of annealing temperature and SYBR green PCR kit was target-specific, suggesting that the sample pool approach is appropriate for evaluating the quality of new primers. Likewise, the efficiency and sensitivity of qPCR amplification was sample-specific and is likely a result of site and date-specific co-extractants. When relativized against calculations based on plasmid curves alone, reaction-specific and sample-specific inhibition influenced calculations of gene copy number. To account for these differences, we present a brief protocol for soil samples that will facilitate comparison of future datasets. [Copyright &y& Elsevier]

Microbial interactions occur in habitats much smaller than those generally captured in homogenized soil cores sampled across a plot or field. This study uses soil aggregates to examine soil microbial community composition and structure of both bacteria and fungi at a microbially-relevant scale. Aggregates were isolated from three land management systems in central Iowa, USA to test if aggregate-level microbial responses were sensitive to large-scale shifts in plant community and management practices. Bacteria and fungi exhibited similar patterns of community structure and diversity among soil aggregates, regardless of land management. Microaggregates supported more diverse microbial communities, and Fimbriimonadales, Acidimicrobiales, Actinomycetales, Alteromonodales, Burkholderiales, Gemmatimonadales, Rhodobacterales, Soligubrobacterales, Sphingobacteriales, Sphingomonodales, Spirobacillaes, Onygenales, Chaetosphaeriales, and Trichosporanales were indicator taxa for microaggregate communities. Large macroaggregates contained greater abundance of Pedosphaerales, Planctomycetales, Syntrophobacterales, and Glomeromycota (arbuscular mycorrhizal fungi). To demonstrate the potential for additional insights into soil microbial diversity, we calculated of a weighted proportional whole soil diversity, which accounted for microbes found in aggregate fractions and resulted in 65% greater bacterial richness and 100% greater fungal richness over independently sampled whole soil (i.e. bulk soil). Our results show microaggregates support highly diverse microbial communities, including several unidentified genera. Isolating aggregates with a microbially sensitive approach provides new opportunities to explore soil microbial communities and the factors shaping them at relevant spatial scales. [ABSTRACT FROM AUTHOR]

Boreal peatlands are important global carbon reservoirs that are vulnerable to increasing CO 2 and associated warming. Soil microbes regulate the balance of carbon that is stored in peat or remineralized to CO 2 ; so characterizing microbial responses to warming and rising CO 2 is critical to predicting how peatlands will feed back to ongoing climate change. To address microbiome responses to changing climate, we examined taxon-specific bacterial growth under elevated CO 2 and across a warming gradient in a peatland using 18O-water quantitative stable isotope probing. Using in situ temperatures, we clustered the responses of bacterial taxa according to excess atom fraction 18O of their genomes, a proxy for growth. Many taxa that showed little to no growth across the temperature range under ambient CO 2 grew rapidly at certain temperatures under elevated CO 2 , highlighting a strong interplay between warming and CO 2 concentrations. The temperature of maximum growth for Proteobacteria shifted higher under elevated CO 2 , while that of Acidobacteria shifted lower. We found support for phylogenetic conservation of growth patterns among Acidobacteria and Proteobacteria under ambient, but not elevated CO 2. Our results suggest that certain taxa may be predisposed for growth under altered climate conditions, with a disproportionate influence on carbon cycling and peatland feedbacks to climate change. • Elevated CO 2 with increased temperature modifies bacterial growth. • Taxa that showed little to no growth under ambient CO 2 grew rapidly under eCO 2. • Proteobacteria and Actinobacteria appear poised to benefit from climate shifts. [ABSTRACT FROM AUTHOR]

The adaptability of soil microbial communities to prolonged periods of drought is influenced by their ability to produce extracellular polymeric substances (EPS) with sufficient water retention properties. Microbial EPS have been extensively investigated as water reservoirs during drought, but it remains unknown how carbon substrate accessibility to soil microbial communities will affect the chemical properties of the EPS they generate, and whether this in turn will alter their water retention characteristics. In this work, we observed that the accessibility of carbon substrates influenced microbial community structure and, consequently, the chemical properties of EPS produced by the microbial communities. Our results demonstrated that an insoluble carbon substrate (i.e., chitin), stimulated microbial communities to produce EPS with measurably better water retention properties in emulated soil microenvironments in comparison to a soluble carbon substrate (i.e., N-acetylglucosamine; NAG). In all, this study demonstrates the importance of carbon substrate accessibility by soil microorganisms in regulating the community structure and consequently, the EPS carbon chemistry, which in turn can greatly influence the adaptability of soil microbial communities to drought. • Carbon substrate accessibility regulates microbial community membership. • The properties of microbial community-produced EPS can be influenced by membership. • Soil microbial community membership influence EPS water retention properties. • Specific carbon substrates may affect microbial community EPS production and properties. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) dynamics are central to soil biogeochemistry and fertility. The retention of SOM is governed initially by interactions with minerals, which mediate the sorption of chemically diverse organic matter (OM) molecules via distinct surface areas and chemical functional group availabilities. Unifying principles of mineral-OM interactions remain elusive because of the multi-layered nature of biochemical-mineral interactions that contribute to soil aggregate formation and the heterogeneous nature of soils among ecosystems. This study sought to understand how soil mineralogy as well as nitrogen (N) enrichment regulate OM composition in grassland soils. Using a multi-site grassland experiment, we demonstrate that the composition of mineral-associated OM depended on the clay content and specific mineral composition in soils across the sites. With increasing abundance of ferrihydrite (Fh) across six different grassland locations, OM in the hydrophobic zone became more enriched in lipid- and protein-like compounds, whereas the kinetic zone OM became more enriched in lignin-like molecules. These relationships suggest that the persistence of various classes of OM in soils may depend on soil iron mineralogy and provide experimental evidence to support conceptual models of zonal mineral-OM associations. Experimental N addition disrupted the accumulation of protein-like molecules in the hydrophobic zone and the positive correlation of lignin-like molecules in the kinetic zone with Fh content, compared to unfertilized soils. These data suggest that mineralogy and clay content together influence the chemical composition not only of mineral-associated OM, but also of soluble compounds within the soil matrix. If these relationships are prevalent over larger spatial and temporal scales, they provide a foundation for understanding SOM cycling and persistence under a variety of environmental contexts. Unlabelled Image • Beyond clay content, mineralogy strongly controlled the composition of MAOM. • Ferrihydrite accumulated proteins and lipids in the hydrophobic zone of MAOM. • The mineral core influenced biochemical persistence of OM in the kinetic zone. • Nitrogen fertilization altered carbon chemistry and organo-mineral interactions. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) form symbioses with roots that can enhance plant nutrition. While AMF have been shown to have a role in soil nitrogen (N) cycling, it is unclear whether AMF affect N cycling microbes such as ammonia-oxidizing bacteria (AOB) and archaea (AOA), which convert ammonium into nitrite in the first step of nitrification. In this study, we examined the effects of AMF on AOA and AOB abundances within the corn rhizosphere and bulk soil of conventional (corn-soybean rotation with inorganic fertilizer) and diversified (corn-soybean-oats/alfalfa-oats rotation with composted manure) systems. We hypothesized that AMF would decrease AOA and AOB abundances in a cropping-system dependent manner, possibly due to competition for ammonium. We grew corn deficient or proficient in AMF symbiosis in microcosms for 10 weeks. At the end of the experiment, both soils planted with the AMF-proficient corn genotype had higher ammonium and lower nitrate pool sizes compared to the same soils planted with the AMF-deficient corn genotype. Likewise, total plant N was higher in the AMF-proficient genotype compared to the AMF-deficient genotype. Despite changes in soil inorganic N pool sizes, AOA and AOB abundances were unaffected by plant AMF-proficiency. Instead, AOA abundance was greater in the rhizosphere than in the bulk soil regardless of cropping system, and AOB abundance was greater in the conventional than the diversified cropping system soil regardless of proximity to the root. These data indicate that 1) AMF did not affect AOA or AOB abundance in these N-rich soils but other factors such as root proximity and inorganic fertilization did and 2) AOA and AOB have differing ecological niches within rhizosphere and bulk soil that should be considered when managing for nitrogen losses. • AMF alter inorganic N pools, but not AOB or AOA abundances in N-rich soils. • AOA abundance was elevated in the maize rhizosphere compared to bulk soil. • AOB were more abundant in soil with increased inorganic N fertilization. • Differing ecological niches exist for AOA and AOB in agricultural soils. [ABSTRACT FROM AUTHOR]

Although most studies of organic matter (OM) stabilization in soils have focused on adsorption to aluminosilicate and iron-oxide minerals due to their strong interactions with organic nucleophiles, stabilization within alkaline soils has been empirically correlated with exchangeable Ca. Yet the extent of competing processes within natural soils remains unclear because of inadequate characterization of soil mineralogy and OM distribution within the soil in relation to minerals, particularly in C poor alkaline soils. In this study, we employed bulk and surface-sensitive spectroscopic methods including X-ray diffraction, 57Fe-Mössbauer, and X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM) methods to investigate the minerology and soil organic C and N distribution on individual fine particles within an alkaline soil. Microscopy and XPS analyses demonstrated preferential sorption of Ca-containing OM onto surfaces of Fe-oxides and calcite. This result was unexpected given that the bulk combined amounts of quartz and Fe-containing feldspars of the soil constitute ~90% of total minerals and the surface atomic composition was largely Fe and Al (>10% combined) compared to Ca (4.2%). Soil sorption experiments were conducted with two siderophores, pyoverdine and enterobactin, to evaluate the adsorption of organic molecules with functional groups that strongly and preferentially bind Fe. A greater fraction of pyoverdine was adsorbed compared to enterobactin, which is smaller, less polar, and has a lower aqueous solubility. Using NanoSIMS to map the distribution of isotopically-labeled siderophores, we observed correlations with Ca and Fe, along with strong isotopic dilution with native C, indicating associations with OM coatings rather than with bare mineral surfaces. We propose a mechanism of adsorption by which organics aggregate within alkaline soils via cation bridging, favoring the stabilization of larger molecules with a greater number of nucleophilic functional groups. Unlabelled Image • Most natural organic carbon associated with Ca-rich coatings in an alkaline soil • Larger more hydrophilic siderophores exhibited greater affinity to soil particles. • Siderophores associated with organic carbon coatings rather than to bare mineral • Cation bridging of organic molecules was a major dominant stabilization mechanism. [ABSTRACT FROM AUTHOR]

Northern latitude peatlands contain large reserves of soil carbon (C) due in part to climatic constraints on decomposition, including low temperatures and water inundated soils. Under future climate change scenarios these peatlands will experience warmer temperatures, extended growing seasons, and a potential draw down of the water table, all of which improve conditions for decomposition, as well as photosynthesis. Increased photosynthesis may feedback to increase decomposition of soil C through increased root exudates to belowground decomposer communities, primarily as low molecular weight carbon compounds (LMWCC). In this study, we examine the interactive effects of climate, a combination of three temperatures and two moisture regimes, and root exudates on microbial decomposer function, measured as CO 2 respiration, biomass, and potential enzyme activity. We had four substrate treatments: two common LMWCC (glycine or glucose + citric acid), chitin to simulate fungal necromass, and DI as the control. Our results support our first hypothesis that increasing temperature will increase C respiration across substrate and moisture treatments. Our second hypothesis was that compared to control and chitin, soluble substrates (i.e., LMWCC) will enhance respiration across all climate treatments. This was only partially supported. As expected, the two LMWCC substrate additions increased C respiration above the two other substrate additions at current recorded growing season average (12 °C, low treatment) and high (20 °C, med treatment) temperature treatments. Surprisingly, when the system was pushed to a higher temperature extreme (28 °C, high treatment), the low moisture controls respired more C than the other substrate × climate treatments. Potential enzyme activity and demand for phosphorus appear to explain these trends as opposed to changes to microbial biomass. Our results indicate that under projected future high temperatures, the peatland microbial community allocates additional labile C resources to enzyme production to meet nutrient demands, and as such, dampens C lost through respiration. Image 1 • At current temperatures soluble exudates stimulate CO 2 respired from boreal peat. • At future higher temperatures more CO 2 was not stimulated with soluble exudates. • Microbial phosphatase enzyme potential explained variation in respiration responses. • Rhizodeposition & nutrient limitation moderate microbial contribution to CO 2 losses. [ABSTRACT FROM AUTHOR]