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Microorganisms play pivotal roles in soil processes. Metabolically related microorganisms constitute functional groups, and diverse microbial functional groups control nutrient cycling in soils. This study explored environmental (i.e., rainfall, temperature) and soil factors driving the distribution of bacterial functional groups involved in soil carbon (C) cycling in paired natural and agricultural ecosystems. Soil samples were collected at a regional scale covering gradients of temperature and rainfall across two orthogonal transects (~1000 km) in New South Wales, Australia. Putative functions of bacteria were linked to two soil C fractions: particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC). We found: (i) temperature and rainfall were important drivers of bacterial functional groups, while soil properties, such as pH, soil C and nitrogen (N), also presented significant contributions; (ii) community structure of bacteria involved in C cycling was mainly related to POC content but not to MAOC; (iii) paired sampling showed that agricultural practices had significant impacts on the composition and responses of soil bacterial functional groups. This study demonstrated the environmental regulation (e.g., temperature and rainfall) of soil microbial functional groups at large scales, which was altered by agricultural practices. Highlights: Soil bacteria involved in C cycling were investigated across two ~1000 km transects.Temperature and rainfall were important drivers of bacterial functional groups at large scale.Paired sampling showed that agriculture led to a significant shift in bacterial functional groups.Community structure of bacterial functional groups were correlated with soil POC but not MAOC. [ABSTRACT FROM AUTHOR]

Purpose: We evaluated the importance of dry season (summer) sunlight on carbon (C) turnover in a Mediterranean-type climate ecosystem in the context of spatially heterogenous distribution of vegetation. Methods: We manipulated summer sunlight exposure of litter and soil at the ecosystem scale in a natural Patagonian woodland over two years and evaluated its effect on C turnover in litter and surface soil. We measured decomposition of standing dead litter of dominant grass and shrub species, changes in labile C pools in soil microsites with and without plant litter, and potential enzyme activity of litter and both soil microsites, evaluating seasonal and legacy effects of summer sunlight exposure. Results: Summer sunlight exposure significantly increased standing litter decomposition for both shrub and grass litter (p < 0.05). Additionally, summer sunlight significantly increased labile carbohydrate availability (saccharification) and potential microbial enzymatic activity of grass (but not shrub) litter. Interestingly, summer sunlight exposure also had effects on soil, reducing by 50% labile organic C while stimulating potential extracellular enzyme activity in both soil microsites, with and without plant litter. Conclusion: Summer sunlight accelerates C loss from standing litter and soil consistently across patches of heterogeneously distributed vegetation. In addition, sunlight exposure demonstrated carryover effects on the acceleration of grass litter decay in the following rainy season, when sunlight increased decomposition five times more than in the dry season. These results suggest that summer sunlight is an important and lasting control of C turnover at the ecosystem scale even in seasonally and spatially heterogenous ecosystems. [ABSTRACT FROM AUTHOR]

High nitrogen (N) input often induces soil carbon (C) limitation, eutrophication of macronutrients, deficiency of base cations, and accumulation of toxic micronutrients. These changes are perceived to be critical factors in regulating soil C mineralization. Previous studies primarily focused on the individual effects of C, macronutrients, exchangeable base cations, and micronutrients on soil C mineralization. However, the relative importance of those factors in regulating soil C mineralization, especially in N-enriched ecosystems, remains unclear. To disentangle the relative contributions of aforementioned factors, lime and/or glucose were added to soils that were collected from a field experiment with historical N addition (6 years) at seven rates (0–50 g N m−2 year−1) in a grassland ecosystem. Lime and glucose were added to improve the soil C and key nutrient conditions. The responses of soil C mineralization rate to changes in soil C and macronutrients (N and P), exchangeable base cations (K+, Na+ and Mg2+), and micronutrients (Fe2+, Mn2+, Cu2+ and Zn2+) were examined. We found that lime addition decreased soil micronutrients, while glucose addition improved the soil available P and exchangeable base cations, especially at high historical N addition rates. The soil C mineralization was weakly associated with changes in soil nutrients, including the availability of N, P, exchangeable base cations, and micronutrients, which were conventionally and previously considered as the vital drivers of soil C mineralization. However, soil C mineralization strongly increased with glucose-induced enhancement of C availability and the subsequent enhancement of microbial biomass under increasing N addition rates. Based on the Structural Equation Model, the standardized total effects of C, macronutrients (N and P), base cations and micronutrients on soil C mineralization were 0.86, − 0.29, 0.15 and − 0.08, respectively. Findings from this study demonstrated that the N-induced significant changes in soil nutrients (e.g., eutrophication of N and P, base cations deficiency and accumulation of toxic macronutrients) mediated soil C mineralization, with C availability being the most critical driver for C mineralization in N-enriched soil. This study provides insight into the mechanistic understanding of the relationship between N input and terrestrial C cycling. • N and P are less important than C for C mineralization in N-enriched soil. • C mineralization is insensitive to base cations and micronutrients in N-enriched soil. • C plays a key role in regulating C mineralization in N-enriched soil. • Effects of soil nutrients on C mineralization highly depend on C availability. [ABSTRACT FROM AUTHOR]

In urban areas, anthropogenic drivers of ecosystem structure and function are thought to predominate over larger‐scale biophysical drivers. Residential yards are influenced by individual homeowner preferences and actions, and these factors are hypothesized to converge yard structure across broad scales. We examined soil total C and total δ13C, organic C and organic δ13C, total N, and δ15N in residential yards and corresponding reference ecosystems in six cities across the United States that span major climates and ecological biomes (Baltimore, Maryland; Boston, Massachusetts; Los Angeles, California; Miami, Florida; Minneapolis‐St. Paul, Minnesota; and Phoenix, Arizona). Across the cities, we found soil C and N concentrations and soil δ15N were less variable in residential yards compared to reference sites supporting the hypothesis that soil C, N, and δ15N converge across these cities. Increases in organic soil C, soil N, and soil δ15N across urban, suburban, and rural residential yards in several cities supported the hypothesis that soils responded similarly to altered resource inputs across cities, contributing to convergence of soil C and N in yards compared to natural systems. Soil C and N dynamics in residential yards showed evidence of increasing C and N inputs to urban soils or dampened decomposition rates over time that are influenced by climate and/or housing age across the cities. In the warmest cities (Los Angeles, Miami, Phoenix), greater organic soil C and higher soil δ13C in yards compared to reference sites reflected the greater proportion of C4 plants in these yards. In the two warm arid cities (Los Angeles, Phoenix), total soil δ13C increased and organic soil δ13C decreased with increasing home age indicating greater inorganic C in the yards around newer homes. In general, soil organic C and δ13C, soil N, and soil δ15N increased with increasing home age suggesting increased soil C and N cycling rates and associated 12C and 14N losses over time control yard soil C and N dynamics. This study provides evidence that conversion of native reference ecosystems to residential areas results in convergence of soil C and N at a continental scale. The mechanisms underlying these effects are complex and vary spatially and temporally. [ABSTRACT FROM AUTHOR]

Litter types and soil properties can affect litter decomposition rate and litter carbon (C) transformation, consequently regulating soil C cycling. Yet, litter C transformations of leaves and roots of invasive plants are poorly understood, which limits our understanding of the role of plant residues in soil C sequestration during plant invasion. In a laboratory incubation experiment lasting for 153 days, we used two types of soil which were collected from invasive S. alterniflora and native Phragmites australis marshlands, and traced the transformation of 13C from leaf and root litter of invasive Spartina alterniflora into CO2, soil-dissolved organic C (DOC), microbial biomass C (MBC), and soil organic C (SOC). The leaf litter of S. alterniflora decomposed faster than root litter, resulting in higher soil respiration and higher transformation of litter-derived 13C into CO2, MBC, and DOC. Although the root litter of S. alterniflora decomposed slowly, SOC comprised up to 24% of litter-derived 13C. Furthermore, the litter C transformations of S. alterniflora showed a positive home-field advantage effect. Soil respiration, MBC, fractions of litter-derived 13C in Gram-negative bacteria, 13C recovered in CO2, DOC, and SOC were higher in the soil colonized by S. alterniflora than in the soil colonized by P. australis, with the home-field advantage effect being more pronounced in root than leaf litter treatments. Therefore, litter type and soil source had differential impacts on litter C transformation patterns of S. alterniflora and the root litter of invasive S. alterniflora played an important role in SOC formation and C sequestration in soils from its invaded ecosystems. [ABSTRACT FROM AUTHOR]

Sustainable forest management should avoid disturbance and volatilization of the soil carbon (C) and nitrogen (N) stocks both under present and projected future climate. Earlier studies have shown that thinning of European beech forests induces a strong initial perturbation of the soil C and N cycles in shallow Rendzic Leptosol, which consists of lower soil N retention and strongly enhanced gaseous losses observed over several years. Persistence of these effects could decrease soil organic matter (SOM) levels and associated soil functions such as erosion protection, nutrient retention, and fertility. Therefore, we resampled untreated control and thinned stands a decade after thinning at sites representing both typical present day and projected future climatic conditions for European beech forests. We determined soil organic C and total N stocks, as well as δ13C and δ15N as integrators of changes in soil C and N cycles. Thinning did not alter these parameters at any of the sampled sites, indicating that initial effects on soil C and N cycles constitute short-term perturbations. Consequently, thinning may be considered a sustainable beech forest management strategy with regard to the maintenance of soil organic C and total N stocks both under present and future climate. [ABSTRACT FROM AUTHOR]

Biochar incorporated into soil has been known to affect soil nutrient availability and act as a habitat for microorganisms, both of which could be related to its particle size. However, little is known about the effect of particle size on soil microbial community structure and function. To investigate short-term soil microbial responses to biochar addition having varying particle sizes and addition rates, we established a laboratory incubation study. Biochar produced via pyrolysis of bamboo was ground into three particle sizes (diameter size < 0.05 mm (fine), 0.05–1.0 mm (medium) and 1.0–2.0 mm (coarse)) and amended at rates of 0% (control), 3% and 9% (w/w) in an intensively managed bamboo ( Phyllostachys praecox ) plantation soil. The results showed that the fine particle biochar resulted in significantly higher soil pH, electrical conductivity (EC), available potassium (K) concentrations than the medium and coarse particle sizes. The fine-sized biochar also induced significantly higher total microbial phospholipid fatty acids (PLFAs) concentrations by 60.28% and 88.94% than the medium and coarse particles regardless of addition rate, respectively. Redundancy analysis suggested that the microbial community structures were largely dependent of particle size, and that improved soil properties were key factors shaping them. The cumulative CO 2 emissions from biochar-amended soils were 2–56% lower than the control and sharply decreased with increasing addition rates and particle sizes. Activities of α-glucosidase, β-glucosidase, β-xylosidase, N -acetyl-β-glucosaminidase, peroxidase and dehydrogenase decreased by ranging from 7% to 47% in biochar-amended soils over the control, indicating that biochar addition reduced enzyme activities involved carbon cycling capacity. Our results suggest that biochar addition can affect microbial population abundances, community structure and enzyme activities, that these effects are particle size and rate dependent. The fine particle biochar may additionally produce a better habitat for microorganisms compared to the other particle sizes. [ABSTRACT FROM AUTHOR]

Sustainable forest management should avoid disturbance and volatilization of the soil carbon (C) and nitrogen (N) stocks both under present and projected future climate. Earlier studies have shown that thinning of European beech forests induces a strong initial perturbation of the soil C and N cycles in shallow Rendzic Leptosol, which consists of lower soil N retention and strongly enhanced gaseous losses observed over several years. Persistence of these effects could decrease soil organic matter (SOM) levels and associated soil functions such as erosion protection, nutrient retention, and fertility. Therefore, we resampled untreated control and thinned stands a decade after thinning at sites representing both typical present day and projected future climatic conditions for European beech forests. We determined soil organic C and total N stocks, as well as δ13C and δ15N as integrators of changes in soil C and N cycles. Thinning did not alter these parameters at any of the sampled sites, indicating that initial effects on soil C and N cycles constitute short-term perturbations. Consequently, thinning may be considered a sustainable beech forest management strategy with regard to the maintenance of soil organic C and total N stocks both under present and future climate.

• Light fraction organic matter increased exponentially with increasing latitude. • Heavy fraction organic matter increased linearly with increasing latitude. • Latitudinal patterns of light fraction organic matter were controlled by climate. • Heavy fraction organic matter was driven by soil physicochemical factors. Semi-arid and arid ecosystems are important for the global C cycle. Despite this, it remains unclear how organic matter fractions vary across latitudinal gradients, and what drives this variation, in dry ecosystems. In this study, we sampled soils from 100 sites across a latitudinal gradient in the dry valleys of southwestern China to explore the latitudinal patterns of light fraction organic matter (LFOM) and heavy fraction organic matter (HFOM) at two soil depths (0–10 cm and 10–20 cm). Across the studied gradient, HFOM accounted for a larger fraction of soil organic matter than LFOM. LFOM increased exponentially with increasing latitude at both 0–10 cm and 10–20 cm depths. Heavy fraction organic C increased linearly with increasing latitude at both depths, while heavy fraction organic N only increased with latitude in soils from 10 to 20 cm depth. Latitudinal patterns of LFOM were mainly explained by climate, with the most important driver being mean annual temperature, followed by mean annual precipitation. Soil physicochemical factors – in particular cation exchange capacity and silt content – explained the most variation in HFOM. Total microbial biomass was also important in explaining variation in HFOM, especially in the 10–20 cm soil layer. Overall, our results shed light on the spatial distribution of organic matter fractions in arid and semi-arid regions. We also identify candidate drivers of the variation in LFOM and HFOM in arid and semi-arid regions, finding that climate primarily explains variation in LFOM while soil physiochemistry primarily explains variation in HFOM. [ABSTRACT FROM AUTHOR]

Soil carbon (C) and nitrogen (N) cycles are inextricably linked, yet the impacts of N availability upon soil C sequestration and turnover are poorly understood. According to stoichiometric theory, in the absence of nutrient limitation substrate decomposition will reach maximum rates, with C assimilated into microbial biomass at the expense of CO2 production. In this study, we added a 14C labelled low molecular weight substrate (glucose) to a sandy soil along with eleven increasing levels of N, phosphorus (P), and sulphur (S) in relative proportions as required for microbial biomass production. Adding a simple soluble substrate allowed us to explicitly examine changes in microbial transformations of added C, rather than changes resulting from extracellular enzyme activity or the extent of substrate decomposition. We hypothesized that as nutrient addition increased, an increasing proportion of the glucose-C provided would be incorporated into microbial biomass at the expense of CO2 production and stabilized as soil organic carbon (SOC). Instead, CO2 production from glucose-C increased significantly with nutrient addition without measurable changes in glucose-derived microbial biomass or SOC. This suggests that if there was greater glucose-derived microbial biomass produced under higher nutrient addition it was offset by a higher rate of microbial biomass turnover. We also found greater soil-derived microbial biomass at lower nutrient addition levels, potentially supporting the concept of microbial mining of soil organic matter (SOM) for nutrients under low nutrient availability. In conclusion, our data suggest that in a sandy soil with low capacity for physical protection of SOM, nutrient addition does not immediately promote C sequestration in the soil microbial community, and that the interaction between C stabilization and nutrient addition requires further work, especially for predicting ecosystem responses. [ABSTRACT FROM AUTHOR]

Straw return to soil is a global field practice for sequestering carbon (C) in agricultural ecosystems, and soil C mineralization depends on the soil microbial metabolic process. However, the variation patterns of microbial respiration (Rs) and associated mechanisms under long-term straw input at different levels remain unclear. Here, this study investigated the changes in Rs and microbial metabolic limitation under straw input at four levels (0, 4, 8, and 12 t ha−1 yr−1) based on a long-term (11-year) field experiment. In addition, the C use efficiency (CUE) and C degradation genes were quantified via an enzyme-based biogeochemical-equilibrium model and high-throughput quantitative PCR-based chip technology, respectively. The results indicated that Rs significantly increased with the amount of straw addition, while its rate of increase dropped when the straw addition amount was greater than 8 t ha−1 yr−1. Interestingly, we also observed an apparent microbial P limitation under straw addition at 0 and 4 t ha−1 yr−1 but a shift to N limitation when the straw addition rate was over 8 t ha−1 yr−1. The shift suggested that Rs changes could be attributed to straw addition leading to soil microbes being increasingly limited by N rather than P. Moreover, straw addition significantly increased microbial biomass, reduced CUE and increased the absolute abundance of genes involved in degrading various organic polymers (e.g., starch, hemicellulose, cellulose, chitin and lignin). Partial least squares path modeling revealed that the variation in Rs was directly attributed to increased microbial biomass and C degradation genes as well as declining CUE, while C degradation genes and CUE were mediated by microbial relative C limitation and N vs. P limitation. This study provides insight into the mechanisms of the Rs response to straw addition by linking the Rs to microbial metabolic limitation, CUE and C degradation genes, highlighting that reducing microbial nutrient limitation by balancing metabolic demand and environmental nutrient supply potentially leads to a higher microbial CUE and lower Rs in agricultural ecosystems. • The rate of increase in Rs dropped when straw addition was greater than 8 t ha−1. • Straw input leads to soil microbes being increasingly limited by N rather than P. • Microbial metabolic limitation mediated CUE and the numbers of C-degrading genes. [ABSTRACT FROM AUTHOR]

Abstract: Our understanding of the mechanisms driving the response of soil organic carbon (SOC) pools to warming, though critical for predicting climate feedbacks, remains limited. Here we report results from a warming experiment using O-horizon soils from two mesic, boreal forest sites with contrasting climate regimes. We replaced extant Oi soil horizons, or litterfall C, with another coniferous Oi possessing a distinct δ13C signature, and tracked the net incorporation of the replaced Oi and, by difference, Oea-C into soil microbial phospholipid fatty acids (PLFA) following 120-day incubations at 15 °C and 20 °C. We demonstrate how regional climate (site effects) and experimental warming (temperature effects) influence microbial incorporation of Oi versus slower-turnover Oea SOC pools. Microbial biomass, estimated from total PLFA, increased by 32–60% with temperature and was 20–42% higher within soils from the warmer versus cooler site, congruent with increased mineralization in those soils. The proportion of Gram-positive bacterial PLFA-C derived from Oi-C more than doubled and coincided with a reduction in the incorporation of Oi-C into fungal relative to bacterial PLFA with warming and in soils from the warmer site. Mirroring the relative decrease in fungal incorporation of Oi-C, warming led to an increase of 22–31% in the proportion of fungal PLFA-C derived from the Oea-C, consistent with the increased incorporation of this slower-turnover SOC pool in soils from the warmer site. The increase in microbial biomass and shift in routing of Oi and Oea pools through PLFA indicate that warming preferentially increases fungal mineralization of more slow-turnover C pools in these boreal organic soils. Shifts in microbial substrate routing and biomass increases with warming observed here underscore the potential importance of changing proportions of microbial biomass remnant contributions to SOC pools with climate warming. [Copyright &y& Elsevier]

Grasslands are globally widespread and capable of storing large amounts of carbon (C) in soils, and are generally experiencing increasing atmospheric CO, nitrogen (N) deposition, and biodiversity losses. To better understand whether grasslands will act as C sources or sinks in the future we measured microbial respiration in long-term laboratory incubations of soils collected from a grassland field experiment after 9 years of factorial treatment of atmospheric CO, N deposition, and plant species richness on a deep and uniformly sandy soil. We fit microbial soil respiration rates to three-pool models of soil C cycling to separate treatment effects on decomposition and pool sizes of fast, slow, and resistant C pools. Elevated CO decreased the mean residence time (MRT) of slow C pools without affecting their pool size. Decreasing diversity reduced the size and MRT of fast C pools (comparing monocultures to plots planted with 16 species), but increased the slow pool MRT. N additions increased the size of the resistant pool. These effects of CO, N, and species-richness treatments were largely due to plant biomass differences between the treatments. We found no significant interactions among treatments. These results suggest that C sequestration in sandy grassland soils may not be strongly influenced by elevated CO or species losses. However, high N deposition may increase the amount of resistant C in these grasslands, which could contribute to increased C sequestration. [ABSTRACT FROM AUTHOR]

part of a multidisciplinary project on carbon (C) dynamics of the ecosystems characterizing the Eastmain Region Watershed (James Bay, Quebec), the objective of this study is to compare the soil C stocks and soil organic matter quality among the main upland vegetation types in a boreal region subjected to a high fire frequency. On average, the organic layer contained twice the amount of C than the mineral soil. Closed canopy vegetation types had greater C stocks both in the mineral and in the organic layers than the other more open canopy vegetation types. Landscape features such as drainage and surficial deposit could not discriminate between vegetation types although closed vegetation types were on average found on wetter site conditions. Average soil C contents varied more than 2-fold across vegetation types. On the opposite, except for the organic layer C:N ratio, which was smaller in closed vegetation types, other measured soil organic matter properties (namely specific rate of evolved C after a long-term incubation, hydrolysis acid-resistant C as well as the rate of changes in soil heterotrophic respiration with increasing temperature ( Q10)) remained within a narrow range between vegetation types. Therefore, total soil C stocks were a major determinant of both labile C and estimated summer soil heterotrophic respiration rate. The homogeneity of soil organic matter quality across the vegetation types could be attributable to the positive relationship between soil C storage and soil C fluxes observed in this landscape experiencing a high fire frequency. The low variability in soil C quality could help simplify the modelling of soil C fluxes in this environment. [ABSTRACT FROM AUTHOR]

We determined soil microbial community composition and function in a field experiment in which plant communities of increasing species richness were exposed to factorial elevated CO2 and nitrogen (N) deposition treatments. Because elevated CO2 and N deposition increased plant productivity to a greater extent in more diverse plant assemblages, it is plausible that heterotrophic microbial communities would experience greater substrate availability, potentially increasing microbial activity, and accelerating soil carbon (C) and N cycling. We, therefore, hypothesized that the response of microbial communities to elevated CO2 and N deposition is contingent on the species richness of plant communities. Microbial community composition was determined by phospholipid fatty acid analysis, and function was measured using the activity of key extracellular enzymes involved in litter decomposition. Higher plant species richness, as a main effect, fostered greater microbial biomass, cellulolytic and chitinolytic capacity, as well as the abundance of saprophytic and arbuscular mycorrhizal (AM) fungi. Moreover, the effect of plant species richness on microbial communities was significantly modified by elevated CO2 and N deposition. For instance, microbial biomass and fungal abundance increased with greater species richness, but only under combinations of elevated CO2 and ambient N, or ambient CO2 and N deposition. Cellobiohydrolase activity increased with higher plant species richness, and this trend was amplified by elevated CO2. In most cases, the effect of plant species richness remained significant even after accounting for the influence of plant biomass. Taken together, our results demonstrate that plant species richness can directly regulate microbial activity and community composition, and that plant species richness is a significant determinant of microbial response to elevated CO2 and N deposition. The strong positive effect of plant species richness on cellulolytic capacity and microbial biomass indicate that the rates of soil C cycling may decline with decreasing plant species richness. [ABSTRACT FROM AUTHOR]

free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO2 on nutrient cycling in terrestrial ecosystems. Using meta-analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N2 fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO2 alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO2 stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO2 by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr−1. Namely, elevated CO2 stimulated overall above- and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO2 respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr−1) and above- and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO2 in experiments receiving the high N treatments. Under low N availability, above- and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO2 only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO2 in the long-term. Therefore, increased soil C input and soil C sequestration under elevated CO2 can only be sustained in the long-term when additional nutrients are supplied. [ABSTRACT FROM AUTHOR]

Rising atmospheric [CO2] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO2] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen ( Populus tremuloides) and sugar maple ( Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO2. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO2]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO2]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO2]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO2] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO2], rather than changes in substrate chemistry. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climate-soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha-1 yr-1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low-N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C-CO2 signature, reflected the turnover times of SOM-C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climate-SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization. [ABSTRACT FROM AUTHOR]