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QUESTIONS: Relative to a landscape with a mosaic of two sagebrush community types and increasing fire frequency, we asked: (a) do vegetation characteristics vary significantly with number of times burned for each sagebrush community; (b) how do vegetation responses to different fire frequencies compare between the two sagebrush communities? LOCATION: Columbia Plateau Ecoregion, Washington, USA. METHODS: We sampled vegetation across a landscape that burned three times over a 10‐year period in two sagebrush community types that are interspersed on unique land forms: big sagebrush (Artemisia tridentata) communities that occur on small “mounds” and scabland sagebrush (A. rigida) communities that occur on surrounding “flats.” Spatially overlapping fires permitted a balanced sampling design to assess unburned and once‐, twice‐, and thrice‐burned locations for each land form/community type. We utilized a suite of statistical analyses to determine differences among plant functional groups and biomass among unburned/burned strata by land form and compared results between land forms. RESULTS: Big sagebrush and scabland sagebrush communities responded uniquely to multiple fires, due to different fuel loadings, fire severities, succession and invasion dynamics. Big sagebrush experienced nearly complete shrub loss and conversion from exotic‐invaded shrubland to exotic annual grassland after only one fire. In contrast, scabland sagebrush retained a minor shrub component and higher relative cover of native herbaceous species, even after three fires. Both communities retained cover of native perennial grasses, including shallow‐ and deep‐rooted species, likely reflecting decreasing fire intensity with number of times burned. CONCLUSIONS: Despite different community‐level responses, increasing fire frequency is transforming the entire landscape to a non‐native/native grassland mix. Quantifying unique ecosystem responses to altered wildfire regimes is critical to understanding the relative resilience of communities to disturbance and their resistance to exotic species invasion (and community type conversion). Management actions may help to maintain spatial heterogeneity of ecosystems and fire‐tolerant native species.

Cheatgrass (Bromus tectorum) invasion can alter community structure of arbuscular mycorrhizal fungi (AMF) in the sagebrush-steppe ecosystem. The feedbacks and underlying mechanisms of a changed AMF community on sagebrush (Artemisia tridentate ssp. wyomingensis) remain unclear. We assessed how ‘own’ versus ‘foreign’ AMF impact plant biomass, C transfer to AMF, and decomposition rates. To evaluate the impact of different AMF communities on plant biomass and C transfer, sagebrush and cheatgrass were grown in sterilized soil amended with ‘own’ or ‘foreign’ AMF. Sagebrush plants were labeled with 13C-CO2 to assess changes in allocation of C belowground (13C-PLFA & NLFA) and decomposition (soil respired 13C-CO2). Community structure and alpha-diversity of AMF were examined in native and cheatgrass-invaded communities. Cheatgrass invasion changed AMF community structure and decreased AMF taxon richness. Sagebrush C transfer and decomposition were not altered, but sagebrush root and cheatgrass shoot production was reduced with ‘foreign’ AMF and no AMF, respectively. Our results from the greenhouse experiment suggest that sagebrush performance declines with cheatgrass invasion. This may be caused by a disadvantageous AMF community shift, where ‘foreign’ AMF received the same amount of C but provided fewer benefits to sagebrush, as shown by decreased root biomass. These findings provide insight into the feedback mechanism that may contribute to decreasing native plant performance upon invasion.

Predators can have strong influences on ecological processes through impacts on individuals at lower trophic levels, and changes in predator-prey dynamics can alter ecosystem functioning. However, much of what we currently know about interactions across trophic levels comes from mesic or relatively fertile systems, with fewer studies examining trophic interactions and resulting ecosystem processes in arid or infertile systems. To address this knowledge gap, we excluded avian predators from shrubs during the growing season using netting in a sagebrush steppe environment. We compared arthropod abundance, shrub herbivory damage, physiology (gas exchange and chlorophyll fluorescence), litter chemistry (C/N ratios and concentrations of phenolic compounds) and decomposition between netted and un-netted (control) sagebrush (Artemisia tridentata ssp. wyomingensis) shrubs across a growing season. While there were clear seasonal patterns in measurements, we observed no statistically significant differences between netting treatments in any of these measurements, though abundances of arthropods in the sap-feeding trophic guilds were appreciably greater (although not significantly) on netted compared to un-netted shrubs. Our results suggest that in the short-term, either top-down effects in this sage-steppe ecosystem are minimal, and/or inter-trophic interactions (vertebrate predators-arthropods-plants) are relatively weak and more dependent on bottom-up processes that are linked with abiotic variables.

Understanding soil systems is critical because they form the structural and nutritional foundation for plants and thus every terrestrial habitat and agricultural system. In this paper, we encourage increased use of mathematical models to drive forward understanding of interactions in soil ecological systems. We discuss several distinctive features of soil ecosystems and empirical studies of them. We explore some perceptions that have previously deterred more extensive use of models in soil ecology and some advances that have already been made using models to elucidate soil ecological interactions. We provide examples where mathematical models have been used to test the plausibility of hypothesized mechanisms, to explore systems where experimental manipulations are currently impossible, or to determine the most important variables to measure in experimental and natural systems. To aid in the development of theory in this field, we present a table describing major soil ecology topics, the theory previously used, and providing key terms for theoretical approaches that could potentially address them. We then provide examples from the table that may either contribute to important incremental developments in soil science or potentially revolutionize our understanding of plant–soil systems. We challenge scientists and mathematicians to push theoretical explorations in soil systems further and highlight three major areas for the development of mathematical models in soil ecology: theory spanning scales and ecological hierarchies, processes, and evolution.

Increased fire frequency in semi-arid ecosystems can alter biochemical soil properties and soil processes that underpin ecosystem structure and functioning, thus threatening native plant communities and the species that rely on them. However, there is much uncertainty about the magnitude of change as soils are exposed to more fires, because soil recovery and changes in fire severity following a first fire mediate the impact of successive fires on soil properties. With this study we aim to evaluate how increased fire frequency affects soil biochemical properties (i.e. soil pH, soil organic matter (SOM), soil organic carbon (SOC), soil structure and mineral N) and processes (i.e. microbial and enzymatic activity) in a sagebrush-steppe ecosystem located in the Columbia Plateau Ecoregion, Washington, USA. During 2016, we collected soils from once (2012), twice (2003 and 2012), and thrice (2003, 2007, and 2012) burned areas, enabling us to test the hypothesis that increasing fire frequency will exacerbate the impact of fire on soil properties and processes. Our study yielded three main results: (1) fire reduced the total soil C concentration and soil C in aggregates relative to unburned soil, but only when soil was exposed to fire once (i.e. the most recent fire), (2) compared to the unburned soils, SOM contents, enzyme activity and microbial CO2 respiration were suppressed in the once and thrice burned soils, but not in the twice burned soils, and (3) fire increased NO3−-N contents across the once and twice burned sites, and reduced enzyme activity associated with N cycling in the thrice burned sites. Taken together, our findings suggest that a one-time fire in this shrub dominated semi-arid ecosystem significantly changes soil biochemical attributes and microbially driven processes. With sufficient time between fires, these structural and functional properties can partially recover, and this may persist even after a second fire, but recovery is limited when a third fire creates an additional disturbance at a shorter time interval. Furthermore, while soil C pools and microbial decomposition processes were able to recover with sufficient time, greater soil resource availability prevailed in soil across all fire frequencies, indicating that fire is likely to promote invasion and reduce ecosystem stability, even when other soil properties recover.

Changing precipitation regimes in semiarid ecosystems will affect the balance of soil carbon (C) input and release, but the net effect on soil C storage is unclear. We asked how changes in the amount and timing of precipitation affect litter decomposition, and soil C stabilization in semiarid ecosystems. The study took place at a long-term (18 years) ecohydrology experiment located in Idaho. Precipitation treatments consisted of a doubling of annual precipitation (+200 mm) added either in the cold-dormant season or in the growing season. Experimental plots were planted with big sagebrush (Artemisia tridentata), or with crested wheatgrass (Agropyron cristatum). We quantified decomposition of sagebrush leaf litter, and we assessed organic soil C (SOC) in aggregates, and silt and clay fractions. We found that: (1) increased precipitation applied in the growing season consistently enhanced decomposition rates relative to the ambient treatment, and (2) precipitation applied in the dormant season enhanced soil C stabilization. These data indicate that prolonged increases in precipitation can promote soil C storage in semiarid ecosystems, but only if these increases happen at times of the year when conditions allow for precipitation to promote plant C inputs rates to soil.

Soil food webs play a key role in the cycling of carbon and nutrients and in sustainably provisioning ecosystem services. Despite the tremendous diversity of organisms that soil food webs harbor, we still know surprisingly little about the role of biodiversity in influencing the processes and services provided by soil food webs. To guide future research in this area, we outline a conceptual framework linking hierarchical levels of soil biodiversity to ecosystem processes and services. Here, we distinguish among different hierarchical levels of diversity: trophic, functional, taxonomic and genetic diversity. We conclude that the levels of food web diversity that matter most vary with the processes or services considered, with functional trait diversity being the most universally influential level of diversity. Increased research emphasis on manipulating diversity across hierarchical levels of biodiversity organization, with an explicit focus on the functional role of the component species, is critical for enhancing our understanding of the role of soil food web diversity in driving ecosystem processes and services.

Environmental perturbations such as agricultural intensification may alter soil biodiversity in a manner that affects ecosystem functioning, but links are not well quantified. With this review we ask: (1) “How does agricultural intensification affect soil biodiversity?” and (2) “How do such changes in soil biodiversity affect ecosystem function?” We used meta-analysis to quantify responses across studies. Our results indicate that agricultural intensification can significantly alter soil biodiversity, with negative impacts of synthetic N fertilization on arbuscular mycorrhizal fungal (AMF) and faunal diversity, and positive effects on fungal- and microbial functional diversity. Bacterial diversity increased with low synthetic N input rates ( 5 years, suggesting that agricultural management practices that promote soil organic matter (SOM) accumulation and retention enhance bacterial biodiversity. Tillage negatively impacted soil faunal and bacterial diversity, but did not affect AMF, fungal or functional diversity, and organic farming relative to conventional farming did not affect soil biodiversity. Biodiversity manipulation studies indicate that changes in soil biodiversity affect ecosystem process rates, although manipulated biodiversity levels tend to exaggerate biodiversity losses and possibly overestimate consequences for ecosystem functioning relative to measured biodiversity losses from environmental perturbations. There is a need for more studies that evaluate how losses in soil biodiversity following environmental perturbations directly affect ecosystem functioning. Advances in analytical techniques to identify soil organisms and an increase in soil biodiversity manipulation experiments should help solidify links between environmental changes, soil biodiversity and ecosystem functioning.

Switchgrass (Panicum virgatum L.), a cellulosic biofuel feedstock, may promote soil C accumulation compared to annual cropping systems by increasing the amount and retention of root-derived soil C inputs. However, these inputs and stabilization thereof may differ by cultivar, and it is uncertain which root traits favor soil C input and stabilization rates. The aim of this study was to assess how different switchgrass cultivars impact soil C inputs and retention, whether these impacts vary with depth, and whether specific root length (SRL) explains these impacts. We collected soil to a depth of 30 cm (10 cm increments) from six switchgrass cultivars with root systems ranging from high to low SRL. The cultivars (C4 species) were grown for 27 months on soils previously dominated by C3 plants, allowing us to quantify both total C and switchgrass-derived C accumulation in the bulk soil and in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), silt-sized, and clay-sized fractions. The study led to two main results: (1) bulk soil C concentrations beneath switchgrass cultivars varied by 40% in the 0–10 cm soil depth and by 70% in the 10–20 cm soil depth, and cultivars with high bulk soil C concentrations tended to have relatively high C concentrations in the mineral soil fractions and relatively low C concentrations in the POM fractions; (2) there were significant differences in switchgrass-derived soil C between cultivars at the 0–10 cm depth, where soil C inputs ranged from 1.2 to 3.2 mg C g− 1 dry soil. In addition, switchgrass-derived C was positively related to SRL when one outlier data point was removed. These results suggest that switchgrass cultivars differentially impact mechanisms contributing to soil C accumulation.

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained

Land-use change for bioenergy production can release greenhouse gases (GHG) through disturbance of soil carbon (C) pools, but use of native species with extensive root systems as bioenergy crops may help mitigate GHG emissions by enhancing soil C sequestration. Here, we investigated how (1) fertilization, (2) plant species and cultivars, and (3) inter- and intra-specific diversity affect soil C and N accumulation five growing seasons after conversion of an old-field dominated by C3 grasses to a grassland dominated by C4 perennial grasses managed for biofuel production. We manipulated diversity at both the species- and cultivar level, and applied nitrogen (N) at two levels (0 and 67 kg ha−1). Establishment of C4 grass treatments on soils that supported C3 pasture grasses for 36 years enabled us to use the natural abundance C isotope ratio technique to estimate the contribution of new C4 plant-derived C to soil organic matter pools. Our study yielded three main results: 1) annual fertilization did not significantly affect soil C and N concentrations after five growing seasons; 2) increasing inter- and intra-specific diversity did not significantly increase soil C and N concentrations; 3) cultivar- and species identity influenced C4-derived C and total soil C concentrations: big bluestem dominated stands exhibited greater soil C accrual relative to stands dominated by switchgrass and mixed-species treatments. Future research is needed to further assess how big bluestem can aid in the sustainable provisioning of second generation biofuel feedstocks.

Semi-arid and arid ecosystems dominated by shrubs (“dry shrublands”) are an important component of the global C cycle, but impacts of climate change and elevated atmospheric CO2 on biogeochemical cycling in these ecosystems have not been synthetically assessed. This study synthesizes data from manipulative studies and from studies contrasting ecosystem processes in different vegetation microsites (that is, shrub or herbaceous canopy versus intercanopy microsites), to assess how changes in climate and atmospheric CO2 affect biogeochemical cycles by altering plant and microbial physiology and ecosystem structure. Further, we explore how ecosystem structure impacts on biogeochemical cycles differ across a climate gradient. We found that: (1) our ability to project ecological responses to changes in climate and atmospheric CO2 is limited by a dearth of manipulative studies, and by a lack of measurements in those studies that can explain biogeochemical changes, (2) changes in ecosystem structure will impact biogeochemical cycling, with decreasing pools and fluxes of C and N if vegetation canopy microsites were to decline, and (3) differences in biogeochemical cycling between microsites are predictable with a simple aridity index (MAP/MAT), where the relative difference in pools and fluxes of C and N between vegetation canopy and intercanopy microsites is positively correlated with aridity. We conclude that if climate change alters ecosystem structure, it will strongly impact biogeochemical cycles, with increasing aridity leading to greater heterogeneity in biogeochemical cycling among microsites. Additional long-term manipulative experiments situated across dry shrublands are required to better predict climate change impacts on biogeochemical cycling in deserts.

Enhanced root-exudate inputs can stimulate decomposition of soil carbon (C) by priming soil microbial activity, but the mechanisms controlling the magnitude and direction of the priming effect remain poorly understood. With this study we evaluated how differences in soil C availability affect the impact of simulated root exudate inputs on priming. We conducted a 60-day laboratory incubation with soils collected (60 cm depth) from under six switchgrass (Panicum virgatum) cultivars. Differences in specific root length (SRL) among cultivars were expected to result in small differences in soil C inputs and thereby create small differences in the availability of recent labile soil C; whereas soil depth was expected to create large overall differences in soil C availability. Soil cores from under each cultivar (roots removed) were divided into depth increments of 0–10, 20–30, and 40–60 cm and incubated with addition of either: (1) water or (2) 13C-labeled synthetic root exudates (0.7 mg C/g soil). We measured CO2 respiration throughout the experiment. The natural difference in 13C signature between C3 soils and C4 plants was used to quantify cultivar-induced differences in soil C availability. Amendment with 13C-labeled synthetic root-exudate enabled evaluation of SOC priming. Our experiment produced three main results: (1) switchgrass cultivars differentially influenced soil C availability across the soil profile; (2) small differences in soil C availability derived from recent root C inputs did not affect the impact of exudate-C additions on priming; but (3) priming was greater in soils from shallow depths (relatively high total soil C and high ratio of labile-to-stable C) compared to soils from deep depths (relatively low total soil C and low ratio of labile-to-stable C). These findings suggest that the magnitude of the priming effect is affected, in part, by the ratio of root exudate C inputs to total soil C and that the impact of changes in exudate inputs on the priming of SOC is regulated differently in surface soil compared to subsoil.

Roots regulate soil carbon (C) input, but fine root decomposition rates and root impacts on soil organic C turnover (SOC) are uncertain. This uncertainty is, partly, caused by the heterogeneity of root systems, which vary in diameter distributions and tissue chemistry. Here, we evaluated how root diameter distributions affect root and SOC decomposition. Roots from eight Panicum virgatum (switchgrass) cultivars were analyzed for root diameter size-class distribution and C:N ratio. Roots from each cultivar were mixed with C 3 soil according to five root diameter treatments: (1) 0–0.5 mm, (2) 0.5–1 mm, (3) 1–2.5 mm, (4) a 1:1:1 mixture of roots from each diameter size class, and (5) a mixture combining diameter classes in proportions representing measured size distributions for each cultivar. All treatments were incubated for 90 days under laboratory conditions. Respired CO 2 was measured throughout and the microbial community structure was measured at termination of the experiment. Carbon-13 isotope techniques were used to partition respiration into root-derived C versus native SOC-derived C. Results indicated: (1) specific root length differed among the cultivars, (2) root decomposition rates within the three size classes varied by cultivar, but were not correlated with cultivar differences in root C:N ratios, (3) root diameter size class affected root and SOC decomposition, and (4) mixing roots of different diameters did not lead to synergistic increases in decomposition. We conclude that intraspecific variation in root architecture is significant and that fine root diameter size class distribution is an important trait for shaping decomposition processes.

Changes in plant species diversity can result in synergistic increases in decomposition rates, while elevated atmospheric CO2 can slow the decomposition rates; yet it remains unclear how diversity and changes in atmospheric CO2 may interact to alter root decomposition. To investigate how elevated CO2 interacts with changes in root-litter diversity to alter decomposition rates, we conducted a 120-day laboratory incubation. Roots from three species (Trifolium repens, Lespedeza cuneata, and Festuca pratense) grown under ambient or elevated CO2 were incubated individually or in combination in soils that were exposed to ambient or elevated CO2 for five years. Our experiment resulted in two main findings: (1) Roots from T. repens and L. cuneata, both nitrogen (N) fixers, grown under elevated CO2 treatments had significantly slower decomposition rates than similar roots grown under ambient CO2 treatments; but the decomposition rate of F. pratense roots (a non-N-fixing species) was similar regardless of CO2 treatment. (2) Roots of the three species grown under ambient CO2 and decomposed in combination with each other had faster decomposition rates than when they were decomposed as single species. However, roots of the three species grown under elevated CO2 had similar decomposition rates when they were incubated alone or in combination with other species. These data suggest that if elevated CO2 reduces the root decomposition rate of even a few species in the community, it may slow root decomposition of the entire plant community.

Summary • Root carbon (C) inputs may regulate decomposition rates in soil, and in this study we ask: how do labile C inputs regulate decomposition of plant residues, and soil microbial communities? • In a 14 d laboratory incubation, we added C compounds often found in root exudates in seven different concentrations (0, 0.7, 1.4, 3.6, 7.2, 14.4 and 21.7 mg C g )1 soil) to soils amended with and without 13 C-labeled plant residue. We measured CO2 respiration and shifts in relative fungal and bacterial rRNA gene copy numbers using quantitative polymerase chain reaction (qPCR). • Increased labile C input enhanced total C respiration, but only addition of C at low concentrations (0.7 mg C g )1 ) stimulated plant residue decomposition (+2%). Intermediate concentrations (1.4, 3.6 mg C g )1 ) had no impact on plant residue decomposition, while greater concentrations of C (> 7.2 mg C g )1 ) reduced decomposition ()50%). Concurrently, high exudate concentrations (> 3.6 mg C g )1 ) increased fungal and bacterial gene copy numbers, whereas low exudate concentrations (< 3.6 mg C g )1 ) increased metabolic activity rather than gene copy numbers. • These results underscore that labile soil C inputs can regulate decomposition of more recalcitrant soil C by controlling the activity and relative abundance of fungi and bacteria.

Soil is the largest reservoir of organic carbon (C) in the terrestrial biosphere and soil C has a relatively long mean residence time. Rising atmospheric carbon dioxide (CO2) concentrations generally increase plant growth and C input to soil, suggesting that soil might help mitigate atmospheric CO2 rise and global warming. But to what extent mitigation will occur is unclear. The large size of the soil C pool not only makes it a potential buffer against rising atmospheric CO2, but also makes it difficult to measure changes amid the existing background. Meta-analysis is one tool that can overcome the limited power of single studies. Four recent meta-analyses addressed this issue but reached somewhat different conclusions about the effect of elevated CO2 on soil C accumulation, especially regarding the role of nitrogen (N) inputs. Here, we assess the extent of differences between these conclusions and propose a new analysis of the data. The four meta-analyses included different studies, derived different effect size estimates from common studies, used different weighting functions and metrics of effect size, and used different approaches to address nonindependence of effect sizes. Although all factors influenced the mean effect size estimates and subsequent inferences, the approach to independence had the largest influence. We recommend that meta-analysts critically assess and report choices about effect size metrics and weighting functions, and criteria for study selection and independence. Such decisions need to be justified carefully because they affect the basis for inference. Our new analysis, with a combined data set, confirms that the effect of elevated CO2 on net soil C accumulation increases with the addition of N fertilizers. Although the effect at low N inputs was not significant, statistical power to detect biogeochemically important effect sizes at low N is limited, even with meta-analysis, suggesting the continued need for long-term experiments.

Elevated CO2 may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO2-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO2. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to 13C labeling at ambient (392 μmol mol−1) and elevated (792 μmol mol−1) CO2 concentrations, in soil containing 15N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-13C was enhanced by 50% under elevated compared to ambient CO2. Concurrently, plant 15N uptake increased (+7%) under elevated CO2 while 15N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO2. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO2 can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO2.

Elevated atmospheric CO2 frequently in- creases plant production and concomitant soil C inputs, which may cause additional soil C sequestra- tion. However, whether the increase in plant produc- tion and additional soil C sequestration under elevated CO2 can be sustained in the long-term is unclear. One approach to study C-N interactions under elevated CO2 is provided by a theoretical framework that centers on the concept of progressive nitrogen limitation (PNL). The PNL concept hinges on the idea that N becomes less available with time under elevated CO2. One possible mechanism underlying this reduction in N availability is that N is retained in long-lived soil organic matter (SOM), thereby limit- ing plant production and the potential for soil C sequestration. The long-term nature of the PNL concept necessitates the testing of mechanisms in field experiments exposed to elevated CO2 over long periods of time. The impact of elevated CO2 and 15 N fertilization on L. perenne and T. repens monocultures has been studied in the Swiss FACE experiment for ten consecutive years. We applied a biological fractionation technique using long-term incubations with repetitive leaching to determine how elevated CO2 affects the accumulation of N and C into more stable SOM pools. Elevated CO2 significantly stimu- lated retention of fertilizer-N in the stable pools of the soils covered with L. perenne receiving low and high N fertilization rates by 18 and 22%, respectively, and by 45% in the soils covered by T. repens receiving the low N fertilization rate. However, elevated CO2 did not significantly increase stable soil C formation. The increase in N retention under elevated CO2 provides direct evidence that elevated CO2 increases stable N formation as proposed by the PNL concept. In the Swiss FACE experiment, however, plant production increased under elevated CO2, indicating that the additional N supply through fertilization prohibited PNL for plant production at this site. Therefore, it remains unresolved why elevated CO2 did not increase labile and stable C accumulation in these systems.

The perennial grass species that are being developed as biomass feedstock crops harbor extensive genotypic diversity, but the effects of this diversity on biomass production are not well understood. We investigated the effects of genotypic diversity in switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) on perennial biomass cropping systems in two experiments conducted over 2008–2014 at a 5.4‐ha fertile field site in northeastern Illinois, USA. We varied levels of switchgrass and big bluestem genotypic diversity using various local and nonlocal cultivars – under low or high species diversity, with or without nitrogen inputs – and quantified establishment, biomass yield, and biomass composition. In one experiment (‘agronomic trial’), we compared three switchgrass cultivars in monoculture to a switchgrass cultivar mixture and three different species mixtures, with or without N fertilization. In another experiment (‘diversity gradient’), we varied diversity levels in switchgrass and big bluestem (1, 2, 4, or 6 cultivars per plot), with one or two species per plot. In both experiments, cultivar mixtures produced yields equivalent to or greater than the best cultivars. In the agronomic trial, the three switchgrass mixture showed the highest production overall, though not significantly different than best cultivar monoculture. In the diversity gradient, genotypic mixtures had one‐third higher biomass production than the average monoculture, and none of the monocultures were significantly higher yielding than the average mixture. Year‐to‐year variation in yields was lowest in the three‐cultivar switchgrass mixtures and Cave‐In‐Rock (the southern Illinois cultivar) and also reduced in the mixture of switchgrass and big bluestem relative to the species monocultures. The effects of genotypic diversity on biomass composition were modest relative to the differences among species and genotypes. Our findings suggest that local genotypes can be included in biomass cropping systems without compromising yields and that genotypic mixtures could help provide high, stable yields of high‐quality biomass feedstocks.

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 (12.2%yr � 1 ) and above- and belowground plant growth (120.1% and 133.7%) 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. Nomenclature FACE 5 free air carbon dioxide enrichment; OTC 5 open top chamber; SOM 5 soil organic matter; SOC 5 soil organic carbon

Soil C sequestration may mitigate rising levels of atmospheric CO2. However, it has yet to be determined whether net soil C sequestration occurs in N-rich grasslands exposed to long-term elevated CO2. This study examined whether N-fertilized grasslands exposed to elevated CO2 sequestered additional C. For 10 years, Lolium perenne, Trifolium repens, and the mixture of L. perenne/T. repens grasslands were exposed to ambient and elevated CO2 concentrations (35 and 60Pa pCO2). The applied CO2 was depleted in d 13 C and the grasslands received low (140kgha � 1 ) and high (560kgha � 1 ) rates of 15 N-labeled fertilizer. Annually collected soil samples from the top 10cm of the grassland soils allowed us to follow the sequestration of new C in the surface soil layer. For the first time, we were able to collect dual-labeled soil samples to a depth of 75cm after 10 years of elevated CO2 and determine the total amount of new soil C and N sequestered in the whole soil profile. Elevated CO2, N-fertilization rate, and species had no significant effect on total soil C. On average 9.4Mg new Cha � 1 was sequestered, which corresponds to 26.5% of the total C. The mean residence time of the C present in the 0‐10cm soil depth was calculated at 4.6 � 1.5 and 3.1 � 1.1 years for L. perenne and T. repens soil, respectively. After 10 years, total soil N and C in the 0‐75cm soil depth was unaffected by CO2 concentration, Nfertilization rate and plant species. The total amount of 15 N-fertilizer sequestered in the 0‐75cm soil depth was also unaffected by CO2 concentration, but significantly more 15 N was sequestered in the L. perenne compared with the T. repens swards: 620 vs. 452kgha � 1 at the high rate and 234 vs. 133kgha � 1 at the low rate of N fertilization. Intermediate values of 15 N recovery were found in the mixture. The fertilizer derived N amounted to 2.8% of total N for the low rate and increased to 8.6% for the high rate of N application. On average, 13.9% of the applied 15 N-fertilizer was recovered in the 0‐75cm soil depth in soil organic matter in the L. perenne sward, whereas 8.8% was recovered under the T. repens swards, indicating that the N2-fixing T. repens system was less effective in sequestering applied N than the non-N2-fixing L. perenne system. Prolonged elevated CO2 did not lead to an increase in whole soil profile C and N in these fertilized pastures. The potential use of fertilized and regular cut pastures as a net soil C sink under longterm elevated CO2 appears to be limited and will likely not significantly contribute to the mitigation of anthropogenic C emissions.

Rising levels of atmospheric CO 2 are thought to increase C sinks in terrestrial ecosystems. The potential of these sinks to mitigate CO 2 emissions, however, may be constrained by nutrients. By using metaanalysis, we found that elevated CO 2 only causes accumulation of soil C when N is added at rates well above typical atmospheric N inputs. Similarly, elevated CO 2 only enhances N 2 fixation, the major natural process providing soil N input, when other nutrients (e.g., phosphorus, molybdenum, and potassium) are added. Hence, soil C sequestration under elevated CO 2 is constrained both directly by N availability and indirectly by nutrients needed to support N 2 fixation.

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 13 C-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.

Summary • With this study, we aimed to determine how elevated CO2 affects rhizodeposition and the cycling of rhizodeposited nitrogen (N) in the soil under C3 and C4 plants. In addition, we examined how cultivated genotypes of wheat (Triticum turgidum) and maize (Zea mays) responded to elevated CO2 in comparison with their wild relatives. • By constructing an N-transfer experiment we could directly assess cycling of the rhizodeposited N and trace the fate of rhizodeposited N in the soil and in receiver plants. • Biomass production, rhizodeposition and cycling of root-borne N in maize genotypes were not affected by elevated CO2. Elevated CO2 stimulated above- and below-ground biomass production of the wheat genotypes on average by 38%, and increased rhizodeposition and immobilization of root-derived N on average by 30%. Concurrently, elevated CO2 reduced mineral 15N and re-uptake of the root-derived N by 50% in wheat. • This study shows that elevated CO2 may enhance N limitation by increasing N rhizodeposition and subsequent immobilization of the root-derived N.

Prolonged elevated atmospheric CO2 might alter decomposition. In a 90-day incubation study, we determined the long-term (9 years) impact of elevated CO2 on N mineralization of Lolium perenne and Trifolium repens plant material grown at ambient and elevated CO2 and low- and high-15N fertilizer additions. No significant differences were observed in N 15 – NO 3 − recovery rates between any of the treatments, except an N addition effect was observed for L. perenne (0.4 versus 0.5% N 15 – NO 3 − day−1 in high versus low N). The results suggest that elevated CO2 did not change plant N mineralization in any of the soils, because of a surplus of available N in the fertilized and leguminous systems, and because of insignificant plant responses to elevated CO2 in the low soil N availability systems.