Soil heterotrophic respiration (Rh) is a key process in the global carbon (C) cycle, strongly influenced by interactions between soil temperature and moisture. It is well documented how soil temperature regulates Rh. However, temperature interacts with soil moisture to regulate Rh via dynamic and complex microbial mechanisms, leading to uncertainties in Rh prediction under altered precipitation. To address this knowledge gap, we conducted a field-based rainfall manipulation experiment to investigate the effects of contrasting rainfall treatments (wet vs. dry) on Rh in root-free soil across multiple seasonal campaigns. We then used the Dual Arrhenius and Michaelis-Menten (DAMM) model to simulate Rh dynamics under both rainfall treatments. Our results showed that, on average, Rh was 13% higher in the wet treatment (mean: 1.32 mu mol m-2 s-1 (range: 0.53, 2.37)) compared to the dry treatment (mean: 1.17 mu mol m-2 s-1 (range: 0.40, 3.19)) across all campaigns. Soil temperature was the primary driver of Rh when sufficient soil moisture was available to support microbial activity. Soil temperature alone explained 46% of the variation in Rh under the wet treatment and 41% under dry. The DAMM model captured seasonal and treatment-driven variation better in the wet than the dry treatment, explaining 80% and 42% of the variance in Rh, respectively. Model parameterization identified an intrinsic temperature sensitivity (EaCO2) for Rh of 72.02 kJ mol-1, with higher microbial basal respiration (alpha CO2) values in wet soils, indicating enhanced microbial respiration under favorable moisture conditions. Our results demonstrate that shifts in rainfall regimes alter microbial respiration responses through complex moisture-temperature interactions, which can be reliably captured using the DAMM model. This highlights the model's value for improving soil C feedback predictions under future climate scenarios.

As climate extremes intensify, interactions among environmental drivers are expected to alter soil respiration (SR) and its response to rewetting, increasing uncertainty in carbon-climate feedbacks. However, the interactive effects of drought and warming, two commonly studied climate stressors, on SR remain elusive due to limited research and a lack of high-resolution data. This study investigated overall SR (SRoverall, encompassing both drying and rewetting phases) and rewetting-induced respiration pulses (SRpulse), along with their apparent sensitivity to temperature and moisture under factorial combinations of rainfall and warming treatments in a field-based climate-manipulation experiment conducted in a temperate pasture system in southeastern Australia. Rainfall extremes were derived from 30 years of regional climate records, while warming was imposed as a continuous, year-round increase of +3°C. An automated flux monitoring system was deployed to measure hourly SR across eight campaigns from October 2023 to November 2024. The drivers of SRoverall and SRpulse were identified by analyzing climatic variables together with soil parameters from rhizosphere and non-rhizosphere zones. Drought and warming consistently suppressed SRoverall but amplified SRpulse and moisture sensitivity. The interactive effects of both treatments on SRoverall varied seasonally, shifting from additive in spring to antagonistic in summer and autumn, and synergistic in winter. Drought suppressed apparent temperature sensitivity (Q10), but warming effects on Q10 varied with moisture conditions. Soil temperature, moisture, and extractable C:N ratio from across rhizosphere and non-rhizosphere soil were consistent predictors of SRoverall and SRpulse but exerted opposing effects on the two components. These findings advance our understanding of how drought-warming interactions shape both overall and pulse-driven SR, providing a process-based foundation for improving predictions of carbon-climate feedbacks under intensifying climate extremes.

Phosphorus (P) limitation may constrain the capacity of forests to remove CO2 from the atmosphere under elevated CO2 (eCa). One key mechanism is that limited P availability requires plants to invest additional fixed carbon belowground, which then returns to the atmosphere via soil respiration. However, the extent and pathways of this response remain unclear. We quantified total soil respiration and inferred its component fluxes before and after alleviating P limitation via fertilization in an established eCa experiment in mature Eucalyptus woodland. eCa increased soil respiration by 22.5% regardless of P availability. Dependence of the stimulation on soil moisture indicated it was driven primarily by root-derived respiration rather than soil organic matter-derived heterotrophic respiration. Fine-root production was stimulated by eCa only after P addition, suggesting that P fertilization triggered a shift from rhizomicrobial respiration, associated with microbial use of root exudates and residues, to autotrophic respiration from root growth and metabolism. These findings indicate that within 9 months of P addition, EucFACE plants may have shifted P-acquisition strategies under eCa. As the first ecosystem-scale study on the interactive effect of eCa and P limitation, this research improves mechanistic understanding of short-term belowground responses and provides insight to motivate follow-up studies.

Changes in the distribution of soil organic carbon (SOC) fractions - particulate organic carbon (POC; unprotected carbon) vs. mineral-associated organic carbon (MAOC; protected carbon) - affect SOC storage and stability. Here, we compile a SOC fraction dataset from 7219 soil samples across six continents. From 2000 to 2022, POC increases by 21.8% while MAOC decreases by 5.3%, leading to a net gain of 11.0% in SOC storage and a 29.1% increase in the POC/MAOC ratio. However, relative to undisturbed natural ecosystems, SOC declines in planted forests, grazed grasslands, and croplands with management practices such as heavy grazing and conventional tillage, primarily due to rapid declines in POC. Our results highlight that the global increase in SOC is mainly driven by POC, but warn of decreasing SOC stability associated with climate change and human activities. Thus, maintaining soil carbon sinks requires targeted strategies focusing on POC.

AimTo compare field-based evidence of plant and animal responses to fire with remotely sensed signals of fire heterogeneity and post-fire biomass recovery.LocationSouth-eastern Australia; New South Wales.Time Period2019-2022.Major Taxa StudiedA total of 982 species of plants and animals, in eight taxonomic groups: amphibians, birds, fish, insects, mammals, molluscs, plants and reptiles.MethodsWe collated 545,223 plant and animal response records from 47 field surveys of 4613 sites that focussed on areas burnt in 2019-2020. For each site, we calculated remotely sensed signals of fire heterogeneity and post-fire biomass recovery, including the delayed recovery index. Meta-regression analyses were conducted separately for species that declined after fire (negative effect sizes) and species that increased after fire (positive effect sizes) for each buffer size (250 m, 500 m, 1 km, 1.5 km, 2 km and 2.5 km radius).ResultsWe found that species exposed to homogenous high-severity fire (i.e., low fire heterogeneity) were more likely to exhibit decreased abundance/occurrence or inhibited recovery. Areas with delayed recovery of biomass also had significant negative on-ground responses, with lower abundance or occurrence in areas where biomass recovery was slower.Main ConclusionsThe fire heterogeneity index and the delayed recovery index are suitable for inclusion in monitoring and reporting systems for tracking relative measures over time, particularly when field survey data is not available at the landscape scales required to support reporting and management decisions. Locations with remotely sensed signals of delayed recovery should be prioritised for protection against further disturbances that may interfere with the recovery process. Research attention must next focus on how cumulative fire heterogeneity patterns of successive fires affect the post-fire recovery dynamics to further inform the application of remote sensing indicators as management tools for biodiversity conservation.

Livestock grazing contributes to greenhouse gas (GHG) emissions, soil degradation and erosion, and loss of biodiversity. Regenerative pasture management includes improvements such as sowing high-diversity seed mixtures with legumes and other deep-rooted forbs in addition to C3 and C4 grasses, alternating intensive grazing with rest periods, bio-based fertilizers, etc. These improvements may alleviate degradation and restore multiple ecosystem services, including soil carbon sequestration and heat wave mitigation. Predictive understanding of management impacts requires process-based models that accurately simulate herbaceous growth and allocation in response to grazing and irrigation events. Moreover, accurate and timely model forecasts depend on well-validated data collected at appropriate temporal and spatial scales, delivered with low latency. We used four years of eddy covariance data in combination with vegetation indices and a process-based model to improve estimates of Net Ecosystem Production (NEP) and energy balance in response to livestock and wildlife grazing in an area with fluctuating soil moisture availability. The enhanced vegetation index (EVI) for the degraded pasture, grazed mainly by native wildlife (kangaroos), demonstrated wide seasonal variations of 0.2 to 0.6, whereas EVI was maintained more consistently close to 0.5 for an improved pasture, grazed intermittently by cattle or sheep. Across three wet years, NEP for the improved pasture averaged 12% higher compared to the degraded one (153 vs. 137 g C m-2 y-1), associated with average 20% greater gross primary production (GPP; 1822 vs. 1521 g C m-2 y-1). However, NEP on the improved pasture was lower than on the degraded pasture in two of those three years, possibly due to grazing-related differences in biomass removal. Sensible heat fluxes were higher from the degraded pasture, especially during hot/dry periods. Ongoing analyses are evaluating soil C storage for benchmarking flux data. Model predictions are also being improved by validating representation of productivity by C3 and C4 species and carbon allocation to roots and crowns. This work contributes to enhancing environmental sustainability in managed grasslands with near-real-time forecasting ability for grazing and irrigation management.

Soils represent both a source of and sink for greenhouse gases (GHG). Elevated temperature (eT) affects both the physical and biological factors that drive GHG emissions from soil and thus understanding the effects of rising global temperatures on terrestrial GHG emission is needed to predict future GHG emissions, and to identify mitigation strategies. However, uncertainty remains about the interactive effects of multiple climate factors across different ecosystems, complicating our ability to develop robust climate change projections. Therefore, a global meta-analysis of 1337 pairwise observations from 150 peer-reviewed publications (1990-2023) was conducted to assess the individual effect of eT and its combined effects with eCO2 (eT + eCO2), drought (eT + drought) and increased precipitation (eT + ePPT) on soil N2O and CH4 fluxes, microbial functional genes, and soil extracellular enzyme activities across grassland, cropland, and forestland ecosystems. Across the dataset, eT significantly increased N2O emissions (21%) and CH4 uptake (36%). Nitrogen cycling was consistently stimulated by eT, with NO3- and NH4+ and the abundance of amoA-AOB gene increasing by 6%, 10%, and 18%, respectively. Soil water content (SWC) was reduced, whereas increases of 9% in soil organic carbon (SOC), 14% in microbial biomass carbon (MBC), and 10% in total plant biomass were found under eT. The stimulation of soil N2O emissions by eT was maintained for all ecosystems when combined with other global change factors (ie., eT + eCO2, eT + ePPT, and eT + drought). By contrast, effects of eT on CH4 uptake and emissions were more variable when combined with other factors; for instance, eT + eCO2 and eT + ePPT suppressed CH4 uptake in grasslands. This study highlights the urgent need to study the microbial mechanisms responsible for combined global change effects on N2O and especially CH4 fluxes.

Soil provides multiple, diverse functions, with these underpinning both planetary and human health. For planetary health, soil contributes to multiple critical processes, including through biomass production, by regulating the carbon pool, providing a habitat for 25% of global biodiversity, cycling the nutrients upon which terrestrial systems depend, and cycling water. Soil also underpins human health; humans use soil to provide 98.8% of our food and sustain our nutrition, regulate pathogens, and supply medicines. However, humans have tended to focus on soil almost solely for producing biomass (food, fiber, and energy) through intensive agriculture, and this narrow focus now causes rapid soil degradation, including through loss of soil organic matter, erosion, and salinization. This degradation directly harms planetary health and reduces the ability of soil to support health of future human generations. We argue that a healthy soil is a soil that is multifunctional and is capable of underpinning human and planetary health. Using this definition, a broad conceptual framework is provided for quantifying soil health, with such an approach enabling a shift in the way that we think about, plan, and manage systems to ensure ongoing planetary and human health.

The forest–atmosphere exchange of carbon and water is regulated by meteorological conditions as well as canopy properties such as leaf area index (LAI, m2 m−2), photosynthetic capacity (PC μmol m−2 s−1), or surface conductance in optimal conditions (Gs,opt, mmol m−2 s−1), which can vary seasonally and inter-annually. This variability is well understood for deciduous species but is poorly characterized in evergreen forests. Here, we quantify the seasonal dynamics of a temperate evergreen eucalypt forest with estimates of LAI, litterfall, carbon and water fluxes, and meteorological conditions from measurements and model simulations. We merged MODIS Enhanced Vegetation Index (EVI) values with site-based LAI measurements to establish a 17-year sequence of monthly LAI. We ran the Community Atmosphere Biosphere Land Exchange model (CABLE-POP (version r5046)) with constant and varying LAI for our site to quantify the influence of seasonal canopy dynamics on carbon and water fluxes. We observed that the peak of LAI occurred in late summer–early autumn, with a higher and earlier peak occurring in years when summer rainfall was greater. Seasonality in litterfall and allocation of net primary productivity (FNPP) to leaf growth (af, 0–1) drove this pattern, suggesting a complete renewal of the canopy before the timing of peak LAI. Litterfall peaked in spring, followed by a high af in summer, at the end of which LAI peaked, and PC and Gs,opt reached their maximum values in autumn, resulting from a combination of high LAI and efficient mature leaves. These canopy dynamics helped explain observations of maximum gross ecosystem production (FGEP) in spring and autumn and net ecosystem carbon loss in summer at our site. Inter-annual variability in LAI was positively correlated with Net Ecosystem Production (FNEP). It would be valuable to apply a similar approach to other temperate evergreen forests to identify broad patterns of seasonality in leaf growth and turnover. Because incorporating dynamic LAI was insufficient to fully capture the dynamics of FGEP, observations of seasonal variation in photosynthetic capacity, such as from solar-induced fluorescence, should be incorporated in land surface models to improve ecosystem flux estimates in evergreen forests.

Global agricultural systems face one of the greatest sustainability challenges: meeting the growing demand for food without leaving a negative environmental footprint. United States (US) and China are the two largest economies and account for 39 % of total global greenhouse gases (GHG) emissions into the atmosphere. No-till is a promising land management option that allows agriculture to better adapt and mitigate climate change effects compared to traditional tillage. However, the efficacy of no-till for mitigating GHG is still debatable. In this metaanalysis, we comprehensively assess the impact of no-till (relative to traditional tillage) on GHG mitigation potential and crop productivity in different agroecological systems and management regimes in the US and China. Overall, no-till in China did not change crop yields, although soil CO2 (8 %) and N2O (12 %) emissions decreased significantly, while soil CH4 emissions increased by 12 %. In contrast to Chinese no-till, a significant improvement in crop yields (up to 12 %) was recorded on US cropland under no -till. Moreover, significant decreases in soil N2O (21 %) and CH4 (12 %) emissions were observed. Of the three cropping systems, only wheat showed significant reduction in CO2, N2O and CH4 emissions in the Chinese no -till system. In the case of US, notill soybean -rice and maize cropping systems demonstrated significant emission reductions for N2O and CO2, respectively. Interestingly, yields of no -till maize in China and rice in US exceeded those of other no -till cereals. In China, no -till on medium -texture soils resulted in significant reductions in GHG emissions and higher crop yields compared to other soil types. In both countries, the relatively higher crop yields under irrigated versus non -irrigated no -till and the significant yield differences on fine textured soils under US no -till are likely due to the substantial N2O reductions. In summary, crop yield disparities from no -till between China and the US were related to the insignificant effects on controlling CH4 emissions and successfully mitigating N2O, respectively. This study comprehensively demonstrates how cropping system and pedoclimatic conditions influence the relative effectiveness of no -till in both countries.

Patchy global data on belowground litter decomposition dynamics limit our capacity to discern the drivers of carbon preservation and storage across inland and coastal wetlands. We performed a global, multiyear study in over 180 wetlands across 28 countries and 8 macroclimates using standardized litter as measures of "recalcitrant" (rooibos tea) and "labile" (green tea) organic matter (OM) decomposition. Freshwater wetlands and tidal marshes had the highest tea mass remaining, indicating a greater potential for carbon preservation in these ecosystems. Recalcitrant OM decomposition increased with elevated temperatures throughout the decay period, e.g., increase from 10 to 20 degrees C corresponded to a 1.46-fold increase in the recalcitrant OM decay rate constant. The effect of elevated temperature on labile OM breakdown was ecosystem-dependent, with tidally influenced wetlands showing limited effects of temperature compared with freshwater wetlands. Based on climatic projections, by 2050 wetland decay constants will increase by 1.8% for labile and 3.1% for recalcitrant OM. Our study highlights the potential for reduction in belowground OM in coastal and inland wetlands under increased warming, but the extent and direction of this effect at a large scale is dependent on ecosystem and OM characteristics. Understanding local versus global drivers is necessary to resolve ecosystem influences on carbon preservation in wetlands.

Mapping in-situ eddy covariance measurements of terrestrial land-atmosphere fluxes to the globe is a key method for diagnosing the Earth system from a data-driven perspective. We describe the first global products (called X-BASE) from a newly implemented up-scaling framework, FLUXCOM-X. The X-BASE products comprise of estimates of CO2 net ecosystem exchange (NEE), gross primary productivity (GPP) as well as evapotranspiration (ET) and, for the first time, a novel fully data-driven global transpiration product (ETT), at high spatial (0.05°) and temporal (hourly) resolution. X-BASE estimates the global NEE at -5.75 ± 0.33 Pg C ⋅ yr-1 for the period 2001–2020, showing a much higher consistency with independent atmospheric carbon cycle constraints compared to the previous versions of FLUXCOM. The improvement of global NEE was likely only possible thanks to the international effort to increase the precision and consistency of eddy covariance collection and processing pipelines, as well as to the extension of the measurements to more site-years resulting in a wider coverage of bio-climatic conditions. However, X-BASE global net ecosystem exchange shows a very low inter-annual variability, which is common to state-of-the-art data-driven flux products and remains a scientific challenge. With 125 ± 2.1 Pg C ⋅ yr-1 for the same period, X-BASE GPP is slightly higher than previous FLUXCOM estimates, mostly in temperate and boreal areas. X-BASE evapotranspiration amounts to 74.7x10³ ± 0.9x10³ km3 globally for the years 2001–2020, but exceeds precipitation in many dry areas likely indicating overestimation in these regions. On average 57 % of evapotranspiration are estimated to be transpiration, in good agreement with isotope-based approaches, but higher than estimates from many land surface models. Despite considerable improvements to the previous up-scaling products, many further opportunities for development exist. Pathways of exploration include methodological choices in the selection and processing of eddy-covariance and satellite observations, their ingestion into the framework, and the configuration of machine learning methods. For this, the new FLUXCOM-X framework was specifically designed to have the necessary flexibility to experiment, diagnose, and converge to more accurate global flux estimates.

With large wildfires becoming more frequent1,2, we must rapidly learn how megafires impact biodiversity to prioritize mitigation and improve policy. A key challenge is to discover how interactions among fire-regime components, drought and land tenure shape wildfire impacts. The globally unprecedented3,4 2019-2020 Australian megafires burnt more than 10 million hectares5, prompting major investment in biodiversity monitoring. Collated data include responses of more than 2,000 taxa, providing an unparalleled opportunity to quantify how megafires affect biodiversity. We reveal that the largest effects on plants and animals were in areas with frequent or recent past fires and within extensively burnt areas. Areas burnt at high severity, outside protected areas or under extreme drought also had larger effects. The effects included declines and increases after fire, with the largest responses in rainforests and by mammals. Our results implicate species interactions, dispersal and extent of in situ survival as mechanisms underlying fire responses. Building wildfire resilience into these ecosystems depends on reducing fire recurrence, including with rapid wildfire suppression in areas frequently burnt. Defending wet ecosystems, expanding protected areas and considering localized drought could also contribute. While these countermeasures can help mitigate the impacts of more frequent megafires, reversing anthropogenic climate change remains the urgent broad-scale solution.

Mapping in situ eddy covariance measurements of terrestrial land-atmosphere fluxes to the globe is a key method for diagnosing the Earth system from a data-driven perspective. We describe the first global products (called X-BASE) from a newly implemented upscaling framework, FLUXCOM-X, representing an advancement from the previous generation of FLUXCOM products in terms of flexibility and technical capabilities. The X-BASE products are comprised of estimates of CO2 net ecosystem exchange (NEE), gross primary productivity (GPP), evapotranspiration (ET), and for the first time a novel, fully data-driven global transpiration product (ETT), at high spatial (0.05 degrees) and temporal (hourly) resolution. X-BASE estimates the global NEE at -5.75 +/- 0.33 Pg C yr(-1) for the period 2001-2020, showing a much higher consistency with independent atmospheric carbon cycle constraints compared to the previous versions of FLUXCOM. The improvement of global NEE was likely only possible thanks to the international effort to increase the precision and consistency of eddy covariance collection and processing pipelines, as well as to the extension of the measurements to more site years resulting in a wider coverage of bioclimatic conditions. However, X-BASE global net ecosystem exchange shows a very low interannual variability, which is common to state-of-the-art data-driven flux products and remains a scientific challenge. With 125 +/- 2.1 Pg C yr(-1) for the same period, X-BASE GPP is slightly higher than previous FLUXCOM estimates, mostly in temperate and boreal areas. X-BASE evapotranspiration amounts to 74.7x10(3) +/- 0.9x10(3) km(3) globally for the years 2001-2020 but exceeds precipitation in many dry areas, likely indicating overestimation in these regions. On average 57 % of evapotranspiration is estimated to be transpiration, in good agreement with isotope-based approaches, but higher than estimates from many land surface models. Despite considerable improvements to the previous upscaling products, many further opportunities for development exist. Pathways of exploration include methodological choices in the selection and processing of eddy covariance and satellite observations, their ingestion into the framework, and the configuration of machine learning methods. For this, the new FLUXCOM-X framework was specifically designed to have the necessary flexibility to experiment, diagnose, and converge to more accurate global flux estimates.

The paucity of studies on nitrous oxide (N2O) dynamics with rising temperatures and nitrogen (N)-based eutrophication makes it challenging to evaluate the role of mangroves in mitigating climate change. Here, a 3year mesocosm experiment was conducted to investigate the effects of climate warming (+3 degrees C) and excessive N input (25 mg N L- 1) on soil N2O emissions from two mangroves (Avicennia marina and Bruguiera gymnorrhiza). We found that warming and N input alone significantly increased soil N2O emissions from both mangroves, while the interactive effects of warming and N input on soil N2O emissions were affected by mangrove species. Warming mitigated the positive effect of N input on soil N2O emissions from A. marina; and amplified the effect of N input on soil N2O emissions from B. gymnorrhiza, suggesting that the response of soil N2O emissions to these global change factors is species-dependent. Stable isotopic signature analysis revealed that both warming and N input significantly increased the relative contribution of nitrification to N2O emissions from A. marina; whereas N input, rather than warming, significantly changed the relative contribution of nitrification in B. gymnorrhiza. This could be attributed to the differential changes in soil environmental conditions, plant growth and the microbial structure of the two mangroves. Overall, this study highlights the role of mangrove species in modifying the effects of warming and N input on soil N2O emissions, which should be considered when accurately projecting N2O emissions from mangroves. Furthermore, considering the low N2O emissions from background sediments and the common N limitation across mangroves, our findings suggest that climate warming and exogenous N input may lead to a surge of N2O emissions from mangroves, especially those that are seriously affected by human activities.

Iron-bound organic carbon (Fe-OC) is a main pathway for the long-term maintenance of soil organic carbon (SOC), but research on its mechanism is still relatively weak. We investigated the coupling relationships among iron (Fe), carbon (C) and Fe-reducing bacteria (FeRB) in the soil of a reclaimed paddy field in comparison with natural Phragmites australis wetland in the Minjiang River estuary in southeastern China. The results showed that conversion of P. australis wetland to paddy cultivation changed the soil redox process. After reclamation, soil Fe(II), Fe(III), HCl-Fet, free iron oxide (Fed) and amorphous iron (Feo) contents and Fe(III)/Fe(II) decreased significantly (p < 0.05), while the content of complexed iron (Fep) increased. Nonmetric multidimensional scaling analysis (NMDS) demonstrated variability in FeRB across soil types (r = 0.900, p = 0.001). The lower Fe-OC concentration in soil after wetland reclamation may be the result of Fe reduction by dissimilatory FeRB (e.g., Bacillus, Anaeromyxobacter). On average, both Fe-OC and SOC contents decreased significantly (p < 0.05), while the contribution of Fe-OC to total SOC (fFe-OC) increased significantly (p < 0.05), after conversion to paddy cultivation. Structural equation modeling (SEM) showed that SOC, dissolved organic C, and Fe-OC were affected by FeRB and the speciation of Fe. In addition, Fe (III) concentration affected SOC concentration (r = 0.60, p < 0.05) and DOC concentration (r = 0.58, p < 0.05), and Fed affected DOC concentration (r = 0.69, p < 0.05). We conclude that after rice field reclamation in estuarine wetlands, Fe-reducing bacteria can mediate iron-bonded organic C decoupling, affecting SOC stabilization.

Plant community biodiversity can be maintained, at least partially, by shifts in species interactions between facilitation and competition for resources as environmental conditions change. These interactions also drive ecosystem functioning, including productivity, and can promote over-yielding- an ecosystem service prioritized in agro-ecosystems, such as pastures, that occurs when multiple species together are more productive than the component species alone. Importantly, species interactions that can result in over-yielding may shift in response to rising C-O2 concentrations and changes in resource availability, and the consequences these shifts have on production is uncertain especially in the context of tropical mixed-species grasslands. We examined the relative performance of two species pairs of tropical pasture grasses and legumes growing in monoculture and mixtures in a glasshouse experiment manipulating CO2. We investigated how over-yielding can arise from nitrogen (N) niche partitioning and biotic facilitation using stable isotopes to differentiate soil N from biological N fixation (BNF) within N acquisition into above-ground biomass for these two-species mixtures. We found that N niche partitioning in species-level use of soil N versus BNF drove species interactions in mixtures. Importantly partitioning and overyielding were generally reduced under elevated CO2. However, this finding was mixture-dependent based on biomass of dominant species in mixtures and the strength of selection effects for the dominant species. This study demonstrates that rising atmospheric CO2 may alter niche partitioning between co-occurring species, with negative implications for the over-yielding benefits predicted for legume-grass mixtures in working landscapes with tropical species. Furthermore, these changes in inter-species interactions may have consequences for grassland composition that are not yet considered in larger-scale projections for impacts of climate change and species distributions.