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Globally pervasive increases in atmospheric CO2 and nitrogen (N) deposition could have substantial effects on plant communities, either directly or mediated by their interactions with soil nutrient limitation. While the direct consequences of N enrichment on plant communities are well documented, potential interactions with rising CO2 and globally widespread phosphorus (P) limitation remain poorly understood. We investigated the consequences of simultaneous elevated CO2 (eCO2) and N and P additions on grassland biodiversity, community and functional composition in P-limited grasslands. We exposed soil-turf monoliths from limestone and acidic grasslands that have received >25 years of N additions (3.5 and 14 g m−2 year−1) and 11 (limestone) or 25 (acidic) years of P additions (3.5 g m−2 year−1) to eCO2 (600 ppm) for 3 years. Across both grasslands, eCO2, N and P additions significantly changed community composition. Limestone communities were more responsive to eCO2 and saw significant functional shifts resulting from eCO2–nutrient interactions. Here, legume cover tripled in response to combined eCO2 and P additions, and combined eCO2 and N treatments shifted functional dominance from grasses to sedges. We suggest that eCO2 may disproportionately benefit P acquisition by sedges by subsidising the carbon cost of locally intense root exudation at the expense of co-occurring grasses. In contrast, the functional composition of the acidic grassland was insensitive to eCO2 and its interactions with nutrient additions. Greater diversity of P-acquisition strategies in the limestone grassland, combined with a more functionally even and diverse community, may contribute to the stronger responses compared to the acidic grassland. Our work suggests we may see large changes in the composition and biodiversity of P-limited grasslands in response to eCO2 and its interactions with nutrient loading, particularly where these contain a high diversity of P-acquisition strategies or deve

Miscanthus x giganteus's efficacy as an energy crop relies on maintaining low greenhouse gas (GHG) emissions. As demand for Miscanthus is expected to rise to meet bioenergy targets, fertilizers and composts may be employed to increase yields, but will also increase GHG emissions. Manipulation experiments are vital to investigate the consequences of any fertilizer additions, but there is currently no way to measure whole‐plant GHG fluxes from crops taller than 2.5 m, such as Miscanthus, at the experimental plot scale. We employed a unique combination of eddy covariance (EC), soil chambers and an entirely new automated chamber system, SkyBeam, to measure high frequency (ca. hourly) fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from a Miscanthus crop amended with green compost. Untreated controls were also monitored in a fully replicated experimental design. Net ecosystem exchange (NEE) of CO2 was partitioned into soil respiration (Rs), gross primary productivity (GPP) and ecosystem respiration, and the crop was harvested to determine the effect of compost on crop productivity. Compost increased NEE emissions by 100% (p < .05), which was the result of a 20% increase of Rs (p < .06) and a 32% reduction in GPP (p < .05) and biomass of 37% (p < .06). Methane fluxes were small and unaffected by compost addition. N2O emissions increased 34% under compost during an emission event; otherwise, fluxes were low and often negative, even under dry conditions. Diurnal variation in N2O fluxes, with uptake during the day and emission at night was observed. These fluxes displayed a negative relationship with soil temperature and a hitherto undescribed diurnal temperature hysteresis. We conclude that compost addition negatively affected the productivity and environmental effects of Miscanthus cultivation during the first year following application.