Mass and energy fluxes were measured over a field of Agave tequilana in Mexico using eddy covariance (EC) methodology. Data were gathered over 252 d, including the transition from wet to dry periods. Net ecosystem exchanges (FN,EC ) displayed a crassulacean acid metabolism (CAM) rhythm that alternated from CO2 sink at night to CO2 source during the day, and partitioned canopy fluxes (FA,EC ) showed a characteristic four-phase CO2 exchange pattern. Results were cross-validated against diel changes in titratable acidity, leaf-unfurling rates, energy exchange fluxes and reported biomass yields. Projected carbon balance (g C m(-2)  year(-1) , mean ± 95% confidence interval) indicated the site was a net sink of -333 ± 24, of which contributions from soil respiration were +692 ± 7, and FA,EC was -1025 ± 25. EC estimated biomass yield was 20.1 Mg (dry) ha(-1)  year(-1) . Average integrated daily FA,EC was -234 ± 5 mmol CO2  m(-2)  d(-1) and persisted almost unchanged after 70 d of drought conditions. Regression analyses were performed on the EC data to identify the best environmental predictors of FA . Results suggest that the carbon acquisition strategy of Agave offers productivity and drought resilience advantages over conventional semi-arid C3 and C4 bioenergy candidates.

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Land-use change to bioenergy crop production can contribute towards addressing the dual challenges of greenhouse gas mitigation and energy security. Realisation of the mitigation potential of bioenergy crops is, however, dependent on suitable crop selection and full assessment of the carbon (C) emissions associated with land conversion. Using eddy covariance-based estimates, ecosystem C exchange was studied during the early-establishment phase of two perennial crops, C 3 reed canary grass (RCG) and C 4 Miscanthus, planted on former grassland in Ireland. Crop development was the main determinant of net carbon exchange in the Miscanthus crop, restricting significant net C uptake during the first 2 years of establishment. The Miscanthus ecosystem switched from being a net C source in the conversion year to a strong net C sink (À411 AE 63 g C m À2 ) in the third year, driven by significant above-ground growth and leaf expansion. For RCG, early establishment and rapid canopy development facilitated a net C sink in the first 2 years of growth (À319 AE 57 (post-planting) and À397 AE 114 g C m À2 , respectively). Peak seasonal C uptake occurred three months earlier in RCG (May) thanMiscanthus (August), however Miscanthus sustained net C uptake longer into the autumn and was close to Cneutral in winter. Leaf longevity is therefore a key advantage of C 4 Miscanthus in temperate climates. Further increases in productivity are projected as Miscanthus reaches maturity and are likely to further enhance the C sink potential of Miscanthus relative to RCG.