• Energy Research

AbstractMass 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.

Abstract Bioenergy crop production can enhance greenhouse gas (GHG) mitigation, whilst producing feedstocks for energy generation. However, the GHG balance of these ecosystems is intimately linked to crop selection, previous and current land management and the effects of land conversion. This study aims to quantify nitrous oxide (N2O) emissions from the early stage of land-use change (LUC) from perennial grassland to two perennial rhizomatous grasses in a temperate climate: Miscanthus and reed canary grass (RCG) in the south of Ireland. Emissions of N2O were measured during the first two years of RCG and Miscanthus establishment. Miscanthus stands emitted 7.7 ± 1.6 and 2.3 ± 0.2 kg N2O-N ha−1 yr−1 in the first and the second year, respectively, while RCG produced 1.1 ± 0.2 kg N2O-N ha−1 yr−1 in the first year following LUC. Temporal fluxes of N2O were generally low, however peak emissions observed in the first year contributed approximately 83% of annual N2O in the Miscanthus treatment. This peak occurred in wet (50 mm rainfall in the week preceding the peak) and warm (>18.5 °C in the top 5 cm of soil) weather conditions and was significantly affected (R2 = 0.77) by the soil moisture deficit. However large, annual N2O losses from Miscanthus and RCG found in this study are well within the range of those from grassland soils in temperate climate, drawing conclusions that any short-term increases in N2O production will soon be offset by the reduced future fertilisation, carbon sequestration and produced bioenergy feedstock.