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.