The use of biomass for energy production is considered a promising way to reduce net carbon emissions and mitigate climate change. However, land‐use change to bioenergy crops can result in carbon emissions from soil and vegetation in amounts that could take decades to compensate. Perennial grasses such as Miscanthus offer a possible solution to this problem as measurements on experimental plots planted with Miscanthus have shown significant carbon sequestration in the soil. It can, however, be expected that sequestration potentials in commercial use might differ from those measured in experimental plots due to different farming practices and soil characteristics. For this study, Miscanthus plantations on 16 farms in SE Ireland as well as on‐farm controls representing the former land‐use (grassland and tillage) have been examined. The Miscanthus plantations were 2–3 years old. Soil organic carbon (SOC) content and a number of soil properties were measured and the amount of Miscanthus‐derived carbon was determined using the ¹³C natural abundance method. On both former tillage fields and grasslands, although there were no significant differences in SOC contents between Miscanthus and control sites, it was shown that 2–3 years after Miscanthus establishment, 1.82 ± 1.69 and 2.17 ± 1.73 Mg ha⁻¹ of the SOC under former‐tilled and former grassland respectively were Miscanthus‐derived. Mixed‐effects models were used to link the total SOC concentrations and Miscanthus‐derived carbon to the land‐use parameters as well as to soil properties. It was shown that on control sites, pH had an effect on total SOC. In the case of Miscanthus‐derived carbon, the initial SOC content, pH, former land‐use and crop age had significant effects.

In Ireland, Miscanthus × giganteus has the potential to become a major feedstock for bioenergy production. However, under current climatic conditions, Ireland is situated on the margin of the geographical range where Miscanthus production is economically feasible. It is therefore important to optimize the yield and other ecosystem services such as carbon sequestration delivered by the crop. A survey of commercial Miscanthus fields showed a large number of areas with no Miscanthus crop cover. These patches can potentially lead to reduced crop yields and soil carbon sequestration and have a significant negative impact on the economic viability of the crop. The aim of this research is to assess patchiness on a field scale and to analyse the impacts on crop yield and soil carbon sequestration. Analysis of aerial photography images was carried out on six commercial Miscanthus plantations in south east Ireland. The analysis showed an average of 372.5 patches per hectare, covering an average of 13.7% of the field area. Using net present value models and a financial balance approach it was shown that patchiness has a significant impact on payback time for initial investments and might reduce gross margins by more than 50%. Total and Miscanthus-derived soil organic carbon was measured in open patches and adjacent plots of high crop density showing significantly lower Miscanthus-derived carbon stocks in open patches compared to high crop-density patches (0.47Mg C ha⁻¹ ± 0.42 SD and 0.91Mg C ha⁻¹ ± 0.55 SD). Using geographic information system (GIS) it was shown that on a field scale Miscanthus-derived carbon stocks were reduced by 7.38% ± 7.25 SD. However, total soil organic carbon stocks were not significantly different between open patches and high crop density plots indicating no impact on the overall carbon sequestration on a field scale over 3–4 years since establishment for these Miscanthus sites.