An interyear comparison of CO 2 flux and carbon budget at a commercial‐scale land‐use transition from semi‐improved grassland to Miscanthus x giganteus

A 6‐ha field at Aberystwyth, UK , was converted in 2012 from semi‐improved grassland to Miscanthus x giganteus for biomass production; results from transition to the end of the first 3 years are presented here. An eddy covariance sensor mast was established from year one with a second mast added from year two, improving coverage and providing replicated measurements of CO 2 exchange between the ecosystem and atmosphere. Using a simple mass balance approach, above‐ground and below‐ground biomass production are combined with partitioned CO 2 fluxes to estimate short‐term carbon deltas across individual years. Years one and two both ended with the site as a net source of carbon following cultivation disturbances, cumulative NEE by the end of year two was 138.57 ± 16.91 g C m −2 . The site became a cumulative net sink for carbon by the end of June in the third growing season and remained so for the rest of that year; NEE by the end of year three was −616.52 ± 39.39 g C m −2 . Carbon gains were primarily found in biomass pools, and SOC losses were limited to years one (−1.43 Mg C ha −1  yr −1 ) and two (−3.75 Mg C ha −1  yr −1 ). Year three saw recoupment of soil carbon at 0.74 Mg C ha −1  yr −1 with a further estimate of 0.78 Mg C ha −1 incorporated through litter inputs over the 3 years, suggesting a net loss of SOC at 3.7 Mg ha −1 from a 0‐ to 30‐cm baseline of 78.61 ± 3.28 Mg ha −1 , down 4.7%. Assuming this sequestration rate as a minimum would suggest replacement of cultivation losses of SOC by year 8 of a potential 15‐ to 20‐year crop. Potential coal replacement per hectare of harvest over the three‐year study would offset 6–8 Mg of carbon emission, more than double the SOC losses.