An interyear comparison of CO2 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 CO2 exchange between the ecosystem and atmosphere. Using a simple mass balance approach, above-ground and below-ground biomass production are combined with partitioned CO2 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.