Abstract
AbstractA 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.
Highlights
There is much debate surrounding the potential for carbon sequestration into soils under perennial energy crops such as Miscanthus x giganteus, when planted into agricultural grasslands (McCalmont et al, 2015)
We present a multiyear source/sink dynamic of an establishing Miscanthus crop on semiimproved grassland and employ a simple mass balance approach to estimate changes in soil carbon stocks
There is no suggestion that all root material has been captured in sampling, fine roots were not recovered so results can only be considered a minimum; increases of BNPP in Eqn 2 that would follow a full capture of fine root material might suggest slightly higher soil organic carbon (SOC) losses than reported
Summary
There is much debate surrounding the potential for carbon sequestration into soils under perennial energy crops such as Miscanthus x giganteus (hereafter Miscanthus), when planted into agricultural grasslands (McCalmont et al, 2015). These challenges are typically compounded by a reliance on adjacent land taken to represent baseline conditions and incompatibility of sampling techniques and depths in comparisons. Sampling to a fixed depth when comparing Miscanthus to grassland or following cultivation can be criticized for not accommodating changes in soil bulk density which can exaggerate changes in carbon stocks (Gifford & Roderick, 2003; Rowe et al, 2015), techniques such as equivalent soil mass (ESM) are increasingly employed (Ferchaud et al, 2015; Richter et al, 2015; Rowe et al, 2015). Soil carbon derived from Miscanthus shows less depletion of this isotope when compared to an atmospheric standard in
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