Carbon balance in land-use change, grassland to Miscanthus

Feb 17, 2014

Turbulent wind structures, created by the interference of a vegetated surface in the wind field moving across(Photo credit: Jon McCalmont) it, are the primary transport mechanism for the exchange of carbon between the canopy and the atmosphere. It is the movement of these circulating structures that ensure a constant supply of carbon dioxide for the utilisation of plants during photosynthesis and, given fast enough sensors and some sophisticated algorithms, this exchange can be captured and quantified across large spatial scales. The eddy covariance technique, coupling high frequency, speed of sound, wind monitoring to infra-red gas analysers, is being employed at the Aberystwyth site to do just that; not only to measure the carbon being taken from the atmosphere by the plants themselves but also to monitor losses from soil carbon pools that may result from cultivation and decomposition of the previous crop; carbon goes up as well as down and, in simple economic terms, the net figures, the difference between the two, determine the “carbon debt” of land-use change.

At Aberystwyth we have now been monitoring our land-use change, agricultural grassland to Miscanthus, for the first two years of what is at least a ten year perennial crop.

The planting year, 2012, was particularly challenging in terms of weather, 235 days of rain meant machinery access to the land was often restricted but, despite this, the change of crop went ahead during April with the first shoots appearing in early June. Miscanthus generally takes three years to mature and this was certainly reflected in the growth figures for the first year. While survivorship in terms of individual plants was good, 14,865 out of 16,000 per hectare planted, the growth of these young plants was not; the grass fought back with weeds representing 50% of the above ground production and Miscanthus producing only a little over half a tonne of dry matter at the end of the season.

Year two, 2013, was very different; better weather, timely weed control and maturing plants produced a much more impressive crop; this time weeds only contributed 5% and the Miscanthus produced a peak yield of nearly 7.5 tonnes of dry matter. Miscanthus is not harvested at peak yield however; moisture content is critical in an energy crop and Miscanthus leaves contain undesirable elements for combustion; ideally the crop is left standing in the field over winter where it can dry out to produce a better fuel and the vast majority of the leaves will be dropped to the ground where they act as a useful mulch and are eventually incorporated into, and potentially increase the size of, soil carbon pools.

(photo credit: Jon McCalmont)

The eddy covariance sensors were in place throughout these first two years (and continue to be so) and, despite the challenges posed by the weather and the somewhat restricted size of the site compared to typical eddy covariance studies, enough data was obtained to produce a reasonable estimate of the carbon dynamics within the system. These dynamics rise and fall with the passing seasons; dormant months, where there is no growth, will see net losses as carbon is respired from the soil in decomposition, growing periods see net gains as uptakes outweigh losses; long term assessments need to consider cumulative sums over a timescale of interest. Preliminary figures, starting from the initial spraying of the grassland a month before ploughing, are suggesting that there was indeed a small net loss of carbon from the site in the first year; in the region of 1.5 tonnes per hectare lost from an existing pool of around 100 tonnes per hectare in the top 30cm of soil. The much stronger growth in the second year seems to show this being slowly recouped with a small net gain in carbon being seen by August and the year possibly finishing with a small carbon credit; not including any possible fossil fuel offset in harvestable take-off. Of course, this small carbon credit is likely to be lost over the winter with dormant season respiration slowly re-building the debt but it will be very interesting to watch in the third year how the expected increases in crop growth combined with fossil fuel offset in harvestable take-off will reduce and repay this debt and whether we will even see a strong, sustainable carbon credit by the end of 2014.


By Jon McCalmont

(Photo credit: Jon McCalmont)