Carbon storage and sequestration under different land uses with a focus on biomass crops

Climate change is caused by rising quantities of greenhouse gases, particularly CO2, in the atmosphere, largely through consumption of fossil fuels. There is interest in sustainable energy generation from renewable resources, particularly biomass crops to reduce reliance on fossil fuels. A key advantage of such energy systems is that they assimilate atmospheric CO2 and thus help mitigate climate change. Soil represents one of the largest pools of C in the biosphere and there is potential to use soil as a sink to sequester C to mitigate climate change. The aim of this project was to investigate soil C storage and sequestration in short rotation coppice (SRC), which is one of the major biomass cropping systems in temperate climates. 14 year old plantations of willow and poplar established at Rothamsted Research in Harpenden were used to investigate how the quantity and quality of organic matter under the SRC compare to that under adjacent land uses, including arable cropping, set-aside grassland and natural woodland. It was shown that change in land use to SRC led to increased C storage in soil relative to alternative agricultural systems, while conversion to setaside had no effect on soil C stocks. There was no difference in C storage under different poplar or willow cultivars. Differences in C storage between arable, SRC and set-aside plots reflected changes in C stocks at 0-30 cm depth with no change occurring to C at 30-60 cm. The quality of C was investigated by analysis of the light fraction organic matter pool which acts as an early indicator of long term changes in total soil organic matter. The work showed that the free LFOM pool responded to land use change while the physically protected intra-aggregate LFOM pool did not. While changes to amounts of LFOM following land use change occurred in soil at 0-30 cm depth, there were no changes at 30-60 cm depth. Fourier Transform Infra Red spectroscopy showed that the chemical composition of free and intra aggregate LFOM was different, and that composition of both pools was affected by land use. Fourier Transform Ion Cyclotron Resonance Mass spectroscopy was shown to provide fine level resolution of the composition of soluble organic matter, and demonstrated that the aromaticity and chain length of C was higher in woodland than arable soil. The potential to sequester C in soil as biochar is a promising option to promote longterm sequestration of C in soil. The potential to use a fluidised bed reactor to produce biochar was investigated. Life Cycle Analysis showed that 4700C was the optimal temperature to promote retention of C in biochar during pyrolysis, while minimising losses of C as bio-oil and gases. Addition of steam during pyrolysis was shown to increase the surface area of the resulting biochar. Addition of biochar to soil reduced net C mineralisation of soil organic matter and litter at a rate of 2 % w/w, but not at 0.5 % w/w. However both concentrations of biochar affected net mineralisation of N. Phospholipid fatty acid analysis showed that both concentrations of biochar altered microbial community structure, with fungal biomass in particular promoted by the addition of biochar. Overall the work demonstrates that SRC promotes significant storage of C within soil organic matter, which has implications for the overall energy budget of SRC biomass. Furthermore, conversion of biomass to biochar for land application provides opportunities for sequestration of C within soil. Application of biochar to soil would provide further benefits to the overall energy balance of SRC by reducing mineralisation rates of native soil organic matter and litter.