• Energy Research

Second-generation bioenergy crops, including Short Rotation Forestry (SRF), have the potential to contribute to greenhouse gas (GHG) emissions savings through reduced soil GHG fluxes and greater soil C sequestration. If we are to predict the magnitude of any such GHG benefits a better understanding is needed of the effect of land use change (LUC) on the underlying factors which regulate GHG fluxes. Under controlled conditions we measured soil GHG flux potentials, and associated soil physico-chemical and microbial community characteristics for a range of LUC transitions from grassland land uses to SRF. These involved ten broadleaved and seven coniferous transitions. Differences in GHGs and microbial community composition assessed by phospholipid fatty acids (PLFA) profiles were detected between land uses, with distinctions between broadleaved and coniferous tree species. Compared to grassland controls, CO2 flux, total PLFAs and fungal PLFAs (on a mass of C basis), were lower under coniferous species but unaffected under broadleaved tree species. There were no significant differences in N2O and CH4 flux rates between grassland, broadleaved and coniferous land uses, though both CH4 and N2O tended to have greater uptake under broadleaved species in the upper soil layer. Effect sizes of CO2 flux across LUC transitions were positively related with effect sizes of soil pH, total PLFA and fungal PLFA. These relationships between fluxes and microbial community suggest that LUC to SRF may drive change in soil respiration by altering the composition of the soil microbial community. These findings support that LUC to SRF for bioenergy can contribute towards C savings and GHG mitigation.

AbstractAn increase in renewable energy and the planting of perennial bioenergy crops is expected in order to meet global greenhouse gas (GHG) targets. Nitrous oxide (N2O) is a potent greenhouse gas, and this paper addresses a knowledge gap concerning soil N2O emissions over the possible “hot spot” of land use conversion from established pasture to the biofuel crop Miscanthus. The work aims to quantify the impacts of this land use change on N2O fluxes using three different cultivation methods. Three replicates of four treatments were established: Miscanthus x giganteus (Mxg) planted without tillage; Mxg planted with light tillage; a novel seed‐based Miscanthus hybrid planted with light tillage under bio‐degradable mulch film; and a control of uncultivated established grass pasture with sheep grazing. Soil N2O fluxes were recorded every 2 weeks using static chambers starting from preconversion in April 2016 and continuing until the end of October 2017. Monthly soil samples were also taken and analysed for nitrate and ammonium. There was no significant difference in N2O emissions between the different cultivation methods. However, in comparison with the uncultivated pasture, N2O emissions from the cultivated Miscanthus plots were 550%–819% higher in the first year (April to December 2016) and 469%–485% higher in the second year (January to October 2017). When added to an estimated carbon cost for production over a 10 year crop lifetime (including crop management, harvest, and transportation), the measured N2O conversion cost of 4.13 Mg CO2‐eq./ha represents a 44% increase in emission compared to the base case. This paper clearly shows the need to incorporate N2O fluxes during Miscanthus establishment into assessments of GHG balances and life cycle analysis and provides vital knowledge needed for this process. This work therefore also helps to support policy decisions regarding the costs and benefits of land use change to Miscanthus.

Abstract Cultivation of maize for biomethane production has expanded rapidly, including on drained peat soils. The resulting soil CO2 emissions at the point of feedstock production are largely overlooked when assessing biogas climate mitigation potential. Based on field-scale flux measurements, we calculate that soil CO2 emissions from biomethane feedstock production on drained peat exceed embodied emissions for an equivalent amount of natural gas by up to a factor of three.

AbstractWhen considering the large‐scale deployment of bioenergy crops, it is important to understand the implication for ecosystem hydrological processes and the influences of crop type and location. Based on the potential for future land use change (LUC), the 10,280 km2 West Wales Water Framework Directive River Basin District (UK) was selected as a typical grassland dominated district, and the Soil & Water Assessment Tool (SWAT) hydrology model with a geographic information systems interface was used to investigate implications for different bioenergy deployment scenarios. The study area was delineated into 855 sub‐basins and 7,108 hydrological response units based on rivers, soil type, land use, and slope. Changes in hydrological components for two bioenergy crops (Miscanthus and short rotation coppice, SRC) planted on 50% (2,192 km2) or 25% (1,096 km2) of existing improved pasture are quantified. Across the study area as a whole, only surface run‐off with SRC planted at the 50% level was significantly impacted, where it was reduced by up to 23% (during April). However, results varied spatially and a comparison of annual means for each sub‐basin and scenario revealed surface run‐off was significantly decreased and baseflow significantly increased (by a maximum of 40%) with both Miscanthus and SRC. Evapotranspiration was significantly increased with SRC (at both planting levels) and water yield was significantly reduced with SRC (at the 50% level) by up to 5%. Effects on streamflow were limited, varying between −5% and +5% change (compared to baseline) in the majority of sub‐basins. The results suggest that for mesic temperate grasslands, adverse effects from the drying of soil and alterations to streamflow may not arise, and with surface run‐off reduced and baseflow increased, there could, depending on crop location, be potential benefits for flood and erosion mitigation.

AbstractTo achieve net zero greenhouse gas emission by 2050 as set out by the 2019 amendment to the 2008 UK Climate Change Act, a major shift towards renewable energy is needed. This includes the development of new methods along with improving and upscaling existing technologies. One example of new methods in bioenergy is developing new Miscanthus cultivars for electricity generation via thermal power station furnaces. Miscanthus is still relatively new compared with other agriculture practices, so market assessments and improvements are needed to reduce the barriers to entry for prospective growers. This publication provides a profile of UK Miscanthus growers and their businesses, their experiences of benefits and drawbacks of the crop, and what they see as potential barriers to entry for prospective farmers. A survey of current Miscanthus growers in England and Wales was conducted and indicated that most farmers were content with the crop and that its environmental and economic benefits were noted. However, it was evident that with a geographically limited UK market, growers wanted to see a better distribution of biomass processing stations to reduce the ongoing costs of transport. With growing demand for renewables, including bio‐energy sources, it was determined important to provide information and support for stable farming operations and to incentivise the adoption of Miscanthus. Such incentives include ongoing development of new cultivars, focussing on traits such as production potential and stressor resilience, and growers indicated preference for an annual planting grant. These developments are predicted to further improve the crop's profit margin, making it a more cost‐effective crop for farmers. Sensitively managed Miscanthus also has the potential to contribute to carbon sequestration, soil health, and aspects of farmland biodiversity. Incentivising such management in government land–based environmental schemes would offer additional income streams and help to promote environmental positive crop planting.

The conversion of agricultural land to bioenergy plantations represents one option in the national and global effort to reduce greenhouse gas emissions whilst meeting future energy demand. Despite an increase in the area of (e.g. perennial) bioenergy crops in the United Kingdom and elsewhere, the biophysical and biogeochemical impacts of large scale conversion of arable and other land cover types to bioenergy cropping systems remain poorly characterised and uncertain. Here, the results of four years of eddy covariance (EC) flux measurements of net ecosystem CO2 exchange (NEE) obtained at a commercial farm in Lincolnshire, United Kingdom (UK) are reported. CO2 flux measurements are presented and compared for arable crops (winter wheat, oilseed rape, spring barely) and plantations of the perennial biofuel crops Miscanthus x. giganteus (C4) and short rotation coppice (SRC) willow (Salix sp., C3). Ecosystem light and temperature response functions were used to analyse and compare temporal trends and spatial variations in NEE across the three land covers. All three crops were net in situ sinks for atmospheric CO2 but were characterised by large temporal and between site variability in NEE. Environmental and biological controls driving the spatial and temporal variations in CO2 exchange processes, as well as the influences of land management, will be analysed and discussed.