Understanding processes determining soil carbon balances under perennial bioenergy crops CARBO-BIOCROP

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In contrast to annual food crops, evidence suggests that biofuels from perennial bioenergy crops have a positive greenhouse gas (GHG) mitigation potential. However, the magnitude of this benefit has been recently questioned, since long-term and indirect effects may considerably reduce any GHG savings generated by the cropping system. Indeed, impacts on soil C have been identified as the weak link in life-cycle analysis of net carbon-equivalent benefit presented by bioenergy. Changes in rates of nitrous oxide (N2O) and methane (CH4) emission are relevant too since they have a large GHG warming potential, but these changes are mostly unquantified for perennial bioenergy cropping systems. Although several soil carbon and trace-gas models have been developed for agricultural and conventional forest systems, these have not been parameterized and validated for transition of land-use to perennial bioenergy crops. To predict the changes in SOC that will occur one to three decades after establishing biomass crops, we need to establish: (a) differences in turnover dynamics and fluxes of carbon under key biomass crops in terms of amount, quality and placement of carbon into the soil from the plant; and (b) mechanisms to overcome short-term loss of pre-existing soil carbon during transition; (c) quantitative, process-based modeling approaches that are predictive, to explore future scenarios for optimum soil carbon management. The overarching aim of this project is to provide improved understanding of fundamental soil processes resulting in changes of soil carbon stocks and pools as a result of land conversion from arable/grassland to land-based renewables. The project focuses on impacts of land use change specifically to perennial bioenergy crops (fast growing SRC trees and grasses) where there is currently a significant knowledge gap. This project will generate new evidence to improve current understanding of how soil carbon processes, sequestration and emission, are affected by the introduction of perennial energy crops. The soil carbon balance is key to informing the debate on whether using these crops for bioenergy and biofuels will result in significant carbon savings compared to land use for food crops and the use of fossil fuels for heat, power and liquid fuels. In the long-term (beyond the life of this project), this will enable dynamic, spatially explicit modeling of GHG (C equivalents, abbreviated here as C) mitigation potential of land-based bioenergy systems across different climates and soil types of the UK. We wish to develop 'Carbon Opportunity Maps' for the UK. The work of the project will be undertaken in three workpackages dealing with data synthesis (WP1), experimental data collection (WP2) and modeling (WP3). Throughout the project we will use leverage of other resources including two flagship sites at Brattleby and Aberystwyth, where commercial-scale plantations are established and where several long-term measuring and monitoring activities are underway funded from elsewhere. Similarly, the modeling resources from within the project are extensive and funded from other sources that will be levered against the work here. They include DNDC, JULES, ROTH C and on-going modeling approaches for miscanthus and SRC. Outputs will include a new database of synthesized data for soil carbon under bioenergy crops. We will have tested and calibrated process-based models that are capable of simulating the dynamics of soil organic carbon, carbon sequestration and greenhouse gas emissions for perennial bioenergy crops in the UK. We will provide increased fundamental understanding of soil processes occurring under bioenergy cropping systems, including the role of mycorrhizal associations and the effectiveness of biochar as a potential to optimize soil carbon and plant growth. We will develop capacity for future 'carbon opportunity' mapping.