Consensus, uncertainties and challenges for perennial bioenergy crops and land use.

Jeanette Whitaker,Patricia Thornley,Reinhart Ceulemans,Niall P Mcnamara,Jon P Mccalmont,Pete Smith,Keith Paustian,Christian A Davies,Rebecca L Rowe,John L Field,Iain S Donnison,Carl J Bernacchi,Carlos E P Cerri,Evan H Delucia

doi:10.1111/gcbb.12488

Abstract

Perennial bioenergy crops have significant potential to reduce greenhouse gas (GHG) emissions and contribute to climate change mitigation by substituting for fossil fuels; yet delivering significant GHG savings will require substantial land‐use change, globally. Over the last decade, research has delivered improved understanding of the environmental benefits and risks of this transition to perennial bioenergy crops, addressing concerns that the impacts of land conversion to perennial bioenergy crops could result in increased rather than decreased GHG emissions. For policymakers to assess the most cost‐effective and sustainable options for deployment and climate change mitigation, synthesis of these studies is needed to support evidence‐based decision making. In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK, EU and internationally. Outcomes from global research on bioenergy land‐use change were compared to identify areas of consensus, key uncertainties, and research priorities. Here, we discuss the strength of evidence for and against six consensus statements summarising the effects of land‐use change to perennial bioenergy crops on the cycling of carbon, nitrogen and water, in the context of the whole life‐cycle of bioenergy production. Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry.

Highlights

Land management decisions and the type of land converted to bioenergy crop production have variable effects on soil carbon stocks and N2O emissions which are difficult to quantify accurately (Gauder et al, 2012; Palmer et al, 2014; Qin et al, 2016), leading to large uncertainties in the life-cycle greenhouse gas (GHG) balance of bioenergy (Rowe et al, 2011; Njakou Djomo & Ceulemans, 2012; Davis et al, 2013)
There are sustainability concerns related to the expansion of bioenergy feedstock cultivation globally, such as potential conflicts with food production through direct and indirect land-use change, excessive nitrous oxide (N2O) emissions due to fertilizer application and land disturbance, and impacts on land and water resources, including soil organic carbon stocks, which could result in undesired outcomes (Crutzen et al, 2008; Smith & Searchinger, 2012; DeCicco, 2013)
Comparing fertilized and unfertilized crops grown on annual cropland and grassland, we found that the net greenhouse gas (GHG) intensity of the biofuel scenarios varied widely from À39 to +54 g CO2-eq MJÀ1, but all delivered significant GHG savings compared to conventional gasoline (Fig. 3)

Summary

Introduction

Land management decisions and the type of land converted to bioenergy crop production have variable effects on soil carbon stocks and N2O emissions which are difficult to quantify accurately (Gauder et al, 2012; Palmer et al, 2014; Qin et al, 2016), leading to large uncertainties in the life-cycle GHG balance of bioenergy (Rowe et al, 2011; Njakou Djomo & Ceulemans, 2012; Davis et al, 2013). At field-scale, the impacts of annual cropland and grassland conversion to perennial bioenergy crops on soil carbon stocks and N2O emissions have been quantified under a variety of scenarios

Results

Conclusion