Chronosequence methodology approaches in literature, including specific recommendations for sampling for ELUM. Report V1.1

This report presents a critical review of paired site and chronosequence approaches in the literature and provides specific sampling recommendations and a UK sampling roadmap for the Ecosystem Land-Use Modelling project (ELUM). It looks beyond bioenergy plantations to uncover the best practices to determine the effects of Land Use Change (LUC) on soil carbon stocks. Work Package 2 (WP2) will adopt these approaches in the context of the most relevant bioenergy transitions across the UK. Consequently, there is a need to consider the criteria for achieving valid and robust comparisons of LUC, changes in the spatial variability of soil carbon and other soil properties during LUC, and how these relate to UK-relevant bioenergy transitions. The findings from this review will inform the development of a statistically robust sampling framework to meet the assumptions of paired site and chronosequence approaches. Paired site and chronosequence soil sampling will now commence on the range of identified fieldsites across the UK in line with our sampling roadmap and developed methodologies. The review begins with a brief background to the project, including an overview of the importance of LUC to soil carbon and the sustainability of bioenergy transitions, and the specific objectives of Work Package 2 (Section 1). The assumptions behind paired site and chronosequence approaches and the need to represent key soil types and climatic zones are then discussed in the context of achieving valid and robust LUC comparisons (Section 2). Paired site and chronosequence approaches have been used extensively by others to examine changes in soil carbon, particularly when considering longer-term changes. An overview of technical issues associated with changes in the spatial variability of soil properties across LUC transitions is presented (Section 3). Firstly, it deals with issues associated with horizontal variability and discusses sampling strategies that may be used to account for such differences between bioenergy land uses. Secondly, it examines issues associated with changes in vertical strata and sampling depth, and in particular, appraises several methods available to account for bulk density changes and their effect on soil carbon measurements. A summary of the sampling approaches used by others is provided in two graphs. Section 4 considers the potential bioenergy LUC transitions that may occur in the UK and outlines soil changes likely to be important in those transitions most relevant in the UK. A summary of the key findings from the review and associated specific recommendations for WP2 sampling is provided in Sections 5 and 6, respectively. The most important of these is the need to adopt a paired site and chronosequence approach to examine transitions to bioenergy crops. Furthermore, these approaches need to adopt a hierarchical spatial soil sampling strategy, must include samples up to 1 m depth, and must quantify soil carbon stocks on a cumulative and/or equivalent soil mass per unit area basis. These key findings and recommendations are presented in the context of a pilot study which was conducted during Q3 2011 to test different spatial sampling strategies (Appendix I). Finally, in Appendix II we provide an overview of our year 1 sampling roadmap for Work Package 2.