The authors thank Drs Goetz Richter and Marianna Cerasuolo (Rothamsted Research) and Drs Iain Donnison and John Clifton-Brown (IBERS) for helpful advice, William Macalpine and Peter Fruen for assistance with experimental work, the Farm support staff at both Rothamsted Research and IBERS for assistance with the field trials and David Steele (Rothamsted Research) for his careful washing of roots from the soil cores. This work was supported by the BBSRC Sustainable Bioenergy Centre (BSBEC) grant (BB/G016216/1) working within the BSBEC BioMASS programme of the centre (http://www.bsbecbiomass. org.uk/). Rothamsted Research is an Institute supported
by the Biotechnological and Biological Sciences research Council of the United Kingdom.