Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 1 2 2004 10.5194/bg-1-123-2004 http://www.biogeosciences.net/1/123/2004/ http://www.biogeosciences.net/1/123/2004/bg-1-123-2004.html http://www.biogeosciences.net/1/123/2004/bg-1-123-2004.pdf 123 131 2004-12-13 Carbon isotope anomaly in the major plant C₁ pool and its global biogeochemical implications F. Keppler R. M. Kalin D. B. Harper W. C. McRoberts J. T. G. Hamilton School of Agriculture and Food Science, Queen’s University Belfast, Newforge Lane, Belfast BT9 5PX, UK Environmental Engineering Research Centres, Queen’s University Belfast, Belfast BT9 5AG, UK Department of Agriculture and Rural Development for Northern Ireland, Newforge Lane, Belfast BT9 5PX, UK We report that the most abundant C₁ units of terrestrial plants, the methoxyl groups of pectin and lignin, have a unique carbon isotope signature exceptionally depleted in ¹³C. Plant-derived C₁ volatile organic compounds (VOCs) are also anomalously depleted in ¹³C compared with C_<i>n</i>+1 VOCs. The results confirm that the plant methoxyl pool is the predominant source of biospheric C₁ compounds of plant origin such as methanol, chloromethane and bromomethane. Furthermore this pool, comprising ca 2.5% of carbon in plant biomass, could be an important substrate for methanogenesis and thus be envisaged as a possible source of isotopically light methane entering the atmosphere. Our findings have significant implications for the use of carbon isotope ratios in elucidation of global carbon cycling. Moreover methoxyl groups could act as markers for biological activity in organic matter of terrestrial and extraterrestrial origin.