The Global Carbon Cycle: a Viewpoint on the Missing Sink

Atmospheric carbon budgets that ignore the possibility of terrestrial ecosystem responses to global atmospheric change do not balance; there is a 'missing sink' of about 0.4 - 4 Gt C yr-1. This paper argues a case that mechanistically it is well within the bounds of possibility that increasing carbon storage in vegetation and soils in response to the globally increasing CO2 concentration, temperature and nitrogen deposition can account for the missing C sink. Global warming conditions considered alone would be unlikely to cause most ecosystems to emit CO2, because the N mineralised by any enhanced soil organic matter decomposition would be largely taken up by plants and reconverted into organic matter having a much higher C:N ratio than that in the soil. Models of the global terrestrial C cycle indicate that an extra 0.5 - 4 Gt C yr-1 could well be being stored in soils and vegetation today in response to the CO2 fertilising effect, having regard for the interactions with growth restricting water, light and nitrogen levels. To obtain direct proof as to whether that this is happening or not is a major challenge.