Boreal forest soil carbon: distribution, function and modelling

Increasing accumulation of CO 2 in the atmosphere has led to calls for terrestrial mechanisms for CO 2 abatement and given that soils represent the largest terrestrial body of C on Earth, there is a great deal of interest in soils as a sink for atmospheric C. This emphasis on sequestration in boreal forest soils is understandable given the sheer mass of this C reservoir (~1700 Pg of C) but diverts our attention from the importance of soil C in soil physical, chemical and biotic functions, and importantly, it ignores the possibility that soils may also represent a source of C. In this review, we address these issues through a discussion of the size and character of boreal forest soil C pool, its role in ecosystem function, the potential impacts of climate change on soil C, efforts to model these processes and the role of soil C in boreal resilience to the impacts of climate change. Soil C is fundamental to ecosystem function in terms of improving soil physical properties, increasing soil biotic activity and enhancing insulation all of which improve site productivity. Managing upland soils for C sequesteration will achieve little in terms of offsetting fossil fuel emissions but would likely improve soil quality. Most of the C stored in the boreal biome is found in permafrost and wetland soils and events related to climatic change could shift these soils from C sink to C source. Melting of permafrost soils with predicted warming trends within the circumpolar region could result in the release of 30-60 Pg C by the year 2040. Such predictions, however, are limited by uncertainty in both climatic changes and soil response to these changes. Prediction of shifts in soil C dynamics with climate change relies on our ability to link C transformations to N dynamics and climatic variables. Improvement in ecosystem models will advance our ability to assess the resilience of the boreal biome under future climatic conditions.