Soil carbon relations : an overview

Soils are localized between base rock, atmosphere and vegetation, and represent a home for numerous organisms and the place of countless biogeochemical transformation and transfer processes. In addition, soils store many substances that are essential to maintain human life and ecosystem processes. Therefore, soils have been a research focus for more than a century and soil science has deep connections to ecology, agriculture and nature conservation. The most important practical applications of soil science are diagnostics and maintenance of soil fertility and, more recently, definition of the role of soils in the terrestrial carbon balance in the context of increasing atmospheric CO2 concentration and the resulting greenhouse effect. Carbon stored in soils represents the largest carbon pool in nearly all terrestrial biomes (Bolin et al., 2001) and thus it has a huge potential for either sequestering or releasing carbon into the atmosphere. Consequently, knowledge of the dynamics of soil carbon is essential for a better understanding of the terrestrial carbon balance. However, inter-annual changes in soil carbon stocks are small compared to the total carbon stored in soils, and thus determining any changes in soil carbon stocks by repeated inventories is difficult. On the other hand, flux measurements also bear inaccuracies and uncertainties, confounding attempts to directly measure and model the CO2 flux from the soil and linking this flux to the underlying processes. There are considerable challenges in monitoring soil fluxes without disturbing the plant–soil carbon flow. Furthermore, we must also deal with considerable spatial and temporal variability inherent to nearly all ecosystems. The above challenges imply some conceptual and technical consequences in the methodology of soil carbon studies. It is the intention of this book to summarize the actual state-of-the-art methods on soil carbon stock and flux measurements and modelling approaches. However, the authors intend to provide more than a ‘manual’ for several methods. The goal is a critical review on the potentials and limitations of different concepts and underlying methodological approaches as well as giving guidance on their informational value and their possible integration. The main aim is, firstly, to provide a more integrated methodology on soil carbon stock and flux measurements at different scales and, secondly, to discuss the relevance of such measurements within the terrestrial carbon cycle and climate system.