Organic matter dynamics in agro‐ecosystems – the knowledge gaps

Knowledge about soil organicmatter (SOM) is necessary for the sustainable use and conservation of natural resources, for which climate change and increasing demands for bio-energy have become crucial global challenges. SOM influences many soil functions and occupies a key position in the global carbon (C) cycle (Lal, 2004). At the agro-ecosystem scale, primary productivity is dependent on the cycling of SOM by the action of decomposers (mainly bacteria and fungi), which mineralize organic compounds and release nutrients necessary for plant growth. Thus any agro-ecosystem of sustainable productivity focuses on goals to maintain and improve SOM content (Gregorich et al., 1994). At a global scale, SOM is an essential component in ecosystems that exert much influence over atmospheric chemistry and the world’s climate through the global biogeochemical cycling of C. The increase in atmospheric CO2 concentration is a major environmental problem. One of the challenges that ‘modern agriculture’ has to face is that of improving cropping systems and soil management strategies in order to remove significant amounts of CO2 from the atmosphere through C sequestration in soil. Jarecki & Lal (2003) point out the need for adoption of ‘recommended management practices’ that will help to fill the large C sink in the world’s agricultural soils. However, the mechanisms that are responsible for the preservation of SOC are not sufficiently understood to adequately develop management strategies to reduce C emissions. Our understanding of SOC sequestration processes in soils is largely based on laboratory experiments, the results of which often deviate from observations made in the field. Further, despite extensive studies devoted to measuring changes in SOM pools over a large area (Bellamy et al., 2005) and under elevated CO2 regimes (Jastrow et al., 2005) considerable uncertainty exists regarding the sink strength of SOM and C sequestration under different forms of land use (e.g. disturbance) and under future climate conditions. The reason for this is that we still do not yet understand fully the retro-action of a number of processes that are involved in SOC sequestration. To overcome such uncertainty, gaps regarding the chemical structures involved in C sequestration in soil with regard to functional microbial diversity, localization of SOMwithin the soil matrix, plant biodiversity and SOCdynamics need to be filled. Information on the chemical structures involved in SOC sequestration