The role of deposition in C sequestration in ecosystems.

Chemical, physical and, biological parameters affecting primary production and decomposition continuously change at different time scales. Currently, atmospheric CO2 concentration is increasing and, as a consequence, temperature may increase appreciably on a timescale of decades. It is not known whether such changes in environmental conditions will increase or decrease storage of C in ecosystems, and thus either buffer or enhance the increase in atmospheric CO2. As one of the largest C pools in ecosystems, organic matter plays a critical role in this issue. It is not likely that fresh litter and stable organic matter will respond to the same degree to changes in temperature and/or CO2. In terrestrial ecosystems, turnover times of organic matter are relatively long: 26 years as a global average (Schlesinger, 1991). The turnover time of different soil organic carbon (SOC) pools varies from a few years for fresh litter to millennia for the most stable organic matter. Little is known as to how increased temperature and CO2 will affect conversion of labile C into stable C. In this article we will treat the factors affecting decomposition. First we will discuss the major pathways along which C is being transformed in terrestrial and aquatic ecosystems. Then we will focus on decomposition and provide experimental data to illustrate the effect of environmental factors on decomposition. Finally, we will use model calculations to show how climate change might affect C storage in the soil in a grassland ecosystem on the basis of our present understanding of decomposition. Based on the experimental data and modelling results we will discuss the possibilities for sequestration of C in soils.