Soil processes and global change

Abstract Contributors to the Intergovernmental Panel on Climate Change (IPCC) generally agree that increases in the atmospheric concentration of greenhouse trace gases (i.e., CO2, CH4, N2O, O3) since preindustrial times, about the year 1750, have led to changes in the earth's climate. During the past 250 years the atmospheric concentrations of CO2, CH4, and N2O have increased by 30, 145, and 15%, respectively. A doubling of preindustrial CO2 concentrations by the end of the twenty-first century is expected to raise global mean surface temperature by about 2  °C and increase the frequency of severe weather events. These increases are attributed mainly to fossil fuel use, land-use change, and agriculture. Soils and climate changes are related by bidirectional interactions. Soil processes directly affect climatic changes through the production and consumption of CO2, CH4, and N2O and, indirectly, through the production and consumption of NH3, NOx, and CO. Although CO2 is primarily produced through fossil fuel combustion, land-use changes, conversion of forest and grasslands to agriculture, have contributed significantly to atmospheric increase of CO2. Changes in land use and management can also result in the net uptake, sequestration, of atmospheric CO2. CH4 and N2O are produced (30% and 70%, respectively) in the soil, and soil processes will likely regulate future changes in the atmospheric concentration of these gases. The soil-atmosphere exchange of CO2, CH4, and N2O are interrelated, and changes in one cycle can impart changes in the N cycle and resulting soil-atmosphere exchange of N2O. Conversely, N addition increases C sequestration. On the other hand, soil processes are influenced by climatic change through imposed changes in soil temperature, soil water, and nutrient competition. Increasing concentrations of atmospheric CO2 alters plant response to environmental parameters and frequently results in increased efficiency in use of N and water. In annual crops increased CO2 generally leads to increased crop productivity. In natural systems, the long-term impact of increased CO2 on ecosystem sustainability is not known. These changes may also result in altered CO2, CH4, and N2O exchange with the soil. Because of large temporal and spatial variability in the soil-atmosphere exchange of trace gases, the measurement of the absolute amount and prediction of the changes of these fluxes, as they are impacted by global change on regional and global scales, is still difficult. In recent years, however, much progress has been made in decreasing the uncertainty of field scale flux measurements, and efforts are being directed to large scale field and modeling programs. This paper briefly relates soil process and issues akin to the soil-atmosphere exchange of CO2, CH4, and N2O. The impact of climate change, particularly increasing atmospheric CO2 concentrations, on soil processes is also briefly discussed.