Inputs to climatic change by soils and agriculture related activities: Present status and possible future trends
Developments in Soil Science
15 - 30
The most important soil borne and land use related greenhouse gases are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). The present annual increase of atmospheric CO2 is 0.5%. The total emission of CO2 is 6.5 to 7.5 Gt C y−1. Fossil fuel combustion contributes 5.7 Gt C y−1. The present global rate of deforestation of 10 to 20 million ha y−1 causes an emission of 1 to 2 Gt C y−1 including the release of CO2 from soil organic matter oxidation. There is uncertainty about the sinks of CO2. The oceanic uptake is less than 1 Gt C y−1, while the atmospheric accumulation accounts for approximately 3.5 Gt C. Increasing net primary production and other terrestrial sinks balance the budget. The emissions from fossil fuel use will probably increase, and projections of efficiency and magnitude of future energy use are rather uncertain. Deforestation will continue in the coming decades to satisfy the growing demand for agricultural land. The atmospheric concentration of CH4 is rising at a rate of 1% y−1. The major part of this fast increment is caused by increasing emissions, while a minor part can be attributed to decreasing destruction in the atmosphere. The total annual emission from all sources is 400 to 640 Tg CH4. Biotic sources make up about 80% of the total annual emission, the rest is from fossil sources. The biotic sources of CH4 are: wetland rice cultivation (20% of the total source), natural wetlands (20%), ruminating animals (15%), landfills (10%), oceans and lakes (5%) and biomass burning (15%). The contribution by termites is very uncertain. Most sources are increasing at present. Nitrous oxide is increasing at a rate of 0.2 to 0.3% and its sources are almost exclusively biogenic. Nitrous oxide is inert in the troposphere, but it destroys stratospheric ozone (O3). The causes of the increase in atmospheric N2O are not well known. Fossil fuel combustion is a minor source of N2O. Increasing use of N-fertilizers in agriculture is a growing source of N2O. The emissions from natural ecosystems are not well known at present.