Agricultural Soils: Sources and Sinks of Greenhouse Gases

Agricultural activities are a significant source for greenhouse gases (GHG), and the release of carbon dioxide from agricultural soils are one of the main source categories within the agricultural sector. Overall agriculture contributes 10-12% of GHG emissions (Smith et al. 2007). Several factors such as soil type, moisture, climate, season, crop type, fertilisation and cultivation methods affect the flow of GHG from agricultural soils. Fertilisers add nitrogen dioxide to soils, thereby increasing natural nitrogen dioxide emissions. Trace gas fluxes are influenced by practices like irrigation, tillage or the fallowing of land. Soils are both sources and sinks for carbon dioxide and carbon monoxide, a sink for methane and a source of nitrogen dioxide. Yet there is a degree of uncertainty surrounding the direction and magnitude of the activities that affect soil emissions as well as the role soils play in mitigating the effects of GHG.

Atmospheric GHG emissions compared to the balance of soil sinks under conventional cropping is highly contentious given future concerns in climate change projections. This is because crop farming removes plant products in the environment and from natural cycles before decomposition can occur, removing essential elements such as carbon, nitrogen and phosphorus. Consequently soil fertility can decline rapidly which requires additional artificial fertilisers that make it more difficult to maintain soil structure and hydrological properties under such a regime. Soil disturbance and increased rates of decomposition result in carbon release to the atmosphere along with increased soil erosion and leaching of soil nutrients further reducing the potential to act as GHG sinks (Jones 2006).

The formulation of climate policies acknowledges the role that agricultural soils play in addressing climate change because soil biomass is a natural GHG sink. The incentives created to raise levels of soil carbon and nitrogen storage will impact on the vitality and productivity of the agricultural industry, reduce the incidence of dryland salinity and soil acidity; and reduce levels of greenhouse gases (Jones 2006). The longer action is delayed, the more difficult it will be to re-sequester soil carbon (nitrogen and methane) and balance the greenhouse equation.

References:

1. Jones, C. (2006). ‘Soil Carbon and Soil Credits’. YLAD Living Soils Seminars: Eurongilly. Available (Online): http://soilcarboncredits.blogspot.com/

2. Smith, P., D. Martino, Z. Cai, D. Gwary, H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, B. Scholes, and O. Sirotenko. (2007). Agriculture In Climate Change Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, and New York, NY, USA.