Fertilization Can Enhance Carbon Sequestration Under Rainfed Mediterranean Cropping Conditions.

At the global level the focus of dryland cropping research has been management to enhance crop yields under limited soil moisture conditions. Soil organic matter (SOM) has emerged as a key factor in the sustainability of such dryland cropping, with implications for carbon sequestration and climate change. As elsewhere, fertilization, especially with nitrogen (N) and phosphorus (P), was shown to be essential for increasing crop yields in the Mediterranean conditions. In most cases, except under the lowest rainfall, responses were shown to N. In addition, crop rotations involving legumes with cereals tended to increase SOM with positive impacts on physical properties such as aggregate stability and related infiltration and permeability, while the traditional cereal-fallow system indicated little or no changes in SOM. Despite evidence for the USA, where the perception is that N fertilization contributes to a decline in SOM, evidence from typical Mediterranean cropping conditions in northern Syria (annual rainfall, 340 mm yr^-1) suggests the contrary. Various long-term trials at the International Center for Agricultural Research in the Dry Areas, ICARDA, examined the common crop rotations practiced in the region, together with options for stubble-grazing management, and N fertilization. In addition to impacting cereal growth and yields in most years, the mean effect of added N was an increase in SOM regardless of the rotation or the stubble management. The increases in SOM are attributed to increased root biomass associated with increased yields along with soil moisture/temperature conditions that limited the mineralization of root biomass organic matter. These results suggest that the impact of N on SOM, or by implication carbon sequestration, are site-specific and depend on the environment that dictates the balance between carbon input and subsequent decomposition. These long-term studies show that N fertilization can increase production with additional benefits to the environment.