The influence of management practices on the greenhouse gas balance of Mediterranean cropping systems : identifying the climate change mitigation potential through quantitative review and life cycle assessment

Abstract

Resource depletion and global change trends require urgent action in order to mitigate the change and adapt to them. In particular, agricultural GHG emissions are significant contributors to climate change. Mediterranean cropping systems are highly vulnerable to climate change and, at the same time, they are an important source of GHG emissions. The scientific information on GHG emissions in Mediterranean systems is growing, but there is need to systematize and integrate the knowledge. In this PhD dissertation, the two main soil processes responsible for the GHG balance, N2O emissions and C sequestration, are studied through qualitative review and meta-analysis of published information under Mediterranean climate conditions (Studies 1 and 2). In studies 3 and 4, a life cycle assessment (LCA) of 80 organic and 80 conventional farms was performed, including all processes involved in the GHG emission balance, and employing climate-specific coefficients derived from the previous meta-analyses for the calculation of N2O emissions and C sequestration. The results show distinct GHG emission patterns in Mediterranean cropping systems. Study 1 shows that N2O emissions from rainfed systems are much lower than the global IPCC value, while drip irrigation systems seem a promising N2O mitigation strategy under irrigation. Solid organic fertilizers are related to lower N2O emissions than synthetic fertilizers. Study 2 shows that carbon sequestration is highly responsive to management changes. Best performing practices are those associated to the highest C input application rates, including organic farming practices. It is possible to achieve relatively high C sequestration rates using internal C inputs such as crop residues and cover crops. Studies 3 and 4 showed that the organic farming systems were generally associated to lower total GHG emissions both on a surface and on a yield-scaled basis, mainly due to the non-use of synthetic fertilizers and to carbon sequestration enhanced by the application of organic fertilizers. In some cases, carbon sequestration under organic farming was enough to offset all other GHG emissions, leading to carbon-neutral cropping systems, while in others, lower yield affected the performance of organic systems. The GHG balance of most studied systems is dominated by energy use (including indirect energy from fertilizer manufacture in conventional systems) and C sequestration, which indicates that these are the two processes in which most mitigation efforts should be focused.