Soil carbon dioxide and methane fluxes as affected by tillage and N fertilization in dryland conditions

The effects of tillage and N fertilization on CO2 and CH4 emissions are a cause for concern worldwide. This paper quantifies these effects in a Mediterranean dryland area. CO2 and CH4 fluxes were measured in two field experiments. A long-term experiment compared two types of tillage (NT, no-tillage, and CT, conventional intensive tillage) and three N fertilization rates (0, 60 and 120 kg N ha(-1)). A short-term experiment compared NT and CT, three N fertilization doses (0, 75 and 150 kg N ha(-1)) and two types of fertilizer (mineral N and organic N with pig slurry). Aboveground and root biomass C inputs, soil organic carbon stocks and grain yield were also quantified. The NT treatment showed a greater mean CO2 flux than the CT treatment in both experiments. In the long-term experiment CH4 oxidation was greater under NT, whereas in the short-term experiment it was greater under CT. The fertilization treatments also affected CO2 emissions in the short-term experiment, with the greatest fluxes when 75 and 150 kg organic N ha(-1) was applied. Overall, the amount of CO2 emitted ranged between 0.47 and 6.0 kg CO2-equivalent kg grain(-1). NT lowered yield-scaled emissions in both experiments, but these treatment effects were largely driven by an increase in grain yield. In dryland Mediterranean agroecosystems the combination of NT and medium rates of either mineral or organic N fertilization can be an appropriate strategy for optimizing CO2 and CH4 emissions and grain yield.