Denitrification in a vertisol under long-term tillage and no-tillage management in dryland agricultural systems: Key genes and potential rates.

The impact of tillage practices on microbial N transformations in Vertisols is poorly understood and data from long-term field experiments are scarce, particularly in semiarid regions. We evaluated the effects of traditional tillage (TT) vs no-tillage (NT) on denitrification in a long-term field experiment under a rainfed crop rotation system (cereal-sunflower-legumes) on a Vertisol (SW Spain). In general, the abundance of denitrifiers and the respective potential denitrification rates was higher under NT compared to TT during the vegetation period, but not after harvesting. However differences in denitrifier numbers were within the same order of magnitude (0.5-3 x 10(7) copies g soil dw). The abundance of nitrite reducers and N2O reducers was relatively similar. In addition, N2O/N-2 ratios between 1 and 2 were found for both treatments. These results emphasize that NT has a limited impact on denitrification in Vertisols under fertilizer regime and legume-crop rotation and thus losses of N2O are expected to be comparable to those of traditional tillage systems