Long-term effect of tillage, crop rotation and N fertilization to wheat on gaseous emissions under rainfed Mediterranean conditions

A field study was conducted to assess the effect of N fertilizer application to wheat (Triticum aestivum L.), tillage system and crop rotation on total denitrification N losses, N₂O and CO₂ emissions under Mediterranean conditions in a long-term trial started 18 years ago on a Vertisol soil. The tillage system consisted of conventional tillage vs. no-tillage and the crop rotation system consisted of two different 2-years rotations: wheat–sunflower (Helianthus annuus L.) (WS) and wheat–faba bean (Vicia faba L.) (WF). Fertilizer rates were 0 and 100 kg N ha^?1 applied to wheat splitted in two amendments of 50 kg N ha^?1 each. Two different fertilization systems were studied. In the old fertilized plots system fertilizer had been applied for 18 years since the beginning of the trial, and in the new fertilized plots system fertilizer was applied for the first time when this experiment was started. Measurements were carried out after fertilizer applications.

In the long term, continued fertilizer application produced a higher soil total N content. Nevertheless, no increase in denitrification potential, N₂O + N₂ production by denitrification, N₂O or CO₂ emissions was observed either by the recent application of N or by the continued application during 18 years. The soil presented a higher potential to denitrify up to N₂ than up to N₂O. So, denitrification was probably occurring mainly in the form of N₂, while N₂O emissions were occurring in a great manner by nitrification, both denitrification and nitrification occurring simultaneously at soil field capacity (60–70%) expressed as water filled pore space (WFPS). Conventional tillage induced an increase in soil total N content and in the potential to denitrify up to N₂ with respect to no-tillage. This higher potential was translated into higher N₂O + N₂ production by denitrification presumably stimulated in the short time by the higher available carbon provided by decomposing roots and by the subsequent creation of soil anaerobic microsites. Contrarily, no effect of tillage was observed on N₂O emissions because of being produced in an important manner by nitrification, which does not depend on carbon availability. The wheat–faba bean rotation induced higher soil nitrate contents than the wheat–sunflower, although the effect in the long time was not observed regarding soil total N content. The same as for the fertilizer effect, this increase in nitrate content was not followed by a higher denitrification potential or higher N₂O + N₂ production by denitrification because of the lack of organic matter, while an increase was observed in N₂O emissions.