| Resumo: | Under Mediterranean conditions, soils under conventional tillage farming usually have very low contents of organic matter. This situation is due to intensive soil tillage, low biomass production under rainfed conditions, and removal of cereal straw for alternative use off the field. In order to study strategies to improve soil organic matter content (SOM), a long term experiment (11 years) was established in 1995 to evaluate the effect of four different tillage systems (conventional tillage (CT) based on moldboard plough (25 cm) + disc harrow with removal of cereal straw; reduced tillage (RT) based on non-inversion tine cultivation with removal of cereal straw; no-till (NT) with removal of cereal straw; and no-till with cereal straw retained (NT+S). The crop rotation was lupine – wheat – forage oat – barley. Soil organic carbon (SOC) (0-30 cm) and crop yields (grain and straw) were measured every year. In addition, yield response of the wheat crop to nitrogen fertilization was studied, in order to evaluate the interactions with SOC accumulation. Under CT and RT, SOC remained almost unchanged over the experimental period. NT improved SOC by 18% in relation to CT and NT+S increased SOC by 62%. There were no significant differences between CT and NT in the amount of residues left in the field over the experimental period, but NT+S significantly increased the amount of residues retained in relation to the other treatments. The results indicate that NT reduced SOM mineralization, but that maximum increase of SOC can only be achieved if crop residues are retained on the field. Nitrogen use efficiency was significantly improved with greater SOC contents, i.e. from 19.1 kg of wheat per kg of applied N at 0.58% SOC to 104 kg of wheat per kg of applied N at 1.74% SOC.
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