Carbon accumulation in soil. Ten-year study of conservation tillage and crop rotation in a semi-arid area of Castile-Leon, Spain

Minimum (MT) or no tillage (NT) and increased cropping intensity can enhance soil structure and raise carbon sequestration in agricultural soils. The effectiveness of these procedures depends on soil type, crops, and tillage management systems. Increases in the organic carbon content may be affected by crop type, crop rotation and the quality and quantity of crop residues left on the soil surface. Soil organic carbon (SOC) is a good indicator of soil quality and conservation. The present study was conducted from 1994 to 2004 at Torrepadierne, Burgos, a cereal farming area in Spain, on Typic Calcixerolls soil with a 1.8% soil organic matter (SOM) content. The average annual rainfall in the area is 448mm. A split-plot experimental design was used, in which the main factor was the tillage system - conventional (CT), minimum (MT) or no-till (NT) - and the sub-factor crop rotation - cereal/cereal (C-C), cereal/fallow (C-F) and cereal/legume (C-L). Fallow/cereal and legume/cereal were added to these sequences to have the same crops every year. The present study was conducted to determine the effect of tillage systems and cropping sequences on SOC patterns after 10 years of soil management. At a depth of 0-10cm, the SOC content was significantly higher with NT than CT or MT, by 58% and 11%, respectively. SOC values were 41% higher with MT, in turn, than with CT. At a depth of 10-20cm, the SOC content was 30% higher with NT than with CT and 7% higher than with MT. And at 20-30cm, it was 7% higher with MT than with CT, 12% higher with NT than CT and 9% higher with no-till than minimum-till. In 2004, at the end of the 10 years period, SOC was 25% greater with NT than CT, 16% greater with NT than MT, and 17% higher with MT than CT. Crop rotation was not observed to have any significant effect on the SOC content in 2004, however. These findings suggest that carbon sequestration in the 30cm layer can be improved if NT or MT are used in lieu of conventional practice. The total crop residue returning to the soil was significantly greater in plots sown with legume after cereal harvest than in plots left fallow. It also enhanced SOC sequestration in non- or minimally tilled soils.