Influence of cover crops and tillage on organic carbon loss in Mediterranean olive orchards

Olive (Olea europaea L.) is one of the main crops in the Mediterranean area, with Spain leading its production. The majority of the plantations are on rainfed land, poor soils with steep slopes. Regarding to soil, tillage (T) is the main management system used by farmers. Consequently, erosion and the resulting soil organic carbon (SOC) losses associated to it are one of the main environmental problems of olive production. Conservation Agriculture (CA) practices in permanent crops comprise the sowing of groundcovers or cover crops (CC) between the rows of olive trees. CC protect the olive orchard from erosion, preventing soil degradation. However, the relationship between the erosion and SOC losses under CC and T need to be better understood. In a four-year period (2003-2007) water, soil and SOC losses by erosion were measured in micro-plots (1 x1 m(2)) that were managed with T and CC in eight study zones in Andalusia (Southern Spain). At the same time, the characteristics of all rainfall events during the study period were registered, and soil cover was estimated from images taken during the field campaigns. SOC output was observed to be statistically positively correlated to runoff and erosion. The loss of water, SOC and sediment were mainly affected by the characteristics of the rainfall events and soil cover. The type of soil or the typology of the olive grove plantation were not so important for the loss of SOC under conservation agriculture management. However, these facts were especially significant for the T conditions. Conversely to T, where only 22.4 % of soil was protected, CC provided a 65.7 % cover throughout the season. This better protection of the soil provided by the CC, resulted in a lower influence of the rainfall events in the loss of water, soil and SOC. This was not the case in the T systems. The conservation management was responsible for a 36.7 % average reduction in runoff and an 85.5 % reduction in erosion compared to T, which decreased SOC loss associated with sediment by 76.4 %.