Runoff and sediment yield of tilled and spontaneous grass-covered olive groves grown on sloping land

Soil erosion in olive groves is a widespread phenomenon in the Mediterranean Basin. Many studies have investigated the effects of tillage and herbaceous ground cover (spontaneous or seeded) in their effectiveness to reduce soil erosion in a wide range of Mediterranean sites under different pedoclimatic and topographic conditions. The present study was performed in Ferrandina, southern Italy: a small drip-type rainfall simulator was used on square microplots (0.25x0.25m) to evaluate the propensity to erosion of a steep rain-fed olive grove (mean slope similar to 10%) with a sandy loam soil by measuring runoff and sediment load under extreme rain events. Two types of soil management were compared: (1) spontaneous grasses as ground cover (GC) providing a maximum ground cover close to 100%; and (2) tillage (T). In the tillage treatment, a further distinction was made between runoff and sediment produced 1 day (T1) and 10 days (T2) after tillage in order to assess the temporal evolution of tillage effects. The results show that GC reduced surface runoff to approximately one-third and soil losses to zero compared with T1. T2 microplots, tested 10 days after tillage, produced only one-tenth the sediment compared with T1 microplots that were subjected to rainfall simulations 1 day after tillage. Total runoff between the two tilled microplots was similar, although runoff on T1 microplots increased steadily over time whereas runoff on T2 microplots remained stable over the duration of simulations. Such findings may be useful to direct and strengthen the policy towards measures to prevent further soil degradation, as clearly requested by the European Union via the cross-compliance concept. A further economic recognition to the olive growers for the achievable environmental benefits could convince them to a shift towards alternative soil management strategies.