Importance of cover crops for weed control in a sustainable Mediterranean agrosystem

From some decades, the usefulness of the cover crop was confirmed by farmers who are increasingly aware that the soil should be kept and that his over-exploitation involves not only a drop-in productivity over the medium to long term, but also the 'pollution of environment and desertification’ (1; 2). Cover crops are perfectly integrated in the perspective of sustainable and environmentally friendly agriculture that can be used in different contexts and in many different areas of production (3). To evaluate the use of Trifolium subterraneum L. (subterranean clover) as cover crop to control weeds, in a Mediterranean 3-years apricot orchard, cvs Pinkcot®, Wonder cot e Big Red®, with a planting density of 400 plant ha–1 and 5x3 m tree spacing, a factorial experiment with 3 factors and 4 replications, in a randomized complete block design, was done in 2015/2016 in Southern Sicily, in province of Caltanissetta (CL) (37°13'36.0"N 14°05'02.4"E, 290 m a.s.l.). The following treatments were compared to the control (conventional management): cover cropping with subterranean clover and weed flora (by hand weeding and residues incorporation). To evaluate the effects of cover crop on weeds, in the central part of each elementary plot, we identified, randomly, permanent squares of 1 m to the side; these plots were used to identify and separate, from subterranean clover, the occurring weed species; the fresh and dry weight of phythomass was calculated after a treatment in a drying thermo-ventilated oven at 105 ° C to a constant weight. Species were identified using Pignatti (1982); the nomenclature of species followed Conti et al. (2005). Before sowing, settlement and during the development of cover crop, a amoA gene-specific quantitative PCR in soil samples was done. Quality and integrity of DNA extracted from soil was verified by electrophoresis on 1% agarose gel and it was subsequently quantified with standard concentrations of calf thymus. Results of the study showed that the number of species and their botanical families were strongly contained by subterranean clover, regardless of its management. This confirms that the positive effects of crop cover on the weeds can be attributed to the sharp reduction in the number of plant species as well as the number of seeds per plant (4). This study represents an initial contribution of knowledge on the role of cover crop in the management of Mediterranean orchards. In the field conditions, the subterranean clover showed very good settlement ability and a good growth and development. In particular, T. subterraneum has always proved able to considerably reduce the soil seed bank that represents the potential flora. This means that this cover crop can significantly reduce the real flora and vegetation that grows in orchards. The highest amount of nucleic acids, which corresponds to the lowest levels of Ct, was constantly higher in presence of cover crop. This would allow, therefore, a reduced use of inputs such as tillage, mowing, herbicides, making the agroecosystem most sustainable. The excellent settlement and growth of T. subterraneum, which has substantially increased the activity of the bacteria associated with the nitrogen cycle, are also a guarantee of release of crop residues in the soil, with a high nitrogen content, given the nitrogen-tacker peculiarities of this species.