Harvest intensity of aromatic shrubs vs. soil erosion: An equilibrium for sustainable agriculture (SE Spain)

In the Mediterranean Europe, where rainfall is scarce and irregular but often of high intensity, wild shrubs protect the soil against erosivity of raindrops. Moreover, some of these plants are the economic income for local farmers. Particularly in SE Spain, soil erosion is a core factor in environmental degradation attributed primarily to the cultivation practices and human pressure on the land. Over a four-year period, soil erosion and runoff were monitored in erosion plots on a mountainside, comparing four harvest intensities of four aromatic shrubs (Lavandula lanata L., Santolina rosmarinifolia L. Origanum bastetanum, and Salvia lavandulifolia V.): 0% (HI-0), 25% (HI-25), 50% (HI-50), and 75% (HI-75). Also, the fresh biomass and essential-oil content were quantified in each treatment. The erosion plots were located in Lanjarón (Granada, SE Spain) on the southern flank of the Sierra Nevada Mountains, on a 20% slope, and of 96 m² in area. The average soil loss for HI-0, HI-25, HI-50, and HI-75 during the study period was 144.6, 187.2, 256.0, and 356.0 kg ha^− 1, respectively, and runoff 2.6, 3.2, 3.4, and 4.7 mm, respectively. The lowest average soil erosion and runoff rates for the study period were recorded with plant cover of S. lavandulifolia V. 67.6 kg ha^− 1 and 1.3 mm, respectively. Since no significant differences were found between HI-25 and HI-50 for soil erosion and runoff, and harvest and distillation of wild-aromatic plants currently persists as an important economic activity in mountainous areas of the study zone, we recommend a rational harvest (HI-50), leaving the 50% of the plant biomass in the field (especially for sage and lavender) to avoid the soil degradation. In this sense, the harvest of 50% of fresh herb of sage, santolina, lavender, and oregano produced reasonable essential-oil yield of 12.7, 14.0, 19.7, and 18.3 L ha^− 1, respectively. The inappropriate harvest of aromatic plants and the intensity farming systems of mountain areas endanger land conservation, and there is an urgent need to implement appropriate land management which has a large-scale perspective but acts at the local level.