Environmental impact of introducing plant covers in the taluses of terraces: Implications for mitigating agricultural soil erosion and runoff

South-eastern Spain, and in particular the coastal areas of Granada and Malaga, feature a large area under subtropical crops, with orchards established on terraces built along the slopes of the mountainous areas. The climate, characterized by periodically heavy rainfall, variable in space and time, and with the common agricultural practice of leaving the taluses with bare soil, are the main factors encouraging soil erosion, runoff, and subsequent transport of pollutants. Over a two-year period, six plant covers were applied [(Thymus mastichina (Th), Lavandula dentata (La), native spontaneous vegetation (Sv), Anthyllis cytisoides (An), Satureja obovata (Sa), Rosmarinus officinalis (Ro)] in comparison to a control of bare soil (Bs) to determine the effectiveness of the covers in reducing soil erosion, runoff, and potential pollution risk by agricultural nutrients (N, P, and K) and heavy metals. Also, carbon losses were monitored in the transported sediments by runoff and in eroded soils. For this purpose, 16 m2 erosion plots (4 m × 4 m) were laid out in the taluses of the terraces. When the yearly data were compared, the control plot (Bs) showed significantly higher soil erosion and runoff rates (26.4 t ha− 1 yr− 1 and 55.7 mm yr− 1, respectively) than the treatments with plant covers. The plant covers studied registered the following results in runoff: Ro > Sa > An > Th ≈ La > Sv (41.7, 38.2, 35.5, 16.9, 16.1, and 12.4 mm yr− 1, respectively) while annual soil erosion gave the following results: Sa > An > Ro > Th > Sv > La (18.0, 13.5, 13.4, 5.5, 4.4, and 3.2 Mg ha− 1 yr− 1, respectively). This means that Sv reduced runoff and soil-erosion rates compared to Bs by not less than 78 and 83%, respectively. Nevertheless, La and Th plots were also very effective plant covers in reducing runoff and soil erosion (71.2 and 87.8; 69.5 and 79.2%, respectively) in comparison with the Bs plot. The heaviest nutrient losses in runoff and eroded soils were found in Bs and the lowest in the La, Th, and Sv plots. Bs and Ro plots registered the highest carbon losses (829.9 and 652.1 kg ha− 1, respectively), the lowest carbon-loss rates being measured in La, Sv, and Th plots (145.2, 140.3, and 109.3 kg ha− 1, respectively). The results indicate that heavy metals (Mn, Cr, Co, Ni, Cu, Zn, Mo, Cd, and Pb) in these types of agroecosystems may also be a potential pollutant due to transport by agricultural runoff. There was a major reduction of heavy-metal transport by plant covers in relation to the control of bare soil. The results of this research support the recommendation of using plant covers with multiple purposes (aromatic–medicinal–culinary) on the taluses of subtropical-crop terraces in order to reduce erosion and pollution risk.

Research highlights
► Aromatic plants and spontaneous vegetation in the taluses of orchard terraces substantially reduced soil erosion and runoff.
► Lavender and thyme were the most effective plant covers in reducing runoff and soil erosion (71.2 and 87.8; 69.5 and 79.2%, respectively) in comparison to the bare soil.
► The plant covers considerably decreased the risk of transport of nutrients, organic carbon, and heavy metals.
► Aromatic plants play an important role in mitigating heavy-metal pollution risk, decreasing its transport by runoff in comparison to the common soil-management practice with bare soil on the taluses of terraces.
► The alternative cultivation of aromatic plants could represent an extra income, and an environment-friendly measure that increases the stability of the taluses in terraces.