The role of forage shrubs in soil erosion prevention

Pasture improvement could be the way of re-establishing pastoral activity and human presence in marginal internal lands of southern Italy, where the hilly areas are prone to soil erosion and environmental degradation. This paper discusses two possible functions of the Medicago arborea: fodder production and protection of the soil from the erosive capacity of rainfall. The research was carried out in the "Murgia", a hilly area in the Apulia region. Shrubs were transplanted in 1991 to an area of 5,000 m 2 (longitudinal slope = 5%) at two plant densities: 2 x 2 m and 2 x 1.5 m. With the use of a rainfall simulator, soil erosion was studied in 50 m 2 plots under the following conditions: bare soil, 2,500 and 3,333 shrubs ha - 1 , both before and after grazing. During each rain simulation, the following parameters were measured: water flow, rainfall intensity, runoff, nutrients and soil losses. Despite low forage production (3 t ha - 1 of green fodder per year) and the short duration of the shrubs grazed, the presence of Medicago arbarea is justified in the forage system of southern Italy because it controls soil erosion: runoff coefficients from a bare soil range were between 3.2 and 30.7% (depending on the water initially stored in the soil profile), while in the presence of the Mediicagostand, runoff coefficients were no higher than 9.7%. The highest figure was obtained in the stand with 2,500 shrubs ha - 1 , after animal grazing. Nitrite and nitrate content in runoff waters (0.11 and 0.04 g l - 1 respectively) did not seem to constitute a real risk of pollution, even on bare soil, where, after a 30 mm rain, the highest nitrate and nitrite values (2.41 and 4.74 kg ha - 1 ) were measured.