اثرات مدیریت خاک بر رواناب، فرسایش خاک و خواص در درختان زیتون جنوب اسپانیا

Rainfall, runoff and soil loss from 6 m × 12 m plots were recorded during 7 years (2000–2006) in an experiment in which three different soil management systems were compared in a young olive grove installed on a heavy clay soil, near Cordoba, Southern Spain. The no-tillage (NT) system, kept weed-free with herbicides, had both the largest soil loss (6.9 t ha−1 year−1) and the highest average annual runoff coefficient (11.9%). By contrast, a cover crop (CC) of barley, reduced the soil losses to 0.8 t ha−1 year−1 and the average annual runoff coefficient to 1.2%. Conventional tillage (CT), had intermediate values of soil loss (2.9 t ha−1 year−1) and an average runoff coefficient of 3.1%. The different treatments were established 4 years after planting the olive trees, and a significant decrease in soil and runoff losses was observed with time as the olive trees grew and their canopies developed. Measurements at the end of the experiment showed a significant improvement in the topsoil properties of the CC treatment as compared to CT and NT. The soil under NT presented a significant degradation with respect to traditional CT management. Organic matter values were 2.0, 1.4 and 1.0%, and stability in water of macroaggregates was 0.452, 0.418 and 0.258 kg kg−1 for CC, CT and NT, respectively. These results indicate that the use of a cover crop can be a simple, feasible soil and water conservation practice in olive groves on rolling lands in the region. A key factor in its practical use is to establish it early enough to protect the soil in the critical initial years of the grove, when most of the soil is unprotected by the small olive canopy.

Olive is one of the most emblematic crops in Spain, and especially in Andalusia, its southernmost region. Andalusia is the main olive cultivation area in the world as it produces 39% of the world's olive oil and 24% of the table olive production (average figures of the period from 2000 to 2003 given by Consejería de Agricultura y Pesca, 2006 and International Olive Oil Council, 2006). This production is achieved in Andalusia by devoting 17% of its total area to the olive. The olive crop represents 25% of the value of the Andalusian agrarian production (CEH-JA, 2006). Although commercial olive cultivation has been present in Andalusia since Greek and Roman times (Semple, 1931, Chapter XIV), its current position as the most important single soil use in Andalusia has been reached through several eras of expansion, from the second half of the 16th century to the early decades of the 20th century (Guzmán, 2005). Historically, olive cropping has been concentrated on hilly lands, where olive trees had to adapt to shallow, stony soils, and to dry conditions, whereas herbaceous crops were cultivated in flat areas with more fertile soils. This explains why, despite a recent trend towards new, more intensive plantations in valley areas, most olive groves in Andalusia are rainfed and planted in sloping areas. To date, 31% of the olive acreage in Andalusia is located on very steep terrains, on slopes of above 15%; 38% of the acreage is on moderate slopes, in the 7–15% range; the rest on slopes of under 7%. Only 16% of the olive acreage in Andalusia is on slopes of below 5% (Consejería de Agricultura y Pesca, 2003). Traditional olive production is based on low tree densities, i.e. about 100 trees ha−1, weed control via frequent tillage and canopy size limited by pruning, to ensure the productivity and survival of the plantation in a limited rainfall environment. These features contribute to understanding why olive production in the region has been associated with severe soil erosion problems ( Gómez et al., 2003) accompanied by fertility depletion and loss of biodiversity ( Beaufoy, 2001), that worsened with the arrival of farm mechanization in the early 1960s. Alternative soil management practices to conventional tillage, hereafter CT, have been developed, partially encouraged by the concern raised by water erosion (Pastor et al., 1999). These alternatives consist basically of no-tillage with herbicides to maintain a bare, weed-free soil, hereafter, NT; or the use of a cover crop grown during autumn and winter, either sown in early autumn or obtained via regeneration of the natural vegetation after the onset of rains, hereafter, CC. The cover crop is controlled by tillage, mowing or spraying with herbicide in early spring to prevent competition with the olive tree for water and nutrients. Several studies have reported the use of runoff plots to evaluate soil erosion in mature olive groves. Raglione et al. (1999), in Southern Italy, measured total soil losses of 0.36 and 41 t ha−1 year−1 for CC and CT, respectively, in a 2-year plot experiment. Kosmas et al. (1996) measured annual soil losses between 0 and 0.03 t ha−1 year−1 in semi-natural olive groves in Greece with 90% of the soil covered by vegetation. In a 2-year study in Andalusia, Spain, Francia et al. (2006) measured soil losses of 25.6, 2.1 and 5.7 t ha−1 from NT, CC and CT, respectively. Also in Andalusia, Gómez et al. (2004) reported average soil losses of 8.5, 1.2 and 4.0 t ha−1 from NT, CC and CT in a 3-year experiment on a heavy clay soil, and Gómez and Giráldez (2007) reported average soil losses of 21.5 and 0.4 t ha−1 for CT and CC in a different 4-year experiment. Bruggeman et al. (2005) measured average soil losses of 41.4 and 11.2 t ha−1 year−1 in orchards under CT and CC, respectively, in Syria in an area with a slope of 24% for a 4-year period. Even though the results of all these trials support the use of CC over other soil management practices for reducing runoff and soil loss, many of them had a limited duration (4 years at the most). The effect of soil management on soil properties in olive orchards has been assessed in a few studies conducted in commercial farms (e.g. Álvarez et al., 2007, Álvarez et al., 2008, Milgroom et al., 2007, Soria et al., 2005, Hernández et al., 2005, Gómez et al., 1999 and Vanderlinden et al., 1998). In general, these studies detected an improvement in the infiltration rate and in the organic matter content in the top soil of groves under CC management. Despite the work described above, long-term studies that evaluate simultaneously the effects of different soil management on soil properties and on soil and runoff losses in olive groves are, to our knowledge, lacking. The results of a 7-year study in which the effects of three soil management methods on soil properties and on runoff and sediment losses were measured in a commercial olive orchard are presented here.

The experiment results link the reduction in soil and runoff losses under CC soil management to significant improvements in key soil properties during 7 years in an intensive olive plantation on a heavy clay soil, typical of the new expansion areas of olive cultivation in southern Spain. These results firmly suggest that CC management should be a requirement for new plantations on sloping lands, given its important effects on resource conservation, and the relatively small costs with regard to the investment required by intensive plantations. Alternative soil management methods such as chemical weed control that maintains the soil bare without tillage, NT, will lead to severe sediment losses and the worsening of the water balance by increasing runoff. It is expected that, under NT, there will be a degradation of soil properties as compared to CT, but the latter management approach will also result in significant soil losses, especially during the first years of the plantation, and lower soil quality as compared to CC. In intensive plantations, where trees cover a significant fraction of the soil, as occurred near the end of our experiment (around 40%), our results suggest that under short slope lengths and low slope steepness an olive grove might be managed under CT with moderate soil and runoff losses, although our results would have to be validated at higher space and time scales. The greater degradation of soil properties under CT as compared to CC indicates that a cover crop may be the preferred management option, either alone, or combined with CT depending on the farm's circumstances.