Soil erosion is intense on steep slopes, where many olive orchards are located in Mediterranean areas. The adoption of cover crops is a promising soil and water conservation practice for these areas. Nevertheless, there has not been enough information to be able to advise farmers on the selection of plant species. The purpose of this report is to assess different plant species as cover crops to reduce erosion and soil organic carbon loss through sediments. Twenty‐four tests were performed in 6 plots on a 20% slope in southern Spain. A gramineous plant (Brachypodium distachyon), 2 leguminous species (Vicia sativa and Vicia ervilia), and a cruciferous plant (Sinapis alba) were sown and compared with spontaneous vegetation and conventional tillage. Simulated rainfall with intensities of 18.1 (±1.6) and 38.8 (±2.3) mm hr−1 was applied during 2 years. All cover crop treatments, in comparison with tillage, significantly reduced soil and soil organic carbon losses by more than 92%, S. alba being the species with the lowest runoff values. The high rate of soil and water losses observed in the tillage treatment emphasizes the need to protect the soil and its fertility. A kinematic wave model considering variable soil infiltration rates was fitted to the runoff data to evaluate relevant soil and surface characteristics. The estimated saturated hydraulic conductivity and length of the capillary scale were not affected by the treatments, but the surface resistance to water flow indicated the efficiency of S. alba, B. distachyon, and V. ervilia in reducing the runoff velocity.

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SUMMARYThe arrival on the market of various types of mulchers and chippers has boosted the use of pruning residues as plant cover among olive growers. In order to increase knowledge regarding the decomposition of these types of residues and their effect on soil fertility, an experiment was performed using different doses and sizes of pruning residues applied on the areas between the lines of olive trees in an organic olive grove.Experiments were conducted over a period of two growing seasons (2009/10 and 2010/11). Treatments consisted of fine (⩽8 cm in diameter) and thick (>8 cm in diameter) pruning residues in the amounts indicated, I=2·65 kg/m2 fine; II=2·65 kg/m2 fine+1·12 kg/m2 thick; III=5·30 kg/m2 fine; IV=5·30 kg/m2 fine+2·24 kg/m2 thick; and a control without residues.As regards the loss of biomass and nutrients during the decomposition of residues, two phases were observed. First, soluble compounds were degraded during a rapid initial phase, while in a second and slower phase, lignocellulosic compounds were decomposed. As a result, the pattern over time of nitrogen (N), phosphorus (P) and potassium (K) release fitted a double exponential model better, regardless of the treatment considered, registering in most cases determination coefficients close to one.The favourable results observed in terms of augmentation in N, P and K soil content following the application of pruning residues confirmed a greater improvement in soil fertility than the soil covered by spontaneous weeds, which is the option most frequently adopted by organic olive growers. The initial amount of pruning residues has influenced the amount of soil nutrients. Considering the entirety of the soil profile (0–40 cm) and the content of these elements in the soil, treatment III, which contained the largest amount of fine residues, was the most efficient in terms of improving soil fertility, recording increases in the concentration of N, P and K of 1805·4, 53·1 and 598·7 kg/ha, respectively. The most unfavourable results were recorded by treatment I, with increases of 480·9 kg/ha in the case of N and a decrease in P content with regard to the control sample. Treatment II increased K (recording 215·2 kg/ha) which was the least in comparison to the control sample.

There are many factors involved in the release of CO2 emissions from the soil, such as the type of soil management, the soil organic matter, the soil temperature and moisture conditions, crop phenological stage, weather conditions, residue management, among others. This study aimed to analyse the influence of these factors and their interactions to determine the emissions by evaluating the environmental cost expressed as the kg of CO2 emitted per kg of production in each of the crops and seasons studied. For this purpose, a field trial was conducted on a farm in Seville (Spain). The study compared Conservation Agriculture, including its three principles (no-tillage, permanent soil cover, and crop rotations), with conventional tillage. Carbon dioxide emissions measured across the four seasons of the experiment showed an increase strongly influenced by rainfall during the vegetative period, in both soil management systems. The results of this study confirm that extreme events of precipitation away from the normal means, result in episodes of high CO2 emissions into the atmosphere. This is very important because one of the consequences for future scenarios of climate change is precisely the increase of extreme episodes of precipitation and periods extremely dry, depending on the area considered. The total of emission values of the different plots of the study show how the soils under the conventional system (tillage) have been emitting 67% more than soils under the conventional agriculture system during the 2010/11 campaign and 25% for the last campaign where the most appreciable differences are observed.