Almond (Prunus dulcis Mill. [D.A. Webb]) is the third most widely spread crop in Spain and has traditionally been cultivated in marginal areas and shallow soils under rainfed conditions. However, it recently has been progressively introduced in flat irrigated areas. The implementation of cover crops in the inter-rows of woody crops has been proven as a suitable strategy to reduce the runoff and soil erosion but they also can boost soil quality and health. A field experiment was conducted during two-monitoring seasons to examine the soil nitrogen and carbon sequestration potential of three seeded cover crops [barley (Hordeum vulgare L.), hairy vetch (Vicia villosa Roth), and a mixture of 65% barley and 35% vetch] and a control of spontaneous flora in irrigated almond orchards (SW Spain). Here, we show that barley provided the highest biomass amount, followed by mixture covers, vetch, and the control treatment. Also, vetch covered the soil faster in the growing stage, but its residues were decomposed easier than barley and mixture treatments during the decomposition period after mowing, providing less soil protection when the risk of water erosion with autumn rainfall is high. On the other hand, vetch improved soil nitrate content by over 35% with respect to barley and mixture treatments at 0–20 cm soil depth throughout the studied period. In addition, a greater carbon input to the soil was determined in the barley plot. That is, the mixture and barley cover crops had higher potential for carbon sequestration, augmenting the soil organic carbon by more than 1.0 Mg ha⁻¹ during the study period. Thus, taking into consideration the findings of the present experiment, the establishment of a seeded cover crop would be more advisable than spontaneous flora to mitigate soil erosion, enhancing soil fertility and carbon sequestration in irrigated almond plantations in Mediterranean semi-arid regions.

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.

SUMMARY: The 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/m²fine; II=2·65 kg/m²fine+1·12 kg/m²thick; III=5·30 kg/m²fine; IV=5·30 kg/m²fine+2·24 kg/m²thick; 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.

Cover crops (CC)s are increasingly employed by farmers in olive groves. Spontaneous soil cover is the most commonly used CC. Its continuous utilization changes ruderal flora. It is necessary to study new CCs. Living CCs provide C and nutrients to soil during decomposition. Information on this issue in olive groves is scarce. A 4-year field study involving grab sampling of Brachypodium distachyon, Sinapis alba and spontaneous CC residues was conducted to study C and nutrient release from cover crop residues. Throughout the decomposition cycles, C, N and P release accounted for 40 to 58% of the C, N and P amounts in the residues after mowing. Most K was released (80–90%). Expressed in kg per hectare, the release of C and N in Brachypodium (C: 4602, N: 181, P: 29, K: 231) and Sinapis (C: 4806, N: 152, P: 18, K: 195) was greater than that in spontaneous CC (C: 3115, N: 138, P: 21, K: 256). The opposite results were observed for K. The Rickman model, employed to estimate the amount of C, N and P in residues, yielded a good match between the simulated and measured values. In comparison to spontaneous CC, the newly proposed CCs have a higher potential to provide soil with C and N.