How binomial (traditional rainfed olive grove-Crocus sativus) crops impact the soil bacterial community and enhance microbial capacities.

Intercropping can favour the yield of the main crop. However, because of the potential competition among woody crops, this system is rarely used by farmers. To increase knowledge about the intercropping system, we have explored three different combinations of alley cropping in rainfed olive groves compared to conventional management (CP): (i) Crocus sativus (D-S); (ii) Vicia sativa/Avena sativa in annual rotation (D-O); and (iii) Lavandula x intermedia (D-L). Different soil chemical properties were analyzed to evaluate the effects of alley cropping, while 16S rRNA amplification and enzymatic activities were determined to study the changes that occurred in soil microbial communities and activity. In addition, the influence of intercropping on the potential functionality of the soil microbial community was measured. Data revealed that the intercropping systems highly affected the microbial community and soil properties. The D-S cropping system increased soil total organic carbon and total nitrogen that were correlated with the bacterial community, indicating that both parameters were the main drivers shaping the structure of the bacterial community. The D-S soil cropping system had significantly higher relative abundances of the phyla Bacteroidetes, Proteobacteria, and Patescibacteria compared to the other systems and the genera Adhaeribacter, Arthrobacter, Rubellimicrobium, and Ramlibacter, related to C and N functions. D-S soil was also related to the highest relative abundances of Pseudoarthrobacter and Haliangium, associated with the plant growth-promoting effect, antifungal activity, and a potential P solubilizer. A potentially increase of C fixation and N fixation in soils was also observed in the D-S cropping system. These positive changes were related to the cessation of tillage and the development of a spontaneous cover crop, which increased soil protection. Thus, management practices that contribute to increasing soil cover should be encouraged to improve soil functionality.

Medium-term evaluation of the 4‰ initiative, soil organic carbon storage and stabilisation in a Mediterranean rainfed olive grove under conventional tillage: A case study.

This study aims to show the effect of conventional tillage (CT) in olive orchards in the medium term (15 years) on carbon (C) storage considering the complete soil profile, on the soil C sequestration and stabilisation capacity and on the viability for the achievement of Objective 4‰. The results obtained showed important losses in soil organic carbon (SOC) and SOC stock (SOC-S), with a significant loss of total SOC-S of 42.3%. Concerning the SOC and the SOC-S linked to the fine soil fraction (<20 µm), the evolution over time led however to a SOC increase in depth (BC and C horizons) of 58.3% and 20.9% and increases in SOC-S of 17.2%, 34.7% and 27.3% for the Ap, BC and C horizons, respectively. Finally, it was seen that the goals set by the 4‰ initiative were not met, as losses of 2.1 Mg C ha-1 yr-1 were found when considering the entire soil profile and 0.8 Mg C ha-1 yr-1 when considering only the first 40 cm. Therefore, we can affirm that medium-term CT has not only conditioned C storage in the soils studied, but also their capacity for sequestration and stabilisation, which has repercussions not only on the failure to meet the objectives of the 4‰ initiative, but also on the amount of C lost in 15 years.

Long-term evaluation of the initiative 4‰ under different soil managements in Mediterranean olive groves.

The 4‰ initiative implementation has increased the emphasis and interest in soil carbon and nitrogen storage in the last few years. This study evaluated the dynamics of soil organic carbon and total nitrogen under rain-fed olive groves over a long-term period (2004-2019). The management practices associated with achieving the 4‰ initiative objectives and the depth of analysis to measure the effectiveness of the initiative have generated uncertainties and wide debate in the scientific community. To contribute to this debate from a farm level, the objective of this study was to analyse the effects of conventional tillage and no-tillage with bare soil by using herbicides (after land management change from conventional tillage) on carbon and nitrogen stocks in complete soil profiles (depth > 100 cm) over 15 years in a Mediterranean olive grove. Soil samples were collected from each farm and analysed for carbon content and physical-chemical characteristics. This study indicates that management practices evaluated resulted in soil organic carbon and total nitrogen contents decreasing in soil, with a reduction >30% in all horizons. Results highlight a significant depletion of soil organic carbon stock with a significant decarbonisation process (-1.8 Mg C ha-1 yr-1) and total nitrogen stock (-0.57 and - 0.41 Mg N ha-1 yr-1) on average under both managements (no-tillage no tillage with herbicide and conventional tillage respectively) as compared to the initial situation. Furthermore, it was demonstrated that deep horizons are significant reservoirs of carbon (>50% in all cases) and in woody crops, its analysis within the dynamics of soil organic carbon stocks proposed by the 4‰ initiative was relevant. With these results, no-tillage with bare soil by using herbicides was demonstrated as an unsustainable agricultural practice and it is proposed to change the current soil management to sustainable management that increases the C inputs to achieve the 4‰ targets.

Short-term effects of land management change linked to cover crop on soil organic carbon in Mediterranean olive grove hillsides.

The spatial distribution of soil organic carbon (SOC) is essential to estimate the SOC reserves. Therefore, the soils ability to store organic carbon is a key factor for climate regulation and for other soil functions. The soil management and the topographic position play an important role in SOC variation, especially when the landscape is not uniform (Mediterranean areas). Many researches have explored the SOC distribution according to topographic position in hillsides for long-term, but very few studies have focused on the short term. Therefore, it is necessary to know, the changes that taking place in the soil due to land management change (LMC) in these irregular surfaces for sustainable agricultural production and its implications on climate change regulation. This study aims to assess the influence of topographic position and LMC on SOC stock (SOC-S) in Mediterranean olive groves (OG) soils in short term (2 years). In this line, three experimental plots were selected in three topographical position (summit - S, backslope - B and toeslope - T). In these plots, the land management was modified from conventional tillage (CT) to no tillage (NT) with application of pruned olive branch chippings branches and vegetation cover (spontaneous vegetation) in the OG streets. The studied soils did not show important changes due to LMC in their physical properties for short term, in addition, these soils were characterized by low organic matter content (<1.2%). LMC caused a SOC reduction in surface, and a SOC increase in the Bw horizon. The N concentrations showed a similar trend to SOC and the C:N ratios were highly variable (4.37: C horizon-NT-S; 13.45 Bw/C horizon -CT-B). Normally, the SOC-S concentrations decreasing in depth. LMC for two years showed soil carbonization (S and T position) and decarbonization (B position) processes. The SOC-S increased 1.88 Mg ha-1 y-1 and 0.47 Mg ha-1 y-1 for S and T topographic position respectively, however the SOC-S decreased in B position 5.27 Mg ha-1 y-1. Therefore, LMC has a positive effect on soil carbon reserves in S and T position, conversely in B position, this effect was negative.