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15 Pág, The plantation of exotic species has been a common practice in (semi-) arid areas worldwide aiming to restore highly degraded habitats. The effects of these plantations on plant cover or soil erosion have been widely studied, while little attention has been paid to the consequences on soil quality and belowground biological communities. This study evaluates the long-term (>60 years) effects of the exotic species Acacia cyclops and Pinus halepensis revegetation on soil properties, including microbiome, in an arid island. Soils under exotic plantation were compared to both degraded soils with a very low cover of native species and soils with well-preserved native plant communities. Seven scenarios were selected in a small area (~25 ha) with similar soil type but differing in the plant cover. Topsoils (0-15 cm) were analyzed for physical, chemical and biochemical properties, and amplicon sequencing of bacterial and fungal communities. Microbial diversity was similar among soils with exotic plants and native vegetation (Shannon's index = 5.26 and 5.34, respectively), while the most eroded soils exhibited significantly lower diversity levels (Shannon's index = 4.72). Bacterial and fungal communities' composition in degraded soils greatly differed from those in vegetated soils (Canberra index = 0.85 and 0.92, respectively) likely due to high soil sodicity, fine textures and compaction. Microbial communities' composition also differed in soils covered with exotic and native species, to a greater extent for fungi than for bacteria (Canberra index = 0.94 and 0.89, respectively), due to higher levels of nutrients, microbial biomass and activity in soils with native species. Results suggest that reforestation succeeded in avoiding further soil degradation but still leading to relevant changes in soil microbial community that may have negative effects on ecosystem stability. Information gained in this research could be useful for environmental agencies and decision makers about the controversial replacement of exotic plants in insular territories., This study has been financed by the Regional Ministry of Ecological Transition, the Fight against Climate Change and Territorial Planning of the Canary Islands Government within the framework of the FEDER Operational Program (2014–2020). The authors would like to thank Sofía Rodríguez Núñez and Carlos Samarín Bello, technicians from the environmental area of the Canary Islands government for their invaluable support throughout this research.We are also grateful to María del Mar Albarrán for her expert guidance and assistance with the analysis of soil biochemical parameters.

RNA-based high-throughput sequencing is a valuable tool in the discernment of the implication of metabolically active bacteria during composting. In this study, “alperujo” composting was used as microbial model for the elucidation of structure-function relationships with physicochemical transformation of the organic matter. DNA and RNA, subsequently retrotranscribed into cDNA, were isolated at the mesophilic, thermophilic and maturation phases. 16S rRNA gene was amplified by quantitative PCR (qPCR) and Illumina MiSeq platform to assess bacterial abundance and diversity, respectively. The results showed that the abundance of active bacteria assessed by qPCR was maximum at thermophilic phase, which confirm it as the most active stage of the process. Concerning diversity, Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the main phyla presented in composts. Concomitantly, three different behaviours were observed for bacterial dynamics: some genera decreased during the whole process meanwhile others proliferated only at thermophilic or maturation phase. Statistical correlation between physicochemical transformations of the organic matter and bacterial diversity revealed bacterial specialisation. This result indicated that specific groups of bacteria were only involved in the organic matter degradation during bio-oxidative phase or humification at maturation. Metabolic functions predictions confirmed that active bacteria were mainly involved in carbon (C) and nitrogen (N) cycles transformations, and pathogen reduction., This work was supported by the ERDF-co-financed projects from Junta de Andalucía (Spain) [P12-AGR-1968], Spanish MINECO [AGL2015-64582-CO3-02, AGL2017-84745-R] and MINECO-CSIC RECUPERA 2020 [20134R070]. G. Tortosa thanks the farmer J. González Almendros for his financial and technical assistance during AL composting. Support by Basque Country Government [IT-583 932-16] and the Spanish Red Sirena [AGL2015-68881-REDT] are also acknowledged. E. Aranda thanks MINECO-ERDF for supporting her Ramón y Cajal project [RYC-2013-12481] and [CTM2017-84332-R].

Mediterranean soils, which are carbonate-rich and organic matter-poor, are prone to erosion and important carbon losses due to seasonal changes associated with dry summers and wet winters. The use of thermophilic digested sewage sludge (TSS) in these agricultural systems, as a soil amendment, has been acknowledged as an interesting way to supply organic matter and nutrients. Data on the long-term evaluation of TSS applied to Mediterranean soils are scarce. Moreover, the effect of the application is unpredictable because of the intrinsic variation in the TSS. The scope of this study was to determine whether the continued application of TSS for 20 years leads to increased carbon sequestration in the soil without affecting emissions of greenhouse gases. To conduct this evaluation, the doses applied since 1992 have been as follows: 40 t ha and 80 t ha every year, and 40 t ha every 3 years, plus annual mineral N fertilization depending on the crop. A control without TSS or mineral fertilization and a treatment with only mineral N fertilizer were also evaluated. In this case, urea was used as the mineral treatment. The TSS doses were applied annually in October, while the mineral was split into one dose around January and another in March. The chemical parameters, greenhouse gas emissions, nitrate and ammonium concentrations of the soil were measured during the crop cycle. The bacterial community and enzymes in the soil were surveyed 15 days after the last annual application and at harvest. Fifteen days after fertilization with TSS and urea, nitrification and denitrification potentials were measured. The 80 t ha yr dose yielded the most significant increase in total carbon, organic matter content, PO, and total nitrogen. This same treatment significantly increased GHG emissions for all gases concerned. Similar results were found in the 40 t ha 3yr and urea for CO and COeq ha. TSS application increased soil enzyme activities. According to the microbial diversity results, 80% of the DNA sequences corresponded to 6 main phyla: (from most to least) Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi and Verrucomicrobia with unclassified material making up an average 10.94% of the sequences. The soil microbial community structure only altered with the 80 t ha yr dose. The highest dose of TSS applied in this study resulted in the irreversible lodging of the crop and a concomitant decrease in yield. In the 40 t ha 3yr treatment, interesting similarities were found with urea alone. In summary, rational application of TSS, such as 40 t ha 3yr dose, along with urea, trigger a beneficial increase in microbial activity in soil, that ultimately activates soil metabolism and enhances carbon sequestration possibilities, while GHG emissions remain at the same level as with urea alone. The results support the hypothesis that TSS can induce carbon sequestration without increasing GHG emissions. TSS has proven to exert beneficial outcomes under Mediterranean conditions; additionally, its application offers a viable opportunity for converting this by-product into a fertilizer. However, application rates must be adjusted or it should be used together with mineral fertilization., This work was supported by the Spanish Commission of Science and Technology under project number AGL2012-37815-C05-05 and project P08-CVI-03549 from the Consejería de Innovación, Ciencia y Empresa of the Junta de Andalucía. The current study involved collaborative work between the INTIA- ITG Division and MCP, as well as Estación Experimental del Zaidín (CSIC) and the Public University of Navarre (UPNA). M.E. Calleja-Cervantes held a grant from the Public University of Navarre. We would particularly like to thank Sandra Blazquez and Alfonso Amorena for their invaluable support.

The main goal of this study was to assess the effect of the inoculation of four autochthonous shrub species with the arbuscular mycorrhizal (AM) fungus Rhizophagus intraradices on the rhizosphere bacterial community and to ascertain whether such an effect is dependent on the host plant species. Additionally, analysis of rhizosphere soil chemical and biochemical properties was performed to find relationships between them and the rhizosphere bacterial communities. Non-metric multidimensional scaling analysis and subsequent permutational multivariate analysis of variance revealed differences in bacterial community composition and structure between non-inoculated and inoculated rhizospheres. Moreover, an influence of the plant species was observed. Different bacterial groups were found to be indicator taxonomic groups of non-inoculated and inoculated rhizospheres, Gemmatimonadetes and Anaerolineaceae, respectively, being the most notable indicators. As shown by distance based redundancy analysis, the shifts in bacterial community composition and structure mediated by the inoculation with the AM fungus were mainly related to changes in plant nutrients and growth parameters, such as the shoot phosphorus content. Our findings suggest that the AM fungal inoculum was able to modify the rhizosphere bacterial community assemblage while improving the host plant performance., R.C.G. acknowledges a predoctoral fellowship (“Ayudas para contratos predoctorales para la formación de doctores 2013”) from the Spanish Ministry of Economy and Competitiveness. This research was supported by the Spanish Plan Nacional-FEDER Project CGL-2015-64168-R.