Impact of cover crops on functional response of soil microbial communities in a citrus orchard in the Canary Islands

Conservative agricultural practices have been identified as pivotal in mitigating the effects of global warming. These practices are essential to maintain soil fertility and ensure the productivity of crops. Ground cover crops are an example of this practice, which can be readily implemented in orchards. They confer numerous benefits to agrosystems, including the prevention of soil mechanical damage and erosion, the reduction of water evaporation, the enhancement of soil carbon sequestration, the facilitation of weed control, the increase of soil microorganism community stability, the promotion of beneficial specific taxa, and the improvement of soil multifunctionality. This study was conducted within the framework of the LIFE Vida for Citrus project (LIFE18 CCA/ES/001109), which had the primary objective of developing sustainable control strategies to enhance the resilience of citrus orchards under the threat of climate change and to prevent the entry of Huanglongbing (HLB), or citrus greening disease, into the European Union. The objective of the present study was to evaluate the benefits of cover crops under the climatic and edaphic conditions of the Canary Islands (Spain), in the functional diversity of edaphic microbiota in a citrus orchard. In the Canary Islands, areas with little or poor soil depth (normally located below 300 meters above sea level) are usually modified by creating terraces with more fertile soil from higher altitudes (300-700 m.a.s.l.) situated in the northern side of the islands. This practice allows for more favorable cultivation, but degrades the original soil. The cover crops that were evaluated included the grass Lolium arundinaceum (Schreb.) Darbysh and a combination of flowering species, such as Lobularia maritima (L.) Desv., Diplotaxis tenuifolia (L.) DC., Calendula arvensis L., Medicago sativa L., Trifolium repens L., and Petroselinum crispum (Mill.) Fuss. The community level physiological profiles were measured, by the MicroResp^TM method, after three years of groundcovers sowing. Total and oxidizable organic matter, as well as total nitrogen content, were also evaluated. The multiple substrate-induced respiration (MSIR) profiles were found to be influenced by the utilization of the cover crop in comparison to the bare soil, which exhibited a substantial impact on the individual respiration rates for 16 of the 18 tested substrates (p < 0.050). The soil under groundcovers exhibited the highest consumption (between 2.2 and 3.0 times higher MSIR) of simple and complex carbohydrates, linear and aromatic carboxylic acids, and amino acids and amino sugars, in comparison to the bare soil (p < 0.050). Additionally, the highest respiratory responses were exhibited after the addition of γ-aminobutyric acid, arabinose and α-ketoglutaric acid, ranging from 0.773 ± 0.370 and 2.34 ± 1.04 μg C-CO₂ · g^-1 · h⁻¹. The diversity of the soil microbial community is a sensitive means to assess soil health in the implementation of conservative agriculture practices in the citrus orchards.