Soil bacterial diversity and activity of olive mycorrhizal symbionts as affected by wool residues soil amendments

Organic matter amendments in agricultural soils have been recommended, together with the reduction of mineral fertilizers and pesticides, in order to improve soil fertility and carbon sequestration and decrease greenhouse gases emissions. Several studies utilized agricultural by-products, such as sheep wool residues, which stimulated plant growth and nutrition, enhanced soil moisture uptake and increased aggregate stability. Only few studies investigated the impact of sheep wool residues (SWR) on soil microbial communities using conventional microbiological analyses. Here, we investigated the impact of two SWR types (scoured residues, white wool, WW, and carbonized scoured residues, black wool, BW), used at 4 different SWR/soil ratios (0, 0.5, 1.0, 2.0%) as soil amendments in pot-grown olive trees, on the diversity and composition of soil bacterial communities by PCR-denaturating gradient gel electrophoresis (PCR-DGGE). Moreover, we evaluated potential detrimental effects of SWR on native arbuscular mycorrhizal fungi (AMF), an important group of beneficial root symbionts of most crop plants. Cluster analysis of DGGE profiles separated the bacterial communities of control soils from those of soils amended with SWR. Band sequencing showed the occurrence of Plant Growth Promoting bacteria, such as Ohtaekwangia sp., Massilia sp. and Chloroflexi in amended soils. The highest levels of BW (2%) significantly decreased the biodiversity index Richness and mycorrhizal colonisation of olive roots. AMF activity, evaluated by mycorrhizal inoculum potential bioassay did not differ significantly among the different treatments. Overall, our findings showed the positive role of SWR as organic soil fertilizers, supporting previous data on their beneficial effects on soil properties and plant nutrition, while detecting the type and concentration of SWR to be avoided in order to maintain bacterial diversity and AMF activity. Further studies will investigate the possible functional roles of the diverse bacteria genera and species developing in the differentially amended soils.