Effect of the conversion of conventional pasture to intensive silvopastoral systems on edaphic bacterial and ammonia oxidizer communities in Colombia

Colombia, as well as many tropical countries, has experienced severe deforestation in the last decades, and millions of acres of native forest areas (F) have been replaced by conventional monoculture pastures (CP), contributing to ecological and soil degradation. In response, multi-canopy intensive silvopastoral systems (ISS), which includes herbs, shrubs and trees, have been developed to provide local fodder sources for livestock while reducing the need for external inputs with a goal to conserve landscapes and improve soil quality. However, there is limited information on the temporal responsiveness of ISS to deliver ecosystem services as reflected in soil microbial properties. Therefore, the objective of this study was to examine the shifts of total and ammonia-oxidizing bacteria (AOB) communities along an ISS chronosequence (ranging from 3 to 15 years since establishment), in comparison to CP and native F and investigate P. juliflora trees as a resource island relative to soil microbial properties. Denaturing gradient gel electrophoresis (DGGE) fingerprints of 16S rRNA gene (total bacteria) as well as amoA gene (ammonia-oxidizing bacteria) (AOB) indicated that soil bacterial communities varied between the land uses, with higher similarities between F and ISS communities, in comparison to CP. The abundance and nitrification potential of ammonia oxidizers were significantly higher in CP and lower in F. In addition, the bacterial communities across ISS chronosequence were more similar between older (ISS-12) and intermediate (ISS-8) systems in comparison with youngest systems (ISS-3). Finally, the canopy of P. juliflora tree did not have an impact on structure of total bacterial community; though, it did have an effect on the structure of AOB communities. Our study suggests that ISS might restore some of the ecosystem services offered by soil microbial communities.