Diversidade e abundância de bactérias totais, amônio-oxidantes e diazotróficos em solos sob sistema silvipastoril com leguminosas arbóreas

Abstract

Intercropping tree legumes with forage grasses in silvopastoral system is an alternative to avoid pastures degradation or recovering degraded pastures. In addition to allowing the nitrogen supply through the legume-rhizobium symbiosis, the improvement of the soil qualities enhances the action of free-living bacteria, fixing N2. Biological nitrogen fixation (BNF) and nitrification are key microbial processes of the N-cycle in soil and both can be studied based on indicator genes, such as the nifH gene, or specific regions of the 16S rRNA. This work evaluated the influence of the silvopastoral system with tree legumes on the structure, diversity and abundance of the community of total bacteria, ammonium-oxidizing bacteria and diazotrophic organisms and their relationship to soil chemical attributes. The study area is located at the Experimental Station from Instituto Agronômico de Pernambuco (IPA), in Itambé city, Pernambuco state, Brazil, which the climate is tropical subhumid and the predominant soils are classified as Ultisols. The experiment had nine plots of one hectare with tree legumes Sabiá (Mimosa caesalpinifolia) and Gliricídia (Gliricidia sepium), and a single Brachiaria (Brachiaria decumbens), in a randomized block design with three treatments: Brachiaria intercropping Sabia; Brachiaria intercropping Gliricídia and single Brachiaria, with three replicates. The samples were collected in wet and dry climatic seasons. In the consortium treatments the samples were collected at zero (0 m), four (4 m) and eight meters (8 m) from the legumes; while in single brachiaria the samples were randomly collected. The structure of the total bacteria communities, ammonium-oxidizing bacteria and diazotrophic organisms was evaluated by DGGE (Denaturing Gradient Gel Electrophoresis) and the genes were quantified by qPCR (real time quantitative PCR). The diversity and equitability of the microbial communities were estimated by the Shannon, Simpson and Pielou indices. The soil structure of the total bacterial, AOB and diazotrophic communities was influenced by the silvopastoral system, presenting different communities among the distances from the legumes, with greater dissimilarities in the wet season. The diazotrophic communities showed greater diversity and equitability under the silvopastoral system. The silvopastoril system presented a greater abundance of microorganisms, which of total bacteria and diazotrophic were more abundant in the dry season. The available phosphorus content correlated positively to the copies number of the nifH gene in the brachiaria withsabia intercropping, during the dry season, while the ammonium content presented negative correlation in the wet season. The cation exchange capacity correlated positively to the AOB abundance in the brachiaria with gliricidia intercropping in the dry season. Silvopastoral system with tree legumes improves the soil biological quality by favoring the microbial community linked to the N cycling.