Spatial and seasonal responses of diazotrophs and ammonium-oxidizing bacteria to legume-based silvopastoral systems

Abstract Legume-based silvopastoral systems drive the soil total bacterial, diazotrophic and ammonium-oxidizing bacterial communities and represent a sustainable strategy for preserving and restoring soil health. These systems increase the input of organic matter and nutrients and can drive the biodiversity and abundance of soil microorganisms, which leads to an intensification of nutrient cycling, mainly nitrogen. However, spatial and seasonal variations in the nitrogen-cycling bacterial community in soils under silvopastoral systems have not been well demonstrated. Therefore, we investigated the influence of silvopastoral systems with shrub-tree legumes on the structure, diversity, and abundance of total bacterial, diazotrophic and ammonium-oxidizing bacterial communities at different sites around the legumes in wet and dry seasons. The experimental design consisted of i) pure signalgrass (Brachiaria decumbens), ii) a silvopastoral system consisting of signalgrass intercropping with gliricidia (Gliricidia sepium) and iii) signalgrass intercropping with Sabia (Mimosa caesalpiniaefolia) in a randomized block design with three replications each. Samples were collected at zero (0 m), four (4 m) and eight meters (8 m) away from the legume rows and randomly in pure signalgrass. We used DGGE (denaturing gradient gel electrophoresis) and real-time quantitative PCR (qPCR) to investigate the influence of treatments on the structure, diversity, and abundance of total bacteria, diazotrophs and ammonium-oxidizing bacteria (AOB). In addition, the samples were subjected to physical and chemical analysis. Soils under silvopastoral systems presented higher acidity, cation-exchange capacity (CEC) and nitrate (NO3−-N) content than those of the soil under pure signalgrass cultivation. The soil community structures of total bacteria, diazotrophs and AOB were influenced by the silvopastoral system and were not similar among all distances from the legume row, particularly in the wet season. The diversity of the diazotrophic community was higher in silvopastoral systems than in pure signalgrass. There was a greater abundance of microbial communities in both silvopastoral systems, in which total bacteria and diazotrophs were more abundant in the dry season than in the wet season. Silvopastoral systems with shrub-tree legumes improved soil biological quality by promoting the abundance and spatial and temporal heterogeneity of the nitrogen-cycling bacterial community.