Silvopastoral systems drive the nitrogen-cycling bacterial community in soil

Abstract

Intercropping tree legumes with forage grasses in a silvopastoral system can avoid pasture degradation benefiting the soil. In such a system, nitrogen (N) is supplied by symbiosis between legumes and bacteria. However, the pasture quality determines the action of free-living nitrogen-fixing bacteria, which possess nifH genes, which encode nitrogenase enzyme. Ammonium-oxidizing bacteria (AOB), involved in the nitrification step, can be evaluated by specific regions of the 16S rRNA corresponding to AOB. Thus, we investigated the influence of the introduction of tree legumes into a silvopastoral system on the community structure and abundance of total bacteria, diazotrophic bacteria and ammonium-oxidizing bacteria by DGGE (denaturing gradient gel electrophoresis) and real-time qPCR (quantitative PCR). The experiment involved nine plots of one hectare each, planted with sabia (Mimosa caesalpinifolia), a Gliricidia species (Gliricidia sepium), and a Brachiaria species (Brachiaria decumbens) in a randomized block design, forming three treatments: I-Brachiaria intercropped with sabia; II-Brachiaria intercropped with Gliricidia and III-Brachiaria only, with three replicates. The structures of the total bacterial and ammonium-oxidizing bacterial communities were influenced by tree legume introduction, possibly through modification of the soil chemical attributes. The copy numbers of total bacteria, ammonium-oxidizing bacteria and diazotrophic bacteria were higher in soils planted with legumes, which provided better conditions for microbial growth compared to planting with the Brachiaria species alone. Silvopastoral management with tree legumes improves the biological quality of soil, favouring the bacterial community linked to N-cycling.

Legume silvopastoral systems enhance soil organic matter quality in a subhumid tropical environment

“…Both silvopastoral systems had significant increases in soil total N content, likely due to biological N fixation in symbiosis with rhizobia, as also seen by Marini, Guimarães, Dartora, Lana, and Pinto Júnior (2015). Barros et al (2018) have shown that, with this experiment at 0 and 4 m away from legume row, the diazotrophic and total bacteria communities were similar and more abundant than under single signalgrass.…”

“…In northeastern Brazil, one such system of intercropping signalgrass (Brachiaria decumbens) with shrub-tree legumes (gliricidia [Gliricidia sepium] and sabia [Mimosa caesalpiniaefolia]) has shown significant amounts of biological N fixation (Apolinário et al, 2016). Barros et al (2018) have also shown this same system type benefits the diazotrophic microbial community.…”

Free-living diazotrophs drive castor bean nitrogen input in tropical semiarid soils

“…GABY & BUCKLEY (2017) used the nifH gene in PCR quantitative (qPCR), finding good representability among their samples; although, our non-quantitative amplification reaction has also been well elaborating. Recently, some studies in tropical and dry lands environments related the contribution of organic nutrients with the energy required to nifH genes express activity (BARROS et al, 2018;PEREG et al, 2018). However, we emphasized that in our research all litter inputs are withdrawn by local farmers and; therefore, only residues from the roots systems of castor bean and maize are contributing with the energy necessary for the nifH gene activity on these soils.…”

Estoques de carbono e nitrogênio no Sistema Silvipastoril com Núcleos: a nucleação aplicada viabilizando a pecuária de baixo carbono

“…Segundo Barros et al, (2018), a produção forrageira pode ser afetada pelo dossel sombreado dos sistemas silvipastoris ao longo do ano, especialmente em climas subtropicais e temperados. Isso pode afetar as características das plantas forrageiras fazendo com que as pastagens sombreadas não apresentem produtividades em níveis equivalentes às das pastagens sem sombra.…”

“…Ao estudarem o Kikuyu (Pennisetum clandestinum) planta com alto potencial de integração de carbono, Barros et al (2018) observaram que as raízes (20 cm), as hastes mortas e as folhas foram os compartimentos com os maiores estoques de carbono em sistemas silvipastoris com valores 4,52, 3,58 e 1,9 Mg C ha -1 ano -1 , respectivamente. Foi observado que biomassa total de raízes finas e grossas, desempenha um papel relevante na redução de GEE, na conservação de solos tropicais, na ciclagem de nutrientes, e consequentemente na massa do dossel forrageiro e carbono.…”