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

Felipe José Cury Fracetto,Felipe Martins do Rêgo Barros,Marcos Siqueira Neto,Giselle Gomes Monteiro Fracetto,Mario de Andrade Lira Junior

doi:10.1590/0103-8478cr20190446

Felipe José Cury Fracetto, Felipe Martins do Rêgo Barros + Show 3 more

Open Access

https://doi.org/10.1590/0103-8478cr20190446

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Abstract

ABSTRACT: There are few studies of microbial diversity in castor bean soils in tropical semiarid environmental. Castor bean products have been widely used around the world justifying the commercial importance of ricinoculture in Brazil’ northeastern semiarid. There is no fertilization or maintenance of litter on the soils from the present study, so we hypothesized that the free-living diazotrophs microorganisms drive the nitrogen (N) input into these soils. We evaluated the communities’ structure and diversity of diazotrophs in tropical semiarid soils with i-Castor bean 50 years of cultivation intercropping with maize (CB-50); ii-Only Castor bean 10 years of cultivation (CB-10) and iii-soil under Caatinga vegetation (CAA). Nitrogenase enzyme activity was performed by the acetylene reduction assay (ARA) and the community profile of nifH gene was separated by denaturing gradient gel polyacrylamide electrophoresis (DGGE). Diazotrophs diversity was estimated by the Shannon (H’) and Simpson (D) indexes. Based on the maintenance of soil N stocks and N-microbial biomass over time, the activity of free-living diazotrophs was determinant in the N-input into these soils. The nifH gene was present and its profiles grouped the CB-50 with CAA treatments, so it reinforced the N-fixing by diazotrophs, which presented a greater diversity in the cultivated soils, even after the land use change for the castor bean implementation.

Highlights

The castor bean (Ricinus communis L) is an oilseed of the Euphorbiaceae family and has xerophilic plant characteristic with good adaptation in different climatic conditions
According to FRACETTO et al (2012) data, soil C and N stocks (0-30 cm) in the same areas of the present study did not show significant variation between CB-10 and CB-50 (~ 40 Mg ha-1 of C and 5 Mg ha-1 of N for both ages of castor bean cultivation) and it remaining constant over time among the different ages of castor bean evaluated
The soil C and N stocks were higher in the soil with native vegetation (~ 90 Mg ha-1 C and 10 Mg ha-1 N) as they expected. These authors emphasized that the land use change caused a reduction of approximately 50% into the soil C and N stocks on the first 10 years of the crop implantation, but the recovery into soil N contents was certified and this has kept soil N stocks for the 40 years

Summary

Introduction

The castor bean (Ricinus communis L) is an oilseed of the Euphorbiaceae family and has xerophilic plant characteristic with good adaptation in different climatic conditions. There are studies involving the carbon and nitrogen cycling in castor bean soils (FRACETTO et al, 2012; FRACETTO et al, 2015), but the structure and diversity of the free-living diazotrophs in these. The diazotrophs species have been identified in diverse taxonomic groups that storage N into the soil and according to WANG et al (2017), N-storage by Archaea domain was limited to the Euryarchaeota phylum, but 13 phyla from Bacteria domain can drive the N-input in the soil. During the biological nitrogen fixation (BNF), the nifH gene encodes the iron-protein or component II, i.e. a homodimer from the nitrogenase complex. The nitrogenase enzyme is necessary to break the N2 triple bond and it reducing this molecule to ammonia and consuming ATP (KACZMAREK et al, 2018)

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