The Impact of Mineral Nitrogen Fertilization on the Occurrence of Native Diazotrophic Bacteria in Kohlrabi (Brassica oleracea) Shoots and Roots

Lempie K Ekandjo ,Silke Ruppel

doi:10.5539/jas.v7n2p1

Abstract

Biological Nitrogen Fixation (BNF) is a process of great importance in crop production systems, as it provides additional natural sources of mineral nitrogen. BNF is catalyzed by diazotrophs that are identified by the nif operon presence comprising the nifH gene that encodes for enzyme nitrogenase synthesis. Thoroughly understanding of factors that influence diazotrophic abundance is crucial for their utilization to enhance sustainability and prevent land degradation in modern agriculture. In this study the impacts of nitrogen fertilization on diazotrophic abundance in Brassica oleracea roots and leaves was investigated in greenhouse experiments by real-time qPCR. One way ANOVA was used to compare means and bivariate Pearson correlation tested for relationships between variables. Increased nitrogen fertilization significantly increased the nitrogen content in leaves but not in roots. No significant changes in nifH gene copy numbers nor in proportion of nifH gene copy numbers were detectable. This indicates no effect of mineral N fertilization on the abundance of total native diazotrophic bacterial numbers in Brassica oleracea plants.

Highlights

Biological nitrogen fixation (BNF) is a process through which the atmospheric inert gas N level 2 (N2) is converted to nutrient ammonia (NH3)
To demonstrate the N-fertilization impact on plant growth and nitrogen content, leaves and roots were sampled and analyzed separately after a growth period of 8 weeks. For both leaves and roots, provision of nitrogen fertilization resulted in a substantially increased plant dry matter between N0-N level 1 (N1), further increase in fertilization did not result in continuous increase in plant dry matter (Figure 1)
Roots did not exhibit the same trend; the lowest mean was recorded at N0 (0.62 ± 0.79 g, n = 8) but the highest mean was at N1 (1.69 ± 0.22 g, n = 8) and values remained essentially constant from N1 to N level 4 (N4) (Figure 2)

Summary

Introduction

Biological nitrogen fixation (BNF) is a process through which the atmospheric inert gas N2 is converted to nutrient ammonia (NH3). The nifH gene codes for dinitrogenase reductase and is part of the nif cluster that produces the nitrogenase catalytic enzyme complex and its regulatory proteins that are responsible for reducing atmospheric N2 to ammonia (Sugitha & Kumar, 2009; Rosado et al, 1998; Poly et al, 2001). This functional gene has been a center of focus for researchers studying detection, dynamics and diversity of bacterial nitrogen fixing communities (Coelho et al, 2007b) and a potential indicator of microorganisms’ ability to carry out BNF. NifH is utilized in phylogenetic, diversity, and abundance studies of diazotrophic microorganisms (Gaby & Buckley, 2012; Sugitha & Kumar, 2009)

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Discussion

Conclusion