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Tropentag, September 20 - 22, 2017 in Bonn

"Future Agriculture: Social-ecological transitions and bio-cultural shifts"

δ15N Leaf Signature in Brachiaria humidicola Reflects the Potential Biological Nitrification Inhibition (BNI)

Hannes Karwat1, Konrad Egenolf1, Jacobo Arango2, Jonathan Núñez2, Danilo Moreta2, Idupulapati Rao2, Frank Rasche1, Georg Cadisch1

1University of Hohenheim, Inst. of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), Germany
2International Center for Tropical Agriculture (CIAT), Colombia


The tropical forage grass Brachiaria humidicola (Bh) reduces soil microbial nitrification by biological nitrification inhibition (BNI) and consequently reduces formation of nitrate (NO3-) in soils. NO3- leaching and nitrous oxide (N2O) emission might therefore be reduced by BNI. Intraspecific contrasting BNI potentials in Bh have been observed but screening methods for field plots need to be further developed to identify high BNI Bh candidates.

Nitrification discriminates against the stable isotope 15N and leads to a 15N enriched ammonium (NH4+) and a 15N depleted NO3- pool. It was hypothesised that high BNI Bh genotypes would mainly feed on NH4+ and lower δ15N values in leaves are expected whereas low BNI (and high nitrification) should cause respective higher δ15N leaf signatures under the assumption that NO3- has been leached out of the rooting zone.

Contrasting BNI genotypes were grown in the Llanos of Colombia for 3 years. Plots were split and either fertilised (+N) or not (-N) with 70 kg N ha-1. Soil was collected and incubated for potential nitrification determination. Leaves were collected from both split plots of two high BNI (CIAT 679 and CIAT 16888) and one low BNI (CIAT 26146) Bh hybrid frequently after fertilisation and 15N was measured with an IRMS. As reference NO3- in shoots were measured simultaneously and NO3- in topsoil determined at 8 DAF.

A strong correlation (P=0.006, R2=0.38) was observed between means of δ15N leaf values and soil NO3- at 8 DAF. High BNI Bh showed respective lower δ15N signatures and less NO3- in stems compared to the low BNI Bh at 11 DAF. Soil incubation indicated lower nitrification for high BNI genotypes compared to the low BNI Bh.

It could be demonstrated that δ15N leaf signatures and BNI are strongly linked. However it needs to be considered that the leaf δ15N might also be influenced by other factors such as N fractionation under high N availability or uptake of both N forms from soil N min pools with different δ15N values. It was concluded that the technique has the potential to screen for contrasting BNI genotypes within Bh.

Keywords: Isotope discrimination, N assimilation, N uptake, nitrate leaching, soil incubation

Contact Address: Hannes Karwat, University of Hohenheim, Inst. of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), Garbenstr.13, 70599  Stuttgart, Germany, e-mail: hannes.karwat@posteo.de

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