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)

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

Abstract

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.karwatposteo.de