Low 15N Natural Abundance in Shoot Tissue of Brachiaria humidicola Is an Indicator of Reduced N Losses Due to Biological Nitrification Inhibition (BNI)

Abstract

The tropical forage grass <i>Brachiaria humidicola</i> (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> ) formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reduced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> losses and enhanced N uptake due to BNI has not been experimentally demonstrated yet. Nitrification discriminates against the ¹⁵N isotope and leads to ¹⁵N depleted <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> , but ¹⁵N enriched <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> in soils. Leaching of ¹⁵N depleted <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> enriches the residual N pool in the soil with ¹⁵N. We hypothesized that altered nitrification and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil ¹⁵N natural abundance (δ¹⁵N), which in turn is reflected in distinct δ¹⁵N in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant δ¹⁵N of Bh. It was our objective to investigate under controlled conditions the link between shoot value of δ¹⁵N in several Bh genotypes and leached <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> amounts and shoot N uptake. Additionally, plant ¹⁵N and N% was monitored among a wide range of Bh genotypes with contrasting BNI potentials in field plots for 3 years. We measured leaf δ¹⁵N of young leaves (regrown after cutback) of Bh and combined it with nitrification rates (NRs) of incubated soil to test whether there is a direct relationship between plant δ¹⁵N and BNI. Increased leached <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> was positively correlated with higher δ¹⁵N in Bh, whereas the correlation between shoot N uptake and shoot δ¹⁵N was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot δ¹⁵N declined and shoot N% increased over time. Leaf δ¹⁵N of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant δ¹⁵N of Bh genotypes over time reflects the long-term effect of BNI as linked to lower <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> formation and reduced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> leaching. Accordingly, a low δ¹⁵N in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.