Abstract
The tropical forage grass Brachiaria humidicola (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate () formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reduced 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 , but 15N enriched in soils. Leaching of 15N depleted enriches the residual N pool in the soil with 15N. We hypothesized that altered nitrification and 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 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 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 formation and reduced leaching. Accordingly, a low δ15N in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.
Highlights
MATERIALS AND METHODSBiological nitrification inhibition (BNI) by the tropical forage grass Brachiaria humidicola (Sylvester-Bradley et al., 1988; Subbarao et al, 2007) is an ecologically evolved trait to compete with nitrifying soil organisms for available ammonium (NH+4 )
We studied the interlinkages of plant δ15N, biological nitrification inhibition (BNI), microbial nitrification, N uptake, and N leaching losses under controlled as well as under field conditions based on a selection of contrasting Brachiaria humidicola (Bh) genotypes
Our main conclusion is that high BNI activity decreases plant δ15N of Bh
Summary
Biological nitrification inhibition (BNI) by the tropical forage grass Brachiaria humidicola (Sylvester-Bradley et al., 1988; Subbarao et al, 2007) is an ecologically evolved trait to compete with nitrifying soil organisms for available ammonium (NH+4 ). By preventing the microbial transformation of less soil mobile NH+4 to more soil mobile nitrate (NO−3 ), BNI is expected to reduce leaching of nitrogen (N) from ecosystems (Subbarao et al, 2009, 2013; Coskun et al, 2017b). The reduction of NO−3 leaching from agroecosystems due to BNI is one of the claimed central features in terms of BNI implementation (Subbarao et al, 2013; Coskun et al., 2017b)
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