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

The tropical forage grass Brachiaria humidicola (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate ([Formula: see text]) formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reduced [Formula: see text] losses and enhanced N uptake due to BNI has not been experimentally demonstrated yet. Nitrification discriminates against the (15)N isotope and leads to (15)N depleted [Formula: see text] , but (15)N enriched [Formula: see text] in soils. Leaching of (15)N depleted [Formula: see text] enriches the residual N pool in the soil with (15)N. We hypothesized that altered nitrification and [Formula: see text] leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil (15)N natural abundance (δ(15)N), which in turn is reflected in distinct δ(15)N in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant δ(15)N of Bh. It was our objective to investigate under controlled conditions the link between shoot value of δ(15)N in several Bh genotypes and leached [Formula: see text] amounts and shoot N uptake. Additionally, plant (15)N and N% was monitored among a wide range of Bh genotypes with contrasting BNI potentials in field plots for 3 years. We measured leaf δ(15)N 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 δ(15)N and BNI. Increased leached [Formula: see text] was positively correlated with higher δ(15)N in Bh, whereas the correlation between shoot N uptake and shoot δ(15)N was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot δ(15)N declined and shoot N% increased over time. Leaf δ(15)N of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant δ(15)N of Bh genotypes over time reflects the long-term effect of BNI as linked to lower [Formula: see text] formation and reduced [Formula: see text] leaching. Accordingly, a low δ(15)N in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.