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

The tropical forage grass (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 N isotope and leads to N depleted , but N enriched in soils. Leaching of N depleted enriches the residual N pool in the soil with N. We hypothesized that altered nitrification and leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil N natural abundance (δN), which in turn is reflected in distinct δN in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant δN of Bh. It was our objective to investigate under controlled conditions the link between shoot value of δN in several Bh genotypes and leached amounts and shoot N uptake. Additionally, plant 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 δN 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 δN and BNI. Increased leached was positively correlated with higher δN in Bh, whereas the correlation between shoot N uptake and shoot δN was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot δN declined and shoot N% increased over time. Leaf δN of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant δN of Bh genotypes over time reflects the long-term effect of BNI as linked to lower formation and reduced leaching. Accordingly, a low δN in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.