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

abstract

  • The tropical forage grass Brachiaria humidicola (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate (NO3-) formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reduced NO3- losses and enhanced N uptake due to BNI has not been experimentally demonstrated yet. Nitrification discriminates against the N-15 isotope and leads to N-15 depleted NO3-, but N-15 enriched NH4+ in soils. Leaching of N-15 depleted NO3- enriches the residual N pool in the soil with N-15. We hypothesized that altered nitrification and NO3- leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil N-15 natural abundance (delta N-15), which in turn is reflected in distinct delta N-15 in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant delta N-15 of Bh. It was our objective to investigate under controlled conditions the link between shoot value of delta N-15 in several Bh genotypes and leached NO3- amounts and shoot N uptake. Additionally, plant N-15 and N% was monitored among a wide range of Bh genotypes with contrasting BNI potentials in field plots for 3 years. We measured leaf delta N-15 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 delta N-15 and BNI. Increased leached NO3- was positively correlated with higher delta N-15 in Bh, whereas the correlation between shoot N uptake and shoot delta N-15 was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot delta N-15 declined and shoot N% increased over time. Leaf delta N-15 of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant delta N-15 of Bh genotypes over time reflects the long-term effect of BNI as linked to lower NO3- formation and reduced NO3- leaching. Accordingly, a low delta N-15 in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.

publication date

  • 2018
  • 2018