Nitrogen and oxygen isotope effects of tissue nitrate associated with nitrate acquisition and utilisation in the moss Hypnum plumaeforme

Abstract

Mosses are effective accumulators and indicators of N deposition, but the mechanisms of moss N utilisation remain unclear. This study monitored nitrate concentrations ([NO3–]) in solutions supplied to Hypnum plumaeforme Wils. to characterise NO3– uptake from rain events. Concentrations and isotopic ratios (δ15N and δ18O) of residual NO3– in moss tissues were measured to interpret induced NO3– reduction. Noninduced NO3– reduction was inferred from endogenous [NO3–] and isotopic variations that occurred during 65 days of N deprivation. H. plumaeforme scavenges NO3– effectively from supplied solutions. The uptake rate increased with substrate [NO3–] (0.4−3.9 mg N L–1) and generally obeyed saturation (Michaelis–Menten) kinetics. The uptake rate was maximised within 60 min after receiving NO3–, irrespective of the initial substrate [NO3–]. Lower tissue [NO3–] and greater isotopic enrichment verified the inducibility of nitrate reductase activity (NRA) by NO3– availability, but short-term darkness did not markedly influence moss NO3– uptake or reduction. Significant reduction and isotopic enrichment were detected in moss NO3– reserves during N deprivation, showing 15ε of 12.1‰ and 18ε of 14.4‰. The Δδ15N : Δδ18O ratios of ~1 : 1 implied that NRA is the single process driving 15N and 18O fractionations. These results provide new isotopic insights into the nitrate reductase dynamics of the moss.