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

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-] (0.4-3.9mgNL-1) and generally obeyed saturation (Michaelis-Menten) kinetics. The uptake rate was maximised within 60min 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.