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[MIS14-04] Tracing the fate of atmospheric nitrate from forest canopy to surface soil using triple oxygen isotopes
Keywords:atmospheric nitrate, triple oxygen isotopes, canopy
This study was conducted in a cool-temperate broadleaved-coniferous mixed forest located in Uryu Experimental Forest of Hokkaido University, northern Hokkaido, Japan. We selected plots under canopies of four trees of Quecus crispula (Oak) and Picea grehnii (Fir) and within four canopy gaps exclusively dominated by dwarf bamboo, Sasa senanensis (Sasa), in a small watershed (ca. three ha) for water collection. Throughfall, litter leachate beneath Oa layer, and soil water at the depth of 10 cm were collected in each plot. Litter leachate and soil water were collected by tension-free lysimeter. Bulk precipitation was collected in a large canopy gap next to the watershed. Water samples were collected after a single rain event in June, September, and October in 2014, and analyzed for inorganic N concentrations and stable isotopic compositions of NO3− (δ15N, δ18O, Δ17O). The stable isotopic compositions of the nitrate were determined using Continuous-Flow Isotope Ratio Mass Spectrometry (CF-IRMS) system in Nagoya University (Tsunogai et al., 2010). The concentration and flux of NO3−atm were calculated from NO3− concentration, Δ17O value, and sample water volume.
The average Δ17O value of precipitation was +23.9 ‰. The average Δ17O values of the throughfall were lower than that of precipitation, and were lower under Oak (+17.7 ‰) and Fir (+18.9 ‰) than under Sasa (+23.3 ‰). These results indicate the occurrence of microbial nitrification on the plant canopies and the magnitude of nitrification in Oak and Fir canopies is larger than in Sasa canopy. The litter leachate and the soil water showed much lower Δ17O value of +2.3 ‰ and +1.4 ‰, respectively. This indicates that remineralizaed NO3− in soil becomes dominant in the leaching NO3− after passing the forest floor, but NO3−atm still exist after passing the forest floor and surface soil where biological activity is very active. The average fluxes of NO3−atm decreased from 0.84 mgN m−2 rain day−1 in the precipitation to 0.26 mgN m−2 rain day−1 in the soil water. The decrease of NO3−atm was larger between precipitation and throughfall than between throughfall and litter leachate or soil water. More than 50 % of NO3−atm in the precipitation was retained or consumed in the canopy, suggesting that forest canopy has a large influence on the fate of NO3−atm in forest ecosystem. Overall, our results suggest that retention of deposited N in the forest canopy is significant and N dynamics in the forest canopy may be very different among plant species.