Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N 2 ) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO 3 − ) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6-30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO 3 − leaching, pointing to widespread dominance of denitrification in removing NO 3 − from forest ecosystems across a range of conditions. Further, we report that the loss of NO 3 − to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.denitrification | nitrate isotopes | nitrogen cycling | forested watersheds

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar d 15 N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha À1 yr À1 , while the throughfall covered a larger range of 11.7-65.1 kg N ha À1 yr À1 . Foliar N concentration, NO 3 À leaching to stream, and soil NO 3 À concentration were low and NO 3 À production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO 3 À leaching to stream, and negatively with pH values in soil and stream water. Foliar d 15 N was between À6.6% and 0.7%, and was negatively correlated with soil NO 3 À concentration and NO 3 À leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest d 15 N values. We proposed several potential mechanism that could contribute to the d 15 N pattern, including (1) increased plant uptake of 15 N-depleted soil NO 3 À , (2) foliage uptake of 15 N-depleted NH 4 1 , (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.

Nitric acid (HNO₃) or nitrate (NO₃^−) is the dominant sink for reactive nitrogen oxides (NO_x = NO + NO₂) in the atmosphere. In many Chinese cities, HNO₃ is becoming a significant contributor to acid deposition. In the present study, we measured nitrogen (N) and oxygen (O) isotopic composition of NO₃^− in 113 precipitation samples collected from Guangzhou City in southern China over a two-year period (2008 and 2009). We attempted to better understand the spatial and seasonal variability of atmospheric NO_x sources and the NO₃^− formation pathways in this N-polluted city in the Pearl River Delta region. The δ¹⁵N values of NO₃^− (versus air N₂) ranged from −4.9 to +10.1‰, and averaged +3.9‰ in 2008 and +3.3‰ in 2009. Positive δ¹⁵N values were observed throughout the year, indicating the anthropogenic contribution of NO_x emissions, particularly from coal combustion. Different seasonal patterns of δ¹⁵N-NO₃^− were observed between 2008 and 2009, which might reflect different human activities associated with the global financial crisis and the intensive preparations for the 16th Asian Games. Nitrate δ¹⁸O values (versus Vienna Standard Mean Ocean Water) varied from +33.4 to +86.5‰ (average +65.0‰ and +67.0‰ in 2008 and 2009, respectively), a range being lower than those reported for high latitude and polar areas. Sixteen percent of δ¹⁸O values was observed lower than the expected minimum of +55‰ at our study site. This was likely caused by the reaction of NO with peroxy radicals; peroxy radicals can compete with O₃ to convert NO to NO₂, thereby donate O atoms with much lower δ¹⁸O value than that of O₃ to atmospheric NO₃^−. Our results highlight that the influence of human activities on atmospheric chemistry can be recorded by the N and O isotopic composition of atmospheric NO₃^− in a N-polluted city

To investigate which of ammonium (NH(4)(+)) or nitrate (NO(3)(-)) is used by plants at gradient sites with different nitrogen (N) availability, we measured the natural abundance of (15)N in foliage and soil extractable N. Hinoki cypress (Chamaecyparis obtusa Endlicher) planted broadly in Japan was selected for use in this study. We estimated the source proportion of foliar N (NH(4)(+) vs. NO(3)(-)) quantitatively using mass balance equations. The results showed that C. obtusa used mainly NH(4)(+) in N-limited forests, although the dependence of C. obtusa on NO(3)(-) was greater in other NO(3)(-)-rich forests. We regarded dissolved organic N (DON) as a potential N source because a previous study demonstrated that C. obtusa can take up glycine. Thus we added DON to our mass balance equations and calculated the source proportion using an isotope-mixing model (IsoSource model). The results still showed a positive correlation between the calculated plant N proportion of NO(3)(-) and the NO(3)(-) pool size in the soil, indicating that high NO(3)(-) availability increases the reliance of C. obtusa on NO(3)(-). Our data suggest the shift of the N source for C. obtusa from NH(4)(+) to NO(3)(-) according to the relative availability of NO(3)(-). They also show the potential of the foliar delta(15)N of C. obtusa as an indicator of the N status in forest ecosystems with the help of the delta(15)N values of soil inorganic and organic N.

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