Lakes are a nitrous oxide (N2O) source to the atmosphere, but the biogeochemical controls and microbial pathways of N2O production are not well understood. To trace microbial N2O production (denitrification, nitrifier denitrification, and nitrification) and consumption (denitrification) in two basins of Lake Lugano, we measured the concentrations and N and O isotope compositions of N2O, as well as the intramolecular 15N distribution, i.e., site preference (SP). Our results revealed differential N2O dynamics in the two lake basins, with N2O concentrations between 12 nmol L−1 and > 900 nmol L−1 in the holomictic South Basin, and significantly lower concentrations in the meromictic North Basin (<13 nmol L−1). In the South Basin, the isotope signatures reflected a complex combination of N2O production by nitrifying bacteria through hydroxylamine (NH2OH) oxidation, N2O production through incomplete denitrification, and N2O reduction to N2, all occurring in close vicinity within the redox transition zone (RTZ). In the North Basin, in contrast, the N2O isotopomer signatures suggested that nitrifier denitrification was the main N2O source. The pronounced decrease in N2O concentrations to undetectable levels within the RTZ, in tandem with an increase in δ15N-N2O, δ18O-N2O, and SP indicated quantitative N2O consumption by microbial denitrification. In the northern basin this was primarily sulfide-dependent. The apparent N and O isotope enrichment factors associated with net N2O consumption were 15ε ≈ 3.2‰ and 18ε ≈ 8.6‰, respectively. The according 18O to 15N enrichment ratio (18ε: 15ε ≈ 2.5) is consistent with previous reports for microbial N2O reduction, underscoring its robust nature across environments.
Abstract. Nitric acid (HNO3) or nitrate (NO3−) is the dominant sink for reactive nitrogen oxides (NOx = NO + NO2) in the atmosphere. In many Chinese cities, HNO3 is becoming a significant contributor to acid deposition. In the present study, we used the denitrifier method to measure 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 NOx 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 NOx 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 altitude and polar areas. Several δ18O values were observed lower than the expected minimum of 50‰ 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 understand how hydrological processes affect biogeochemical and methane (CH 4 ) cycles in temperate riparian wetlands, we measured CH 4 fluxes and dissolved chemical constituents and CH 4 concentrations in groundwater, and monitored several environmental factors in wetlands located within a forested headwater catchment in a warm, humid climate in Japan. Variation in redox components dissolved in groundwater, including nitrate (NO 3 − ), Mn, Fe, and sulfate (SO 4 2− ), depended on temperature and soil‐water conditions. Strongly reducing conditions usually occurred in the high‐temperature months of July, August, and September. Dissolved CH 4 in groundwater changed with redox conditions and was highest in summer and lowest in winter. CH 4 emissions from riparian wetlands were observed almost throughout the year and displayed clear seasonality. Occasionally in summer, emission rates were more than 4 orders of magnitude greater than hillslope uptake rates. Although CH 4 emissions increased markedly during most of the summer, they were constrained by (1) fluctuation of the water table, which when lowered can shift the subsurface zone to a more oxidized condition, and (2) the oxygen‐rich water such as precipitation and lateral subsurface flow from the hillslope. These results suggest that hydrological processes in forest headwater catchments play an important role in supplying oxygen to soils and consequently affect biogeochemical cycles, including CH 4 formation, and that small wetlands in forest watersheds function as large sources of CH 4 , especially in regions with warm humid summers.
Abstract Increased deposition of reactive atmospheric N has resulted in the nitrogen saturation of many forested catchments worldwide. Isotope‐based studies from multiple forest sites report low proportions (mean = ∼10%) of unprocessed atmospheric nitrate in streams during baseflow, regardless of N deposition or nitrate export rates. Given similar proportions of atmospheric nitrate in baseflow across a variety of sites and forest types, it is important to address the postdepositional drivers and processes that affect atmospheric nitrate transport and fate within catchments. In a meta‐analysis of stable isotope‐based studies, we examined the influence of methodological, biological, and hydrologic drivers on the export of atmospheric nitrate from forests. The δ 18 O‐ values in stream waters may increase, decrease, or not change with increasing discharge during stormflow conditions, and δ 18 O‐ values are generally higher in stormflow than baseflow. However, δ 18 O‐ values tended to increase with increasing baseflow discharge at all sites examined. To explain these differences, we present a conceptual model of hydrologic flowpath characteristics (e.g., saturation overland flow versus subsurface stormflow) that considers the influence of topography on landscape‐stream hydrologic connectivity and delivery of unprocessed atmospheric nitrate to streams. Methodological biases resulting from differences in sampling frequency and stable isotope analytical techniques may further influence the perceived degree of unprocessed atmospheric nitrate export. Synthesis of results from numerous isotope‐based studies shows that small proportions of unprocessed atmospheric nitrate are common in baseflow. However, hydrologic, topographic, and methodological factors are important drivers of actual or perceived elevated contributions of unprocessed atmospheric nitrate to streams.
Abstract Soil nitrogen (N) transformations between labile N forms (extractable organic N [EON], ammonium [NH 4 + ], and nitrate [NO 3 − ]) regulate soil N availability. However, it has long been difficult to quantify the transformations of total soil organic and labile N forms in soils, which has left large uncertainties in evaluating atmospheric N deposition effects on soil N dynamics. Based on concentrations and natural abundances of N isotopes of soil organic N, EON, NH 4 + , and NO 3 − across 11 forests with variant N deposition levels, we established a quantitative isotopic framework to estimate the fractions of soil N depolymerization ( f D ), mineralization ( f M ), nitrification ( f N ), and of NO 3 − losses ( f L ) via denitrification and leaching. Based on the fractions, the gross production and storage of corresponding soil labile N were estimated for forests of China and Japan. We found that f D , f M , and f N increased, while f L decreased with the increase of N deposition among the study forests. And the contribution of denitrification (relative to the NO 3 − leaching) to total NO 3 − losses also increased with increasing N deposition. Our method provides new and straightforward insights into the present soil N transformations and allows to evaluate the soil N status. These findings are useful for modeling forest N cycles under different N deposition regimes.
We tested the ecosystem functions of microbial diversity with a focus on ammonification (involving diverse microbial taxa) and nitrification (involving only specialized microbial taxa) in forest nitrogen cycling. This study was conducted on a forest slope, in which the soil environment and plant growth gradually changed. We measured the gross and net rates of ammonification and nitrification, the abundance of predicted ammonifiers and nitrifiers, and their community compositions in the soils. The abundance of predicted ammonifiers did not change along the soil environmental gradient, leading to no significant change in the gross ammonification rate. On the other hand, the abundance of nitrifiers and the gross nitrification rate gradually changed. These accordingly determined the spatial distribution of net accumulation of ammonium and nitrate available to plants. The community composition of predicted ammonifiers gradually changed along the slope, implying that diverse ammonifiers were more likely to include taxa that were acclimated to the soil environment and performed ammonification at different slope locations than specialized nitrifiers. Our findings suggest that the abundance of ammonifiers and nitrifiers directly affects the corresponding nitrogen transformation rates, and that their diversity affects the stability of the rates against environmental changes. This study highlights the role of microbial diversity in biogeochemical processes under changing environments and plant growth.
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