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Denitrification characteristics of forest soils from eight headwater catchments in China were investigated in this study, along a climatic gradient from the tropics in the South to the temperate zones. Within each catchment, different landscape positions along hydrological flow paths were also considered, including well-drained soils on hill slopes and poorly drained soils in groundwater discharge zones. The results showed that instantaneous denitrification rates were much greater in soils from the northern sites than those from the southern sites (with the average of 110.0 and 25.4nmolNg -1 drysoilh. - 1 , respectively). Large potentials for nitrous oxide (N 2 O) loss (evaluated as maximum N 2 O accumulation before it was reduced to dinitrogen (N 2 )) were observed in the six tropical and subtropical catchments, particularly in soils with high carbon (C) and nitrogen (N). Meanwhile high N 2 O/(N 2 ) stoichiometries were displayed in soils from these southern sites. Within catchments, soils from the groundwater discharge zones showed greater potential denitrification rates but smaller N 2 ) stoichiometries were displayed in soils from these southern sites. Within catchments, soils from the groundwater discharge zones showed greater potential denitrification rates but smaller N 2 O/(N 2 ) ratios in comparison with those on the hill slopes, implying large N removal potentials of soils from the groundwater discharge zones. Furthermore, our findings suggest soil pH is the key controller for the potential denitrification rates and the N 2 ) ratios in comparison with those on the hill slopes, implying large N removal potentials of soils from the groundwater discharge zones. Furthermore, our findings suggest soil pH is the key controller for the potential denitrification rates and the N 2 O/(N 2 O+N 2 O loss synergistically. Our findings not only pinpoint the denitrification phenotypes of soils along the climatic gradient, but also confirm the small-scale variations within catchments which reflect the in situ habitat of the denitrifiers. These indicate the importance of discrimination related to different landscape positions when modeling N 2 O loss synergistically. Our findings not only pinpoint the denitrification phenotypes of soils along the climatic gradient, but also confirm the small-scale variations within catchments which reflect the in situ habitat of the denitrifiers. These indicate the importance of discrimination related to different landscape positions when modeling N 2 O emissions and N removals from regional N loading., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Rationale: Acidic subtropical forest soils that receive high atmospheric nitrogen (N) deposition have been identified as important sources of nitric oxide (NO). The relative importance of major processes producing NO is unclear. Methods: To partition NO sources, we conducted an in situ tracing experiment with 15NH4NO3 and NH415NO3 in well‐drained acid soils of an N‐saturated subtropical forest in Chongqing, southwest China. Results: In the 15NH4NO3 treatment, the 15N signature of NO emitted from the foot of the hillslope (Lower site) was similar to that of the NH4+ pool, indicating predominant autotrophic nitrification for NO formation. In the NH415NO3 treatment, the 15N enrichment of NO was smaller than that of the NO3− pool, suggesting minor contribution of denitrification to NO production (~15%). Conclusions: Nitrification is the main process responsible for NO emissions, even in monsoonal summers when soil water‐filled pore space values are relatively high. [ABSTRACT FROM AUTHOR]

Increasing nitrogen (N) deposition in subtropical forests in south China causes N saturation, associated with significant nitrate (NO3−) leaching. Strong N attenuation may occur in groundwater discharge zones hydrologically connected to well‐drained hillslopes, as has been shown for the subtropical headwater catchment "TieShanPing", where dual NO3− isotopes indicated that groundwater discharge zones act as an important N sink and hotspot for denitrification. Here, we present a regional study reporting inorganic N fluxes over two years together with dual NO3− isotope signatures obtained in two summer campaigns from seven forested catchments in China, representing a gradient in climate and atmospheric N input. In all catchments, fluxes of dissolved inorganic N indicated efficient conversion of NH4+ to NO3− on well‐drained hillslopes, and subsequent interflow of NO3− over the argic B‐horizons to groundwater discharge zones. Depletion of 15N‐ and 18O–NO3− on hillslopes suggested nitrification as the main source of NO3−. In all catchments, except one of the northern sites, which had low N deposition rates, NO3− attenuation by denitrification occurred in groundwater discharge zones, as indicated by simultaneous 15N and 18O enrichment in residual NO3−. By contrast to the southern sites, the northern catchments lack continuous and well‐developed groundwater discharge zones, explaining less efficient N removal. Using a model based on 15NO3− signatures, we estimated denitrification fluxes from 2.4 to 21.7 kg N ha−1 year−1 for the southern sites, accounting for more than half of the observed N removal. Across the southern catchments, estimated denitrification scaled proportionally with N deposition. Together, this indicates that N removal by denitrification is an important component of the N budget of southern Chinese forests and that natural NO3− attenuation may increase with increasing N input, thus partly counteracting further aggravation of N contamination of surface waters in the region. [ABSTRACT FROM AUTHOR]

Abstract Nitrous oxide (N 2 O) is a major greenhouse gas, with elevated emission being reported from subtropical forests that receive high nitrogen (N) deposition. After 10 years of monthly addition of ammonium nitrate (NH 4 NO 3) or sodium nitrate (NaNO 3) to a Mason pine forest at Tieshanping, near Chongqing city in Southwest China, the simulated N deposition was stopped in October 2014. The results of soil N 2 O emissions monitoring in different seasons during the nitrogen application period showed that nitrogen addition significantly increased soil N 2 O emission. In general, the N 2 O emission fluxes were positively correlated to nitrate (NO 3 −) concentrations in soil solution, supporting the important role of denitrification in N 2 O production, which was also modified by environmental factors such as soil temperature and moisture. After stopping the application of nitrogen, the soil N 2 O emissions from the treatment plots were no longer significantly higher than those from the reference plots, implying that a decrease in nitrogen deposition in the future would cause a decrease in N 2 O emission. Although the major forms of N deposition, NH 4 + and NO 3 −, had not shown significantly different effects on soil N 2 O emission, the reduction in NH 4 + deposition may decrease the NO 3 − concentrations in soil solution faster than the reduction in NO 3 − deposition, and thus be more effective in reducing N 2 O emission from N-saturated forest soil in the future. Graphical abstract Image 1 Highlights • Effects of increasing/decreasing N deposition on soil N 2 O emission were studied. • N deposition increases soil N 2 O emission from an N-saturated subtropical forest. • Soil N 2 O emissions were positively correlated to NO 3 − concentrations in soil water. Soil nitrate availability, other than soil temperature and moisture, is key factor determining soil N 2 O emission from an N-saturated subtropical forests. [ABSTRACT FROM AUTHOR]

Chronically elevated deposition of reactive nitrogen (N), as ammonium (NH4+) and nitrate (NO3−), in subtropical forests with monsoonal climate has caused widespread N leaching in southern China. So far, little is known about the effect of further increases in N input and changes in the relative proportion of NH4+ and NO3− on turnover rate and fate of atmogenic N. Here we report a 15N tracer experiment in Tieshanping (TSP) forest, SW China, conducted as part of a long-term N fertilization experiment, using NH4NO3 and NaNO3, where effects of a doubling of monthly N inputs were compared. In June 2012, the regular N fertilizers were replaced by their 15N-labeled forms, viz., 15NH4NO3 and Na15NO3, as a single-dose addition. Mass balances of N for the initial 1.5 years following label addition showed that for both treatments, 70% to 80% of the annual N input was leached as NO3−, both at ambient and at double N input rates. This confirms the earlier reported extreme case of N saturation at TSP. The 15N, added as Na15NO3, showed recoveries of about 74% in soil leachates, indicating that NO3− input at TSP is subject to a rapid and nearly quantitative loss through direct leaching as a mobile anion. By contrast, recoveries of 15N in soil leachates of only 33% were found if added as 15NH4NO3. Much of the 15N was immobilized in the soil and to a lesser extent in the vegetation. Thus, immobilization of fresh N input is significantly greater if added as NH4+, than as NO3−. [ABSTRACT FROM AUTHOR]

Chronically elevated nitrogen (N) deposition has led to severe nutrient imbalance in forest soils. Particularly in tropical and subtropical forest ecosystems, increasing N loading has aggravated phosphorus (P) limitation of biomass production, and has resulted in elevated emissions of nitrous oxide (N2O) and reduced uptake of methane (CH4/, both of which are important greenhouse gases. Yet, the interactions of N and P and their effects on greenhouse gas emissions remain elusive. Here, we report N2O and CH4 emissions together with soil N and P data for a period of 18 months following a single P addition (79 kg P ha--1, as NaH2PO4 powder) to an N-saturated, Masson pine-dominated forest soil at TieShanPing (TSP), Chongqing, south-western (SW) China. We observed a significant decline in both nitrate (NO--3 ) concentrations in soil water (5 and 20 cm depths) and in soil N2O emissions, following P application. We hypothesise that enhanced N uptake by plants in response to P addition, resulted in less available NO-3 for denitrification. By contrast to most other forest ecosystems, TSP is a net source of CH4. P addition significantly decreased CH4 emissions and turned the soil from a net source into a net sink. Based on our observation and previous studies in South America and China, we believe that P addition relieves N inhibition of CH4 oxidation. Within the 1.5 years after P addition, no significant increase of forest growth was observed and P stimulation of forest N uptake by understorey vegetation remains to be confirmed. Our study indicates that P fertilisation of N-saturated, subtropical forest soils may mitigate N2O and CH4 emissions, in addition to alleviating nutrient imbalances and reducing losses of N through NO--3 leaching. [ABSTRACT FROM AUTHOR]

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH ) and nitrate (NO ). Previous long-term studies indicate efficient turnover of atmogenic NH to NO in weathered, acidic soils of the subtropics, leading to excessive NO leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. NHNO, NHNO and N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (NO) were monitored for nine days. Incorporation of NO into soil organic N was negligible and 80% of the applied label was lost from the top soil (0-15 cm) primarily by leaching within 9 days. In contrast, NH was largely retained in soil organic N. However, instant production of NO in the NH treatment suggested active nitrification. In both the NH and N-glutamic acid treatments, the N enrichment in the NO pool exceeded that in the NH pool one day after N application, suggesting preferential nitrification of added NH with subsequent dilution of the NH pool and/or immobilization of NH followed by heterotrophic nitrification. The cumulative recovery of N in NO after 9 days ranged from 2.5 to 6.0% in the NO treatment, confirming the previously reported significant response of NO emission to N deposition. Source partitioning of NO demonstrated a measurable contribution of nitrification to NO emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO and NH ) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO in the soil and (ii) rapid immobilization-mineralization of NH followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and NO emission. [ABSTRACT FROM AUTHOR]

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH4+), causes significant nitrate (NO3−) leaching from well-drained acid forest soils on hill slopes (HS), whereas significant retention of NO3− occurs in near-stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ15N and δ18O) of nitrate (NO3−) in soil pore water along a hydrological continuum in the N-saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH4+ and production of NO3− in soils on HS were associated with a significant decrease in δ15N-NO3−, suggesting efficient nitrification despite low soil pH. The concentration of NO3− declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ15N and δ18O of residual NO3−, providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent 15N enrichment factor ( ε) of NO3− of about −5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ15N and δ18O-NO3− in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ15N-NO3− values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N-saturated, subtropical forests with important implications for regional N budgets. [ABSTRACT FROM AUTHOR]

Biochar (BC) application to soil suppresses emission of nitrous- (N2O) and nitric oxide (NO), but the mechanisms are unclear. One of the most prominent features of BC is its alkalizing effect in soils, which may affect denitrification and its product stoichiometry directly or indirectly. We conducted laboratory experiments with anoxic slurries of acid Acrisols from Indonesia and Zambia and two contrasting BCs produced locally from rice husk and cacao shell. Dose-dependent responses of denitrification and gaseous products (NO, N2O and N2) were assessed by high-resolution gas kinetics and related to the alkalizing effect of the BCs. To delineate the pH effect from other BC effects, we removed part of the alkalinity by leaching the BCs with water and acid prior to incubation. Uncharred cacao shell and sodium hydroxide (NaOH) were also included in the study. The untreated BCs suppressed N2O and NO and increased N2 production during denitrification, irrespective of the effect on denitrification rate. The extent of N2O and NO suppression was dose-dependent and increased with the alkalizing effect of the two BC types, which was strongest for cacao shell BC. Acid leaching of BC, which decreased its alkalizing effect, reduced or eliminated the ability of BC to suppress N2O and NO net production. Just like untreated BCs, NaOH reduced net production of N2O and NO while increasing that of N2. This confirms the importance of altered soil pH for denitrification product stoichiometry. Addition of uncharred cacao shell stimulated denitrification strongly due to availability of labile carbon but only minor effects on the product stoichiometry of denitrification were found, in accordance with its modest effect on soil pH. Our study indicates that stimulation of denitrification was mainly due to increases in labile carbon whereas change in product stoichiometry was mainly due to a change in soil pH. [ABSTRACT FROM AUTHOR]

Long-term elevated atmogenic deposition (~5 g m −2 year −1) of reactive nitrogen (N) causes N saturation in forests of subtropical China which may lead to high nitrous oxide (N 2O) emissions. Recently, we found high N 2O emission rates (up to 1,730 μg N 2O–N m −2 h −1) during summer on well-drained acidic acrisols (pH = 4.0) along a hill slope in the forested Tieshanping catchment, Chongqing, southwest China. Here, we present results from an in situ 15N–NO 3− labeling experiment to assess the contribution of nitrification and denitrification to N 2O emissions in these soils. Two loads of 99 at.% K 15NO 3 (equivalent to 0.2 and 1.0 g N m −2) were applied as a single dose to replicated plots at two positions along the hill slope (at top and bottom, respectively) during monsoonal summer. During a 6-day period after label application, we found that 71–100 % of the emitted N 2O was derived from the labeled NO 3− pool irrespective of slope position. Based on this, we assume that denitrification is the dominant process of N 2O formation in these forest soils. Within 6 days after label addition, the fraction of the added 15N–NO 3− emitted as 15N–N 2O was highest at the low-N addition plots (0.2 g N m −2), amounting to 1.3 % at the top position of the hill slope and to 3.2 % at the bottom position, respectively. Our data illustrate the large potential of acid forest soils in subtropical China to form N 2O from excess NO 3− most likely through denitrification. [ABSTRACT FROM AUTHOR]

Abstract: Subtropical forests in SW China, receiving high nitrogen (N) deposition, have been reported to show high N retention. N removal by denitrification may be one important process favored by warm and wet soils during monsoonal summer when most of the atmogenic N input occurs. Due to low pH soils which are common in this region, N removal by denitrification may entail substantial N2O emissions. In this study, we explored intrinsic denitrification characteristics of soils from different landscape elements of a forested headwater catchment in SW China (Tieshanping, Chongqing). Laboratory incubations were used to measure instantaneous denitrification rates, apparent specific denitrifier growth rates, denitrification product stoichiometries (N2O/(N2O + N2)) and response to carbon-addition as a function of soil depth and landscape position. The results revealed that potential denitrification and denitrifier growth rates were highest in top soils along a hill slope, and decreased strongly with soil depth due to C-limitation. A hydrologically connected, colluvial groundwater discharge zone showed equally high instantaneous denitrification rates which were more equally distributed with depth. Denitrifying communities in the top horizons of the hill slope were less efficient in expressing N2O reductase in response to anoxia resulting in higher N2O/(N2 + N2O) product ratios than found in soils of the groundwater discharge zone, suggesting that a significant share of the deposited N can be lost as N2O from the hill slopes. Differences in denitrification traits appeared to be linked to eco-hydrological conditions in the two landscape elements and to substrate availability along the hydrological flowpath. Our study supports the notion that denitrification plays an important role for observed N removal in SW Chinese forest ecosystems and illustrates how habitat functions constrain the amount of N2O emitted during N removal. [Copyright &y& Elsevier]

Acidic deposition used to cause severe surface water acidification in Europe and the North America, but no damage of surface water acidification was reported under continuous heavy acid deposition in China. We present the first detailed study on regional surface water acidification in southern and northeastern China based on a survey of 255 forested headwater streams, which were investigated during 2010–2015 to explore the status and mechanism of surface water acidification. South China has a subtropical climate with high acid deposition, whereas northeast China is located in a cold temperature zone with relatively low acid deposition. High pH and acid neutralizing capacity (ANC) were observed for almost all of the streams, indicating that surface water acidification may not be a serious regional issue in China both at present and in the future. In northeast China, where soil types are similar in low weathering rate to those in Northern Europe and North America, the ANC of surface waters was lower than that in South China, indicating a higher acid-sensitivity of the surface water. In comparison, the high pH and ANC of streams in southwest China resulted from high soil weathering and atmospheric calcium (Ca) deposition, despite elevated atmospheric acid inputs due to the high sulfur (S) and nitrogen (N) deposition. Comparison of stream nitrate (NO 3 − ) and sulfate (SO 4 2 − ) concentrations with modeled N and S deposition showed significant N and possible S sinks in the catchments in south China, probably due to denitrification of NO 3 − in groundwater discharge zone and SO 4 2 − adsorption by acid soils respectively, which may also buffer the acidifying effect of S and N depositions. It seems that considerable NO 3 − leaching to stream waters does not occur in China unless N deposition is higher than 1.8 keq·ha − 1 ·yr − 1 (2.50 g N·m − 2 ·yr − 1 ) (or greater values in certain regions). [ABSTRACT FROM AUTHOR]

Emission of NO and N2O from a full-scale two-reactor nitritation–anammox process was determined during a measurement campaign at the Dokhaven–Sluisjesdijk municipal WWTP (Rotterdam, NL). The NO and N2O levels in the off-gas responded to the aeration cycles and the aeration rate of the nitritation reactor, and to the nitrite and dissolved oxygen concentration. Due to the strong fluctuations in the NO and N2O levels in both the nitritation and the anammox reactor, only time-dependent measurements could yield a reliable estimate of the overall NO and N2O emissions. The NO emission from the nitritation reactor was 0.2% of the nitrogen load and the N2O emission was 1.7%. The NO emission from the anammox reactor was determined to be 0.003% of the nitrogen load and the N2O emission was 0.6%. Emission of NO2 could not be detected from the nitritation–anammox system. Denitrification by ammonia-oxidizing bacteria was considered to be the most probable cause of NO and N2O emission from the nitritation reactor. Since anammox bacteria have not been shown to produce N2O under physiological conditions, it is also suspected that ammonia-oxidizing bacteria contribute most to N2O production in the anammox reactor. The source of NO production in the anammox reactor can be either anammox bacteria or denitrification by heterotrophs or ammonia-oxidizing bacteria. Based on the results and previous work, it seems that a low dissolved oxygen or a high nitrite concentration are the most likely cause of elevated NO and N2O emission by ammonia-oxidizing bacteria. The emission was compared with measurements at other reject water technologies and with the main line of the Dokhaven–Sluisjesdijk WWTP. The N2O emission levels in the reject water treatment seem to be in the same range as for the main stream of activated sludge processes. Preliminary measurements of the N2O emission from a one-reactor nitritation–anammox system indicate that the emission is lower than in two-reactor systems. [Copyright &y& Elsevier]