Evaluating four mathematical models for nitrous oxide production by autotrophic ammonia‐oxidizing bacteria

There is increasing evidence showing that ammonia‐oxidizing bacteria (AOB) are major contributors to N₂O emissions from wastewater treatment plants (WWTPs). Although the fundamental metabolic pathways for N₂O production by AOB are now coming to light, the mechanisms responsible for N₂O production by AOB in WWTP are not fully understood. Mathematical modeling provides a means for testing hypotheses related to mechanisms and triggers for N₂O emissions in WWTP, and can then also become a tool to support the development of mitigation strategies. This study examined the ability of four mathematical model structures to describe two distinct mechanisms of N₂O production by AOB. The production mechanisms evaluated are (1) N₂O as the final product of nitrifier denitrification with NO as the terminal electron acceptor and (2) N₂O as a byproduct of incomplete oxidation of hydroxylamine (NH₂OH) to NO. The four models were compared based on their ability to predict N₂O dynamics observed in three mixed culture studies. Short‐term batch experimental data were employed to examine model assumptions related to the effects of (1) NH concentration variations, (2) dissolved oxygen (DO) variations, (3) NO accumulations and (4) NH₂OH as an externally provided substrate. The modeling results demonstrate that all these models can generally describe the NH, NO, and NO data. However, none of these models were able to reproduce all measured N₂O data. The results suggest that both the denitrification and NH₂OH pathways may be involved in N₂O production and could be kinetically linked by a competition for intracellular reducing equivalents. A unified model capturing both mechanisms and their potential interactions needs to be developed with consideration of physiological complexity. Biotechnol. Bioeng. 2013; 110: 153–163. © 2012 Wiley Periodicals, Inc.     

Effect of oxic and anoxic conditions on nitrous oxide emissions from nitrification and denitrification processes

A lab-scale sequencing batch reactor fed with real municipal wastewater was used to study nitrous oxide (N(2)O) emissions from simulated wastewater treatment processes. The experiments were performed under four different controlled conditions as follows: (1) fully aerobic, (2) anoxic-aerobic with high dissolved oxygen (DO) concentration, (3) anoxic-aerobic with low DO concentration, and 4) intermittent aeration. The results indicated that N(2)O production can occur from both incomplete nitrification and incomplete denitrification. N(2)O production from denitrification was observed in both aerobic and anoxic phases. However, N(2)O production from aerobic conditions occurred only when both low DO concentrations and high nitrite concentration existed simultaneously. The magnitude of N(2) O produced via anoxic denitrification was lower than via oxic denitrification and required the presence of nitrite. Changes in DO, ammonium, and nitrite concentrations influenced the magnitude of N(2)O production through denitrification. The results also suggested that N(2)O can be produced from incomplete denitrification and then released to the atmosphere during aeration phase due to air stripping. Therefore, biological nitrogen removal systems should be optimized to promote complete nitrification and denitrification to minimize N(2)O emissions.     

Multiyear dual nitrate isotope signatures suggest that N‐saturated subtropical forested catchments can act as robust N sinks

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.     

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

This study examines the role of tree canopies in processing atmospheric nitrogen (N_dep) for four forests in the United Kingdom subjected to different N_dep: Scots pine and beech stands under high N_dep (HN, 13–19 kg N ha^?1 yr^?1), compared to Scots pine and beech stands under low N_dep (LN, 9 kg N ha^?1 yr^?1). Changes of NO₃‐N and NH₄‐N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ¹⁸O, Δ¹⁷O and δ¹⁵N in NO₃^? and δ¹⁵N in NH₄⁺, were used to assess N transformations by the canopies. Generally, HN sites showed higher NH₄‐N and NO₃‐N concentrations in RF compared to the LN sites. Similar values of δ¹⁵N‐NO₃^? and δ¹⁸O in RF suggested similar source of atmospheric NO₃^? (i.e. local traffic), while more positive values for δ¹⁵N‐NH₄⁺ at HN compared to LN likely reflected the contribution of dry NH_x deposition from intensive local farming. The isotopic signatures of the N‐forms changed after interacting with tree canopies. Indeed, ¹⁵N‐enriched NH₄⁺ in TF compared to RF at all sites suggested that canopies played an important role in buffering dry N_dep also at the low N_dep site. Using two independent methods, based on δ¹⁸O and Δ¹⁷O, we quantified for the first time the proportion of NO₃^? in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ¹⁷O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of N_dep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ¹⁷O.     

Dynamics of nitrous oxide in groundwater at the aquatic–terrestrial interface

Few data are available to validate the Intergovernmental Panel on Climate Change's (IPCC) emission factors for indirect emissions of nitrous oxide (N₂O). In particular the N₂O emissions resulting from nitrogen leaching and the associated groundwater and surface drainage (EF5-g) are particularly poorly characterized. In situ push–pull methods have been used to identify the fate of NO₃⁻ in the groundwater. In this study, we adapted a previously published in situ denitrification push–pull method to examine the fate of ¹⁵N₂O introduced into the subsoil–groundwater matrix. Enriched ¹⁵N₂O was manufactured, added to groundwater via a closed system in the laboratory, and then introduced into the groundwater–subsoil matrix in an upland-marsh transition zone of a salt marsh and a forested alluvial riparian zone. Conservative tracers (SF₆ and Br⁻) and ¹⁵N₂O were injected into the groundwater and left for 1–4 h after which the groundwater was sampled. Added ¹⁵N₂O behaved in a conservative manner at one site while the other site showed variability with some injections showing significant consumption (3–8 μg N₂O-¹⁵N kg⁻¹ soil day⁻¹) of ¹⁵N₂O. Our results show that the fate and dynamics of N₂O in groundwater are complex and variable and that these dynamics should be considered in the development of improved IPCC inventory calculations.     

冻融循环期间土壤氧化亚氮排放影响因素

全球气候变化可能会提高冻融循环时间、强度以及频率, 从而可能显著影响土壤氧化亚氮(N₂O)排放。N₂O是一种重要的温室气体, 但目前对冻融循环期间土壤N₂O排放规律以及影响因素的了解还有限。为此, 该研究采用整合分析方法, 从已发表文献中收集了30篇关于冻融循环对土壤N₂O通量和累积排放量影响的文献, 探究冻融循环在不同生态系统对N₂O排放的影响, 从试验设置、土壤基本理化性质以及冻融循环格局等角度全面综合地探究其排放影响因素。该研究得出, 冻融循环能显著增加N₂O通量、N₂O累积排放量和硝化作用速率, 全球平均增幅分别为72.34%、143.25%和124.63%; 冻融循环也可增加反硝化作用速率, 全球平均增幅为162.56%; 与之相反, 冻融循环显著减少微生物生物量氮含量, 全球平均减幅为6.39%。不同生态系统土壤水热条件和基本理化性质差异可显著影响冻融循环对N₂O排放的影响。当年平均气温超过5 ℃时, 冻融循环作用可显著提高N₂O通量104.13%, 显著高于年平均气温为0-5 ℃ (25.56%)和小于0 ℃ (55.29%)时; 土壤湿度大于70%时, N₂O通量增加109.17%, 显著高于土壤湿度为50%-70% (65.67%)和小于50% (20.37%)时的通量。土壤黏粒和养分含量越高的土壤区域, 冻融循环对N₂O排放的提高幅度越大。在有植物存在时, 冻融循环可显著提高土壤N₂O通量达91.21%, 高于无植物存在时的54.43%。土壤过筛和在冻融循环期间采集土壤都会增加冻融循环对N₂O排放的影响。另外, 融化时间长, 冻结强度大和冻融循环频率高均可显著提高土壤N₂O累积排放量对冻融循环的响应。当冻结温度低于-10 ℃时, 冻融循环对土壤N₂O排放通量的增幅可达100.73%, 显著高于在冻结温度为-10- -5 ℃ (47.74%)和高于-5 ℃ (70.25%)时。主要原因是冻结强度高可促进土壤微生物和土壤结构释放更多的养分, 从而提高N₂O的产生和排放。该研究结果有助于更好地理解土壤N₂O对冻融循环的响应及其影响因素, 为更准确地预测未来全球气候变化对N₂O排放影响提供科学数据支撑。     

Mitigation of nitrous oxide emissions from acidic soils by Bacillus amyloliquefaciens,a plant growth‐promoting bacterium

Nitrous oxide (N₂O) is a long‐lived greenhouse gas that can result in the alteration of atmospheric chemistry and cause accompanying changes in global climate. To date, many techniques have been used to mitigate the emissions of N₂O from agricultural fields, which represent one of the most important sources of N₂O. In this study, we designed a greenhouse pot experiment and a microcosmic serum bottle incubation experiment using acidic soil from a vegetable farm to study the effects of Bacillus amyloliquefaciens (BA) on plant growth and N₂O emission rates. The addition of BA to the soil promoted plant growth enhanced the soil pH and increased the total nitrogen (TN) contents in the plants. At the same time, it decreased the concentrations of ammonium (NH₄⁺), nitrate (NO₃^?) and TN in the soil. Overall, the addition of BA resulted in a 50% net reduction of N₂O emissions compared with the control. Based on quantitative PCR and the network analysis of DNA sequencing, it was demonstrated that BA partially inhibited the nitrification process through the significant reduction of ammonia oxidizing bacteria. Meanwhile, it enhanced the denitrification process, mainly by increasing the abundance of N₂O‐reducing bacteria in the treatment with BA. The results of our microcosm experiment provided evidence that strongly supported the above findings under more strictly controlled laboratory conditions. Taken together, the results of our study evidently demonstrated that BA has dual effects on the promotion of plant growth and the dramatic reduction of greenhouse emissions, thus suggesting the possibility of screening beneficial microbial organisms from the environment that can promote plant growth and mitigate greenhouse trace gases.     

Suppression of nitrification and nitrous oxide emission by the tropical grass Brachiaria humidicola

Nitrification by soil nitrifiers may result in substantial losses of applied nitrogen through NO₃ ^– leaching and N₂O emission. The biological inhibition of nitrification by crop plants or pasture species is not well known. This study was conducted to evaluate the ability of three pasture species, Brachiaria humidicola, B. decumbens and Melinis minutiflora to inhibit nitrification. Plants were grown in a growth chamber for sixty days, fertilized with (NH₄)₂SO₄. After harvesting, the soil was incubated with (NH₄)₂SO₄ for 24 days. Ammonium oxidizing bacteria (AOB), NH₄-N levels, and N₂O emission were monitored at 4 d intervals. Among the species studied, B. humidicola inhibited nitrification and maintained NH₄-N in soil to a much greater extent than the other two species. This nitrification inhibition lasted for 12 days after initiation of soil incubation study (i.e. from 60 DAS when the plants were harvested). The AOB populations and N₂O emission from the soil were significantly lower in the soils where B. humidicola has been grown compared to the other two species. Root exudates and soil extracts of B. humidicola suppressed AOB populations, whereas those of B. decumbens and M. minutiflora did not. The results are in consistence with the hypothesis that B. humidicola suppressed nitrification and N₂O emissions through an inhibitory effect on the AOB population.     

Diurnal patterns of denitrification, oxygen consumption and nitrous oxide production in rivers measured at the whole-reach scale

1. Denitrification, net oxygen consumption and net nitrous oxide flux to the atmosphere were measured in three small rivers (discharge approximately 2–27 m³ s^?1) at the whole reach scale during Spring and Summer, 2002. Two of these rivers (Iroquois River and Sugar Creek in north‐west Indiana – north‐east Illinois, U.S.A.) drained agricultural catchments and the other (Millstone River in central New Jersey, U.S.A.) drained a mixed suburban–agricultural catchment. 2. Denitrification, oxygen consumption and N₂O flux were measured based on net changes in dissolved gas concentrations (N₂, O₂, and N₂O) during riverine transport, correcting for atmospheric exchange. On each date, measurements were made during both light and dark periods. 3. Denitrification rates in these rivers ranged from 0.31 to 15.91 mmol N m^?2 h^?1, and rates within each river reach were consistently higher during the day than during the night. This diurnal pattern could be related to cyclic patterns of nitrification driven by diurnal variations in water column pH and temperature. 4. Oxygen consumption ranged from 2.56 to 241 mmol O₂ m^?2 h^?1. In contrast to denitrification, net oxygen consumption was generally higher during the night than during the day. 5. River water was consistently supersaturated with N₂O, ranging from 102 to 209% saturated. Net flux of N₂O to the atmosphere ranged from 0.4 to 60 μmol N m^?2 h^?1. Net flux of N₂O was generally higher at night than during the day. The high flux of N₂O from these rivers strengthens the argument that rivers are an important contributor to anthropogenic emissions of this greenhouse gas.     

A test of a field‐based 15N–nitrous oxide pool dilution technique to measure gross N2O production in soil

Soils are both a major source and sink of nitrous oxide (N₂O), but the proportion of soil N₂O production released to the atmosphere (termed the N₂O yield) is poorly constrained due to the difficulty in measuring gross N₂O production. The quantification of gross N₂O fluxes would greatly improve our ability to predict N₂O dynamics across the soil‐atmosphere interface. We report a new approach, the ¹⁵N₂O pool dilution technique, to measure rates of gross N₂O production and consumption under laboratory and field conditions. In the laboratory, gross N₂O production and consumption compared well between the ¹⁵N₂O pool dilution and acetylene inhibition methods whereas the ¹⁵NO₃^? tracer method measured significantly higher rates. In the field, N₂O emissions were not significantly affected by increasing chamber headspace concentrations up to 100 ppb ¹⁵N₂O. The pool dilution model estimates of ¹⁴N₂O and ¹⁵N₂O concentrations as well as net N₂O fluxes fit observed data very well, suggesting that the technique yielded robust estimates of gross N₂O production. Estimated gross N₂O consumption rates were underestimated relative to rates calculated as the difference between gross and net N₂O production rates, possibly due to heterogeneous and/or inadequate tracer diffusion to deeper layers in the soil profile. Gross N₂O production rates were high, averaging 8.4 ± 3.2 mg N m^?2 day^?1, and were most strongly correlated to mineral nitrogen concentrations and denitrifying enzyme activity (R² = 0.73). Gross N₂O production rates varied spatially, with the highest rates in soils with the best drainage and the highest mineral N availability. Estimated and calculated N₂O consumption rates constrained the average N₂O yield from 0.70 to 0.84. Our results demonstrate that the ¹⁵N₂O pool dilution technique can provide well‐constrained estimates of N₂O yields and field rates of gross N₂O production correlated to soil characteristics, improving our understanding of terrestrial N₂O dynamics.     

Explaining geographical variation in the isotope composition of mouse lemurs (Microcebus)

Aim We sought to quantify geographical variation in the stable isotope values of mouse lemurs (Microcebus) and to determine whether this variation reflects trophic differences among populations or baseline isotopic differences among habitats. If the latter pattern is demonstrated, then Microcebus can become a proxy for tracking baseline habitat isotopic variability. Establishing such a baseline is crucial for identifying niche partitioning in modern and ancient communities. Location We studied five species of Microcebus from eight distinct habitats across Madagascar. Methods We compared isotopic variation in C₃ plants and Microcebus fur within and among localities. We predicted that carbon and nitrogen isotope values of Microcebus should: (1) vary as a function of abiotic variables such as rainfall and temperature, and (2) covary with isotopic values in plants. We checked for trophic differences among Microcebus populations by comparing the average difference between mouse lemur and plant isotope values for each locality. We then used multiple regression models to explain spatial isotope variation in mouse lemurs, testing a suite of explanatory abiotic variables. Results We found substantial isotopic variation geographically. Ranges for mean isotope values were similar for both Microcebus and plants across localities (carbon 3.5–4.0‰; nitrogen 10.5–11.0‰). Mean mouse lemur and plant isotope values were lowest in cool, moist localities and highest in hot, dry localities. Rainfall explained 58% of the variation in Microcebus carbon isotope values, and mean plant nitrogen isotope values explained 99.7% of the variation in Microcebus nitrogen isotope values. Average differences between mouse lemur and plant isotope values (carbon 5.0‰; nitrogen 5.9‰) were similar across localities. Main conclusions Isotopic data suggest that trophic differences among Microcebus populations were small. Carbon isotope values in mouse lemurs were negatively correlated with rainfall. Nitrogen isotope values in Microcebus and plants covaried. Such findings suggest that nitrogen isotope values for Microcebus are a particularly good proxy for tracking baseline isotopic differences among habitats. Our results will facilitate future comparative research on modern mouse lemur communities, and ecological interpretations of extinct Holocene communities.     

Lower–Middle Ordovician carbon and oxygen isotope chemostratigraphy at Hällekis,Sweden: implications for regional to global correlation and palaeoenvironmental development

A high‐resolution chemostratigraphical (coupled δ¹³C_carb and δ¹⁸O_carb) study of the topmost Floian through the middle Darriwilian (Ordovician) succession at the Hällekis quarry, Kinnekulle, southern Sweden, shows relatively steady isotopic values with overall minor changes, although some notable short‐ and long‐term shifts are discernible. A pronounced positive shift in δ¹³C in the uppermost part of the study succession is identified as the Middle Darriwilian Isotopic Carbon Excursion (MDICE), representing the only named global isotopic excursion in the data set. Regional and global comparisons suggest that few details in the different carbon and oxygen isotope curves can be confidently correlated, but longer‐term patterns appear quite consistent. Trends in the isotope data are in agreement with palaeogeographical reconstructions. Differences in stratigraphical patterns of both carbon and oxygen isotopes between localities suggest strong secular development at several spatiotemporal scales; any global signal involving relatively minor isotopic shifts is often masked/subdued by local and regional overprinting and care should be taken not to overinterpret data sets. Collectively, the data suggest rising sea levels and cooling climates through the studied time interval, but detailed interpretations remain problematic.     

Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Global soil nitrous oxide emissions since the preindustrial era estimated by an ensemble of terrestrial biosphere models: Magnitude,attribution, and uncertainty

Our understanding and quantification of global soil nitrous oxide (N₂O) emissions and the underlying processes remain largely uncertain. Here, we assessed the effects of multiple anthropogenic and natural factors, including nitrogen fertilizer (N) application, atmospheric N deposition, manure N application, land cover change, climate change, and rising atmospheric CO₂ concentration, on global soil N₂O emissions for the period 1861–2016 using a standard simulation protocol with seven process‐based terrestrial biosphere models. Results suggest global soil N₂O emissions have increased from 6.3 ± 1.1 Tg N₂O‐N/year in the preindustrial period (the 1860s) to 10.0 ± 2.0 Tg N₂O‐N/year in the recent decade (2007–2016). Cropland soil emissions increased from 0.3 Tg N₂O‐N/year to 3.3 Tg N₂O‐N/year over the same period, accounting for 82% of the total increase. Regionally, China, South Asia, and Southeast Asia underwent rapid increases in cropland N₂O emissions since the 1970s. However, US cropland N₂O emissions had been relatively flat in magnitude since the 1980s, and EU cropland N₂O emissions appear to have decreased by 14%. Soil N₂O emissions from predominantly natural ecosystems accounted for 67% of the global soil emissions in the recent decade but showed only a relatively small increase of 0.7 ± 0.5 Tg N₂O‐N/year (11%) since the 1860s. In the recent decade, N fertilizer application, N deposition, manure N application, and climate change contributed 54%, 26%, 15%, and 24%, respectively, to the total increase. Rising atmospheric CO₂ concentration reduced soil N₂O emissions by 10% through the enhanced plant N uptake, while land cover change played a minor role. Our estimation here does not account for indirect emissions from soils and the directed emissions from excreta of grazing livestock. To address uncertainties in estimating regional and global soil N₂O emissions, this study recommends several critical strategies for improving the process‐based simulations.     

Variability in nitrogen stable isotope ratios of macroalgae: consequences for the identification of nitrogen sources

In our research, we collected and analyzed numerous macroalgal specimens (738) for isotopic analysis sampled over a year at monthly intervals across 20 sites within the Urías lagoon complex, a typical subtropical coastal ecosystem located in the Gulf of California. We quantified and characterized (chemically and isotopically) the N loads received by Urías throughout a year. We studied the spatial‐temporal variation of the chemical forms and isotopic signals of the available N in the water column, and we monitored in situ different environmental variables and other hydrodynamic parameters. Multiple N sources (e.g., atmospheric, sewage, seafood processing, agriculture and aquaculture effluents) and biogeochemical reactions related to the N cycle (e.g., ammonia volatilization, nitrification and denitrification) co‐occurring across the ecosystem, result in a mixture of chemical species and isotopic compositions of available N in the water column. Increased variability was observed in the δ¹⁵N values of macroalgae (0.41‰–22.67‰). Based on our results, the variation in δ¹⁵N was best explained by spatio‐temporal changes in available N and not necessarily related to the N sources. The variability was also explained by the differences in macroalgal biology among functional groups, species and/or individuals. Although the δ¹⁵N‐macroalgae technique was a useful tool to identify N sources, its application in coastal ecosystems receiving multiple N sources, with changing environmental conditions influencing biogeochemical processes, and high diversity of ephemeral macroalgal species, could be less sensitive and have less predictive power.     

Increased snowfall weakens complementarity of summer water use by different plant functional groups

Winter snowfall is an important water source for plants during summer in semiarid regions. Snow, rain, soil water, and plant water were sampled for hydrogen and oxygen stable isotopes analyses under control and increased snowfall conditions in the temperate steppe of Inner Mongolia, China. Our study showed that the snowfall contribution to plant water uptake continued throughout the growing season and was detectable even in the late growing season. Snowfall versus rainfall accounted for 30% and 70%, respectively, of the water source for plants, on the basis of hydrogen stable isotope signature (δD) analysis, and accounted for 12% and 88%, respectively, on the basis of oxygen stable isotope signature (δ¹⁸O) analysis. Water use partitioning between topsoil and subsoil was found among species with different rooting depths. Increased snowfall weakened complementarity of plant water use during summer. Our study provides insights into the relationships between precipitation regimes and species interactions in semiarid regions.     

Short-term dynamics of the carbon isotope composition of CO2 emitted from a wheat agroecosystem--physiological and environmental controls

Understanding environmental and physiological controls of the variations in δ(13) C of CO(2) respired (δ(13) C(R)) from different compartments of an ecosystem is important for separation of CO(2) fluxes and to assess coupling between assimilation and respiration. In a wheat field, over 3 days we characterised the temporal dynamics of δ(13) C(R) from shoots and roots, from the soil and from the whole agroecosystem. To evaluate the basis of potential variations in δ(13) C(R), we also measured δ(13) C in different organic matter pools, as well as meteorological and gas exchange parameters. We observed strong diel variations up to ca. 6% in shoot, root and soil δ(13) C(R), but not in δ(13) C of the putative organic substrates for respiration, which varied by not more than ca. 1% within 24 h. Whole ecosystem-respired CO(2) was least depleted in (13) C in the afternoon and most negative in the early morning. We assume that temporally variable respiratory carbon isotope fractionation and changes in fluxes through metabolic pathways, rather than photosynthetic carbon isotope fractionation, governs the δ(13) C of respired CO(2) at the diel scale, and thus provides insights into the metabolic processes related to respiration under field conditions.     

Simultaneous nitrification,denitrification, and phosphorus removal in a lab-scale sequencing batch reactor

Simultaneous nitrification and denitrification (SND) via the nitrite pathway and anaerobic-anoxic-enhanced biological phosphorus removal (EBPR) are two processes that can significantly reduce the energy and COD demand for nitrogen and phosphorus removal. The combination of these two processes has the potential of achieving simultaneous nitrogen and phosphorus removal with a minimal requirement for COD. A lab-scale sequencing batch reactor (SBR) was operated in alternating anaerobic-aerobic mode with a low dissolved oxygen (DO) concentration (0.5 mg/L) during the aerobic period, and was demonstrated to accomplish nitrification, denitrification, and phosphorus removal. Under anaerobic conditions, COD was taken up and converted to polyhydroxyalkanoates (PHAs), accompanied by phosphorus release. In the subsequent aerobic stage, PHA was oxidized and phosphorus was taken up to <0.5 mg/L by the end of the cycle. Ammonia was also oxidized during the aerobic period, but without accumulation of nitrite or nitrate in the system, indicating the occurrence of simultaneous nitrification and denitrification. However, off-gas analysis showed that the final denitrification product was mainly nitrous oxide (N(2)O), not N(2). Further experimental results demonstrated that nitrogen removal was via nitrite, not nitrate. These experiments also showed that denitrifying glycogen-accumulating organisms (DGAOs), rather than denitrifying polyphosphate-accumulating organisms (DPAOs), were responsible for the denitrification activity.     

Diverse diet compositions among harpaline ground beetle species revealed by mixing model analyses of stable isotope ratios

1. Species diversities of some insect lineages have been attributed to differentiation of feeding habits among species. Our objective was to determine variation in diet composition among harpaline ground beetle species occurring in a riverside grassland. 2. We examined the diet compositions of 14 species from six genera in the spring and 10 species from two genera in the autumn. We performed measurements of nitrogen and carbon stable isotope ratios in consumers and in their potential food items, and estimated relative contributions of different food items with two mixing models, IsoSource and MixSIR. 3. IsoSource and MixSIR software gave similar results, but IsoSource tended to calculate higher contributions of principal food items and smaller percentile ranges than MixSIR. Among harparine beetle species, there were diverse food utilisation patterns among four food categories (detritivorous invertebrates, herbivorous invertebrates, C3 plants, and C4 plants). Detritivores comprised the main diets of abundant harpaline species in the spring, whereas abundant harpaline species in the autumn were primarily herbivores feeding on C4 plants, or omnivores feeding on herbivorous invertebrates and C3 plants. Seasonal changes in food use were related to seasonal changes in the abundance of each food resource. 4. Mixing model analysis of stable isotope ratios is a convenient and effective method for roughly estimating diets of many species with diverse food habits (such as ground beetles). This method can contribute to determining the trophic relationships of related insects in one ecosystem.