Microbial N2O consumption in and above marine N2O production hotspots

The ocean is a net source of N₂O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N₂O via microbial N₂O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N₂O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O₂ tolerance, and community composition of N₂O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N₂O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N₂O cycling. Microbes from the oxic layer consume N₂O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N₂O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O₂ and N₂O gradients right above the ODZ is a previously ignored potential gatekeeper of N₂O and should be accounted for in the marine N₂O budget.Subject terms: Water microbiology, Biogeochemistry, Microbial ecology     

土壤含水量与N2O产生途径研究

土壤含水量变化对Ｎ２Ｏ产生和排放影响的研究表明,不同含水量情况下,Ｎ２Ｏ排放也不相同．特别是用乙炔抑制技术证明了在播种前后,气候干燥而土壤含水量较低的情况下,Ｎ２Ｏ产生主要来自于硝化过程;降雨后,土壤含水量较高时,Ｎ２Ｏ主要是通过反硝化过程产生;而在农田中等含水量情况下,土壤微生物的硝化和反硝化作用产生的Ｎ２Ｏ大约各占一半．指出旱作农田Ｎ２Ｏ产生途径主要取决于土壤水分的控制和调节．     

Effect of Siberian tree species on N2O production and consumption

The effect of six Siberian tree species on two stages of denitrification—N₂O production and consumption—was studied. Broadleaf species (aspen and birch) proved to have lower rates of N₂O consumption compared to coniferous species. The factors influencing production and consumption of N₂O were also evaluated. The replacement of coniferous forests with broadleaf trees will double the N₂O/N₂ ratio in the denitrification end-products. Doubled N₂O emission from Siberian forest soils to the atmosphere can be expected due to changes in tree species composition of forest ecosystems even without considering changes in water and temperature regimes in soil.     

Interactions between N application rate, CH4 oxidation and N2O production in soil

Here we report on a controlled environment experiment in which we applied ¹³C- and ¹⁵N-enrichment approaches to quantify methane oxidation rates and source partition N₂O production in a silt loam soil following application of NH₄NO₃, enabling us to look for potential interactions between methane oxidation and nitrifier-N₂O production. ¹⁵N-N₂O, ¹⁴⁺¹⁵N-N₂O and CO₂ fluxes and mineral N concentrations were measured over a 23-day period after application of NH₄NO₃ (5 at.% excess ¹⁵N) at rates of 0, 5, 10, 20, 30 and 40 g N m^?2 to a silt loam soil. Change in ^12/13C-CH₄ concentrations (as indicative of ¹³C-CH₄ oxidation rates) and production of ¹³C-CO₂ were monitored over the first 72 h after addition of 1.7 ??l ¹³C-CH₄ l^?1 (10 at.% excess ¹³C) to these N treatments. Oxidation of applied ¹³C-CH₄ was slower in the 5, 10, 20 and 30 g N m^?2 (5 at.% excess ¹⁵N) treatments (0.24?C0.32 ??g ¹³C-CH₄ l^?1 day^?1) than in the control (0.40 ??g ¹³C-CH₄ l^?1 day^?1), suggesting that these N loadings inhibited oxidation. N₂O production was raised after N addition, and in the 10, 20 and 30 g N m^?2 treatments nitrification was the predominant source of N₂O accounting for 61, 83 and 57% of the total ¹⁵N-N₂O produced, respectively. Our results point towards the possibility of methylotrophs switching function to oxidise ammonia in the presence of N, which may result in greater atmospheric loading of both CH₄ and N₂O.     

Effects of tree species composition on the CO2 and N2O efflux of a Mediterranean mountain forest soil

Background and Aims

Tree species composition shifts can alter soil CO₂ and N₂O effluxes. We quantified the soil CO₂ and N₂O efflux rates and temperature sensitivity from Pyrenean oak, Scots pine and mixed stands in Central Spain to assess the effects of a potential expansion of oak forests.

Methods

Soil CO₂ and N₂O effluxes were measured from topsoil samples by lab incubation from 5 to 25 °C. Soil microbial biomass and community composition were assessed.

Results

Pine stands showed highest soil CO₂ efflux, followed by mixed and oak forests (up to 277, 245 and 145 mg CO₂-C m^?2 h^?1, respectively). Despite contrasting soil microbial community composition (more fungi and less actinomycetes in pine plots), carbon decomposability and temperature sensitivity of the soil CO₂ efflux remain constant among tree species. Soil N₂O efflux rates and its temperature sensitivity was markedly higher in oak stands than in pine stands (70 vs. 27 μg N₂O-N m^?2 h^?1, Q₁₀, 4.5 vs. 2.5).

Conclusions

Conversion of pine to oak forests in the region will likely decrease soil CO₂ effluxes due to decreasing SOC contents on the long run and will likely enhance soil N₂O effluxes. Our results present only a seasonal snapshot and need to be confirmed in the field.     

Simultaneous measurement of ethylene and CO2 production and of O2 consumption in soil and pure microbial cultures

Summary The method described here makes it possible to determine gaseous and volatile compounds produced by stationary and shaken cultures of microorganisms, and by soil samples. In a closed system oxygen consumed by a biological sample is electrolytically replenished and CO₂ produced is trapped in a KOH solution. Oxygen from the electrolyzer was replenished through a distribution bottle with the aid of a system of polyethylene tubings equipped with injection needles at both ends. The amount of the oxygen consumed is permanently reflected by volume of the electrolytically released hydrogen. The reverse flow of atmosphere from the cultivation flasks was prevented by a liquid seal formed by a KOH solution or the cultivation medium itself. Suba-seal type flasks (125 ml) with rubber caps served for the cultivation. Glass cylinders placed inside cultivation flasks or penicillin flasks (20 ml) connected with cultivation flasks by polyethylene tubings served as adsorption vessels. By exchanging the KOH solution it is possible to follow CO₂ production even during individual cultivation phases. Volatile acids adsorbed in the KOH solution could be determined after their release by mineral acid. Samples of atmosphere were analyzed by gas chromatography. The whole system had to be temperature equilibrated.     

The effect of grass maturing and root decay on N2O production in soil

The N₂O flux from the surface of grass-covered pots was only significant following grass maturing. Removal of the above-ground plant material resulted in an immediate and long-lasting increase in N₂O production in the soil. The results suggest that easily available organic matter from the roots stimulates the denitrification when the plants are damaged. Grass cutting might therefore result in a marked nitrogen loss through denitrification. The quantitative effect was equal in soil with and without succinate added. The size of the anaerobic zone around the roots is therefore sufficient to allow for denitrification activity mediated by increased organic matter availability because of plant cutting.     

In situ 15N-N2O site preference and O2 concentration dynamics disclose the complexity of N2O production processes in agricultural soil

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N₂O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N₂O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N₂O fluxes in an arable soil, combined with in situ high-frequency measurements of soil matrix oxygen (O₂) and N₂O concentrations, in situ ¹⁵N labeling, and N₂O ¹⁵N site preference (SP). The in situ O₂ concentration and further microcosm visualized spatiotemporal distribution of O₂ both suggested that O₂ dynamics were the proximal determining factor to matrix N₂O concentration and fluxes due to quick O₂ depletion after N fertilization. Further SP analysis and in situ ¹⁵N labeling experiment revealed that the main source for N₂O emissions was bacterial denitrification during the hot-wet summer with lower soil O₂ concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold-dry winter with higher soil O₂ concentration. The robust positive correlation between O₂ concentration and SP values underpinned that the O₂ dynamics were the key factor to differentiate the composite processes of N₂O production in in situ structured soil. Our findings deciphered the complexity of N₂O production processes in real field conditions, and suggest that O₂ dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N₂O production processes in undisturbed structure soils. Our results help to develop targeted N₂O mitigation measures and to improve process models for constraining global N₂O budget.     

Bamboo invasion of broadleaf forests altered soil fungal community closely linked to changes in soil organic C chemical composition and mineral N production

Plant and Soil - Loss of biodiversity caused by intensive agriculture is a major worldwide concern. Crop rotation can enhance crop productivity and increase soil microbial diversity. However, the...     

Influence of carbon availability on the production of NO, N2O, N2 and CO2 by soil cores during anaerobic incubation

Net productions of permanent soil atmosphere gases (N₂, CO₂, O₂) and temporary gases (N₂O, NO) were monitored in soil cores using a non-interfering, fully automated measuring technique allowing highly time resolved measurements over prolonged periods. The influence of changes in available organic carbon on CO₂, N₂O, NO and N₂ production was studied by changing the soil carbon content through aerobic preincubations of different length, up to 21 days.The aerobic preincubation caused an increase in NO₃ ^- concentration and a decrease in available carbon content. Available carbon content dominated both CO₂ and total N gas (N₂+N₂O+NO) production during anaerobiosis. Both CO₂ and total N gas production rates decreased with increasing length of the previous aerobic preincubation, this in spite of the higher initial NO₃ ^- concentration.Total denitrification rates were closely related to the anaerobic CO₂ production rates. No relation was found between water soluble carbon content and total denitrification. The N₂O/N₂ ratio could be explained by an interaction of carbon availability, NO₃ ^- concentration and enzyme status. Net N₂O consumption was monitored. The balance between cumulative total N gas production and NO₃ ^- consumption varied according to the different treatments. Cumulative N₂O production exceeded cumulative N₂ production for 0 up to 5 days.     

Denitrification and N2O emission from urine-affected grassland soil

Denitrification and N₂O emission rates were measured following two applications of artificial urine (40 g urine-N m^–2) to a perennial rye-grass sward on sandy soil. To distinguish between N₂O emission from denitrification or nitrification, urine was also applied with a nitrification inhibitor (dicyandiamide, DCD). During a 14 day period following each application, the soil was frequently sampled, and incubated with and without acetylene to measure denitrification and N₂O emission rates, respectively.Urine application significantly increased denitrification and N₂O emission rates up to 14 days after application, with rates amounting to 0.9 and 0.6 g N m^–2 day^–1 (9 and 6 kg N ha^–1 day^–1), respectively. When DCD was added to the urine, N₂O emission rates were significantly lower from 3 to 7 days after urine application onwards. Denitrification was the main source of N₂O immediately following each urine application. 14 days after the first application, when soil water contents dropped to 15% (v/v) N₂O mainly derived from nitrification.Total denitrification losses during the 14 day periods were 7 g N m^–2, or 18% of the urine-N applied. Total N₂O emission losses were 6.5 and 3 g N m^–2, or 16% and 8% of the urine-N applied for the two periods. The minimum estimations of denitrification and N₂O emission losses from urine-affected soil were 45 to 55 kg N ha^–1 year^–1, and 20 to 50 kg N ha^–1 year^–1, respectively.     

Steady-state nitrogen isotope effects of N2 and N2O production in Paracoccus denitrificans

Nitrogen stable-isotope compositions (delta15N) can help track denitrification and N2O production in the environment, as can knowledge of the isotopic discrimination, or isotope effect, inherent to denitrification. However, the isotope effects associated with denitrification as a function of dissolved-oxygen concentration and their influence on the isotopic composition of N2O are not known. We developed a simple steady-state reactor to allow the measurement of denitrification isotope effects in Paracoccus denitrificans. With [dO2] between 0 and 1.2 microM, the N stable-isotope effects of NO3- and N2O reduction were constant at 28.6 per thousand +/- 1.9 per thousand and 12.9 per thousand +/- 2.6 per thousand, respectively (mean +/- standard error, n = 5). This estimate of the isotope effect of N2O reduction is the first in an axenic denitrifying culture and places the delta15N of denitrification-produced N2O midway between those of the nitrogenous oxide substrates and the product N2 in steady-state systems. Application of both isotope effects to N2O cycling studies is discussed.     

土壤动物与N素循环及对N沉降的响应

以主要的生态过程之一——N循环为对象,论述了土壤动物不仅对凋落物的分解有重要影响,而且在N素矿化和植物对N的吸收过程中也起着重要作用。同时,日益严重的全球变化问题之一——N沉降对土壤动物的多样性及其在生态系统中的功能构成了极大的威胁。另还对土壤动物与N循环研究的方法、土壤动物在N循环过程中的作用机制、热带地区的需求及N沉降下土壤动物的响应作了探讨,并提出,开展大尺度的专类研究及长期定位研究成为下一步研究的需要。     

Characterization of microbial NO production, N2O production and CH4 oxidation initiated by aeration of anoxic rice field soil

Intermittent drainage of rice fields isdiscussed as an option to mitigate emission ofCH₄, an important greenhouse gas. HoweverN₂O, a potentially more effective greenhouse gas,may be emitted during the aeration phase. Therefore,the metabolism of NO, N₂O, NH ,NO and NO and the kinetics ofCH₄ oxidation were measured after aeration ofmethanogenic rice field soil. Before aeration, thesoil contained NH in relatively highconcentrations (about 4 mM), while NO andNO were almost undetectable. Immediatelyafter aeration both NO and N₂O were produced withrates of about 15 pmol h^-1 gdw^-1 and 5 pmolh^-1 gdw^-1, respectively. Simultaneously,NH decreased while NO accumulated. Later on, NO was depletedwhile NO concentrations increased.Characteristic phases of nitrogen turnover wereassociated with the activities of ammonium oxidizers,nitrite oxidizers and denitrifiers. Oxidation ofNH and production of NO and N₂O wereinhibited by 10 Pa acetylene demonstrating thatnitrification was obligatory for the initiation ofnitrogen turnover and production of NO and N₂O.Ammonium oxidation was not limited by the availableNH and thus, concomittant production of NOand N₂O was not stimulated by addition ofNH . However, addition of NO stimulated production of NO and N₂O in bothanoxic and aerated rice soil slurries. In this case,10 Pa acetylene did not inhibit the production of NOand N₂O demonstrating that it was due todenitrification which was obviously limited by theavailability of NO . In the aerated soilslurries CH₄ was only oxidized if present atelevated concentrations >50 ppmv CH₄). Atatmospheric CH₄ concentrations (1.7 ppmv)CH₄ was not consumed, but was even slightly produced.CH₄ oxidation activity increased afterpreincubation at 20% CH₄, and then CH₄was also oxidized at atmospheric concentrations. CH₄oxidation kinetics exhibited sigmoid characteristicsat low CH₄ concentrations presumably because ofinhibition of CH₄ oxidation by NH .     

Importance of denitrifiers lacking the genes encoding the nitrous oxide reductase for N2O emissions from soil

Analyses of the complete genomes of sequenced denitrifying bacteria revealed that approximately 1/3 have a truncated denitrification pathway, lacking the nosZ gene encoding the nitrous oxide reductase. We investigated whether the number of denitrifiers lacking the genetic ability to synthesize the nitrous oxide reductase in soils is important for the proportion of N₂O emitted by denitrification. Serial dilutions of the denitrifying strain Agrobacterium tumefaciens C58 lacking the nosZ gene were inoculated into three different soils to modify the proportion of denitrifiers having the nitrous oxide reductase genes. The potential denitrification and N₂O emissions increased when the size of inoculated C58 population in the soils was in the same range as the indigenous nosZ community. However, in two of the three soils, the increase in potential denitrification in inoculated microcosms compared with the noninoculated microcosms was higher than the increase in N₂O emissions. This suggests that the indigenous denitrifier community was capable of acting as a sink for the N₂O produced by A. tumefaciens. The relative amount of N₂O emitted also increased in two soils with the number of inoculated C58 cells, establishing a direct causal link between the denitrifier community composition and potential N₂O emissions by manipulating the proportion of denitrifiers having the nosZ gene. However, the number of denitrifiers which do not possess a nitrous oxide reductase might not be as important for N₂O emissions in soils having a high N₂O uptake capacity compared with those with lower. In conclusion, we provide a proof of principle that the inability of some denitrifiers to synthesize the nitrous oxide reductase can influence the nature of the denitrification end products, indicating that the extent of the reduction of N₂O to N₂ by the denitrifying community can have a genetic basis.     

Contrasting fine-root production,survival and soil CO2 efflux in pine and poplar plantations

Tree root activity, including fine-root production, turnover and metabolic activity are significant components of forest productivity and nutrient cycling. Differences in root activity among forest types are not well known. A 3-year study was undertaken in red pine (Pinus resinosa Ait.) and hybrid poplar (Populus tristis X P. balsamifera cv `Tristis no. 1') plantations to compare belowground root dynamics. We measured fine-root production, mortality and standing crop, as well as soil CO₂ efflux. Pine fine-root production was only 2.9% of that of poplar during three years; 85 pine roots were observed in minirhizotron tubes compared with 4088 poplar roots. Live-root density oscillated seasonally for both species with late winter minimum and autumn maximum. Poplar reached constant maximum live-root length within the first growing season, but pine continued to increase observed fine-root length for three growing seasons. Within the first 100 days following initial appearance, 22% of the pine roots disappeared and 38% of the poplar roots disappeared. Median fine-root longevity of pine was 291 days compared with 149 days for poplar roots. Fine-root longevity increased with depth in the soil, and was greater for roots with initial diameter >0.5 mm. The probability of poplar root death from late February to May was more than three times that in any other season, regardless of root age. Despite the greater poplar root production and live-root length, fine-root biomass and soil CO₂ efflux was greater in pine. Greater metabolic activity in the pine stand may be due to greater fine-root biomass or greater heterotrophic respiration. This revised version was published online in June 2006 with corrections to the Cover Date.