Field method to determine N2O emission from nitrification and denitrification

 Nitrous oxide (N₂O) emissions via the nitrification (I _nit) and denitrification (I _den) pathways were successfully measured with in-field incubation of soil cores in preserving jars at 0 Pa and 5–10 Pa acetylene. From the incubations, fractions of nitrification – N₂O over total N₂O (I _nit / I _tot) – and denitrification – N₂O over total N₂O (I _den / I _tot) – were obtained. Actual field emissions of N₂O via nitrification (F _nit) and denitrification (F _den) were calculated by multiplying the fractions from the incubation technique with the daily N₂O emission (F _day) determined with a direct soil cover method. The approach presented here was successful for a whole range of soil moisture conditions in intensive grassland. F _nit and F _den followed the trends of soil ammonium and soil nitrate. Received: 31 October 1997     

Increase in nitrous oxide production in soil induced by ammonium and organic carbon

We observed that soil cores collected in the field containing relatively high NH _inf4 ^sup+ and C substrate levels produced relatively large quantities of N₂O. A series of laboratory experiments confirmed that the addition of NH _inf4 ^sup+ and glucose to soil increase N₂O production under aerobic conditions. Denitrifying enzyme activity was also increased by the addition of NH _inf4 ^sup+ and glucose. Furthermore, NH _inf4 ^sup+ and glocose additions increased the production of N₂O in the presence of C₂H₂. Therefore, we concluded that denitrification was the most likely source of N₂O production. Denitrification was not, however, directly affected by NH _inf4 ^sup+ in anaerobic soil slurries, although the use of C substrate increased. In the presence of a high substrate C concentration, N₂O production by denitrifiers may be affected by NO _inf3 ^sup- supplied from NH _inf4 ^sup+ through nitrification. Alternatively, N₂O may be produced during mixotrophic and heterotrophic growth of nitrifiers. The results indicated that the NH _inf4 ^sup+ concentration, in addition to NO _inf3 ^sup- , C substrate, and O₂ concentrations, is important for predicting N₂O production and denitrification under field conditions.     

Relationships between soil moisture content and nitrous oxide production during nitrification and denitrification

Summary The effect of soil water content [60%–100% water-holding capacity (WHC)] on N₂O production during autotrophic nitrification and denitrification in a loam soil was studied in a laboratory experiment by selectively inhibiting nitrification with a low C₂H₂ concentration (2.1 Pa). Nitrifiers usually produced more N₂O than denitrifiers. During an initial experimental period of 0–6 days the nitrifiers produced more N₂O than the denitrifiers by a factor ranging from 1.4 to 16.5, depending on the water content and length of incubation. The highest N₂O production rate by nitrifiers was observed at 90% WHC, when the soil had become partly anaerobic, as indicated by the high denitrification rate. At 100% WHC there were large gaseous losses from denitrification, while nitrification losses were smaller except for the first period of measurement, when there was still some O₂ remaining in the soil. The use of 10 kPa C₂H₂ to inhibit reduction of N₂O to N₂ stimulated the denitrification process during prolonged incubation over several days; thus the method is unsuitable for long-term studies.     

Influence of nitrification inhibitors on nitrification and nitrous oxide (N2O) emission from a clay loam soil fertilized with urea

Laboratory incubation experiments were conducted to compare the effects of the nitrification inhibitors 3,4-dimethylpyrazole phosphate (DMPP) and 2-Chloro-6-(trichloromethyl)-pyridine (N-serve) on nitrification and nitrous oxide (N₂O) emission from a Vertosol from southern Australia, under controlled moisture and temperature. Nitrification rates in the control soil were strongly influenced by the temperature and moisture, increasing by a factor of 3.6 for each 10 °C increase between 5 and 25 °C. DMPP inhibited nitrification effectively for 42 days at 5-15 °C and 40-60% water filled pore space (WFPS). DMPP also slowed nitrification appreciably at 25 °C when the soil was at 40% WFPS, but was less effective at 60% water filled pore space. N-serve inhibited nitrification effectively for 42 days under all test conditions. Emissions of N₂O from the urea treatment (no inhibitors) significantly increased with increasing temperature and moisture. The ratio of total N₂O emission to total nitrification was not constant and varied from around 0.03% at 5 °C and 40% WFPS to 0.12% at 25 °C and 60% WFPS. DMPP and N-serve reduced cumulative N₂O emission over 42 days by more than 65% under all the imposed conditions.     

Nitrous oxide production by nitrification and denitrification in soil aggregates as affected by O2 concentration

Nitrous oxide emitted by soils can be produced either by denitrification in anoxic conditions or by nitrification in presence of O₂. The relative importance of the two processes, particularly under varied partial pressures of O₂, is not always known. This paper focuses on the influence of O₂ concentration on N₂O production by nitrification and denitrification in an arable Orthic Luvisol. Soil aggregates (2-3 mm size), water unsaturated, received 116 mg N kg⁻¹ as ammonium sulphate labelled with ¹⁵N and were incubated during 14 days at different O₂ partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A ¹⁵N tracing technique was used to quantify nitrification and denitrification rates. ¹⁵N₂O and ¹⁵N₂ were measured. Oxygen pressure appeared to strongly influence both nitrification and denitrification rates and also N₂O emissions. Nitrification rates were reduced by a factor of 6-9 when O₂ decreased from 20.4 to 0.35 kPa. They were highly correlated with O₂ consumption rates. Denitrification mainly occurred in complete anoxic conditions. The proportion of N₂O emitted by denitrification was estimated by two independent methods: one based on ¹⁵N tracing using isotope composition of NH₄, NO₃ and N₂O, the other based on the measurement of the ¹⁵N₂O:¹⁵N₂ ratio. The two methods gave close results. The highest N₂O emissions were obtained under complete anoxic conditions and were due to denitrification. However, N₂O emissions almost as important were obtained at day 14 with 1.5 kPa O₂ pressure, and they were due to nitrification. Nitrification was the main source of N₂O at O₂ concentrations greater than 0.35 kPa. The amounts of N₂O-N emitted by nitrification were linearly related to the amounts of N nitrified, but the slope of the regression was highly dependent on O₂ concentration: it varied from 0.16 to 1.48% when O₂ concentration was reduced from 20.4 to 0.76 kPa. Emissions of N₂O by nitrification may then be quite significant if nitrification occurs at a reduced O₂ concentration.     

Estimation of simultaneous nitrification and denitrification in grassland soil associated with urea-N using 15N and nitrification inhibitor

 A low efficiency of use of N fertilisers has been observed in mid-Wales on permanent pasture grazed intensively by cattle. Earlier laboratories studies have suggested that heterogeneity in redox conditions at shallow soil depths may allow nitrification and denitrification to occur concurrently resulting in gaseous losses of N from both NH₄ ⁺ and NO₃ ^–. The objective of the investigation was to test the hypothesis that both nitrification and denitrification can occur simultaneously under simulated field capacity conditions (∼5 kPa matric potential). Intact soil cores were taken from grassland subjected to both grazing and amenity use. The fate of applied NH₄ ⁺ was examined during incubation. ¹⁵N was used as a tracer. Nitrapyrin was used as a nitrification inhibitor and acetylene was used to block N₂O reductase. More than 50% of N applied as NH₄ ⁺ disappeared over a period of 42 days from the soil mineral-N pool. Some of this N was evolved as N₂O. Accumulation of NO₃ ^––N in the surface 0–2.5 cm indicated active nitrification. Addition of nitrapyrin increased N recovery by 26% and inhibited both the accumulation of NO₃–N and emission of N₂O. When intact field cores were incubated after addition of ¹⁵N-urea, all of the N₂O evolved was derived from added urea-N. It was concluded that nitrification and denitrification do occur simultaneously in the top 7.5 cm or so, of the silty clay loam grassland topsoils of mid-Wales at moisture contents typical of field capacity. The quantitative importance of these concurrent processes to N loss from grassland systems has not yet been assessed. Received: 15 December 1998     

Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space

A combination of stable isotope and acetylene (0.01% v/v) inhibition techniques were used for the first time to determine N₂O production during denitrification, autotrophic nitrification and heterotrophic nitrification in a fertilised (200 kg N ha^–1) silt loam soil at contrasting (20–70%) water-filled pore space (WFPS). ¹⁵N-N₂O emissions from ¹⁴NH₄¹⁵NO₃ replicates were attributed to denitrification and ¹⁵N-N₂O from ¹⁵NH₄¹⁵NO₃ minus that from ¹⁴NH₄¹⁵NO₃ replicates was attributed to nitrification and heterotrophic nitrification in the presence of acetylene, as there was no dissimilatory nitrate reduction to ammonium or immobilisation and remineralisation of ¹⁵N-NO₃^–. All of the N₂O emitted at 70% WFPS (31.6 mg N₂O-N m^–2 over 24 days; 1.12 g N₂O-N g dry soil^–1; 0.16% of N applied) was produced during denitrification, but at 35–60% WFPS nitrification was the main process producing N₂O, accounting for 81% of ¹⁵N-N₂O emitted at 60% WFPS, and 7.9 g ¹⁵N-N₂O m^–2 (0.28 ng ¹⁵N-N₂O g dry soil^–1) was estimated to be emitted over 7 days during heterotrophic nitrification in the 50% WFPS treatment and accounted for 20% of ¹⁵N-N₂O from this treatment. Denitrification was the predominant N₂O-producing process at 20% WFPS (2.6 g ¹⁵N-N₂O m^–2 over 7 days; 0.09 ng ¹⁵N-N₂O g dry soil^–1; 85% of ¹⁵N-N₂O from this treatment) and may have been due to the occurrence of aerobic denitrification at this WFPS. Our results demonstrate the usefulness of a combined stable isotope and acetylene approach to quantify N₂O emissions from different processes and to show that several processes may contribute to N₂O emission from agricultural soils depending on soil WFPS.     

Nitrous oxide production in grassland soils: assessing the contribution of nitrifier denitrification

Nitrifier denitrification is the reduction of NO₂⁻ to N₂ by nitrifiers. It leads to the production of the greenhouse gas nitrous oxide (N₂O) as an intermediate and possible end product. It is not known how important nitrifier denitrification is for the production of N₂O in soils. We explored N₂O production by nitrifier denitrification in relation to other N₂O producing processes such as nitrification and denitrification under different soil conditions. The influence of aeration of the soil, different N sources, and pH were tested in four experiments. To differentiate between sources of N₂O, an incubation method with inhibitors was used [Biol. Fertil. Soils 22 (1996) 331]. Sets of four incubations included controls without addition of inhibitors, incubations with addition of small concentrations of C₂H₂ (0.01-0.1 kPa), large concentrations of O₂ (100 kPa), or a combination of C₂H₂ and O₂. The results indicate that the availability of NO₂⁻ stimulated the apparent N₂O production by nitrifier denitrification. A decreasing O₂ content increased the total N₂O production, but decreased N₂O production by nitrifier denitrification. No significant effect of pH could be found. The study revealed problems concerning the use of the inhibitors C₂H₂ and O₂. Almost one-third of all incubations with inhibitors produced more N₂O than the controls. Possible reasons for the problems are discussed. The inhibitors C₂H₂ and O₂ need to be tested thoroughly for their effects on different N₂O producing processes before further application.     

Mechanistic model for nitrous oxide emission via nitrification and denitrification

Seasonal and annual N₂O fluxes from urine-affected pasture were approximated with a mechanistic model based on Michaelis-Menten kinetics. The model combined the effects of soil nitrate-N, soil ammonium-N, soil temperature and soil moisture (all from the top 5cm) to calculate N₂O emissions from nitrification (F _nit ) and denitrification (F _den ), with total N₂O emission being the sum of the two (F _tot =F _nit +F _den ). Best results were obtained when different kinetic parameters were used for periods of constant soil moisture conditions and after heavy rainfalls when a rapid change of the soil moisture status occurred. Modelled N₂O emissions over a year were within the range of uncertainties of measured N₂O emissions. Results indicate that the spatial variability of N₂O emissions at times when all the model inupt variables were constant may be related to microorganism growth dynamics or enzyme production rates. Received: 2 October 1995     

Laboratory investigations into the effects of the pesticides mancozeb, chlorothalonil, and prosulfuron on nitrous oxide and nitric oxide production in fertilized soil

Independent soil microcosm experiments were used to investigate the effects of the fungicides mancozeb and chlorothalonil, and the herbicide prosulfuron, on N₂O and NO production by nitrifying and denitrifying bacteria in fertilized soil. Soil cores were amended with NH₄NO₃ or NH₄NO₃ and pesticide, and the N₂O and NO concentrations were monitored periodically for approximately 48 h following amendment. Nitrification is the major source of N₂O and NO in these soils at soil moistures relevant to those observed at the field site where the cores were collected. At pesticide concentrations from 0.02 to 10 times that of a standard single application on a corn crop, N₂O and NO production was inhibited by all three pesticides. Generally N₂O production was inhibited by the pesticides from 10 to 62% and 20 to 98% at the lowest and highest dosages, respectively. Nitric oxide production was generally inhibited from about 5 to 47% and by 20 to 97% at the lowest and highest dosages, respectively. Nitrous oxide and nitric oxide production by nitrification was more susceptible to inhibition by these pesticides than denitrification. Production of both N₂O and NO by nitrification was inhibited by as much as 99%, at the highest concentration of pesticide applied. The net production of N₂O increased as soil moisture increased. The rate of NO production was greatest at the intermediate moistures investigated, between 14 and 19% gravimetric soil moisture, suggestive that nitrification is the dominant source of NO.     

Potency of nitrification inhibitors following their repeated application to soil

Summary Nitrification inhibitors were applied to a field experiment on loamy sand soil each autumn for 4 years, immediately prior to sowing winter cereals. Laboratory experiments demonstrated that repeated application of the inhibitor dicyandiamide (DCD) to a soil had little effect either on the rate of DCD decomposition or the ability of DCD to inhibit nitrification. Repeated field application of the inhibitors DCD, nitrapyrin or etridiazole resulted in increased sensitivity of ammonium-oxidizing bacteria to nitrapyrin or etridiazole, but not to DCD. The rate of decomposition of etridiazole was unaffected by four annual applications of this inhibitor, but decomposition of nitrapyrin was somewhat slower in soil that had received nitrapyrin annually for 4 years than in soil that had never been treated with an inhibitor.     

Denitrification and nitrous oxide to nitrous oxide plus dinitrogen ratios in the soil profile under three tillage systems

There is a growing interest in the adoption of conservation tillage systems [no-till (NT) and reduced tillage (RT)] as alternatives to conventional tillage (CT) systems. A 2-year study was conducted to investigate possible environmental consequences of three tillage systems on a 2.4-ha field located at Macdonald Research Farm, McGill University, Montreal. The soil was a sandy loam (0.5 m depth) underlain by a clay layer. Treatments consisted of a factorial combination of CT, RT, and NT with the presence or absence of crop residue. Soil NO₃^--N concentrations tended to be lower in RT than NT and CT tillage treatments. Denitrification and N₂O emissions were similar among tillage systems. Contrary to the popular assumption that denitrification is limited to the uppermost soil layer (0–0.15 m), large rates of N₂O production were measured in the subsurface (0.15–0.45 m) soil, suggesting that a significant portion of produced N₂O may be missed if only soil surface gas flux measurements are made. The N₂O mole fraction (N₂O:N₂O+N₂) was higher in the drier season of 1999 under CT than in 2000, with the ratio occasionally exceeding 1.0 in some soil layers. Dissolved organic C concentrations remained high in all soil depths sampled, but were not affected by tillage system.     

Influence of thiosulfate on nitrification,denitrification, and production of nitric oxide and nitrous oxide in soil

Thiosulfate and CS₂ inhibit nitrification. The effect of the addition of thiosulfate on the turnover of inorganic N compounds was tested in an Egyptian and a German arable soil under nitrifying and denitrifying conditions. For nitrification, the soils were amended with NH _inf4 ^sup+ and incubated under aerobic conditions. For denitrification, the soils were amended with NO _inf3 ^sup- and incubated under anaerobic conditions. In both cases, the thiosulfate decreased with time while tetrathionate accumulated to an intermediate extent. Both compounds disappeared completely after <25 days. Production of CS₂ was not observed. Carbonyl sulfide was produced only in the Egyptian soil, but production decreased with increasing amounts of added thiosulfate. Under nitrifying conditions, the addition of increasing amounts of thiosulfate (25, 50, and 100 g S g^-1 dry weight) resulted in decreasing rates of NH _inf4 ^sup+ oxidation to NO _inf3 ^sup- ; it also resulted in an increasing intermediate accumulation of NO _inf2 ^sup- and NO, and in an increasing production of N₂O. Under denitrifying conditions, the addition of increasing amounts of thiosulfate did not significantly affect the rate of NO _inf3 ^sup- reduction, and resulted in an increasing intermediate accumulation of NO _inf2 ^sup- and of NO only in the German soil in which the production of N₂O was slightly inhibited by thiosulfate. These results demonstrate that the nitrification of NH _inf4 ^sup+ and NO _inf2 ^sup- was inhibited by increasing concentrations of thiosulfate and/or tetrathionate without involving the formation of volatile S compounds as potential nitrification inhibitors. Denitrification was not affected by the addition of thiosulfate.     

菜地氮肥用量与N2O排放的关系及硝化抑制剂效果

通过连续种植四季蔬菜近一年的大田试验,探究高施氮水平和低氮肥利用率的蔬菜生产系统中,N2O排放量与氮肥施用量之间的定量关系及其机理,并研究硝化抑制剂减少菜地N2O排放的效果.结果表明,在氮肥施用水平为N 0～1 733 kg hm-2a-1间,无论氮肥中是否添加硝化抑制剂,N2O总排放量与氮肥施用量均呈指数函数关系,即氮肥施用量高时,N2O排放率也高.在各氮肥水平处理下,硝化抑制剂均能降低N2O排放,抑制率为8.75％ ～ 25.28％,且这种减排效果随着施氮量增加而增加.在氮肥施用量为N 300或400 kg hm-2季-1时,施用硝化抑制剂减少N2O排放所带来的效益略高于其成本,因此,即使不考虑氮肥利用率的提高等因素,施用硝化抑制剂仍是一种有利的选择.     

Small-scale heterogeneity in carbon dioxide, nitrous oxide and methane production from aggregates of a cultivated sandy-loam soil

Spatial variability in carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) emissions from soil is related to the distribution of microsites where these gases are produced. Porous soil aggregates may possess aerobic and anaerobic microsites, depending on the water content of pores. The purpose of this study was to determine how production of CO₂, N₂O and CH₄ was affected by aggregate size and soil water content. An air-dry sandy loam soil was sieved to generate three aggregate fractions (<0.25 mm, 0.25–2 mm and 2–6 mm) and bulk soil (<2 mm). Aggregate fractions and bulk soil were moistened (60% water-filled pore space, WFPS) and pre-incubated to restore microbial activity, then gradually dried or moistened to 20%, 40%, 60% or 80% WFPS and incubated at 25 °C for 48 h. Soil respiration peaked at 40% WFPS, presumably because this was the optimum level for heterotrophic microorganisms, and at 80% WFPS, which corresponded to the peak N₂O production. More CO₂ was produced by microaggregates (<0.25 mm) than macroaggregate (>0.25 mm) fractions. Incubation of aggregate fractions and soil at 80% WFPS with acetylene (10 Pa and 10 kPa) and without acetylene showed that denitrification was responsible for 95% of N₂O production from microaggregates, while nitrification accounted for 97–99% of the N₂O produced by macroaggregates and bulk soil. This suggests that oxygen (O₂) diffusion into and around microaggregates was constrained, whereas macroaggregates remained aerobic at 80% WFPS. Methane consumption and production were measured in aggregates, reaching 1.1–6.4 ng CH₄–C kg⁻¹ soil h⁻¹ as aggregate fractions and soil became wetter. For the sandy-loam soil studied, we conclude that nitrification in aerobic microsites contributed importantly to total N₂O production, even when the soil water content permitted denitrification and CH₄ production in anaerobic microsites. The relevance of these findings to microbial processes controlling N₂O production at the field scale remains to be confirmed.     

Effects of nitrification inhibitors on germination of various seeds in soil

Summary The effects of 19 nitrificiation inhibitors on germination of seeds in soil were investigated. The nitrification inhibitors tested were sodium azide, potassium azide, potassium ethyl xanthate, nitrapyrin (N-Serve), etridiazole (Dwell), 3-mercapto-1,2,4-triazole (MT), 2-amino-4-chloro-6-methylpyrimidine (AM), 2,4-diamino-6-trichloromethyl-s-triazine, 2-mercaptobenzothiazole (MBT), 4-amino-1,2,4-triazole (ATC), sodium thiocarbonate (STC), guanylthiourea (ASU), thiourea (TU), dicyandiamide (DCD), sulfathiazole (STC), phenylacetylene, 2-ethynyl-pyridine, 3-methylpyrazole-l-carboxamide (MPC), and ammonium thiosulfate (ATS). Germination tests were performed with seeds of alfalfa (Medicago sativa L.), wheat (Triticum aestivum L.), rye (Secale cereale L.), barley (Hordeum vulgare L.), sorghum [Sorghum bicolor (L.) Moench], oats (Avena sativa L.), and corn (Zea mays). Only 2 of the 19 nitrification inhibitors studied (potassium azide and sodium azide) reduced germination of the seeds tested when applied at the rate of 12.5 g g^–1. The other inhibitors studied had no effect on the germination of wheat, alfalfa, barley, corn, oat, rye, or sorghum seeds when they were applied at the rate of 125 g g^–1 soil, and most of them had no effect on seed germination when applied at the rate of 625 g g^–1 soil.     

Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

 Two versions of the acetylene inhibition (AI)/soil core method were compared for the measurement of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha^–1. With AI/soil core method A, the denitrification rate was measured by analysing the headspace N₂O, followed by estimation of N₂O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N₂O entrapped in the soil was measured in addition to that released from soil cores into the headspace of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N₂O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO₂, the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N₂O was significantly correlated with the water-filled pore space (WFPS) and with the N₂O concentration in the headspace, whereas CO₂ entrapment was dependent on the headspace CO₂ concentration but not on the WFPS. Due to the entrapment of N₂O and CO₂ in soil, the denitrification rate on several (18 of the 41) sampling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha^–1 day^–1) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha^–1 day^–1) was 1.6 times the rate measured with method A. Results of this study suggest that the N₂O and CO₂ entrapped in soil should also be measured to ensure the recovery of the gaseous products of denitrification by the soil core method. Received: 12 May 1998     

Temporal and spatial patterns of denitrification enzyme activity and nitrous oxide fluxes in three adjacent vegetated riparian buffer zones

The aim of this study was to investigate temporal and spatial patterns of denitrification enzyme activity (DEA) and nitrous oxide (N₂O) fluxes in three adjacent riparian sites (mixed vegetation, forest and grass). The highest DEA was found in the surface (0–30 cm) soil and varied between 0.7±0.1 mg N kg^–1 day^–1 at 5°C and 5.9±0.4 mg N kg^–1 day^–1 at 15°C. There was no significant difference (P >0.05) between the DEA in the uppermost (0–30 cm and 60–90 cm) soil depths under different vegetation covers. In the two deepest (120–150 cm and 180–210 cm) soil depths the DEA varied between 0.0±0.0 mg N kg^–1 day^–1 at 5°C and 4.4±0.9 mg N kg^–1 day^–1 at 15°C and was clearly associated with the accumulation of buried organic carbon (OC). Two threshold values of OC were observed before DEA started to increase significantly, namely 5 and 25 g OC kg^–1 soil at 10–15°C and 5°C, respectively. In the three riparian sites N₂O fluxes varied between a net N₂O uptake of –0.6±0.4 mg N₂O-N m^–2 day^–1 and a net N₂O emission of 2.5±0.3 mg N₂O-N m^–2 day^–1. The observed N₂O emission did not lead to an important pollution swapping (from water pollution to greenhouse gas emission). Especially in the mixed vegetation and forest riparian site highest N₂O fluxes were observed upslope of the riparian site. The N₂O fluxes showed no clear temporal trend.     

Nitrate removal, denitrification and nitrous oxide production in the riparian zone of an ephemeral stream

Riparian zones are important features of the landscape that can buffer waterways from non-point sources of nitrogen pollution. Studies of perennial streams have identified denitrification as one of the dominant mechanisms by which this can occur. This study aimed to assess nitrate removal within the riparian zone of an ephemeral stream and characterise the processes responsible, particularly denitrification, using both in-situ and laboratory techniques. To quantify rates of groundwater nitrate removal and denitrification in-situ, nitrate was added to two separate injection-capture well networks in a perched riparian aquifer of a low order ephemeral stream in South East Queensland, Australia. Both networks also received bromide as a conservative tracer and one received acetylene to inhibit the last step of denitrification. An average of 77 ± 2% and 98 ± 1% of the added nitrate was removed within a distance of 40 cm from the injection wells (networks with acetylene and without, respectively). Based on rates of N₂O production in the network with added acetylene, denitrification was not a major mechanism of nitrate loss, accounting for only 3% of removal. Reduction of nitrate to ammonium was also not a major pathway in either network, contributing <4%. Relatively high concentrations of oxygen in the aquifer following recent filling by stream water may have reduced the importance of these two anaerobic pathways. Alternatively, denitrification may have been underestimated using the in-situ acetylene block technique. In the laboratory, soils taken from two depths at each well network were incubated with four nitrate-N treatments (ranging from ambient concentration to an addition of 15 mg N l⁻¹), with and without added acetylene. Potential rates of denitrification, N₂O production and N₂O:N₂ ratios increased with nitrate additions, particularly in shallow soils. Potential rates of denitrification observed in the laboratory were equivalent in magnitude to nitrate removal measured in the field (mean 0.26 ± 0.12 mg N kg of dry soil⁻¹ d⁻¹), but were two orders of magnitude greater than denitrification measured in the field with added acetylene. The relative importance of assimilatory vs. dissimilatory processes of nitrate removal depends on environmental conditions in the aquifer, particularly hydrology and its effects on dissolved oxygen concentrations. Depending on seasonal conditions, aquifers of ephemeral streams like the study site are likely to fluctuate between oxic and anoxic conditions; nevertheless they may still function as effective buffers. While denitrification to N₂ is a desirable outcome from a management perspective, assimilation into biomass can provide a rapid sink for nitrate, thus helping to reduce short-term delivery of nitrate downstream. Longer-term studies are needed to determine the overall effectiveness of riparian buffers associated with ephemeral streams in mitigating nitrate loads reaching downstream ecosystems.     

Nitrous oxide production of heavy metal contaminated soil

Arsenic (As), lead (Pb), copper (Cu) and zinc (Zn) can be found in large concentrations in mine spills of central and northern Mexico. Interest in these heavy metals has increased recently as they contaminate drinking water and aquifers in large parts of the world and severely affect human health, but little is known about how they affect biological functioning of soil. Soils were sampled in seven locations along a gradient of heavy metal contamination with distance from a mine in San Luis Potosí (Mexico), active since about 1800 AD. C mineralization and N₂O production were monitored in an aerobic incubation experiment. Concentrations of As in the top 0-10 cm soil layer ranged from 8 to 22,992 mg kg⁻¹, from 31 to 1845 mg kg⁻¹ for Pb, from 27 to 1620 mg kg⁻¹ for Cu and from 81 to 4218 mg kg⁻¹ for Zn. There was a significant negative correlation between production rates of CO₂ and concentrations of As, Pb, Cu and Zn, and there was a significant positive correlation with pH, water holding capacity (WHC), total N and soil organic C. There was a significant negative correlation (P<0.05) between production rate of nitrous oxide (N₂O) attributed to nitrification by the inhibition method in soil incubated at 50% WHC and total concentrations of Pb and Zn, and there was a significant positive correlation (P<0.05) with pH and total N content. There was a significant negative correlation (P<0.05) between the production rate of N₂O attributed to denitrification by the inhibition method in soil incubated at 100% WHC and total concentrations of Pb, Cu and Zn, and a significant positive correlation (P<0.01) with pH; there was a significant positive correlation (P<0.05) between the production of N₂O attributed to other processes by the inhibition method and WHC, inorganic C and clay content. A negative value for production rate of N₂O attributed to nitrifier denitrification by the inhibition method was obtained at 100% WHC. The large concentrations of heavy metals in soil inhibited microbial activity and the production rate of N₂O attributed to nitrification by the inhibition method when soil was incubated at 50% WHC and denitrification when soil was incubated at 100% WHC. The inhibitor/suppression technique used appeared to be flawed, as negative values for nitrifier denitrification were obtained and as the production rate of N₂O through denitrification increased when soil was incubated with C₂H₂.