Measured and simulated denitrification activity in a cropped sandy and loamy soil

Summary Denitrification activities were measured over a 3-year period in a coarse sandy soil and a sandy loam soil. In all years the crops were spring barley in combination with Italian ryegrass as a catch crop. The denitrification loss was measured using the acetylene inhibition technique on soil cores. Furthermore, a simple model was developed, based on daily values of soil moisture and soil temperature, to calculate the denitrification loss. Soil temperatures for the model were measured, whereas soil moisture was derived from a water-balance model. Measurements of denitrification gave an annual loss of 0.6 kg N ha^-1, and the model calculated a loss of 1–2 kg N ha^-1 in the coarse sandy soil. In the sandy loam soil annual losses were measured as 1.5, 3.0, and 13.0 kg N ha^-1 in 1988, 1989, and 1990, respectively. The corresponding values from the model simulation were 14, 9 and 14 kg N ha^-1.     

Regulation of potential denitrification by soil pH in long-term fertilized arable soils

 The denitrifying enzyme activity (DEA), denitrification potential (DP) and anaerobic respiration (RESP) together with chemical characteristics were measured in three contrasting soils collected from experimental arable plots that had been subjected to long-term (21–23 years) fertilizer treatments. The plots sampled were either unfertilized or had received either annual inorganic NPK, manure and lime, or inorganic NPK and manure treatments. Addition of inorganic NPK, manure and lime led to large increases in the DEA for two of the three soils, but in the absence of lime, inorganic NPK and manure caused only small increases in DEA compared to unfertilized soils. Both DP and RESP were increased by the addition of inorganic NPK, manure and lime, but were substantially decreased by fertilizer treatments without lime. In most cases there was a simple relationship between soil pH and either DEA and DP, with those treatments that reduced soil pH also leading to reduced denitrification and vice versa. The effects of artificially increasing the pH to a value close to the pH in unfertilized soils (6.3) by addition of NaOH to the soils that had received inorganic NPK, and which had the lowest soil pH values, were to increase substantially DEA, DP and RESP. In soil from one of the sites that had been stored for 5 weeks, the DP values responded differently between the fertilizer treatments. The DP value was lowest in the soil that had inorganic NPK and manure, higher in the soil that received inorganic NPK, manure and lime and it was the highest in unfertilized (control) soil. The soil pH values for these treatments were 4.47, 5.79 and 6.58, respectively. However, when the soil pHs were adjusted by addition of either H₂SO₄ or NaOH to give a range between pH 2 and 12, the DP values from all three fertilizer treatments showed almost identical responses. The optimum pH value for DP was between 7 and 8 for all three fertilizer treatments. Substrate-induced respiration values from all fertilizer treatments showed a similar trend to DP when the soil pHs were modified. The results show that soil pH was an important factor which in the studied soils controls the microbial community in general and the community of denitrifiers in particular. However, denitrifiers showed a high pH resilience leading to no marked change of the pH optimum for potential denitrification. Received: 10 September 1998     

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     

Effects of vegetation regime on denitrification potential in two tropical volcanic soils

Effects of vegetation and nutrient availability on potentail denitrification rates were studied in two volcanic, alluvial-terrace soils in lowland Costa Rica that differ greatly in weathering stage and thus in availability of P and base cations. Potential denitrification rates were significantly higher in plots where vegetation had been left undisturbed than in plots where all vegetation had been removed continuously, and were higher on the less fertile of the two soils. The potential denitrification rates were correlated strongly with respiration rates, levels of mineralizable N, microbial biomass, and moisture content, and moderately well with concentrations of extractable NH _inf4 ^sup+ , Kjeldahl N, and total C. In all plots, denitrification rates were stimulated by the removal of O₂ and by the addition of glucose but not by the addition of water or NO _inf3 ^sup- .This is Paper 2772 of the Forest Research Laboratory, Oregon State University     

Site of nitrous oxide production in field soils

Summary Nitrous oxide (N₂O) fluxes at the soil surface and concentrations at 0.1, 0.2, and 0.3 m were determined in a 40-year-old planted tallgrass (XXX) prairie, a 40-year-old white pine (Pinus strobus) plantation, and field plots treated annually for 18 years either with 33 metric tons of manure ha^–1 (330 kg N ha^–1) and NH₄NO₃ (80 kg N ha^–1) or with only NH₄NO₃ (control). Nitrous oxide fluxes from the prairie, forest, manure-amended, and control sites from 13 May to 10 November 1980 ranged from 0.2 to 1.3, 3.5 to 19.5, 3.7 to 79.0, and 1.7 to 24.8 ng N₂O-N m^–2s^–1, respectively. We observed periods when there was no apparent relationship between the N₂O flux from the surface and N₂O concentrations in the soil profile. This was generally the case in the prairie and in the field sites following the application of N fertilizer. The N₂O concentrations in the soil profile increased markedly and coincided with increased soil water content following periods of heavy rainfall for all sites except the prairie. Nitrous oxide concentration gradients indicate that following heavy rainfalls the site of N₂O production was moved from the surface deeper into the soil profile. We suggest that the source of N₂O production near the surface is nitrification and that N₂O is produced by denitrification of NO₃ leached into the soil following heavy rainfall.     

长白山森林土壤反硝化潜力及产物组成

采用淹水厌氧密闭培养-乙炔抑制法测定了长白山4种森林土壤的反硝化势以及气态产物N2O、NO和N2的产生量。供试4种森林土壤均呈酸性,pH(H2O)4.5～5.3,有机碳含量24.6～83.0 g kg-1。结果表明,长白山4种典型森林土壤反硝化势及反硝化产物中氮氧化物的比例差异很大。土壤反硝化势强弱顺序依次为:阔叶红松>白桦>红松云杉冷杉>兴安落叶松,与土壤有机碳和全氮含量呈显著的正相关关系(p<0.01),与土壤pH的相关性不显著。比较加乙炔和不加乙炔处理发现,在培养过程中,N2O始终是反硝化的主要产物,占反硝化产物的比例变化于50%～85%之间,不随培养时间而发生显著的变化,与土壤pH呈弱负相关关系(p=0.22)。反硝化产物中,NO仅占0.2%～2.4%,随培养时间也未发生有规律的变化,与土壤有机碳含量呈显著的对数负相关关系(p<0.05)。上述结果表明,长白山森林土壤反硝化过程并不能有效地将活性氮转化为惰性氮,反硝化作用的生态环境意义需要重新评估。     

Soil nitrogen and denitrification potential as affected by land use and stand age following agricultural abandonment in a headwater catchment

Changes in vegetation and soil properties because of agricultural abandonment may affect soil nitrogen (N) and associated processes. We investigated soil N (total N: TN, inorganic N: NH₄–N and NO₃–N) and denitrification potential in cropland, pine plantations and abandoned agricultural land along a secondary succession sequence (grassland→shrubland→secondary forest) in a headwater catchment in the Qinling Mountains, northwest China. The results show that the soil denitrification potential differed significantly among the five land‐use types with the highest potential in the secondary forest, followed by grassland, shrubland, cropland and plantations. The denitrification potential of the 20‐ to 40‐cm layer was significantly lower compared with the topsoil (0–20 cm) across all land‐use types. TN, soil organic matter (SOM) and NH₄–N increased significantly with stand age, whereas there was an opposite trend in soil pH. However, the denitrification potential did not relate to stand age in a linear manner. We conclude that changes in soil TN, SOM and pH during vegetation succession following agricultural abandonment are critical controls on the denitrification potential.     

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.     

Enhanced denitrification in plots of N2-fixing faba beans compared to plots of a non-fixing legume and non-legumes

Denitrification rates were studied using the C₂H₂ inhibition technique in a 2-year field experiment within plots of nodulated and non-nodulated faba beans, ryegrass, and cabbage. Denitrification rates ranged from 14.40 to 0.02 ng N₂O–N g^–1 soil dry weight h^–1. Mean denitrification increased fourfold in plots of N₂–fixing Vicia faba compared to non-nodulated V. faba mutant F48, Lolium perenne, and Brassica oleracea. The results with and without C₂H₂ treatment indicate that in the field the major part of this enhanced denitrification led to the endproduct N₂ rather than to the ozone-degrading N₂O. Higher denitrification rates of plots with N₂–fixing plants in September seemed to be caused by an increase in soil NO _inf3 ^sup- of about 20 kg ha^–1 found between July and August. Soil NO _inf3 ^sup- and soil moisture explained 67% of the variation in denitrification rates of the different soil samples over the growing seasons in the 2 years. Soil moisture explained 44% of the variation for soil planted with N₂–fixing plants and 62% for soil planted with non-fixing plants. Positive exponential relationships were obtained between denitrification rates and soil nitrate (r=0.71) and soil moisture (r=0.82).     

Influence of the water regime on denitrification and aerobic respiration in soil

Summary The influence of soil moisture on denitrification and aerobic respiration was studied in a mull rendzina soil. N₂O formation did not occur below –30 kPa matric water potential (_m), above 0.28 air-filled porosity (a) and below 0.55 fractional water saturation (_v/PV volumetric water content/total pore volume). Half maximum rates of N₂O production and O₂ consumption were obtained between _m = –1.2 and –12 kPa,a = 0.05 and 0.23, and _v/PV = 0.63 and 0.92. No oxygen consumption was measured at _v/PC 1.17. O₂ uptake and denitrification occurred simultaneously arounda = 0.10 (at _m = –10 kPa and _v/PV = 0.81) at mean rates of 3.5 µl O₂ and 0.3 µl N₂ h^–1g^–1 soil. Undisturbed, field-moist soil saturated with nitrate solution showed constant consumption and production rates, respectively, of 0.6 µl O and 0.22 µl N₂O h^–1g^–1 soil, whereas the rates of air-dried remoistened soil were at least 10 times these values. The highest rates obtained in remoistened soil amended with glucose and nitrate were 130 µl O₂ and 27 µl N₂O h^–1g^–1 soil.     

Denitrification under different cultivated plants: effects of soil moisture tension,nitrate concentration,and photosynthetic activity

Summary Plant effects on the denitrification rate were investigated in pot experiments at different soil moisture tensions and nitrate concentrations. Nitrate concentrations and the soil moisture tension were regulated immediately before each measurement. The effects of the plants on denitrification rates were dependent on the soil moisture tension. At a low soil moisture tension (–7 cm H₂O), there was a 10-fold increase in the denitrification rate (planted versus unplanted soil). At a medium moisture tension (–30 cm H₂O) the plants had practically no effect, and at the highest tension (–60 cm H₂O) the effect was slightly negative. Large differences in denitrification rates under different plant species were observed. At a low soil moisture tension, the average denitrification rate (g N kg^–1 soil h^–1) was 39–42 under small grains (barley, wheat, and oats), 47–82 under the grasses (cocksfoot, meadow grass, meadow fescue, and timothy) and 18 under red clover. The differences between the monocots were attributable to differences in plant growth rates, rather than to any specific difference in stimulation or inhibition of denitrification, since the variations in photosynthetic activity fairly well predicted the differences in denitrification rates under different monocots. Clover, however, gave much lower denitrification rates than those predicted by the photosynthetic activity.     

Denitrification potential of intermittently saturated floodplain soils from a subtropical perennial stream and an ephemeral tributary

Denitrification has the potential to remove excess nitrogen from groundwater passing through riparian buffers, thus improving water quality downstream. In regions with markedly seasonal precipitation, transient stream flow events may be important in saturating adjacent floodplain soils and intermittently providing the anaerobic conditions necessary for denitrification to occur. In two experiments we characterised the denitrification potential of soils from two contrasting floodplains that experience intermittent saturation. We quantified under controlled laboratory conditions: 1) potential rates of denitrification in these soils with depth and over time, for a typical period of saturation; and 2) the influences on rates of nitrate and organic carbon. Treatments differed between experiments, but in each case soil-water slurries were incubated anaerobically with differing amendments of organic carbon and nitrate; denitrification rates were measured at selected time intervals by the acetylene-block technique; and slurry filtrates were analysed for various chemical constituents. In the first experiment (ephemeral tributary), denitrification was evident in soils from both depths (0-0.3 m; 0.3-1.1 m) within hours of saturation. Before Day 2, mean denitrification rates at each depth were generally comparable, irrespective of added substrates; mean rates (Days 0 and 1) were 5.2 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m) and 1.6 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.1 m). Rates generally peaked on Days 2 or 3. The availability of labile organic carbon was a major constraint on denitrification in these soils. Acetate addition greatly increased rates, reaching a maximum in ephemeral floodplain soils of 17.4 ± 1.8 mg N kg dry soil⁻¹ day⁻¹ on Day 2: in one deep-soil treatment (low nitrate) this overcame differences in rates observed with depth when acetate was not added, although the rate increase in the other deep-soil treatment (high nitrate) was significantly less (P ≤ 0.01). Without acetate, peak denitrification rates in this experiment were 6.9 ± 0.4 and 2.8 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ in surface and deep soils, respectively. Differences in rates were observed with depth on all occasions, despite similar initial concentrations of dissolved organic carbon (DOC) at both depths. Levels of substrate addition in the second experiment (perennial stream) more closely reflected natural conditions at the site. Mean denitrification rates were consistently much higher in surface soil (P ≤ 0.001), while the source of water used in the slurries (surface water or groundwater from the site) had little effect on rates at any depth. Mean rates when all treatments retained nitrate were: 4.5 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m depth); 0.8 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.0 m); and 0.6 ± 0.1 mg N kg dry soil⁻¹ day⁻¹ (1.8-3.5 m). For comparable treatments and soil depths, denitrification potentials at both sites were similar, apart from higher initial rates in the ephemeral floodplain soils, probably associated with their higher DOC content and possibly also their history of more frequent saturation. The rapid onset of denitrification and the rates measured in these soils suggest there may be considerable potential for nitrate removal from groundwater in these floodplain environments during relatively short periods of saturation.     

Soil organic carbon changes in particle-size fractions following cultivation of Black soils in China

Soil texture can be an important control on soil organic carbon (SOC) retention and dynamics. The (clay + silt)-sized SOC pool (SOC < 20 μm) in non-cultivated or grassland soils has been proposed to reach an equilibrium or maximum level named protective capacity. Proper knowledge of SOC in this size fraction in non-cultivated and cultivated Black soils is important to evaluate management-induced changes in SOC in NE China. Twenty-seven paired soil samples (non-cultivated vs. cultivated) were collected in the Black soil zone in Heilongjiang and Jilin provinces. Bulk soil was dispersed in water with an ultrasonic probe and then soil size fractions were collected using the pipette technique for SOC analyses. Soil organic carbon in bulk soil and size fractions was measured by dry combustion. Average content of SOC < 20 μm was 23.2 g C kg⁻¹ at the 0–30 cm depth for the non-cultivated soils, accounting for 75.1% of the total SOC at the same depth. There was significant positive relationship between soil clay plus silt content and SOC < 20 μm in non-cultivated soils. Accordingly, a model of the maximum SOC < 20 μm in 0–30 cm depth of non-cultivated Black soils was developed: y = 0.36x where y is the maximum SOC < 20 μm pool (g C kg⁻¹) and x is the percentage of clay + silt (<20 μm) content. The average content of SOC < 20 μm was 18.7 g C kg⁻¹ at 0–30 cm depth for cultivated soils, accounting for 81.5% of total SOC. This average value of SOC was 4.4 g C kg⁻¹ less than the maximum value (23.1 g C kg⁻¹) and accounted for 55.0% of the difference of SOC between non-cultivated and cultivated Black soils. Cultivation resulted in 45.0% loss of sand-sized (>20 μm) SOC concentration relative to SOC < 20 μm. This result indicates that SOC < 20 μm and sand-sized SOC both play important roles in SOC dynamics resulting from management practices. This model can be applied to calculate the actual potential to restore SOC for cultivated Black soils under conservation tillage in NE China.     

基于大比例尺数据库的福建省耕地土壤固碳速率和潜力研究

明确土壤固碳速率和潜力是制定耕地固碳减排措施的基础。以我国典型亚热带地区—福建省不同地理位置的闽侯、浦城、同安、武平和永定5个县为研究区,运用生物地球化学过程模型DNDC（DeNitrification and Decomposition）,模拟这5个县在目前区域尺度最详细的1:5万土壤数据库下1980─2009年和2010─2039年有机碳动态变化,并运用尺度上推的方法估算出全省耕地土壤固碳速率和潜力。结果表明,福建省耕地土壤1980─2009年的固碳总量为7.37 Tg,而2010─2039年的固碳潜力为7.04 Tg,两个时段的年均固碳速率分别为：190 kg·hm-2和176 kg·hm-2,说明目前的农田管理措施有利于研究区长期固碳。其中,水稻土和盐渍水稻土分别在土类和亚类级别中固碳速率最大,不同时段均大于175 kg·hm-2·a-1;而红壤在土类和亚类级别中固碳速率皆最小,不同时段均小于3 kg·hm-2·a-1。总体来看,1980─2009年和2010─2039年水稻土的固碳总量均占全省耕地固碳总量的92%以上,是今后制定固碳减排措施的重点。     

Evaluation of denitrification losses by the acetylene inhibition technique in a permanent ryegrass field (Lolium perenne L.) fertilized with animal slurry or ammonium nitrate

In a field experiment, the effect of animal slurry, (with and without the nitrification inhibitor dicyandiamide on total denitrification losses estimated by the C₂H₂ inhibition technique was measured over 2 years (1989–1990). During this period, four different plots (each with four replicates) were fertilized six times with 150 kg N ha^-1 in the form of cattle-pig slurry or NH₄NO₃. Soil samples (0–20 cm) were analysed at regular intervals for NH _inf4 ^sup+ and NO _inf3 ^sup– concentrations. The soil water content was determined gravimetrically. During the first year (1989) total denitrification losses from unfertilized, mineral-fertilized, and animal slurry-amended plots (with or without dicyandiamide) were estimated as 0.2, 3.1, 0.7, and 0.6 kg N ha^-1, respectively. During the second year (1990) the denitrification losses were 0.4, 1.3, 0.7, and 0.7 kg N ha^-1, respectively. There was a clear relationship between the NO _inf3 ^sup– concentration or soil water content and the denitrification rate. The results are siteund experiment-specific and cannot be generalized so far.     

Effect of an increasing carbon: nitrate-N ratio on the reliability of acetylene in blocking the N2O-reductase activity of denitrifying bacteria in soil

Summary In model experiments with a silty loam soil the effect of different C : NO _inf3 ^sup- -N ratios on the reliability of C₂H₂ (1% v/v) in blocking N₂O-reductase activity was examined. The soil was carefully mixed with different amounts of powdered lime leaves (Tilia vulgaris) to obtain organic C contents of about 1.8, 2.3, and 2.8%, and of NO _inf3 ^sup- solution to give C : NO _inf3 ^sup- -N ratios of 84, 107, 130, 156, 200, and 243. The soil samples were incubated in specially modified anaerobic jars (22 days, 25°C, 80% water-holding capacity, He atmosphere) and the atmosphere was analysed for N₂, N₂O, CO₂, and C₂H₂ by gas chromatography at regular intervals. Destruction jars were used to analyse soil NO _inf3 ^sup- , NH ₄ ⁺ and C. The results clearly showed that N₂O-reductase activity was completely blocked by 1% (v/v) C₂H₂ only as long as NO _inf3 ^sup- was present. In the presence of C₂H₂, NO _inf3 ^sup- was apparently entirely converted into N₂O. The C₂H₂ blockage of N₂O-reductase activity ceased earlier in soils with a wide C : NO _inf3 ^sup- -N ratio (156, 200, and 243) than in those with closer C : NO _inf3 ^sup- -N ratios (84, 107, and 130). As soon as NO _inf3 ^sup- was exhausted, N₂O was reduced to N₂ in spite of C₂H₂. The wider the C : NO _inf3 ^sup- -N ratio, the earlier the production of N₂ and the less the reliability of the C₂H₂ blockage. In the untreated control complete inhibition of N₂O-reductase activity by C₂H₂ lasted for 7–12 days. In the field, estimates of total denitrification losses by the C₂H₂ inhibition technique should be considered reliable only as long as NO _inf3 ^sup- is present. Consequently, NO _inf3 ^sup- monitoring in the field is essential, particularly in soils supplied with easily decomposable organic matter.     

Comparison of two different methods to measure nitric oxide turnover in soils

 The NO turnover in soils was measured in two different experimental set-ups, a flow-through system, which is very time-consuming and needs rather sophisticated equipment, and a closed system using serum bottles. We compared the NO turnover parameters (NO consumption rate constant, NO production rate, NO compensation concentration) that were measured with both systems in different soils, under different conditions and in the presence of 10 Pa acetylene to inhibit nitrification. The values of the NO turnover parameters that were measured with the two systems under oxic conditions were usually comparable. The addition of acetylene did not affect the NO consumption rate constants of the soils with the exception of soil G1. However, the NO production rates and the NO compensation concentrations decreased significantly in the presence of acetylene, indicating that nitrification was the main source of NO in these soils. Only one soil (Bol) showed no nitrifying activity. Increasing soil moisture content resulted in decreasing NO consumption rate constants and NO production rates. Even at a high soil moisture content of 80% water holding capacity, nitrification was the main source of NO. The values of the NO turnover parameters that were measured with the two systems were not comparable under anoxic conditions. The NO consumption rate constants and the NO production rates were much lower in the closed than in the flow-through system, indicating that the NO consumption activity became saturated by the high NO concentrations accumulating in the closed system. Under oxic conditions, however, closed serum bottles were a cheap, easy and reliable tool with which to determine NO turnover parameters and to distinguish between nitrification and denitrification as sources of NO. Received: 21 April 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.     

Increase in denitrification capacity of soils due to addition of alkylbenzene sulfonates

 In semi-arid regions wastewater irrigation is a valuable resource for agricultural production. The contamination of irrigated soils with surfactants is one of the ecological risks related to irrigating with untreated wastewater. In this study, the effects of branched alkylbenzene sulfonates (ABS) on microbial biomass, respiratory activity, and denitrification capacity of soil samples (eutric vertisols) taken from an irrigation district in the Mexico City area were investigated in laboratory experiments. Increasing concentrations of ABS lead to a decrease in soil microbial biomass and an increase in soil respiratory activity as well as in the metabolic quotient (qCO₂) of the soils. Denitrification capacity was lowest without the addition of ABS and highest at a medium ABS concentration of 50 μg g^–1. Denitrification capacity seems to be highly sensitive to ABS addition at moderate concentrations. From the laboratory results, high rates of denitrification and N₂O evolution from fields irrigated with wastewater containing ABS are expected. Received: 11 November 1997     

Effect of bulk density and soil water tension on denitrification in the rhizosphere of spring wheat (Triticum vulgare

Summary Pot experiments were carried out to study the influence of bulk density (D _b), soil water tension (pF) and presence of plants (spring wheat) on denitrification in a low-humus B_t-horizon of a udalf. Pots of only 5-cm depth were found to be most suitable for the experiments when using the acetylene inhibition method. Almost homogeneous soil compaction between 1.1 and 1.6g soil cm^–3 was achieved by a Proctor tamper. Water tensions were adjusted by means of ceramic plates on which negative pressure was applied. No denitrification was detected in unplanted pots. With planted pots and increasing bulk density denitrification increased more in pots with 14-day-old plants than in pots with 7-day-old plants. With 14-day-old plants N₂O emission pot^–1 increased steadily from 2 mol at D _b 1.1 to 8 mol at D _b 1.6, when soil moisture was adjusted to pF 1.5, although root growth was impaired by higher bulk density. From an experiment with different bulk densities and water tensions it could be deduced that the air-filled porosity ultimately determined the rate of denitrification. When low water tension was applied for a longer period, water tension had an overriding effect on total denitrification. Denitrification intensity, however, i.e. the amount of N₂O g^–1 root fresh weight, was highest when low water tension was accompanied by high bulk density. The results suggest that the increase in denitrification intensity at oxygen stress is partly due to higher root exudation.