Nitrous oxide emissions and controls as influenced by tillage and crop residue management strategy

Mixed responses of soil nitrous oxide (N₂O) fluxes to reduced tillage/no-till are widely reported across soil types and regions. In a field experiment on a Danish sandy loam soil we compared N₂O emissions during winter barley growth following five years of direct drilling (DD), reduced tillage (RT) or conventional tillage (CT). Each of these tillage treatments further varied in respect to whether the resulting plot crop residues were retained (+Res) or removed (−Res). Sampling took place from autumn 2007 to the end of spring 2008. Overall N₂O emissions were 27 and 26% lower in DD and RT, respectively, relative to N₂O emissions from CT plots (P < 0.05). We observed that in residue removal scenarios N₂O emissions were similar for all tillage treatments, but in residue retention scenarios N₂O emissions were significantly higher in CT than in either DD or RT (P < 0.05). Irrespective of residue management, N₂O emissions from DD and RT plots never exceeded emissions from CT plots. Retention of residue was estimated to reduce emissions from DD plots by 39% and in RT plots by 9%, but to increase N₂O emissions from the CT plots by 35%. Relative soil gas diffusivity (Rdiff), soil NO₃-N, soil temperature, tillage and residue were important driving factors for N₂O emission (P < 0.05). A multiple linear regression model using Rdiff to represent the water factor explained N₂O emissions better than a water-filled pore space (WFPS) based model, suggesting a need for review of the current use of WFPS in N₂O prediction models. We conclude that on light textured soils, no-till has the potential for reducing N₂O emissions when crop residues are returned to the soil.     

Nitrous oxide emission from urine-treated soil as influenced by urine composition and soil physical conditions

Urine patches from cattle and sheep on pastures represent considerable, highly localized N applications. Subsequent nitrification and denitrification of the nitrogenous compounds may result in high nitrous oxide (N₂O) emissions. Not much is known about the extent of these emissions, or about possible mitigation options. The aims of this study were to experimentally quantify the effects of urine composition, dung addition, compaction and soil moisture on N₂O emissions from urine patches. For an incubation study at 16 °C, soil was collected from a typic Endoaquoll, and N₂O production was monitored during a 103-day period. Emissions for the whole period averaged 0.3 and 0.9% of the applied urine-N for dry and moist soil, respectively. When compacted or when dung was added, emissions from moist soils increased to 4.9 and 7.9%, respectively. Both addition of dung and soil compaction resulted in a delay of the peak N₂O emission of approximately 10-15 days. No significant effect of amount of urine-N on emission percentages was detected. Changing the volume of urine with equal amounts of urine-N resulted in highly significant effects, peaking with an emission of 2.3% at a water-filled pore space (WFPS) of 78%. When the soil was water-saturated, N₂O production was delayed until evaporation had decreased moisture contents. We concluded that denitrification was the main N₂O forming process in the incubation study. Emission factors for urine reported in the literature do not generally include the potentially considerable effects of compaction or combination with dung. We conclude that realistic emission factors should take into account such an effect, together with estimates for the occurrence of camping areas in pastures. From our results, the best mitigation strategies appear to be increasing the volume of urine through feed additives, and avoiding compaction and promoting more homogeneous application of N through a lower cattle stocking rate. Also, research efforts may be targeted at management practices to avoid camping areas in pastures.     

Nitrous oxide emission as affected by changes in soil water content and nitrogen fertilization

Nitrous oxide emission was measured in laboratory incubations of an alluvial soil (58% clay, pH 7.4). The soil was amended with 40 mg N kg⁻¹ as NaNO₃ or NH₄Cl, or with NaCl as a control. Each fertilization treatment was adjusted to three different water contents: constant 60% WHC (water-holding capacity), constant 120% WHC, and water content alternating between 60 and 120% WHC. During an 8-day incubation period N₂O emission rates and inorganic nitrogen concentrations in soil (NH₄⁺, NO₂⁻, NO₃⁻) were determined at regular intervals. In the control and after nitrate application small N₂O emission rates occurred with only minor variations over time, and no differences between the water treatments. In contrast, with ammonium application N₂O emission rates were much higher during the first two days of incubation, with peaks in the constant 60% WHC and 120% WHC at day 1 and in the changing-water treatment at day 2, when the first wet period (120% WHC) was completed. This N₂O peak in the changing-water treatment was 4 to 9 times higher than with constant WHC and occurred when both, NH₄⁺ and NO₂⁻ concentrations declined sharply. Thus, this N₂O emission flush can be attributed to nitrifier denitrification. After the second rewetting of the NH₄⁺-amended soil no further N₂O emission peak was observed, being in accordance with small NH₄⁺ and NO₂⁻ concentrations in soil at that time. The unexpectedly small N₂O fluxes in the constant 120% WHC treatment after nitrate application were probably caused by the reduction of N₂O to N₂ under the prevailing conditions. It can be concluded that continuous wetting or flooding of a soil is an effective measure to reduce N₂O emissions immediately after the application of NH₄⁺ fertilizers.     

Properties of soil aggregates as influenced by tillage practices

Abstract. Changes in aggregate stability, density, and porosity as well as the water retention and nutrient contents of different aggregate size fractions due to intensive tillage were investigated. Three soils from Vicarello, Fagna and Gambassi in North Central Italy which had been under permanent vegetation, minimum or conventional tillage for more than seven years were studied. The aggregates on conventionally tilled plots were slightly denser and less porous than those on the untilled or minimum-tilled plots. The aggregates were less stable under conventional tillage on all soils. Conventional tillage reduced the proportion or macro-aggregates by 22% at Vicarello and 35% at Gambassi. There were no differences in macro-aggregate proportions between minimum- and conventionally tilled plots at Fagna. The potential of the dry aggregates to distintegrate upon contact with water was greatest in the conventionally tilled and least in the untilled treatments. The proportions of dry macro-aggregates (> 0.25 mm) in the untilled and tilled plots were 90 and 71%, respectively. The soil of the tilled plots contained less carbon and nitrogen than that of the untilled plots in all aggregate size fractions. The silt-plus-clay contents of the aggregates accounted for between 65 and 93% of variability in the water they retained at small potentials while organic carbon contents accounted for between 71 and 90% of variability in the stability of the aggregates irrespective of the tillage treatments.     

水分状态、土壤类型和氮素种类对氧化亚氮和甲烷排放的影响

Specific management of water regimes, soil and N in China might play an important role in regulating N2O and CH4 emissions in rice fields. Nitrous oxide and methane emissions from alternate non-flooded/flooded paddies were monitored simultaneously during a 516-day incubation with lysimeter experiments. Two N sources （^15N-（NH4）2SO4 and ^15N-labeled milk vetch） were applied to two contrasting paddies： one derived from Xiashu loess （Loess） and one from Quaternary red clay （Clay）. Both N2O and CH4 emissions were significantly higher in soil Clay than in soil Loess during the flooded period. For both soil, N2O emissions peaked at the transition periods shortly after the beginning of the flooded and non-flooded seasons. Soil type affected N2O emission patterns. In soil Clay, the emission peak during the transition period from non-flooded to flooded conditions was much higher than the peak during the transition period from flooded to non-flooded conditions. In soil Loess, the emission peak during the transition period from flooded to non-flooded conditions was obviously higher than the peak during the transition period from non-flooded to flooded conditions except for milk vetch treatment. Soil type also had a significant effect on CH4 emissions during the flooded season, over which the weighted average flux was 111 mg C m^-2 h^-1 and 2.2 mg C m^-2 h^-1 from Clay and Loess, respectively. Results indicated that it was the transition in the water regime that dominated N2O emissions while it was the soil type that dominated CH4 emissions during the flooded season. Anaerobic oxidation of methane possibly existed in soil Loess during the flooded season.     

Nitrate leaching as influenced by soil tillage and catch crop

Because of public and political concern for the quality of surface and ground water, leaching of nitrate is of special concern in many countries. To evaluate the effects of tillage and growth of a catch crop on nitrate leaching, two field trials were conducted in spring barley (Hordeum vulgare L.) under temperate coastal climate conditions. On a coarse sand (1987–1992), ploughing in autumn or in spring in combination with perennial ryegrass (Lolium perenne L.) as a catch crop was evaluated. Furthermore, rotovating and direct drilling were included. The experiment was conducted on a 19-year-old field trial with continuous production of spring barley. On a sandy loam (1988–1992), ploughing in autumn or in spring in combination with stubble cultivation and perennial ryegrass, in addition to minimum tillage, was evaluated in a newly established field trial. For calculation of nitrate leaching, soil water isolates from depths of 0.8 or 1.0 m were taken using ceramic cups. No significant effect of tillage was found on the coarse sand; however, a significant effect of tillage was found on the sandy loam, where leaching from autumn ploughed plots without stubble cultivation was 16 kg N ha⁻¹ year⁻¹ higher than leaching from spring ploughed plots. Leaching was significantly less when stubble cultivation in autumn was omitted. Leaching on both soil types was significantly reduced by the growth of a catch crop which was ploughed under in autumn or in spring. It was concluded that soil cultivation increased leaching on the sandy loam but not on the coarse sand, and that the growth of perennial ryegrass as a catch crop reduced leaching on both soil types, particularly when ryegrass was ploughed under in spring.     

Nitrous oxide emission from a transplanted rice field in alluvial soil as influenced by management of nitrogen fertiliser

We investigated nitrous oxide (N₂O) emission from an irrigated rice field over two years to evaluate the management of nitrogenous fertiliser and its effect on reducing emissions. Four forms of nitrogenous fertilisers: NPK at the recommended application rate, starch–urea matrix (SUM) + PK, neem‐coated urea + PK and urea alone (urea without coating) were used. Gas samples were collected from the field at weekly intervals with the static chamber technique. N₂O emissions from different treatments ranged from 11.58 to 215.81 N₂O‐N μg/m²/h, and seasonal N₂O emissions from 2.83 to 3.89 kg N₂O‐N/ha. Compared with other fertilisers, N₂O emissions were greatest after the application of the conventional NPK fertiliser. Moreover, SUM + PK reduced total N₂O emissions by 22.33% (P < 0.05) compared with NPK during the rice‐growing period (P < 0.05). The results indicate a strong correlation between N₂O emissions and soil organic carbon, nitrate, ammonium, above‐ and below‐ground plant biomass and photosynthesis (P < 0.05). The application of SUM + PK in rice fields is suitable as a means of reducing N₂O emissions without affecting grain production.     

Nitrous oxide emission from a soil under permanent grass: Seasonal and diurnal fluctuations as influenced by manuring and fertilization

The emission of nitrous oxide (N₂O) from soil under grass was measured, following applications of cow slurry and NH₄NO₃ fertilizer. The N₂O-flux from untreated soil averaged 0.58 mg Nm^?2 day^?1 through April to August. Application of slurry at the end of April and at the middle of July caused increases in the daily N₂O-flux of up to 40-fold, compared to untreated grass. Applications of NH₄NO₃ increased the N₂O-flux up to 5 times during the same period. The N₂O-flux often showed marked diurnal fluctuations. These fluctuations are not solely associated with change in temperature, but may also be related to grass root activity and to photosynthesis.     

Soil porosity and water infiltration as influenced by tillage methods

The relations between soil pore structure induced by tillage and infiltration play an important role in flow characteristics of water and solutes in soil. In this study, we assessed the effect of long-term use of various tillage systems on pore size distribution, areal porosity, stained (flow-active) porosity and infiltration of silt loam Eutric Fluvisol. Tillage treatments were: (1) ploughing to the depth of 20 cm (conventional tillage (CT)); (2) ploughing to 20 cm every 6 years and to 5 cm in the remaining years (S/CT); (3) harrowing to 5 cm each year (S); (4) sowing to the uncultivated soil (no tillage (NT)), all in a micro-plot experiment. Equivalent pore size distribution was derived from the water retention curve, areal porosity – from resin-impregnated blocks (8 cm × 9 cm × 4 cm) and stained porosity – from horizontal sections (every 2 cm) of column samples (diameter: 21.5 cm, height: 20 cm) taken after infiltration of methylene blue solution. The pore size distribution curves indicated that the textural peaks of the pore throat radius of approximately 1 μm were mostly defined under NT, whereas those in the structural domain of radii of 110 μm radius—under CT. The differences among the tillage treatments were more pronounced at depth 0–10 cm than 10–20 cm. At both depths, the differences in pore size distribution between the tillage treatments were relatively greater in structural than those in the matrix domain. CT soil had the greatest areal porosity and stained porosity. The stained porosity as a function of depth could be well described by logarithmic equations in all treatments. Cumulative infiltration (steady state) as measured by the double ring infiltrometer method was the highest under CT (94.5 cm) and it was reduced by 62, 36 and 61% in S/CT, S and NT soil, respectively. Irrespective of tillage method, cumulative infiltration rates throughout 3 h most closely correlated with stained porosity in top layers (0–6 cm). Overall, the results indicate that soil pore system under CT with higher contribution of large flow-active pores compared to reduced and no tillage treatments enhanced infiltration and water storage capacity.     

Soil structures produced by tillage as affected by soil water content and the physical quality of soil

Tillage experiments were carried out in order to study the effect of water content on the aggregate size distribution produced by tillage, and to investigate the relationship between the soil structures produced by tillage and Dexter's index of soil physical quality, S. Tillage with a mouldboard plough was done on four different soils over a range of naturally occurring water contents. The aggregate size distribution and the specific surface area produced by tillage were obtained by sieving. We define the optimum water content for tillage, θ_OPT, as the water content at which the specific surface area of the aggregates produced is maximum. This is consistent with the water content at which the amount of small aggregates produced is greatest and the proportion of clods produced is smallest. For the four investigated soils, θ_OPT was found to be close to the water content at the inflection point of the water retention curve, and in the vicinity of 0.8θ_PL (where θ_PL is the lower plastic limit). At water contents either lower or higher than θ_OPT, the specific surface area produced was smaller. The specific surface area produced at θ_OPT was found to be strongly correlated with the index of soil physical quality, S. The specific surface area produced is larger the greater S, i.e. the better the soil physical quality. Consistently, the proportion of small aggregates produced at θ_OPT is larger and the proportion of clods produced at θ_OPT smaller, the greater S. No clods (>50 mm) are produced on soils with good physical quality.     

Virus adsorption and inactivation in soil as influenced by autochthonous microorganisms and water content

Virus adsorption and inactivation in soil matrix are crucial processes controlling the potential of viruses to contaminate water resources. These two processes behave interactively and are controlled by various factors. In this study, batch and incubation experiments were conducted at 4 °C to compare the adsorption of bacteriophage MS2 in different soils, and to examine its inactivation behavior at different soil water content. Soils with presence/absence of autochthonous microorganisms were used. The interactive effects of sterilization and soil water content on virus inactivation were also evaluated. The Ustisandic Primosols showed no virus adsorption and minimal differences in activation regardless of the presence or absence of soil autochthonous microorganisms. For the Ferriudic Cambosols, however, sterilization increased adsorption and enhanced inactivation, and inactivation was accentuated by decreasing soil water content. The results indicate that soil water content and sterilization had additive effects on virus inactivation, and reveal that the enhanced “adsorption” by sterilization was mainly due to greater die-off in the Ferriudic Cambosols. The greater inactivation observed in the Ferriudic Cambosols, which has relative high contents of Fe- and Al-oxides and low pH, resulting from the additive effect of sterilization and decreasing soil water content was mainly due to increased reactions at the solid–water interface. We conclude that the effect of sterilization and soil water content on virus inactivation depends on soil type, and the extent of inactivation is likely controlled by the content of metal oxides.     

Nitrous oxide release by soil fungi

Fusarium oxysporum and F. solani reduced nitrite in growing cultures and in resting cell experiments at low oxygen tensions with the simultaneous release of nitrous oxide. Nitrate, however, was not transformed during growth under aerobic or partially anaerobic conditions. No nitrous oxide was generated from nitrate, ammonium or hydroxylamine by resting cells. There are no plausible explanations for the physiological reaction involved in the release of nitrous oxide, but the finding that soil fungi are able to volatilize nitrogen implies a new factor in the disappearance of nitrogen from soil.     

Nitrous oxide emissions from a fallow and wheat field as affected by increased soil temperatures

 In order to determine the effects of increased soil temperature resulting from global warming on microbiological reactions, a 21-month field experiment was carried out in the Bavarian tertiary hills. The major objective was to focus on N₂O releases as either a positive or negative feedback in response to global warming. The soils of a fallow field and a wheat field were heated 3  °C above ambient temperature and N₂O fluxes were measured weekly from June 1994 to March 1996. During the experimental period, measured temperature differences between the control plots and the heated plots were 2.9±0.3  °C at a depth of 0.01 m and 1.0–1.8  °C at a depth of 1 m. Soil moisture decreased with the elevated soil temperatures of the heated plots. The mean differences in soil moisture between the treatments were 6.4% (fallow field) and 5.2%_DW (wheat field dry weight, DW), respectively. Overall N₂O releases during the experimental period from the fallow field were 4.8 kg N₂O–N ha^–1 in the control plot against 5.0 kg N₂O–N ha^–1 in the heated plot, and releases from the wheat field were 8.0 N₂O–N ha^–1 in the control plot and 7.6 N₂O–N kg ha^–1 in the heated plot. However, on a seasonal basis, cumulated N₂O emissions differed between the plots. During the summer months (May–October), releases from the heated fallow plot were 3 times the rates from the control plot. In the winter months, N₂O releases increased in both the fallow and wheat fields and were related to the number of freezing and thawing cycles. Received: 1 December 1997     

Fate of spilled xylene as influenced by soil moisture content

Barrel size undisturbed monoliths of Weswood silt loam soil (Fluventic Ustochrept) were collected, instrumented, equilibrated at desired moisture contents, and treated with xylene by spilling it on the soil surface. Volatilization of xylene was measured using a chamber equipped with a granular activated carbon (GAC) vapor trap. Leachate was collected daily under ? 33 kPa tension with a GAC vapor trap between each collection bottle and the vacuum source. Residual xylene was determined by collecting soil samples at the end of the leaching period and analyzing them for xylene. Degradation was estimated as the xylene applied less the sum of the xylene which remained, leached, and volatilized. Most of the observed volatilization occurred immediately after application and was greatest from the driest soil. An application rate of 7.2 × 10^?2 m depth of xylene at the intermediate moisture content resulted in four times more volatilization than occurred from the 3.6 × 10^?2 m application. Xylene appeared in the leachate collected at a depth of 0.78 m from all treatments within 12 hr after the xylene was applied. Initial soil moisture content greatly influenced the amount which passed through the soil. An average of 34% and less than 0.5% of the applied xylene moved through the wettest and driest soils, respectively. Doubling the xylene application depth resulted in a 10 to 17-fold increase in the amount of xylene in the leachate. Vapor traps in line with soil pore liquid samplers were essential, since for some treatments, an average of 95.1% of the xylene collected in the leachate was recovered from the vapor trap. The xylene which remained in the soil after 67 days ranged from 6.7 to 12.8%. Estimated degradation rates ranged from 45.7 to 137.8 g day^?1 with the greatest degradation occurring in the soil with the highest application rate and the least degradation in the wettest soil with the lowest application rate.     

Nitrous oxide flux from soil amino acid mineralization

Nitrous oxide (N₂O) is a greenhouse gas produced during microbial transformation of soil N that has been implicated in global climate warming. Nitrous oxide efflux from N fertilized soils has been modeled using NO₃⁻ content with a limited success, but predicting N₂O production in non-fertilized soils has proven to be much more complex. The present study investigates the contribution of soil amino acid (AA) mineralization to N₂O flux from semi-arid soils. In laboratory incubations (−34 kPa moisture potential), soil mineralization of eleven AAs (100 μg AA-N g⁻¹ soil) promoted a wide range in the production of N₂O (156.0±79.3 ng N₂O-N g⁻¹ soil) during 12 d incubations. Comparison of the δ¹³C content (‰) of the individual AAs and the δ¹³C signature of the respired AA-CO₂-C determined that, with the exception of TYR, all of the AAs were completely mineralized during incubations, allowing for the calculation of a N₂O-N conversion rate from each AA. Next, soils from three different semi-arid vegetation ecosystems with a wide range in total N content were incubated and monitored for CO₂ and N₂O efflux. A model utilizing CO₂ respired from the three soils as a measure of organic matter C mineralization, a preincubation soil AA composition of each soil, and the N₂O-N conversion rate from the AA incubations effectively predicted the range of N₂O production by all three soils. Nitrous oxide flux did not correspond to factors shown to influence anaerobic denitrification, including soil NO₃⁻ contents, soil moisture, oxygen consumption, and CO₂ respiration, suggesting that nitrification and aerobic nitrifier denitrification could be contributing to N₂O production in these soils. Results indicate that quantification of AA mineralization may be useful for predicting N₂O production in soils.     

Wheel traffic impact on soil conditions as influenced by tillage system in Central Anatolia

The increased limiting effects of soil compaction on Central Anatolian soils in the recent years demonstrate the need for a detailed analysis of tillage system impacts. This study was undertaken to ascertain the effects of seven different tillage systems and subsequent wheel traffic on the physical and mechanical properties of typical Central Anatolian medium textured clay loam soil (Cambisol), south of Ankara, Turkey. Both tillage and field traffic influenced soil bulk density, porosity, air voids and strength significantly except the insignificant effect of traffic on moisture content. Traffic affected the soil properties mostly down to 20 cm. However, no excessive compaction was detected in 0–20 cm soil depth. The increases of bulk density following wheel traffic varied between 10–20% at 0–5 cm and 6–12% at 10–15 cm depth. In additions, traffic increased the penetration resistance by 30–74% at 0–10 cm and 7–33% at 10–20 cm. Less wheel traffic-induced effects were found on chisel tilled plots, compared to ploughed plots. Soil stress during wheel passage was highly correlated with soil strength. Also, both tillage and traffic-induced differences were observed in mean soil aggregate sizes, especially for mouldboard ploughed plots. The obtained data imply that chisel+cultivator-tooth harrow combination provides more desirable soil conditions for resisting further soil compaction.     

Nitrous oxide emissions as influenced by amendment of plant residues with different C:N ratios

To investigate the influence of plant residues decomposition on N₂O emission, laboratory incubations were carried out for a period of 21 days using urea and five plant residues with a wide range of C:N ratios from 8 to 118. Incorporation of plant residues enhanced N₂O and CO₂ emissions. The two gas fluxes were significantly correlated (R²=0.775, p<0.001). Cumulative emissions of N₂O and CO₂ were negatively correlated with the C:N ratio in plant residues (R²=0.783 and 0.986 for N₂O, and 0.854 for CO₂, respectively). A negative relationship between the N₂O-N/NO₃⁻-N ratio and the C:N ratio was observed (R²=0.867) when residue plus urea was added. We calculated the changes in dissolved organic C (DOC) and the relevant changes in N₂O emission. The incorporation of residues increased DOC when compared with the control, while the incorporation of residue plus urea decreased DOC. Cumulative emissions of N₂O and CO₂ were positively correlated with DOC concentration measured at the end of the incubation. In addition, the N₂O emission fraction, defined as N₂O-N emissions per unit N input, was not found to be a constant for either residue-N or urea-N amendment but dependent on C:N ratio when plant residue was incorporated.     

Total and labile soil organic nitrogen as influenced by crop rotations and tillage in Canadian prairie soils

Crop rotations and tillage practices influence the quantity and quality of soil organic N (SON). We evaluated the impact of crop rotations and tillage practices on SON and mineralizable N at a depth of 0–15 cm in six field experiments, varying in duration over 8–25 years, that were being conducted in three Chernozemic soil zones in Saskatchewan, Canada. In a Brown Chernozem, continuous wheat increased SON at 0–15 cm by 7–17 kg N ha^–1year^–1 more than fallow/wheat. In a Dark Brown Chernozem, continuous cropping increased SON by 30 kg N ha^–1year^–1, compared with cropping systems containing fallow once every 3 years; and, in a Rego Black Chernozem, the increase in SON was 29 kg N ha^–1 year^–1, compared with cropping systems containing fallow once every 4 years. The increase in SON due to increased cropping frequency was accompanied by an increase in the proportion of mineralizable SON in the Brown Chernozem, but not in the Dark Brown and Black Chernozems. In the Brown Chernozemic soil zone, no-tillage management increased SON, compared with conventional tillage, varying from 16 kg N ha^–1year^–1 to 28 kg N ha^–1year^–1. In the Dark Brown Chernozemic soil zone, it increased SON by 35 kg N ha^–1year^–1 and, in the Black Chernozemic soil zone, by about 40 kg N ha^–1year^–1. Increases in SON at a depth of 0–7.5 cm due to no-tillage management was accompanied by a greater increase in the mineralizable N for Hatton fine sandy loam, Melfort silty clay and Indian Head clay than for other soils, indicating that the material responsible for the increased SON due to no-tillage was more labile than the soil humus N. However, the increased SON under no-till in Swinton loam, Sceptre clay and Elstow clay loam was not associated with an increase in the mineralizable N, indicating that this increased SON was no more susceptible to decomposition than the soil humus N. Therefore, increases in SON under improved management practices, such as conservation tillage and extended crop rotations, do not necessarily increase the potential soil N availability.     

Nitrous oxide emission from wetland rice soil as affected by the application of controlled-availability fertilizers and mid-season aeration

 N₂O emission from a wetland rice soil as affected by the application of three controlled-availability fertilizers (CAFs) and urea was investigated through a pot experiment. N₂O fluxes from the N fertilized paddy soil averaged 44.8–69.3 μg N m^–2 h^–1 during the rice growing season, accounting for 0.28–0.51% of the applied N. The emission primarily occurred during the mid-season aeration (MSA) and the subsequent re-flooding period. Fluxes were highly correlated with the NO₃ ^– and N₂O concentrations in the soil water. As there were relatively large amounts of NH₄ ⁺-N present in the soil of the CAF treatments at the beginning of MSA, leading to large amounts of NO₃ ^–-N during the MSA and the subsequent re-flooding period, the tested CAFs were not effective in reducing N₂O emission from this paddy soil. The potential of applied CAFs to reduce N₂O emissions from paddy soil is discussed. Received: 25 May 1999     

Nitrous oxide emission as affected by alternate anaerobic and aerobic conditions from soil suspensions enriched with ammonium sulfate

The effect of several anaerobic and aerobic cycles of varying duration on N₂O emission and labelled N loss was investigated in (¹⁵NH₄)₂SO₄ amended soil suspensions. No N₂O was evolved from the continuously-anaerobic treatment. The continuously-aerobic treatment produced approximately 0.8 μg N₂O-N g^?1 dry soil in 56 days. Alternate anaerobic-aerobic cycles increased the net N₂O evolution with 7.2 μg N₂O-N g^?1 dry soil produced in 56 days from the 7-day anaerobic, 7-day aerobic treatment. The net N₂O evolution increased further when the duration of the anaerobic and aerobic periods was increased from 7-7 days to 14-14 days (15.7μg N₂O-N g^?1 dry soil in 56 days), although the total ¹⁵N loss from the system was approximately the same for the two treatments. The results of this study show that N₂O evolution from soils is likely to be greater under fluctuating moisture conditions than under either continuously well-aerated conditions, or continuously excess-moisture conditions.