Increase In Emission Rate Research Articles

Weather radar observations of the Hekla 2000 eruption cloud, Iceland

The Hekla eruption cloud on 26–27 February 2000 was the first volcanic cloud to be continuously and completely monitored advecting above Iceland, using the C-band weather radar near the Keflavik international airport. Real-time radar observations of the onset, advection, and waning of the eruption cloud were studied using time series of PPI (plan-position indicator) radar images, including VMI normal, Echotop, and Cappi level 2 displays. The reflectivity of the entire volcanic cloud ranges from 0 to >60 dBz. The eruption column above the vent is essentially characterised by VMI normal and Cappi level 2 values, >30 dBz, due to the dominant influence of lapilli and ash (tephra) on the overall reflected signal. The cloud generated by the column was advected downwind to the north-northeast. It is characterised by values between 0 and 30 dBz, and the persistence of these reflections likely result from continuing water condensation and freezing on ash particles. Echotop radar images of the eruption onset document a rapid ascent of the plume head with a mean velocity of ~30 to 50 m s−1, before it reached an altitude of ~11–12 km. The evolution of the reflected cloud was studied from the area change in pixels of its highly reflected portions, >30 dBz, and tied to recorded volcanic tremor amplitudes. The synchronous initial variation of both radar and seismic signals documents the abrupt increase in tephra emission and magma discharge rate from 18:20 to 19:00 UTC on 26 February. From 19:00 the >45 dBz and 30–45 dBz portions of the reflected cloud decrease and disappear at about 7 and 10.5 h, respectively, after the eruption began, indicating the end of the decaying explosive phase. The advection and extent of the reflected eruption cloud were compared with eyewitness accounts of tephra fall onset and the measured mass of tephra deposited on the ground during the first 12 h. Differences in the deposit map and volcanic cloud radar map are due to the fact that the greater part of the deposit originates by fallout off the column margins and from the base of the cloud followed by advection of falling particle in lower level winds.

Estimation of annual Mn emissions from MMT source in the Canadian environment and the Mn pollution index in each province

Methylcyclopentadienyl manganese tricarbonyl (MMT), is an organic derivative of Mn used as an anti-knock agent in Canadian unleaded gasoline since 1976. This study aims to estimate Mn emissions from MMT source in the Canadian environment as well as the Mn pollution index per capita in each of the Canadian provinces. In concomitance, lead emissions will also be addressed. The quantities of Mn and Pb used in gasoline were calculated from data on annual gasoline sales in Canada supplied by Statistics Canada. Obviously, the Pb emission profile shows a substantial decrease with an annual average of −28% until 1991, when MMT totally replaced Pb in Canadian gasoline. The case of Mn is quite different. Since emission rates of Mn at the tailpipe vary between 6 and 45%, the quantities released into the atmosphere were calculated for each of these rates, as well as for 100 and 25% emission rate scenarios. For 1981 and 1999, the quantities of Mn used in gasoline were estimated at 101 122 and 344 880 kg, respectively. Based on the realistic 25% emission rate scenario and a concentration of 0.009 g/l of Mn in gasoline, Mn emissions from car exhausts were estimated to 25 280 and 86 220 kg, respectively. The mean annual increase in emission rate between 1981 and 1999 is 7.34%. The atmospheric Mn pollution index seems to be stable over time, and it varies from 0.0024 kg/capita to 0.0041 kg/capita in 1999. In the context of a notable decrease in Mn emissions from industrial sources, these results suggest that the combustion of MMT used in gasoline may be an important factor contributing to maintaining stable atmospheric Mn concentrations instead of a significant decrease due to reduced industrial emissions.

Stimulation of nitrous oxide production resulted from soil fumigation with chloropicrin

Abstract Agricultural soils are a major source of the atmospheric greenhouse gas nitrous oxide (N2O). Agronomic practices such as tillage and fertilizer applications can significantly affect the production and consumption of N2O because of alteration in soil physical, chemical, and biochemical activities. Soil fumigation is an agronomic practice used to control soil-borne disease pathogens, weeds, plant-parasitic nematodes, and fungi. The strong impact of fumigants on soil microorganisms can indirectly affect the production and/or consumption of N2O and would potentially alter net emissions from agricultural soils. Laboratory incubation and field soil fumigation studies were conducted to determine the potential impact of soil fumigation on the dynamics of N2O production. Laboratory soil incubations showed an eight-fold increase in the production rate of N2O as a consequence of chloropicrin (CP) fumigation. This stimulation effect was confirmed by a seven-fold increase in N2O emission rates in field plots following CP fumigation. The mechanism of N2O production appeared to be microbial related; however, additional work is needed to fully elucidate the pathways.

Geomagnetic storm heating effects on the low-latitude dayside thermosphere from WINDII observations at equinox

The response of the neutral thermosphere to geomagnetic storms is an important aspect of the study of “space weather”. Yet, this response above the magnetic dip equator has rarely been measured on the dayside because a satellite-borne instrument is required. WINDII, the WIND Imaging Interferometer on board UARS, NASA's Upper Atmosphere Research Satellite, measured emission rates and apparent temperatures to altitudes as high as 270 km using its O( 1 S) 558 nm green line channel several days each week during 1993. On 2 days close to equinox in 1993 the maximum K p index exceeded 7, and these days were selected for comparison with surrounding days of lesser magnetic activity. Evidence for thermospheric heating is seen in the Doppler broadening increase of 100 K , the green line volume emission rate increase of 30–78%, and the emission rate scale height increase of 5 km , all measured at 250 km in comparisons between storm and non-storm conditions.

Characteristics of fluoride emission from five clay minerals as affected by temperature, heating time and addition of calcium compounds

Characteristics of fluoride emission from five clay minerals (montmorillonite, kaolinite, vermiculite, geothite, and allophane) as affected by temperature, heating time and addition of calcium compounds were studied. Marked increase of the fluoride emission rate was noticed with increase of temperature. The fluoride release, began at 500°C–600°C, and the main bulk of the fluoride emission occurred at the temperature of about 800°C. The loss of crystalline water was primarily responsible for the increase of fluoride emission. When minerals were heated at 800°C, The fluoride emission rate from the clay minerals reached the highest after heating for 1 hour. The samples treated by CaO, CaCO3, Ca(OH)2, Ca3(PO4)2, and CaSO4 had 55.45%, 59.58%, 46.45%, 54.31%, 31.25% reduction in the fluoride emission from montmorillonite at the temperature of 800°C, respectively. CaCO3 had the highest fluoride fixing capacity compared to other calcium compounds.

Effect of moisture, texture and aggregate size of paddy soil on production and consumption of CH4

Laboratory experiments were conducted to find out under which conditions the soil from Italian rice fields could change from a source into a sink of atmospheric CH4. Moist (30% H2O=68% of the maximum water holding capacity (whc)) rice field soil oxidized CH4 with biphasic kinetics, exhibiting both a low (145ppmv CH4) and a high (20,200ppmv CH4) Km value and Vmax values of 16.8 and 839nmol gdw−1h−1, respectively. The activity with the low Km allowed the oxidation of atmospheric CH4. Uptake rates of high CH4 concentrations (16.5% v/v) and of O2 linearly decreased with aggregate size of soil between 2 and 10mm. Atmospheric CH4 (1.8 ppmv) was consumed in soil aggregates <6mm, but soil aggregates >6mm released CH4 into the atmosphere. Similarly, net uptake of atmospheric CH4 turned into net release of CH4 when the soil moisture was decreased below a water content of about 20% whc. The uptake rate of atmospheric CH4 increased threefold when the soil was amended with sterile quartz sand. Flooded microcosms with non-amended and quartz-amended soil emitted CH4 into the atmosphere. The CH4 emission rate increased when the flux was measured under an atmosphere of N2 instead of air, indicating that 30–99% of the produced CH4 was oxidized in the oxic soil surface layer. Removal of the flood water resulted in increase of CH4 emission rates until a water content of about 75–82% whc was reached, and subsequently in a rapid decrease. However, the soil microcosms never showed net uptake of atmospheric CH4. Our results show that the microorganisms consuming atmospheric CH4 were inactivated at an earlier stage of drainage than the microorganisms producing CH4, irrespective of the soil porosity which was adjusted by addition of quartz sand. Hence, it is unlikely that the Italian rice fields can act as a net sink for atmospheric CH4 even when drained.

Natural perturbations, drying–wetting and freezing–thawing cycles, and the emission of nitrous oxide, carbon dioxide and methane from farmed organic soils

We investigated the effects of drying–wetting and freezing–thawing cycles on the emission of nitrous oxide, carbon dioxide and methane from intact soil cores from farmed organic soils at sites in Germany, Sweden and Finland. During the first week following wetting or thawing, cores from the German and Swedish sites produced an up to 1000-fold increase in N2O emission rates. The total surplus N2O emission due to the first wetting event ranged between 3 and 140mg N–N2O m−2, and between 13 and 340mg N–N2O m−2 due to the first thawing event but declined following two successive freeze–thaw events. Wetting and thawing produced a greater surplus emission of N2O from grassland sites compared to arable sites. Following wetting, denitrification was responsible for the majority of N2O emission from the German grassland soil while in the German ploughed soil and the Swedish soils denitrification was responsible for less than 60% of the N2O emission. In contrast, following thawing, denitrification was responsible for <40% of N2O emission from the German grassland soil while in the remaining German and Swedish soils denitrification was responsible for most of the emission. Wetting or thawing of soil cores from the Finnish sites did not result in any significant increase in N2O emission rates perhaps because a prolonged drought at the time of soil core collection had changed soil properties considerably. The CO2 emission rates increased up to 5-fold following wetting or thawing. The total surplus CO2 emission ranged from 0.0 to 13g C–CO2 m−2, differed between locations and land use, and decreased during successive cycles of freezing and thawing. The total surplus CO2 emission from grassland sites following thawing was generally higher than from arable and forest sites probably due to decomposition of carbon sources liberated from stressed grass roots. Methane emission rates were very small and often below our detection limit (<1μg C–CH4 m−2 h−1); no effects of wetting or thawing on CH4 emission rates were observed at any of the sites. A comparison with annual emission rates of N2O and CO2 showed that even a single wetting or thawing event may account for a large proportion of the N2O emission from farmed organic soils.

The relationship between degassing and ground deformation at Soufriere Hills Volcano, Montserrat

We examine the correlations between SO2 emission rate, seismicity and ground deformation in the month prior to the 25 June 1997 dome collapse of the Soufriere Hills Volcano, Montserrat. During this period, the volcano exhibited a pattern of cyclic inflation and deflation with an 8–14h period. We find that SO2 emission rates, measured by COSPEC, correlate with the amplitude of these tilt cycles, and that higher rates of SO2 emission were associated with stronger ground deformation and enhanced hybrid seismicity. Within tilt cycles, degassing peaks coincide with maximum deformation gradients. Increases in the amount of gas in the magma conduit feeding the dome, probably due to increases in volatile content of ascending magma volume can account for the observed increases in tilt amplitude, hybrid seismicity and SO2 emission rate.

Crop management affecting methane emissions from irrigated and rainfed rice in Central Java (Indonesia)

Methane (CH4) emissions were determined from 1993 to 1998 using an automated closed chamber technique in irrigated and rainfed rice. In Jakenan (Central Java), the two consecutive crops encompass a gradient from low to heavy rainfall (wet season crop) and from heavy to low rainfall (dry season crop), respectively. Rainfed rice was characterized by very low emission at the onset of the wet season and the end of the dry season. Persistent flooding in irrigated fields resulted in relatively high emission rates throughout the two seasons. Average emission in rainfed rice varied between 19 and 123 mg CH4 m−2 d−1, whereas averages in irrigated rice ranged from 71 to 217 mg CH4 m−2 d−1. The impact of organic manure was relatively small in rainfed rice. In the wet season, farmyard manure (FYM) was completely decomposed before CH4 emission was initiated; rice straw resulted in 40% increase in emission rates during this cropping season. In the dry season, intensive flooding in the early stage promoted high emissions from organically fertilized plots; seasonal emissions of FYM and rice straw increased by 72% and 37%, respectively, as compared with mineral fertilizer. Four different rice cultivars were tested in irrigated rice. Average emission rates differed from season to season, but the total emissions showed a consistent ranking in wet and dry season, depending on season length. The early-maturing Dodokan had the lowest emissions (101 and 52 kg CH4 ha−1) and the late-maturing Cisadane had the highest emissions (142 and 116 kg CH4 ha−1). The high-yielding varieties IR64 and Memberamo had moderately high emission rates. These findings provide important clues for developing specific mitigation strategies for irrigated and rainfed rice.

Effects of elevated CO2 and temperature on methane production and emission from submerged soil microcosm

Incubation experiments were conducted under controlled laboratory conditions to study the interactive effects of elevated carbon dioxide (CO2 ) and temperature on the production and emission of methane (CH4) from a submerged rice soil microcosm. Soil samples (unamended soil; soil + straw; soil + straw + N fertilizer) were placed in four growth chambers specifically designed for a combination of two levels of temperature (25 ° C or 35 ° C) and two levels of CO2 concentration (400 or 800 μmol mol-1) with light intensity of about 3000 Lx for 16 h d-1. At 7, 15, 30, and 45 d after incubation, CH4 flux, CH4 dissolved in floodwater, subsurface soil-entrapped CH4 and CH4 production potential of the subsurface soil were determined. The results are summarized as follows: 1) The amendment with rice straw led to a severalfold increase in CH4 emission rates, especially at 35 “C. However, the CH4 flux tended to decrease considerably after 15 d of incubation under elevated CO2 . 2) The amount of entrapped CH4 in subsurface soil and the CH4 production potential of the subsurface soil were appreciably larger in the soil samples incubated under elevated CO2 and temperature during the early incubation period. However, after 15 d, they were similar in the soil samples incubated under elevated or ambient CO2 levels. These results clearly indicated that elevated CO2 and temperature accelerated CH4, formation by the addition of rice straw, while elevated CO2 reduced CH4 emission at both temperatures.

Kinetics of leaf temperature fluctuation affect isoprene emission from red oak (Quercus rubra) leaves

Because the rate of isoprene (2-methyl-1,3-butadiene) emission from plants is highly temperature-dependent, we investigated natural fluctuations in leaf temperature and effects of rapid temperature change on isoprene emission of red oak (Quercus rubra L.) leaves at the top of the canopy at Harvard Forest. Throughout the day, leaves often reached temperatures as much as 15 degrees C above air temperature. The highest temperatures were reached for only a few seconds at a time. We compared isoprene emission rates measured when leaf temperature was changed rapidly with those measured when temperature was changed slowly. In all cases, isoprene emission rate increased with increasing leaf temperature up to about 32 degrees C and then decreased with higher temperatures. The temperature at which isoprene emission rates began to decrease depended on how quickly measurements were made. Isoprene emission rates peaked at 32.5 degrees C when measured hourly, whereas rates peaked at 39 degrees C when measurements were made every four minutes. This behavior reflected the rapid increase in isoprene emission rate that occurred immediately after an increase in leaf temperature, and the subsequent decrease in isoprene emission rate when leaf temperature was held steady for longer than 20 minutes. We concluded that the observed temperature response of isoprene emission rate is a function of measurement protocol. Omitting this parameter from isoprene emission models will not affect simulated isoprene emission rates at mild temperatures, but can increase isoprene emission rates at high temperatures.

Assessment of Benzene and Toluene Emissions from Automobile Exhaust in Bangkok

The use of unleaded gasoline, together with an increase in the number of vehicles in Bangkok, has significantly influenced benzene and toluene concentrations in vehicular emissions and contributes to the air pollution problem. As a matter of practical necessity, a quick test program is done for the measurement of emission concentrations/rates for vehicles driven on the road. Exhaust emission measurement at idle mode was conducted in a fleet of 12 vehicles of different model years and manufacturers. The study revealed that the benzene and toluene concentrations in the exhaust effluent averaged 4.4–22.02 and 12.24–44.75 mg/m3, respectively for 1990–1992 cars and decreased to 0.76–4.14 and 0.89–6.26 mg/m3, respectively for 1994–1995 cars. In another study, exhaust emission measurement on a chassis dynamometer was carried out in a fleet of nine selected, in-use cars. It was observed that benzene and toluene emission rates were considerably higher—in the range of 70.84–85.82 and 354.15– 429.00 mg/km, respectively, for 1990–1991 model year cars. Lower benzene and toluene emission rates of 0.43–95.07 and 2.15–475.35 mg/km, respectively, were represented by newer cars with model years 1994–1995. These results indicated that there was a significant increase in benzene and toluene emission concentrations and rates with increasing car mileage and model year. The finding also revealed that only 28% of the tested vehicles complied to the approved emission standard.

A New Test Chamber To Measure Material Emissions under Controlled Air Velocity

A new 20-L glass chamber for the determination of VOC emissions from construction materials and consumer products under controlled air velocity and turbulence is described. Profiles of air velocity and turbulence, obtained with precisely positioned hot wire anemometric probes, show that the velocity field is homogeneous and that air velocity is tightly controlled by the fan rotation speed; this overcomes the problem of selecting representative positions to measure air velocity above a test specimen. First tests on material emissions show that the influence of air velocity on the emission rate of VOCs is negligible for sources limited by internal diffusion (e.g., PVC tile) and strong for sources limited by evaporation. In a velocity interval from 0.15 to 0.30 m s-1, an emission rate increase of 50% has been observed for pure n-decane and 1,4-dichlorobenzene and of 30% for 1,2-propanediol from a water-based paint. In contrast, no measurable influence of turbulence could be observed during vaporization of 1,...

Spontaneous lifetime control of quantum dot emitters in apertured microcavities

Data are presented on modified spontaneous emission rates in quantum dot microcavity arrays. Three-dimensional optical confinement is achieved using apertured microcavities with square aperture sizes varied from 5 to 1 μm on a side. A maximum measured increase in the on-resonance spontaneous emission rate over that off resonance is ∼50%, and we identify this enhancement with apertured modes of the microcavity. Oscillations in the spontaneous emission rate versus wavelength correspond to spectral resonances in the higher order aperture modes.

Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming

AbstractIt was hypothesized that high CO2 availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO2 concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO2 concentrations. Ponderosa pine grown at 700 μmol mol–1 CO2 exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol–1 CO2. Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol–1 CO2 exhibited increased photosynthetic rates as compared to growth at 350 μmol mol–1 CO2 in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO2 on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO2 plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO2 concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO2 may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO2 on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate.

Volatile organic compounds and isoprene oxidation products at a temperate deciduous forest site

Biogenic volatile organic compounds (BVOCs) and their role in atmospheric oxidant formation were investigated at a forest site near Oak Ridge, Tennessee, as part of the Nashville Southern Oxidants Study (SOS) in July 1995. Of 98 VOCs detected, a major fraction were anthropogenic VOCs such as chlorofluorocarbons (CFCs), alkanes, alkenes and aromatic compounds. Isoprene was the dominant BVOC during daytime. Primary products from BVOC oxidation were methylvinylketone, methacrolein and 3‐methylfuran. Other compounds studied include the BVOCs α‐pinene, camphene, β‐pinene, p‐cymene, limonene and cis‐3‐hexenyl acetate and a series of light alkanes, aromatic hydrocarbons and seven of the CFCs. The correlation of meteorological parameters, with the mixing ratios of these different compounds, reveals information on atmospheric oxidation processes and transport. Long‐lived VOCs show very steady mixing ratio time series. Regionally and anthropogenically emitted VOCs display distinct diurnal cycles with a strong mixing ratio decrease in the morning from the breakup of the nocturnal boundary layer. Nighttime mixing ratio increases of CFCs and anthropogenic VOCs are suspected to derive from emissions within the Knoxville urban area into the shallow nocturnal boundary layer. In contrast, the time series of BVOCs and their oxidation products are determined by a combination of emission control, atmospheric oxidation and deposition, and boundary layer dynamics. Mixing ratio time series data for monoterpenes and cis‐3‐hexenyl acetate suggest a temporarily emission rate increase during and after heavy rain events. The isoprene oxidation products demonstrate differences in the oxidation pathways during night and day and in their dry and wet deposition rates.

The performance and emission characteristics of C1-C5 alcohol-gasoline blends with matched oxygen content in a single-cylinder spark ignition engine

Alcohols with carbon numbers ranging from C1 to C5 were individually blended with unleaded test gasoline. All the alcohol-gasoline blends had the same oxygen mass content. The performance characteristics of the blends were quantified using a single-cylinder spark ignition engine. The knock-limiting spark timing was determined by analysis of the third derivative of the measured in-cylinder gas pressure versus crank angle. The engine operating conditions were optimized for each (C1-C5) blend with two different values of matched oxygen mass content (2.5 and 5.0 per cent). Emission mass rates of carbon monoxide (CO), nitric oxides (NOx), total unburned hydrocarbons (THCs), alcohols and aldehydes were quantified. The brake power specific rate emissions were compared with that of neat gasoline. Adding lower alcohols (C1, C2 and C3) to gasoline improved the knock resistance. Further improvement was achieved by increasing the oxygen content of the fuel blend. Blends with higher alcohols (C4 and C5) showed degraded knock resistance when compared with neat gasoline. Generally, all alcohol-gasoline blends showed reduction in CO emissions. Higher alcohol-gasoline blends with an oxygen mass content of 5.0 per cent showed a pronounced increase in NOx emission rates when operating at high compression ratios and 5° before top dead centre timing. This is attributed to their lower enthalpy of vaporization and higher flame temperature. All blends tested at optimized operating conditions showed reduction in THC emission rates. Unburned alcohol emission rates were higher for blends with higher content of alcohol, and aldehyde emissions were higher for all blends with formaldehyde as the main constituent.

Effect of rice variety on methane emission from an Indonesian paddy field

Variations in CH4 emission from a Sumatra paddy field in which 8 popular modern varieties in Indonesia were grown were compared in the 1994/1995 rainy season. Total amounts of CH4 emitted during the period of rice growth were in the ranges of 32.6-41.7 and 51.3–64.6 g CH4 m-2 for the plots amended with chemical fertilizer only and those amended with both rice straw and chemical fertilizer, respectively. The mean CH4 emission rate was highest in the plot with the variety Bengawan solo and lowest in the plots with the varieties Atomita-4 and Way seputih among the plots which received chemical fertilizer, while highest in the plot with Way seputih and lowest in the plot with Bengawan solo among the plots amended with both rice straw and chemical fertilizer. The increase in the mean CH4 emission rates by rice straw application was higher for the plots planted with Way seputih (1.98 times) and Atomita-4 (1.77 times) than for the plots with Bengawan solo (1.23 times) and IR-64 (1.35 times). The plots with Walanai and Cisanggarung recorded intermediate mean emission rates and the increase in CH4 emission by rice straw application was also intermediate (1.57–1.64 times). It was noteworthy that Way seputih and Atomita-4 were derived from the variety Cisadane, Bengawan solo and IR-64 from the variety IR-54, and Walanai and Cisanggarung from the varieties IR-36 and Pelita 1-1, respectively. The amounts of CH. emitted for 1 kg grain production ranged from 53 (Atomita-4) to 74 (Kapuas and Walanai) and from 89-93 (IR-64, Bengawan solo, and Atomita-4) to 121 (Kapuas) g CH4 kg-1 of grain for the plots amended with chemical fertilizer and those amended with rice straw and chemical fertilizer, respectively.

Biogenic NO emissions from savanna soils as a function of fire regime, soil type, soil nitrogen, and water status

A study of NOx emissions from soils representative of nutrient‐poor and nutrient‐rich savannas and their response to burning and soil water content was carried out in the southern Kruger National Park, South Africa. The study spanned the end of the dry season and the beginning of the wet season (September–December 1992). Nitrogen mineralization rates were measured using an in situ technique simultaneously with measurements of NOx emissions. NOx emissions were almost entirely as NO. The relationship between NO emission rate and soil moisture was parabolic regardless of soil type and management practice, with the lowest NO emission rates being measured at low (&lt;0.087) and high (&gt;0.542) water‐filled pore space values. The initial increase in NO emission rates with increasing soil moisture are paralleled by increases in the nitrate concentration in the soil. The highest NO emission rates (20 ng N‐NO m−2 s−1—excluding the brief initial peak) were measured on plots from which fire had been excluded for 35 years. The next highest rates (8 ng N‐NO m−2 s−1) were measured on the more fertile soils. Infertile soils, burned every second year, had rates of 3.5 ng N‐NO m−2 s−1. The NO emission rates show a positive correlation with soil total N content and N nitrification rate. The effect of excluding fire from a savanna is to increase the soil nitrogen content through increased litter inputs, which in turn increases nitrification rates and soil NO emissions.

SIMULTANEOUS EFFECT OF TWO TOXICANTS ON BIOLOGICAL SPECIES: A MATHEMATICAL MODEL

In this paper, a mathematical model to study the simultaneous effect of two toxicants (one is more toxic than the other) on the growth and survival of a biological species is proposed. The cases of instantaneous spill, constant and periodic emissions of each of the toxicant into the environment are considered. It is shown that in the case of an instantaneous spill of each of the toxicant into the environment, the species after its initial decrease in density may recover to its original level after a period of time, the magnitude of which depends on the toxicity and washout rate of each of the toxicant. However, if both the toxicants are emitted with constant rates, the species in the habitat is doomed to extinction sooner than the case of a single toxicant having the same influx and washout rates as one of them, the extinction rate becoming faster with the increase in toxicity and emission rate of the other toxicant. It is also shown that for a small amplitude periodic emission of the toxicant with a constant mean, the stability behavior of the system is same as that of the case of the constant emission. It is found further through the model study that if suitable efforts are made to reduce the emission rate of each of the toxicant at the source and its concentration in the environment by some removal mechanism, an appropriate level of species density can be maintained.