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Soil Science Society of America Journal 63:858-865 (1999)
© 1999 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Effect of Temperature and Water on Gaseous Emissions from Soils Treated with Animal Slurry

Michael Maaga and Finn P. Vinthera

a Dep. of Crop Physiology and Soil Science, Danish Inst. of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark

finn.vinther{at}agrsci.dk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Microbial respiration and denitrification are greatly affected by abiotic factors, but they are difficult to assess in natural environments. Under controlled conditions the interactions between temperature and soil water content on microbial respiration, N2O production, and denitrification in soil amended with animal slurries were studied. The effects of the abiotic factors on the biological processes were monitored for 8 wk in repacked soil cores amended with pig or cattle slurry. The soil cores were incubated at 43, 57, and 72% water-filled pore space (WFPS) and at 10, 15, and 20°C with or without addition of 10% acetylene. The amount of N2O lost at 72% WFPS corresponded to 8 to 22% of the slurry's NH+4 content, but to only 0.01 to 0.9% at 43 to 57% WFPS. Denitrification losses at 72% WFPS accounted for 17 to 58% of the slurry's NH+4 content, but for only 0.01 to 1.2% at 43 to 57% WFPS. The amount of available C accounted for by denitrification was 8 to 16% of total respiration at 72% WFPS, but only 0.03 to 0.4% at 43 to 57% WFPS. Both N2O production and denitrification peaked earlier in the cattle-slurry treated soil than in the pig-slurry treated soil, whereas the total N loss was greatest from the latter. Neither amendments nor soil water contents seemed to affect the Q10-values for the CO2 production, resulting in values between 1.6 and 2.6. At 72% WFPS, N2O production and denitrification had Q10-values ranging between 3.3 and 5.4. High temperatures enhanced both aerobic respiration and denitrification, and aerobic respiration further enhanced denitrification by consuming oxygen, resulting in strong sensitivity of denitrification to temperature.

Abbreviations: CS, cattle slurry • FC, field capacity • MSE, mean square errors • PS, pig slurry • TAN, total ammoniacal nitrogen • VFA, volatile fatty acids • WFPS, water-filled pore space • WSC, water soluble carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
CONCERN FOR ENVIRONMENTAL QUALITY has prompted research to develop agricultural management systems that mitigate nutrient losses to the environment. Insufficient knowledge of the proper application time for manures and slurries in relation to crop needs and minimal nutrient loss often limit current agricultural management systems. Improper handling of slurry may result in NO-3 leaching into ground and surface waters (Drommerhausen et al., 1995; Lind et al., 1995), or it may result in gaseous loss of N to the atmosphere (Comfort et al., 1990; Maag, 1995; Mosier et al., 1996).

Nitrous oxide is formed during the nitrification process when O2 is limiting (Firestone and Davidson, 1989) and during denitrification (Firestone et al., 1980; Aulakh et al., 1992). Nitrous oxide contributes to global warming and to the chemical destruction of ozone in the stratosphere (Bouwman, 1990). Besides contributing to atmospheric pollution, the denitrification process lowers fertilizer use efficiency and negatively impacts on groundwater quality (Aulakh et al., 1992; Granli and Bøckman, 1994).

The intensity of nitrate reduction in soils depends mainly on soil parameters that control the oxygen state of soils (Tiedje et al., 1989). Among these parameters, available C, temperature, and the soil water content seem to be the most important (Linn and Doran, 1984; Beauchamp et al., 1989; Aulakh et al., 1992). In relation to water content, laboratory studies (Doran et al., 1988; Sexstone et al., 1988) with several soil types revealed that denitrification increased exponentially at WFPS above 70 to 75%. Although such a WFPS normally exceeds the field capacity (FC) of many soils, several studies have shown that even a brief increase in soil water content may cause a dramatic increase in denitrification (Aulakh et al., 1984; Sexstone et al., 1985; Mosier et al., 1986; Vinther, 1992). In a recent study, Renault and Sierra (1994) showed that the link between the anaerobic soil volume and the soil water content was highly influenced by soil temperature and microbial activity, which in turn is partly controlled by the availability of organic C.

Several studies, for example, Cabrera et al. (1994), have shown that the use of animal manures as fertilizers enhance N2O and CO2 emissions, but research on the combined effects of soil water content and temperature on N2O production is limited (Nugroho and Kuwatsuka, 1990, 1992). The threshold value of WFPS at which N2O production and denitrification production rates can occur and the ratio of the gases emitted may be lowered by the addition of manures, beause its application will influence C and N.

The biological processes of denitrification and microbial respiration are greatly affected by abiotic factors, but they are difficult to assess in natural environments; therefore, research under controlled conditions is required to study these interactions. We investigated the interactions between temperature and soil water content on microbial respiration, N2O production, and denitrification in a sandy loam soil amended with pig (Sus scrofa) or cattle (Bos taurus) slurry.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Soil Sampling and Treatment
The soil used in our study was a sandy loam from Askov experimental station (Typic Hapludalf; 36.4% coarse sand, 25.2% fine sand, 14.5% coarse silt, 10.8% fine silt, 10.8% clay, 1.34% total organic C, and 0.12% total N). Dissing Nielsen and Møberg (1984) have described the soil in detail. Bulk samples from the upper 20 cm of the soil was sieved (<2 mm), air-dried to a water content of 90 g kg-1, and stored at 2°C for no more than 1 mo.

Soil treatments consisted of the addition of either pig or cattle slurry (Table 1) or an unamended control. Slurry was added to the soil at a rate of 20 mL kg-1 soil and subsamples (50 g dry wt.) of the mixture were placed in plastic cores (3.2 cm i.d., length 10 cm) closed at the bottom by a 0.2-mm nylon mesh. The soil cores were then gently compressed to a bulk density of 1.4 Mg m-3 using a hand held piston. The soil water content was adjusted to water contents equivalent to 43, 57 (FC), and 72% WFPS by adding water to the soil surface. The soils were incubated at 10°, 15°, and 20°C. Each combination (or batch) of slurry amendment, temperature, and WFPS consisted of {approx}100 repacked soil cores. Soil cores were placed in groups of three in 1-L preservation jars (Luminarc, France) with a 100-mL flask filled with water to avoid excessive water loss from the soils. Punctured parafilm was used to seal the openings of the jars.


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  		Table 1 Characteristics of the animal slurries used

 
Periodically (on Day 1, 2, 3, 4, 7, 14, 20, 28, 32, 42, 55) six randomly chosen soil cores from each batch were placed in six jars, each sealed with a vacuum greased o-ring and a lid with a septum for gas sampling. Acetylene (100 mL L-1) was injected into three of these jars for determination of denitrificaton using the acetylene block assay (Yoshinari et al., 1977). Prior to the injection of acetylene, a corresponding volume of air was withdrawn from the same three jars. Nitrous oxide concentrations were measured three times during incubation to calculate the ratio of N2O production and denitrification. Treatments with the lowest denitrifying potential (43% WFPS and unamended samples at 10°C and 57% WFPS) were sampled at 24, 48, and 72 h after acetylene addition; otherwise, samples were taken 1, 2, and 3 h after addition.

Gas and Soil Analysis
Air samples (1 mL) were taken from the jar headspace using an airtight syringe, replaced by equal volume of N2, and then analyzed by gas chromatography. The concentration of N2O was determined using an electron capture detector (Hewlett-Packard, Avondale, PA), and concentration of CO2 was determined using a thermal conductivity detector (Varian, Walnut Creek, CA). Concentration of N2O and CO2 in the jars were corrected for their solubility in water (Tiedje, 1994) and for the dilution caused by addition of N2. Details of the gas chromatographic analysis of N2O and CO2 are given in Maag et al. (1997). Denitrification rates were calculated by linear regression between the amount of N2O in the jars with acetylene and the time for gas sampling. Likewise, N2O production rates were calculated from jars without acetylene. The headspace volume of the jars was calculated by subtracting the soil and water volume. The procedure gave a sensitivity of 0.05 µg N2O–N kg-1 h-1 for the low potential activity samples (i.e., 43 and 57% WFPS and unamended) and 1 µg N2O–N kg-1 h-1 for all the other samples. CO2 evolution rates were calculated from the increase in CO2 concentration between the first measurement and the concentration in ambient air. After gas sampling, the samples not treated with acetylene were transferred back to the batches, while the acetylene treated samples were used for determination of ammonium and nitrate with a flow injection analyzer (FIA Star 5002, Tecator, Höganäs, Sweden) after extraction with 2 M KCl and filtration. Soil samples were not continuously exposed to acetylene because of the limited amount of time (4–7 d) during which acetylene is able to inhibit N2O reductase (Yeomans and Beauchamp, 1978). The N2 evolved was calculated using the difference between N2O produced in acetylene-amended and in unamended cores (Ryden et al., 1979).

Statistical Analysis
Because the standard deviation increased with the mean, data were log transformed prior to analysis. Statistical analyses were performed with the general linear model procedure (GLM) in SAS (SAS Inst., 1994). Nitrous oxide evolution and denitrification was analyzed as a split-plot experiment with three replications in which day of sampling was regarded as main plot and the treatments as the split plots, or each sampling day was analyzed separately. Statistical analyses were performed on data until Day 28, after which the rates in all treatments were close to zero and their differences negligible.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Significant differences (P < 0.05) in N2O production, denitrification rates, and soil respiration were observed in the interactions between the treatments and measurement days (Table 2) . However, the treatments seemed to have a much more pronounced overall effect on N2O production and denitrification than on CO2 production, as indicated by the considerably lower mean square errors (MSE) of the latter (Table 2). Likewise, when comparing each day separately, the MSE values were considerably lower for CO2 production than for N2O production and denitrification.


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  		Table 2 Analysis of variance results for effects of temperature (T), water-filled pore space (WFPS), sampling day (D), and amendments (AM) on denitrification, N2O production CO2 production{dagger}

 
Nitrous oxide production and denitrification rates for PS and CS-amended soils were low (<4 µg N kg-1 h-1) at water contents at 43% WFPS (not shown) and at 57% WFPS with N2O comprising a minor part of the total gaseous emission (Fig. 1A and 2A) . In slurry-amended soils wetted to 72% WFPS, N2O production and denitrification ranged from 40 to 170 µg N kg-1 h-1 during the initial 20 d, but decreased thereafter to a level of <20 µg N kg-1 h-1.



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  		Fig. 1 Net N2O production (mean + SE) in soil with or without incorporated pig (PS) and cattle slurry (CS) incubated at 20°C, and (A) 57% and (B) 72% water-filled pore space (WFPS). Notice different scales on y-axis

 


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  		Fig. 2 Net denitrification (mean + SE) in soil with or without incorporated pig (PS) and cattle slurry (CS) incubated at 20°C, and (A) 57% and (B) 72% water-filled pore space (WFPS). Notice different scales on y-axis

 
Denitrification and N2O and CO2 emissions from soil started <1 d after slurry addition and peaked between Day 1 and Day 15 at 10 or 15°C (not shown) and between Day 1 and Day 5 at 20°C (Fig. 1 and 2). At 57% WFPS, PS-amended soils were able to sustain N emissions until Day 40 (P < 0.05), whereas N2O emission and denitrification from CS-amended soil fell to background levels after Day 20 (P < 0.05, AFig. 1A–2A). Soil incubated at 72% WFPS had higher N emissions than soil incubated at 57% WFPS, and CS-amended soils were able to sustain N emissions above the background level for a significantly (P < 0.05) longer period than the PS-amended soils (Fig. 1B and 2B). Furthermore, at 72% WFPS the denitrification tended to increase again toward the end of the incubation period in both the PS- and CS-amended soils (Fig. 2B). The PS- and CS-amended soils differed during the early stages of the incubation period. Both the N2O production and the denitrification seemed to peak earlier (between Day 1 and 5) in the CS treated soil than in the PS treated soil (between Day 5 and 15).

The concentration of nitrate was initially low, ranging from 5 to 20 mg N kg-1 in all treatments (Fig. 4) , and increased only slightly in soil amended with cattle slurry. On Day 55 the small amount of exchangeable NH+4 (<15 mg N kg-1) that could be measured in PS-amended soil at a WFPS < 72% presumably reflected the higher nitrification activity and lower denitrification activity in this soil.



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  		Fig. 4 Ammonium and nitrate (mean + SE) in soil with or without incorporated pig (PS) and cattle slurry (CS) incubated at (A) 43%, (B) 57% or (C) 72% water-filled pore space (WFPS), and at 20°C

 
The average daily N2O production at 72% WFPS in amended soils ranged from 3 to 9 µg N kg-1 h-1 (data not shown). This was much greater than the average rates from amended soil at 43% WFPS (0.06 µg N2O–N kg-1 h-1) and at 57% WFPS (0.03 to 0.20 µg N2O–N kg-1 h-1). Average denitrification rates from amended soils at 72% WFPS ranged from 8 to 21 µg N kg-1 h-1. Total loss of N2O (without acetylene inhibition) during the 55-d incubation at 72% WFPS from amended soils ranged from 4 to 13 mg N kg-1, while denitrification losses under the same condition ranged from 13 to 33 mg N kg-1 (Table 3) .


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  		Table 3 Cumulative losses of CO2–C, N2O–N, (N2O + N2)–N, the amount of C required by denitrifiers, the fraction of total CO2 used, and the fraction of slurry-NH+4 evolved as N gas. Soils were incubated during 55 d with cattle (CS) or pig slurry (PS) at different water-filled porosities (WFPS)

 
Soil respiration in amended soils ranged from 4 to 9 mg C kg-1 h-1 during the initial 20 d with the highest rates seen in CS-amended soil (Fig. 3) . After Day 20 respiration rates fell below 2 mg C kg-1 h-1. The cumulative amount of CO2 evolved at 20°C during the incubation period (55 d) was >329 mg C kg-1, and no differences were seen among amendments or WFPS (Table 3).



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  		Fig. 3 Rates of CO2 production (mean + SE) from soil with or without incorporated pig (PS) and cattle slurry (CS) incubated at 20°C, and at (A) 43%, (B) 57% or (C) 72% water-filled pore space (WFPS). Notice different scales on y-axis

 
Calculation of the amount of available C used by denitrification, using the stoichiometric relationship developed by Burford and Bremner (1975), showed that an amount corresponding to 11 to 29 mg CH2O–C kg-1 was used at 72% WFPS during the 55-d incubation (Table 3). This amount of C accounted for 8 to 16% of total soil respiration at 72% WFPS and was much higher than the amount of C accounted for by denitrification at 43 to 57% WFPS (0.03–0.4%).

The N2O losses at 72% WFPS represented 8 to 22% of the applied NH+4–N, and denitrification losses represented 17 to 58% of applied NH+4–N (Table 3). Cumulative loss of N2O and denitrification from soils incubated at 43% and 57% WFPS were much lower and ranged from 0.01 to 1.2% of applied NH+4–N (Table 3).

Results concerning the effects of temperature on gas-producing processes are presented in Table 4 as Q10-values; i.e., the numerical factor by which a rate process is changed with a 10°-change in temperature. Q10-values for the N2O production and the denitrification processes were not estimated for the 43% and 57% WFPS because of the very low level of activities at these soil water contents, which implies large inaccuracies in the calculations. Neither amendments nor soil water contents seemed to affect the Q10-values for the CO2 production, resulting in values between 1.6 and 2.6. The effect of temperature on N2O production and denitrification resulted in higher Q10-values, ranging between 3.3 and 5.4.


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  		Table 4 Estimated Q10 values for the microbial respiration (CO2 production), N2O production, and denitrification at different amendments and soil moisture contents in percentage of water-filled pore space (WFPS)

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Differences Between Cattle and Pig Slurry
The changes in concentrations of NO-3 and NH+4 were very different in cattle and pig slurry-amended soil. Bernal and Roig (1993) noticed a substantial immobilization of inorganic N during the first 3 d of incubation of soil with added pig slurry. They noticed that the concentration increased until Day 7 and fell steadily thereafter. Kirchmann and Lundvall (1993) reported a very high correlation between the initial content of volatile fatty acids (VFA) and immobilization under nondenitrifying conditions. In spite of the lower content of VFA in their cattle slurry, they found that only pig slurry showed a net release of N after 70 d at 25°C, whereas cattle slurry caused a net immobilization of N despite extensive N mineralization and CO2 evolution. This has also been observed by Diaz-Fierros et al. (1988), who noted that >40 d of incubation at 28°C was required to induce net mineralization from cattle slurry-amended soil. This immobilization might explain the low amount of nitrate in cattle slurry-amended soil obtained in our study.

From samples incubated at 57% WFPS, PS-amended soils lost more N than CS-amended soil, especially in the initial period, despite more CO2 emission from the latter during Days 0 to 5. In a study of denitrification in soil supplemented with nine different manures, Paul and Beauchamp (1989) reported that denitrification was twice as high during the Day 1 to Day 2 period in pig slurry-amended soil than in cattle slurry-amended soil, but denitrification in the CS-amended soil was still significant during Days 4 to 7, as opposed to the PS-amended soil. In a soil incubated at a water content of 10 kPa and amended with 13 Mg ha-1 pig or cattle slurry, Petersen and Andersen (1996) found higher rates of denitrification on Days 3 and 6 in cattle slurry treatment than in pig slurry treatment. They argued that cattle slurry had a higher potential for denitrification than pig slurry because it has a higher content of organic matter than pig slurry. The observations made by Petersen and Andersen (1996) are in agreement with the results of the present study, which show that N2O production and denitrification from cattle slurry-amended soil peaked earlier in the incubation period (between Day 1 and 5) than did those processes in the pig slurry-amended soil, where N-emissions peaked between Day 5 and 15. While cattle slurry normally has a higher content of organic matter (Cooper and Cornforth, 1978), results suggest that it contains more recalcitrant substances than pig slurry because of more aggressive decomposition by ruminants (Kirchman and Lundwall, 1993).

Measurements of the amount of VFA in slurries have shown that pig slurry contains three to four times more VFA and two to three times as much water soluble C (WSC) as dairy cattle slurry (Cooper and Cornforth, 1978; Paul and Beauchamp, 1989; Kirchman and Lundwall, 1993). Strong correlations between the N2O and CO2 production rates during the 24- to 48-h period and between the amount of VFA and WSC have been found (Paul and Beauchamp, 1989). They concluded that WSC in cattle and pig slurry was the primary C source for the denitrifying bacteria during the first days after application. In our experiment the cumulative N loss from pig slurry-amended soils was higher than from cattle slurry-amended soils, despite the observation of high, but short-lived, peaks of N2O evolution from cattle slurry-amended soil. This may suggest that the cattle slurry we used contained a small amount of a very degradable organic substance. This suggestion was substantiated by the large initial flush of CO2. However, as for the N emission results, Paul and Beauchamp (1989) found that pig slurry-amended soil gave a higher initial CO2 flush than the cattle slurry-amended soil, but they found no difference in the respiration rates later on during the incubation period.

Role of Carbon or Nitrate Availability
As NO-3 is formed in the aerobic part of the soil, N availability to denitrifiers is governed by the length of the diffusion path from the aerobic to the anaerobic site and the resulting concentration gradient. Calculation of the Thiele modulus {phi} (Myrold and Tiedje, 1985) can indicate whether denitrification is nitrate-diffusion limited:

where ran is the aggregate radius, F is a C limitation factor ranging from 0 to 1, Vmax and Km are the true physiological values, and D is the intra-aggregate diffusion coefficient (Myrold and Tiedje, 1985). For values of {phi} < 1, reaction rates are controlled by biological kinetics, while diffusion becomes the rate limiting factor as {phi} becomes >1.

Using a Vmax of 2.1 mg N kg-1 h-1 (unpublished data, 1997), the value of 5 µg N kg-1 for Km (Christensen and Tiedje, 1988), ran = 0.1 cm, a C limitation factor F = 1, and a diffusion coefficient for NO-3 of 5 x 10-6 cm2 s-1 (Myrold and Tiedje, 1985), a value of {phi} = 5 can be calculated for soil amended with cattle and pig slurry at 72% WFPS. The actual denitrification rate is a function of both {phi} and the bulk concentration of NO-3. At {phi} = 5, no diffusion limitations are expected for NO-3 concentrations > 100 x Km (Myrold and Tiedje, 1985). In the present experiment, the bulk concentration of NO-3 was greater than 200 x Km (1 mg N kg-1) from Day 1 onwards, thus, denitrification was not dependent upon the bulk concentration of soil NO-3 and was likely limited by O and/or C availability.

Role of Soil Moisture Content
The general time course patterns of denitrification and CO2 production, in which the rates peaked within a few days after amendment, were very similar to the patterns observed by Nugroho and Kuwatsuka (1992). The N2O emission rates at 57% WFPS (100% FC) corresponded to those measured by Paul et al. (1993) in a silt loam at 85% FC. They measured rates of up to 0.5 and 1.5 µg N kg-1 soil h-1 from soil amended with 50 g kg-1 dairy cattle slurry. Furthermore, they measured rates of denitrification of up to 200 µg N kg-1 soil h-1 from waterlogged soil amended with 100 g kg-1 dairy cattle slurry, and they found rates that were comparable to the rates of up to 150 µg N kg-1 soil h-1 from soil amended with 20 g kg-1 pig or cow slurry incubated at 72% WFPS (1.25 x FC) in the present experiment.

In our study, total losses of N2O from soils over the course of 55 d ranged from 2 to 13 mg N kg-1 from soils at 72% WFPS, but only from 0.01 to 0.1 mg N kg-1 from soils at 43 to 57% WFPS. At 72% WFPS this loss of N2O accounted for 7 to 22% of the slurry's total ammoniacal N (TAN), whereas it amounted to 0.01 to 0.9% of applied TAN at 43 to 57% WFPS. An evaluation of data from several field experiments concluded that 0.01 to 2% of applied N in organic fertilizers was lost as N2O (Bouwman, 1990). Granli and Bøckman (1993) have summarized data for N2O emission from soils and found that median N loss varied from 0.1 to 0.17% of NH+4–N fertilizers.

Estimates of total denitrification losses in the present experiment showed that approximately 17 to 58% of NH+4–N was lost from slurry-amended soils at 72% WFPS, but that only 0.01 to 1.2% was lost at 43 to 57% WFPS. This compares with results of Comfort et al. (1990), who determined that 2.5 to 3.2% of TAN was lost as (N2O + N2)–N from a silt loam incubated at 0.23 kg H2O kg-1 soil and 12°C with liquid dairy cattle manure. Similar losses have been measured in field experiments. Maag (1989) reported that yearly denitrification in a sandy loam soil cropped with spring barley did not exceed 4% of TAN in pig slurry that was surface broadcasted and incorporated. Van der Abbeel (1989) also reported a similar result. On the other hand, denitrification loss as high as 42 to 54% of TAN from autumn or winter injected slurry have been reported; e.g., by Thompson (1989).

The small amount of CO2–C derived from denitrification in relation to the total amount of respiration (0.05–0.6% of total C respiration) in soil incubated at or below 57% WFPS was also seen in a field experiment (Qian et al., 1997). They found that denitrification accounted for only 0.7% of the total soil-CO2 flux from irrigated corn during a dry year, but accounted for 1 to 3.7% during a wetter year. These relatively low values suggest that concentrations of available C were more than sufficient to sustain microbial denitrification in parts of the soil volume, and that oxygen was limiting denitrification. Higher rates of C decomposition would have created larger spatial or temporal zones of anaerobiosis (Rice et al., 1988). In the present experiment, the highly significant relationship between log(denitrification) and log(CO2) suggested that aerobic respiration was the main factor responsible for the creation of anaerobic microsites (Qian et al., 1997).

Role of Soil Temperature
Rate processes are exponentially affected by temperature, and the standard Arrhenius equation can be used within moderate temperature limits (15–35°C) to describe the effect of temperature on the rates. A phenomenon that applies for all biochemical systems is that at lower temperatures (generally below 12–15°C) the rate processes are much more drastically affected by temperature; i.e., Q10-values decrease with increasing temperatures (Ingraham, 1962). In earlier studies (Vinther, 1992; Maag and Vinther, 1996) we have found that the average Q10-value for the denitrification process was 9.3 when estimated in the range of 5 to 15°C and 4.8 for the range of 15 to 25°C. This is in accordance with the findings in the present study, where the Q10-values ranged between 3.3 and 5.4 for N2O production and the denitrification process. Also, Smid and Beauchamp (1976) found that the Q10 values decreased (from 8.3 to 2.2) as the temperature range increased (from 5–15°C to 15–25°C). The Q10-values for the microbial respiration (CO2 production) ranged between 1.6 and 2.6, which is in agreement with the general assumption that Q10 = 2 for biochemical processes (Ingraham, 1962). Increasing temperatures cause an increase in microbial activity, which implies an increase in oxygen consumption and the development of anaerobic sites in the soil. This is probably the reason for the higher Q10-values found for the N2O production and the denitrification process than for microbial respiration. Thus, the Q10 for N2O production and the denitrification process do not express the temperature effect only, but rather a combined effect of temperature and the development of anaerobic microsites in the soil.SAS Institute 1994; Van den Abbeel Claes Vlassak 1989; Yeomans Beauchamp 1978

Received for publication March 31, 1998.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
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 Results
 Discussion
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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome