- Email: [email protected]
Potential Denitrification in Relation to Irrigation
Zentralbl. Mikrobiol. 143 (1988), 447-451 VEB Gustav Fischer Verlag Jena
[Research Institute for Soil Science and Agricultural Chemistry. Hungarian Academy of Sciences. Budapest. Hungary]
Potential Denitrification in Relation to Irrigation!) TIBOR SZILI KOVAcs, J6ZSEF SZEGI and GtZA KovAcs With 2 Figures
Summary The potential rate of denitrification and number of nitrifying bacteria were studied under laboratory conditions for samples collected from irrigated plots. Both number of nitrifying bacteria and potential denitrification decreased with the soil depth. Irrigation had an appreciable effect on potential denitrification. it did not succeded in finding any correlation between its values and investigated soil parameters.
Zusammenfassung Die potentielle Denitrifikationsrate und die Anzahl nitrifizierender Bakterien wurde unter Laborbedingungen in bewasserten Parzellen untersucht. Sowohl die nitrifizierenden Bakterien als auch die potentielle Denitrifikation nahmen mit zunehmender Bodentiefe abo Die Bewasserung hatte einen beachtlichen Effekt auf die potentielle Denitrifikation; es gelang aber nicht. eine KorreJation zwischen jenen Werten und den untersuchten Bodenparametem zu finden.
Increasing soil moisture acts as a factor decreasing the rate of oxygen diffusion through the soil matrix. Oxygen concentration is the primary ecological factor limiting denitrification. We have tried to approach the ecological aspect of this problem and to establish the trend of this process in laboratory experiment. We studied physiological stress caused by water on winter wheat (it is not discussed here) and whether it affects potential denitrification or not. Potential denitrification: maximum rate of denitrification under oxygen free conditions with original nitrate, moisture and organic matter content of the soil.
Materials and Methods The study site was located at the experimental station of our institute. Soil is a loamy calcareous chemozem formed on loess, sand: clay == 16:9. Characteristics of the soil: pH (KCn == 7.1, organic-C(%) == 1.8, CaC0 3 (%) == 5, N (%) == 0.23, NH4 + - N (ppm) == 18, N0 3 - - N (ppm) == IS. The following treatments were layout: I. control. 2. 50 mm irrigation. 3. 5 X 10 mm irrigation (I0mm a day during 5 d), 4. 150 mm irrigation. 5. 150mm irrigation + 50kglha N, as NH 4 N0 3 . Irrigation was carried out with a special sprinkling apparatus made of PVC tubes. PVC tubes were perforated ten by 10 cm. Watering lasted for 2 days 23th and 24th May, 1986. Fertilization consisted of90kg N, 100 kg P and IOOkg K per hectare in the autumn of 1985 and 90 kg/ha N in the spring of 1986. Experimental plant was winter wheat. Soil samples were collected weekly after irrigation. Sampling depths were 0-5cm, 5-lOcm. 1O-25cm. 25-50cm and 50-75cm. From every plot samples were taken at 20 points and mixed. Potential denitrification and number of nitrifying bacteria were established on the day following the sampling day. Potential denitrification investigations i) 30*
Presented at the Reinhardsbrunn Symposium, May 1987.
448
T. S.
KOVACS,
1. SZEGI and G.
KOVACS
were carried out in flasks which contained to gram soil of a sample. Incubation was in nitrogen atmosphere containing 10% acetylene. Incubation temperature was 28°C, incubation time 24 h. Produced nitrous o)(ide was measured using a CHROM-5 gas chromatograph, equipped with hot wire detector. Injections were carried out with a Hamilton microliter gas-tight syringe. Scotty gas standard was used to prepare ~tandard curves. The amount of nitrous oxide in water was calculated using the Bunsen absorption coefficient. Most probable number method was used enumerating ammonia o)(idizer and nitrite oxidizer populations. Subsamples were removed for soil chemical analysis. Soil water content, organic carbon, nitrate, ammonium and pH were determined.
Results Table I shows the distribution of potential denitrification according to soil depth. The highest amount of evolved nitrous oxide can be found in the upper 2 layers, while in case of the fifth sampling depth (50-75cm) potential denitrification is below the detection limit. The number of denitrifiers was estimated in one case before the irrigation. There are no significant differences among the numbers of denitrifiers the upper 3 layers, but they were less frequent in the 25 - 50 em layer. The number of nitrifiers decreased with the depth, and number of ammonia oxidizers was about ten times higher than the number of nitrite oxidizers in all cases. Distribution of ammonium-N is not affected by depth. Organic carbon content of the soil samples was similar in the upper three layers, but higher than one in 2S-50cm layer. The amount of nitrate-N is higher in the upper layers than in those below them (Table 1). Table I. Microbiological data obtained per g dry soil and soil physical properties for depth and treatments
Depth (em) 0- 5 5-10 10-25
25-50 n = 20 Treatments Control 50 mm water 5 x IO mm water 150 mm water 150 mm Water +50 kg/ha N n = 16
fAg N2O-N Log MPN Log MPN Log MPN Moisture produced ofNH 4 + - of N02 +- of denitri- (%) oxidizers oxidizers fiers
Organic-C NO}- -N (mgl (fAg)
NH4 +-N (fAg)
1.67 1.77 0.96 0.10
4.74 n.d. 4.00 3.77
3.60 n.d. 3.02 2.65
11\.8 18.8 1S.6 19.3
18.3 18.1 lS.3 15.8
15.9 15.0 9.9 7.8
19.3 18.7 14.9 17.6
0.43 0.95 1.17 1.09 2.00
4.18 3.99 n.d. 4.09 4.42
3.21 3.22 n.d. 2.93 3.15
15.1 20.7 17.4 21.5 21.2
17.3 17.0 IS.3 17.8 17.7
10.2
18.1 18.9 16.4 16.6 18.0
5.1 5.5 5.2 4.5
14.0 Il.I
11.9 15.6
Irrigation had a high effect on potential denitrification. The least values ,were obtained the control samples. There were no significant differences among treatments of 50 mm, 5 X 10 mm and 150mm irrigation. The highest values could be observed for samples treated with 150mm water plus 50 kglha N. The treatments did not affect the number of nitrifiers, organic carbon, pH and ammonium-N. In Table 2 the first column shows the values of potential denitrification 2 weeks before irrigation. We got the highest potential denitrification values on the second sampling date (2 weeks) after irrigation (Table 2). There was hardly any rainfall before first sampling. Between first and second sampling dates the amount of rainfall was 28 mm. Between the second and third sampling the amount of rainfall was 17 mm. Between the third and fourth sampling dates rainfall amount was
Plltential Denitrification
449
Table 2. Potential denitrification data tilr depth and treatment (~lg N,O-N g . r dry soilld r) Sampling dates 05. 08.
05.28.
06.03.
06. 10.
06. 17.
0.22 1.48 1.45 0.05 LSDo 05
1.07 0.82 O.SS 0.10 LSDll
2.7h 3.08 1.39 0.21 LSDu05
= 0.24
1.30 1.62 0.82 0.00 LSDl105 = 0.13
\.56 \.57 0.75 0.11 LSDo05 = 0.13
0.74 1.17 272 0.8/1 3.80 LSDo05 = 0.27
0.52 105 0.88 0.73 1.48 LSDo05 = 0.14
0.43 0.70 \.08 I. 12 1.64 LSDo.05 = 0.15
Depth (em) 0- 5 5-10 10-25 25-50
= O. \0
= 0.12
Treatments Control 50 mm water 5 x 10 mm water 150 mm water 150 mm water +SO kg/ha N
-
0.01 0.86 0.00 1.63 I.(l9 LSD o 05 = 0.13
37 mm. In case of the third and fourth sampling dates potential denitrification values were very similar to each other and stated between the values of first and second sampling date·s. The number of ammonia oxidizers and nitrite oxidizers is fairly correlated. Regression coefficients were about 0.65, but at the second sampling date this value increased to 0.81. It is interesting that the highest change both among potential denitrification values and the rates of nitrifying bacteria was at the second sampling date. There was no linear correlation between potential denitrification and the investigated soil parameters.
Discussion Soil moisture, nitrate concentration and organic matter are known to be the main ecological factors affecting denitrification. The role of soil moisture is to determine oxygen diffusion. We could not expect any response to soil moisture in relation when controlling oxygen diffusion because of oxygen free incubation (Fig. I). The maximum peaks of potential denitrification can be found between 17- 23 % moisture content. At the least moisture level, the soil is too dry for effective microbiological activity. In addition aerobic microsites may occur within soil matrix in spite of anaerobic incubation. Increased moisture can reduce the range of these sites encouraging denitrification process. At the higher moisture level, N-flux decreases perhaps due to less nitrate concentration or more dissolved N20 in the water. The maximum N-fluxes were observed for suctions between 50 to 100 mbars in studies of RYDEN and LUND (1980). A similar scatter of denitrification values can be seen in Fig. 1 as on their paper. Peak fluxes appeared 24 to 48 h after irrigation (RYDEN and LUND 1980). However, SEXTONE et al. (1985) observed quicker response to water, but they applied different methods. It indicates that establishing negligible concentration of oxygen takes much time. As denitrification is a respiratory process it requires organic or inorganic H-donor substrates. Adding organic matter to the soil, the rate of denitrification markedly increases (BOWMAN and FOCHT 1974, CHRISTENSEN 1983). There were no significant differences in organic-C content at various treatment in our experiment which could have intluenced the potential denitrification, but we determined only the total organic-C content of the soil. We have to emphasize that the different organic-C determining methods reflect variously the availability of soil organic matter (STANFORD et al. 1975). There is hardly any correlation between soil nitrate-N and potential denitrification in wide range of nitrate concentration (Fig. 2). The rate of denitrification was not increased by soil nitrate-N
450
T.
S. KOVACS,
1.
SZEGI
and G.
KOVACS
POTENTIAL DENITRIFICATION
(pg N20-N g-I soil day-I) 7
0
•
S
•
0
r
A
0
J
-
•
0
Dr!
A
o
0
+t:. .nt:.. t:. t:..j..j..j. t.j.
I
8
6
t:.
I
I
I
/0
/2
.0
DO
o. 0
0
A
t:.t:.t:.
+
I
A
;.
O-A
0
.j.
•
: A-t o·
••
•
1
•
0
+
11t /8 16 MO/STURE (%)
+
•
t:. t:.
t:.+ 0 t:.tR+ ++ + I
I
22
20
+ I
+
26
21,
Fig. I. Relationship between potential denitrification and soil moisture at 4 sampling depth.
POTENTIAL DENITRIFle ATlON (P9 N20-N g-1soil day-I)
·0-5 05-10
A10-25
+25-S0cm
7
0
I-
•
5 0
e A
I-
0
3
0
e A
-
"0
0 A 1 A
• 0
e
0
.O".A
A
•
,.
I
5
70
0
0
15
0
• e
-.
~I:-A-DA .j.D a .t.,..o.. ok'+-
a
III
+
0
I
20
25
30
SOIL NITRATE
•
eA
35
'to
+
t,s
50
fpg Ng-I}
Fig. 2. Relationship between potential denitrification and soil nitrate at 4 sampling depth.
55
Potential Denitrification
451
>20[tgig soil (STEFANSON 1972). RYDEN and LUND (1980) did not get differences in denitrification flux for nitrate-N levels of 2 to 33 [tg/g soil. The major role of soil nitrate concentration is to affect the distribution of denitrification products between dinitrogen and nitrous oxide (BLACKMER and BREMNER 1978). BURTON and BEACHAMP (1985) investigated 12 soil parameters as to be mentioned affecting denitrification but they could not detect any correlations between organic-C, water content and nitrate-N, but they found it in air-filled pore space. The distribution of potential denitrification for soil depth is markedly expressed (Fig. 1 and 2). It is perhaps caused by 'vertical distribution of soil humus and/or readily available organic matter. Similar results can be found in the paper of HUSSEY et al. (1985).
References BLACKMER, A. M., BREMNER, J. M. : Inhibitory effect of nitrate on reduction of N20 to N2 by soil microorganisms. Soil BioI. Biochem. 10 (1978), 187-191. BOWMAN, R. A., FOCHT, D. D.: The influence of glucose and nitrate concentrations upon denitrification rates in sandy soils. Soil BioI. Biochem. 6 (1974) 297- 30!. BURTON, D. L., BEAUCHAMP, E. G.: Denitrification rate relationships with soil parameters in the field. Commun. in Soil Sci. Plant Anal. 16 (1985), 539-549. CHRISTENSEN, S. : Nitrous oxide emission from a soil under permanent grass: Seasonal and diurnal fluctuations as influenced by manuring and fertilization. Soil BioI. Biochem. 15 (1983),531-536. HUSSEY, M. R., SKINNER, Q. D., ADAMS, J. C., HARVEY, A. J. : Denitrification and bacterial numbers in riparian soils of a Wyoming mountain watershed. J. Range Man. 38 (1985), 492-496. RYDEN, 1. C., LUND, L. J.: Nature and extent of directly measured denitrification losses from some irrigated vegetable crop production units. Soil Sci. Soc. Am. 1. 44 (1980),505-511. SEXSTONE, A. J., PARKIN, T. B., TIEDJE, 1. M.: Temporal response of soil denitrification rates to rainfall and irrigation. Soil Sci. Soc. Am. 1. 49 (1985), 99-103. STANFORD, G., VANDER POL, R. A., DZIENIA, S.: Denitrification rates in relation to total and extractable soil carbon. Soil Sci. Soc. Amer. Proc. 39 (1975),284-289. STEFANSON, R. C. : Soil denitrification in sealed soil-plant systems. I. Effects of plants, soil water content and soil organic matter content. Plant and Soil 33 (1972),113-127. Author's address: T. SZlU KOVACS, Research Institute for Soil Science and Agricultural Chemistry, Herman O. u. 15. Budapest, H - 1022.
[Research Institute for Soil Science and Agricultural Chemistry. Hungarian Academy of Sciences. Budapest. Hungary]
Potential Denitrification in Relation to Irrigation!) TIBOR SZILI KOVAcs, J6ZSEF SZEGI and GtZA KovAcs With 2 Figures
Summary The potential rate of denitrification and number of nitrifying bacteria were studied under laboratory conditions for samples collected from irrigated plots. Both number of nitrifying bacteria and potential denitrification decreased with the soil depth. Irrigation had an appreciable effect on potential denitrification. it did not succeded in finding any correlation between its values and investigated soil parameters.
Zusammenfassung Die potentielle Denitrifikationsrate und die Anzahl nitrifizierender Bakterien wurde unter Laborbedingungen in bewasserten Parzellen untersucht. Sowohl die nitrifizierenden Bakterien als auch die potentielle Denitrifikation nahmen mit zunehmender Bodentiefe abo Die Bewasserung hatte einen beachtlichen Effekt auf die potentielle Denitrifikation; es gelang aber nicht. eine KorreJation zwischen jenen Werten und den untersuchten Bodenparametem zu finden.
Increasing soil moisture acts as a factor decreasing the rate of oxygen diffusion through the soil matrix. Oxygen concentration is the primary ecological factor limiting denitrification. We have tried to approach the ecological aspect of this problem and to establish the trend of this process in laboratory experiment. We studied physiological stress caused by water on winter wheat (it is not discussed here) and whether it affects potential denitrification or not. Potential denitrification: maximum rate of denitrification under oxygen free conditions with original nitrate, moisture and organic matter content of the soil.
Materials and Methods The study site was located at the experimental station of our institute. Soil is a loamy calcareous chemozem formed on loess, sand: clay == 16:9. Characteristics of the soil: pH (KCn == 7.1, organic-C(%) == 1.8, CaC0 3 (%) == 5, N (%) == 0.23, NH4 + - N (ppm) == 18, N0 3 - - N (ppm) == IS. The following treatments were layout: I. control. 2. 50 mm irrigation. 3. 5 X 10 mm irrigation (I0mm a day during 5 d), 4. 150 mm irrigation. 5. 150mm irrigation + 50kglha N, as NH 4 N0 3 . Irrigation was carried out with a special sprinkling apparatus made of PVC tubes. PVC tubes were perforated ten by 10 cm. Watering lasted for 2 days 23th and 24th May, 1986. Fertilization consisted of90kg N, 100 kg P and IOOkg K per hectare in the autumn of 1985 and 90 kg/ha N in the spring of 1986. Experimental plant was winter wheat. Soil samples were collected weekly after irrigation. Sampling depths were 0-5cm, 5-lOcm. 1O-25cm. 25-50cm and 50-75cm. From every plot samples were taken at 20 points and mixed. Potential denitrification and number of nitrifying bacteria were established on the day following the sampling day. Potential denitrification investigations i) 30*
Presented at the Reinhardsbrunn Symposium, May 1987.
448
T. S.
KOVACS,
1. SZEGI and G.
KOVACS
were carried out in flasks which contained to gram soil of a sample. Incubation was in nitrogen atmosphere containing 10% acetylene. Incubation temperature was 28°C, incubation time 24 h. Produced nitrous o)(ide was measured using a CHROM-5 gas chromatograph, equipped with hot wire detector. Injections were carried out with a Hamilton microliter gas-tight syringe. Scotty gas standard was used to prepare ~tandard curves. The amount of nitrous oxide in water was calculated using the Bunsen absorption coefficient. Most probable number method was used enumerating ammonia o)(idizer and nitrite oxidizer populations. Subsamples were removed for soil chemical analysis. Soil water content, organic carbon, nitrate, ammonium and pH were determined.
Results Table I shows the distribution of potential denitrification according to soil depth. The highest amount of evolved nitrous oxide can be found in the upper 2 layers, while in case of the fifth sampling depth (50-75cm) potential denitrification is below the detection limit. The number of denitrifiers was estimated in one case before the irrigation. There are no significant differences among the numbers of denitrifiers the upper 3 layers, but they were less frequent in the 25 - 50 em layer. The number of nitrifiers decreased with the depth, and number of ammonia oxidizers was about ten times higher than the number of nitrite oxidizers in all cases. Distribution of ammonium-N is not affected by depth. Organic carbon content of the soil samples was similar in the upper three layers, but higher than one in 2S-50cm layer. The amount of nitrate-N is higher in the upper layers than in those below them (Table 1). Table I. Microbiological data obtained per g dry soil and soil physical properties for depth and treatments
Depth (em) 0- 5 5-10 10-25
25-50 n = 20 Treatments Control 50 mm water 5 x IO mm water 150 mm water 150 mm Water +50 kg/ha N n = 16
fAg N2O-N Log MPN Log MPN Log MPN Moisture produced ofNH 4 + - of N02 +- of denitri- (%) oxidizers oxidizers fiers
Organic-C NO}- -N (mgl (fAg)
NH4 +-N (fAg)
1.67 1.77 0.96 0.10
4.74 n.d. 4.00 3.77
3.60 n.d. 3.02 2.65
11\.8 18.8 1S.6 19.3
18.3 18.1 lS.3 15.8
15.9 15.0 9.9 7.8
19.3 18.7 14.9 17.6
0.43 0.95 1.17 1.09 2.00
4.18 3.99 n.d. 4.09 4.42
3.21 3.22 n.d. 2.93 3.15
15.1 20.7 17.4 21.5 21.2
17.3 17.0 IS.3 17.8 17.7
10.2
18.1 18.9 16.4 16.6 18.0
5.1 5.5 5.2 4.5
14.0 Il.I
11.9 15.6
Irrigation had a high effect on potential denitrification. The least values ,were obtained the control samples. There were no significant differences among treatments of 50 mm, 5 X 10 mm and 150mm irrigation. The highest values could be observed for samples treated with 150mm water plus 50 kglha N. The treatments did not affect the number of nitrifiers, organic carbon, pH and ammonium-N. In Table 2 the first column shows the values of potential denitrification 2 weeks before irrigation. We got the highest potential denitrification values on the second sampling date (2 weeks) after irrigation (Table 2). There was hardly any rainfall before first sampling. Between first and second sampling dates the amount of rainfall was 28 mm. Between the second and third sampling the amount of rainfall was 17 mm. Between the third and fourth sampling dates rainfall amount was
Plltential Denitrification
449
Table 2. Potential denitrification data tilr depth and treatment (~lg N,O-N g . r dry soilld r) Sampling dates 05. 08.
05.28.
06.03.
06. 10.
06. 17.
0.22 1.48 1.45 0.05 LSDo 05
1.07 0.82 O.SS 0.10 LSDll
2.7h 3.08 1.39 0.21 LSDu05
= 0.24
1.30 1.62 0.82 0.00 LSDl105 = 0.13
\.56 \.57 0.75 0.11 LSDo05 = 0.13
0.74 1.17 272 0.8/1 3.80 LSDo05 = 0.27
0.52 105 0.88 0.73 1.48 LSDo05 = 0.14
0.43 0.70 \.08 I. 12 1.64 LSDo.05 = 0.15
Depth (em) 0- 5 5-10 10-25 25-50
= O. \0
= 0.12
Treatments Control 50 mm water 5 x 10 mm water 150 mm water 150 mm water +SO kg/ha N
-
0.01 0.86 0.00 1.63 I.(l9 LSD o 05 = 0.13
37 mm. In case of the third and fourth sampling dates potential denitrification values were very similar to each other and stated between the values of first and second sampling date·s. The number of ammonia oxidizers and nitrite oxidizers is fairly correlated. Regression coefficients were about 0.65, but at the second sampling date this value increased to 0.81. It is interesting that the highest change both among potential denitrification values and the rates of nitrifying bacteria was at the second sampling date. There was no linear correlation between potential denitrification and the investigated soil parameters.
Discussion Soil moisture, nitrate concentration and organic matter are known to be the main ecological factors affecting denitrification. The role of soil moisture is to determine oxygen diffusion. We could not expect any response to soil moisture in relation when controlling oxygen diffusion because of oxygen free incubation (Fig. I). The maximum peaks of potential denitrification can be found between 17- 23 % moisture content. At the least moisture level, the soil is too dry for effective microbiological activity. In addition aerobic microsites may occur within soil matrix in spite of anaerobic incubation. Increased moisture can reduce the range of these sites encouraging denitrification process. At the higher moisture level, N-flux decreases perhaps due to less nitrate concentration or more dissolved N20 in the water. The maximum N-fluxes were observed for suctions between 50 to 100 mbars in studies of RYDEN and LUND (1980). A similar scatter of denitrification values can be seen in Fig. 1 as on their paper. Peak fluxes appeared 24 to 48 h after irrigation (RYDEN and LUND 1980). However, SEXTONE et al. (1985) observed quicker response to water, but they applied different methods. It indicates that establishing negligible concentration of oxygen takes much time. As denitrification is a respiratory process it requires organic or inorganic H-donor substrates. Adding organic matter to the soil, the rate of denitrification markedly increases (BOWMAN and FOCHT 1974, CHRISTENSEN 1983). There were no significant differences in organic-C content at various treatment in our experiment which could have intluenced the potential denitrification, but we determined only the total organic-C content of the soil. We have to emphasize that the different organic-C determining methods reflect variously the availability of soil organic matter (STANFORD et al. 1975). There is hardly any correlation between soil nitrate-N and potential denitrification in wide range of nitrate concentration (Fig. 2). The rate of denitrification was not increased by soil nitrate-N
450
T.
S. KOVACS,
1.
SZEGI
and G.
KOVACS
POTENTIAL DENITRIFICATION
(pg N20-N g-I soil day-I) 7
0
•
S
•
0
r
A
0
J
-
•
0
Dr!
A
o
0
+t:. .nt:.. t:. t:..j..j..j. t.j.
I
8
6
t:.
I
I
I
/0
/2
.0
DO
o. 0
0
A
t:.t:.t:.
+
I
A
;.
O-A
0
.j.
•
: A-t o·
••
•
1
•
0
+
11t /8 16 MO/STURE (%)
+
•
t:. t:.
t:.+ 0 t:.tR+ ++ + I
I
22
20
+ I
+
26
21,
Fig. I. Relationship between potential denitrification and soil moisture at 4 sampling depth.
POTENTIAL DENITRIFle ATlON (P9 N20-N g-1soil day-I)
·0-5 05-10
A10-25
+25-S0cm
7
0
I-
•
5 0
e A
I-
0
3
0
e A
-
"0
0 A 1 A
• 0
e
0
.O".A
A
•
,.
I
5
70
0
0
15
0
• e
-.
~I:-A-DA .j.D a .t.,..o.. ok'+-
a
III
+
0
I
20
25
30
SOIL NITRATE
•
eA
35
'to
+
t,s
50
fpg Ng-I}
Fig. 2. Relationship between potential denitrification and soil nitrate at 4 sampling depth.
55
Potential Denitrification
451
>20[tgig soil (STEFANSON 1972). RYDEN and LUND (1980) did not get differences in denitrification flux for nitrate-N levels of 2 to 33 [tg/g soil. The major role of soil nitrate concentration is to affect the distribution of denitrification products between dinitrogen and nitrous oxide (BLACKMER and BREMNER 1978). BURTON and BEACHAMP (1985) investigated 12 soil parameters as to be mentioned affecting denitrification but they could not detect any correlations between organic-C, water content and nitrate-N, but they found it in air-filled pore space. The distribution of potential denitrification for soil depth is markedly expressed (Fig. 1 and 2). It is perhaps caused by 'vertical distribution of soil humus and/or readily available organic matter. Similar results can be found in the paper of HUSSEY et al. (1985).
References BLACKMER, A. M., BREMNER, J. M. : Inhibitory effect of nitrate on reduction of N20 to N2 by soil microorganisms. Soil BioI. Biochem. 10 (1978), 187-191. BOWMAN, R. A., FOCHT, D. D.: The influence of glucose and nitrate concentrations upon denitrification rates in sandy soils. Soil BioI. Biochem. 6 (1974) 297- 30!. BURTON, D. L., BEAUCHAMP, E. G.: Denitrification rate relationships with soil parameters in the field. Commun. in Soil Sci. Plant Anal. 16 (1985), 539-549. CHRISTENSEN, S. : Nitrous oxide emission from a soil under permanent grass: Seasonal and diurnal fluctuations as influenced by manuring and fertilization. Soil BioI. Biochem. 15 (1983),531-536. HUSSEY, M. R., SKINNER, Q. D., ADAMS, J. C., HARVEY, A. J. : Denitrification and bacterial numbers in riparian soils of a Wyoming mountain watershed. J. Range Man. 38 (1985), 492-496. RYDEN, 1. C., LUND, L. J.: Nature and extent of directly measured denitrification losses from some irrigated vegetable crop production units. Soil Sci. Soc. Am. 1. 44 (1980),505-511. SEXSTONE, A. J., PARKIN, T. B., TIEDJE, 1. M.: Temporal response of soil denitrification rates to rainfall and irrigation. Soil Sci. Soc. Am. 1. 49 (1985), 99-103. STANFORD, G., VANDER POL, R. A., DZIENIA, S.: Denitrification rates in relation to total and extractable soil carbon. Soil Sci. Soc. Amer. Proc. 39 (1975),284-289. STEFANSON, R. C. : Soil denitrification in sealed soil-plant systems. I. Effects of plants, soil water content and soil organic matter content. Plant and Soil 33 (1972),113-127. Author's address: T. SZlU KOVACS, Research Institute for Soil Science and Agricultural Chemistry, Herman O. u. 15. Budapest, H - 1022.