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Denitrification at sub-optimal temperatures in soils from different climatic zones
CoP>n@t
0033-0'17 SS 53.00 -c 0.00 _ 1388 Pergamon Press plc
DENITRIFICATION AT SUB-OPTIMAL TEMPERATURES IN SOILS FROM DIFFERENT CLIMATIC ZONES D.
Soils
S. POWLSOS,
and Plan: Nutrition
P. G.
SAFFIGNA*
and
MOKIQLT
KRAGT-COTTAAR+
Department. Rothamsted Experimental Herts, AL5 2JQ. U.K.
(Accepred
I December
Station.
Harpcnden.
1987)
Summary-The effectof temperature on denitrification was compared in a temperate zone soil. a Paieudalf cropped to wheat in south-east England, and a sub-tropical soil, a Vertisol (Typic Pellustert) cropped to sorghum in Queensland, Australia. The 2 soils were incubated anaerobically in the presence of KNOt and of the added nitrate was measured glucose at 5, IO. 15, 20 or 25’C for either 2 d or 7 d. Immobilization in parallel incubations using K” NO!. After 2 d more nitrate was lost from the English soil than from the Australian at every temperature. After 7 d at IO C, all nitrate had been reduced in the English soil. 92oio denitrified and 8% immobilized, whilst in the Australian soil 91% of the nitrate still remained. Only at 2O.C or above was all of the nitrate reduced (89% denitrified. I I% immobilized) in the Australian soil. The results indicate that the denitrifying populations in each soil was adapted to its own environment. The dcnitrifiers in the temperate soil reduced nitrate at lower temperatures than did those in the sub-tropical soil. The occurrence of considerable denitrification at IO C in the temperate soil. and a sharp increase between 5 and IO-C, shows that dcnitrification could be a major cause of N loss in temperate areas in spring. when much N fertilizer is applied. Even though soil temperatures can fall to similar values in the subtropics in winter, much less denitrification would be expected.
ISTRODl’CTION Recent ticld experiments in the temperate zone indicate that denitrification can occur at fairly low tempcratures in soils that are at. or near, field capacity. “I\; balance cxpcriments in England, in which labelled fertilizer was applied in spring to winter wheat, indicated losses of up to 300/o in some years (Powlson rt ul.. 198-l. 1986). These losses were greater than would be expected from leaching alone and suggested that denitrification was occurring, even though soil temperatures were generally less than IO’C. Ryden (1983) measured the evolution of N,O from a grassland soil in southern England throughout the year and found that N,O evolution from moist soil began when the temperature exceeded about 8-C. In laboratory experiments with soils from a temperate region of the U.S.A.. some dsnitrification occurred at 7-C though much less than at IS’C (Jacobson and Alexandcr, 1980). In sub-tropical soils, however. denitrification seems less likely to occur even at soil temperatures above IO C. For example, in Queensland, Australia. SatIigna ef al. (1984) found no loss of “N-labclled fertilizer several weeks after application to a fallow soil at field capacity and at an ambient temperature of about IS-C, although when nitrogen was applied in a warmer period 35% was lost. Craswell (1978) observed no loss of “N-labelled nitrate from cores of Queensland soils held under waterlogged
conditions
at
IO-C.
‘Present address: School of Australian Environmental Studies, Griffith Unixersit). Brisbane, Queensland 41 I I. Australia. tPresent address: Department of Soil Science and Plant Nutrition. Agricultural University. De Dreyen 3. 6703 BC Wageningen. The Netherlands.
These observations suggest that the denitrifying population in temperate soils is better adapted to function at lower temperatures than that in tropical sdils. This possibility was investigated by comparing denitrification. over a range of temperatures, in a temperate and a sub-tropical soil incubated under identical anaerobic conditions in the presence of nitrate and glucose. Glucose was included so that temperature would be the dominant factor limiting denitrification.
.VATERIALS
AND METHODS
Soils
Details of the soils used are given in Table I. The English soil was a Paleudalf from the Broadbalk Continuous Wheat Experiment at Rothamsted Experiment Station (latitude 51-48’ N, longitude 0’21’ W). It was sampled @-IOcm in November 1983. The Australian soil was a Vertisol (Typic Pellustert) from an experiment on tillage and sorghum residue management near Biloela in central Queensland (latitude 24-21’ S. longitude 150-30’ E). The soil used (&lOcm) was a bulked sample from 4 plots under conventional cultivation without incorporation of residues. It was sampled in October 1983 and sent to England by air. Both soils were sieved < 2 mm and then stored moist, under aerobic conditions, for about 3 weeks at temperatures close to the field temperatures at the time of sampling: 5-C for the English soil and 25-C for the Australian soil. Incubation
procedure
and analyricul
methods
The anaerobic incubation procedure was based on that described by Burford and Bremner (1975). Triplicate portions of moist soil. containing 5 g oven-dry 71’)
D. S. POWLSOSe:
710
iile g -’ o\rn Cr&pin* Slsxl iirmUdl temperature Sfean annuBl temperature
df) soli)
5011 ( C) ;11r ( Cl
7. -2 Wheat -wo ZPI frrtrlrzer
I> Sorghum gtvcn no ferttliter
Y 2’
ND
89
(1000 R for IO min) and the soil residue extracted a further 4 times to remove all traces of inorganic 5. These additional extractions were each for 20 min with 70 ml 0.5 Y K:SO,. .After the tinal K$O, extraction and centrifugation the soil residue was dried at SO-C. Labelled N remaining in the soil was determined by Kjeldahl digestion and steam distillation followed by “N ‘% ratio analysis of the distilate using a VG ?f~icromass 602D mass spectrometer (Pruden er al.. 1955).
20. I
‘Lfsasured daily ;Lt ;t depth oi IOcm un&r
RESULTS
yrass at 09.00 h C.M.T.
soil, were incubated in sealed bottles (30 ml McCartney bottles) with 15 ml of a solution containing 2 mg N as KNO, and IO mg C as glucose (i.e. 400 pg N and 1000 pg C g - ’ oven-dry soil). They were incubated for either 2 or 7 d at 5. IO. IS, 20 or 25’C. After incubation, 5 ml of 4 M KCI were added to each bottle to give a final concentration of approx. I Y KCI. The bottles were shaken for I h and the filtered extracts analyscd for nitrate (including any nitrite) and ammonium by steam distillation with MgO and Dsvada’s alloy (Bremner, 1965). Zero time controls u-crc also extracted: recovery of nitrate was 96% in the English soil and 100% in the Australian soil. An additional replicate, containing ‘%-labclled KNO, (11 atom % excess) was included with each treatment in order to measure immobilization of added nitrate. After incubation. the “N-labellcd soils were cxtructed for I h with I M KCI, centrifuged
Afler
2 days
in!.
There were clear differences in the amounts of nitrate remaining in the :! soils following anaerobic incubation at different temperatures in the presence of glucose (Fig. I). After 2 d significantly more nitrate remained in the Australian than in the English soil at all temperatures above 5 C. After 7 d even more striking differences had emerged. In the English soil all nitrate had been lost after incubation at IO’C or above but, in the Australian soil, much nitrate remained after incubation at IO or IS’C. Only at 2O’C or above was all nitrate lost from the Australian soil. Although the losses of nitrate shown in Fig. I were mainly due to dcnitrification, some immobilization of the added N occurred. Figure 2 shows the percentage of the added labelled N that remained in the soils after extraction with I M KC1 and 0.5 M K2S0,. The amounts of N immobilized were fairly small: a maximum of 7.5% after ?. d and 15% after 7 d at the highest temperatures (Fig. 2). At every temperature
After
incubation
7 days
mcubalion
_
Australian
o--d
English I
Incubation
Fig. I. Effect of
temperature
of glucose.
on nitrate
(&here
no error
remaining
lemoeratuie
soil erfof
“C
in solIs durtng
bar is shown.
Standard
solI
anaerobic
mcubatlon
in the presence
SE KU less than size of symbol.)
Denitrificatmn A!:er
at sub-optimal
!erqxratures
2 days Incubation
After
7 :iays incuballon
Aas:ralian soil
?ercenta;e added
of
Englisn 531
lsN-labelled
nixale-N
immobilized
L
5
1
10
15
20
I
25 Incubation
Fig. 2. Effect of temperature
temperature
10
15
20
25
OC
on immobilization of “N-labelled nitrate-N incubation in the presence of glucose.
and time. slightly more N was immobilized in the English soil than in the Australian. WC assume that this retained N is predominantly in organic microbial assimilates. although it could include N that was reduced to ammonium and then held on nonexchangeable sites on clays. The percentage of the initial nitrate-?I lost by dcnitrification was calculated as: (decrcasc in total KCI-cxtractablc nitrate-N) less (amount of N immobilized as measured with ‘“N). After 2 d anaerobic incubation more nitrate had been denitrified in the English soil than in the Australian at each tcmpcraturc (Fig. 3). After 7d at 5 C. a 10% denitritication loss had occurred from the English soil Afler
1
5
in soils during
anaerobic
but none in the Australian. At 1O’C more than 90% of the initial nitrate in the English soil had been dcnitrificd, but the corresponding loss from the Australian soil was less than 100/o. Even at 15-C dcnitrification was not complete in the Australian soil within 7d. The small dccrcasc in the loss of nitrate due to denitrification in the English soil between 10 and 25-C (from 91 to 85%: Fig. 3) reflected an increase in immobilization of N over this temperature range (Fig. 2). At the three highest temperatures, about 3Oj~g ammonium-N g ’ soil were present after 7 d incubation. This could have been forrncd from nitrate by dissimilatory reduction; we did not check this with
2 days incubation
After 7 days incubation
oo-
iOO-
ao-
80-
60-
40 ~
c---r
Australian
M
English soil
soil
20
0I I
5
IO
15
20
25 Incubation
Fig. 2. Effect
of temper;iture
on don~trification
temperature
OC
in soils during glucose.
anaerobic
mcubation
in the presence of
7?2
D. S. Powtsos
“X as insufficient KCI extract was available. However. if dissimilatory reduction of nitrate did occur it uas a very minor cause of nitrate loss. If all of the ammonium present resuited from this process it would account for only 5% of the added nitrate. Much of this ammonium was almost certainly formed by anaerobic decomposition of soil organic N.
We interpret the results as indicating that the denitrifying population in the English soil was better adapted to reduce nitrate at low temperatures than was the population in the Australian soil. A possible alternative explanation is that glucose was decomposed more rapidly in the English soil because the organisms responsible for decomposition were better adapted to function at the lower temperatures. If this was so, the concentration of dissolved oxygen in the English soil would fail more rapidly than in the Australian soil, leading to an earlier onset of denitrification. We did not measure the rate of disappearance of glucose in the Z soils and so cannot cxcludc the possibility of a diffcrcncc. Slightly more S was immobilized in the English soil than in the Australian at each time and temperature (Fig. 2) and this might reflect a diffcrcncc in overall microbial activity. However. differences in the dcplction ol dissolved oxygen could have made only a minor contribution to observed ditTerence in the denitritication. The water in each incubation would have contained about 0.1 ml dissolved 02, less than 1% of the 0: rcquircd for complctc aerobic decomposition of the glucose. Thus. dissolved 0: would have been rapidly dcplctcd even if glucose decomposition was extremely slow. Furthermore, if there was ;1 significant lag in the onset of anacrobiosis in the Australian soil cornpatto the English, it would be expected to have caused the greatest difference in denitritication between the soils at the shorter incubation time: in fact the opposite was observed (Fig. I). The English soil may have contained a larger population of dcnitrihcrs than the Australian soil because it had received nitrogenous fertilizers for many years. If this was the major factor causing a ditTcrencc in dsnitrification between tht two soils at a given temperature. the expected result would bc a lag in the onset of denitrification in the Australian soil. Again, this would lcad to a greater difTerence between the soils at the shorter incubation time, the opposite of the observed result (Figs I and 3). Although there have &en many laboratory studiss on the cffcct of tcmpcrature on denitrificatton (see reviews by Fillery. 1983; Firestone, 1982). the experiments reported here appear to bs the first direct comparison of denitrihcation in soils from diffcrcnt climatic zones. Ho&ever the microbiological study of Gamble er al. (1977) indicated that differences such as those dcmonstratcd hcrc wcrc to be expected. They isolated dcntrifying bacteria from soils of different climatic zones and found that them were differences in the characteristics of the populations prsssnt. Of the 95 dcnitriticrs isolated from temperate soil samples (mean annual tcmpcraturc < 20 C). 68?b vvcre able to divide in a culture incuhatcd at 4 C. In
et nl
contrast, none of the 33 species isolated from tropical samples (mean annual temperature > 2OC) could divide at 4-C. Our results show that denitrification measurements made on soils in one climatic zone should only be extrapolated to conditions in other zones with caution. The existence of different denitrifying populations in soils developed under different climatic conditions may explain, at least in part, the differences in the temperature dependence of denitrification observed by workers in various parts of the world. For example. the reported minimum temperatures for the occurrence of denitrification in soil range from < 4-C in soils from Alberta. Canada, to IO-C in Queensland soils (Crasu-ell, 1978; Cho rr nl., 1979: Firestone, 1982). It may also explain differences that have been noted between soils within Crasuell (1978) found very little Australia. denitrification in a Queensland soil (sub-tropical) incubated under flooded conditions at ZO’C for 7d. By contrast, Stefanson and Greenland (1970) found considerable denitrification in a non-flooded soil from South Australia (Mediterranean climate) incubated at 10-C for the same period. Craswell and Martin (1975) discussed some possible reasons for such ditTerences: we suggest the major reason was that the denitrifying population in each soil was adapted to its own environment. At Rothamsted, mean soil temperatures for April, measured at 09.00 h G.hl.T. at a depth of IOcm under grass. arc in the range 5.8-7.5 C; higher temperatures are reached during the course of most days. Similar temperatures will occur in many temperate zone soils in spring and our results show that denitrification can occur at such temperatures, provided other conditions arc suitable. Figures I and 3 show that there was a sharp increase in denitrification over the temperature range 5-IOC. Thus, small changes in soil temperature due to weather, soil conditions’or crop cover, could have a major effect on the total quantity of N lost by denitrification. This is potentially serious in agricultural soils in the temperate zone which often contain much nitrate in spring following application of fertilizer N. A similar sharp increase In denitrification occurred in the subtropical soil. but over the higher temperature range of IO-20 C. For this reason preplant applications of nitrogen fertilizer for winter crops in sub-tropical areas should be made in autumn. when soil temperatures are lower, rather than in late summer. This the possibility of strategy should minimize denitrilication losses occurring.
Quernslan~ Department of Primary Industries for providinr the soil from Biloela and Dr D. S. Jenkinson for helpful d&ussion. We are Indebted to the late P.G.S. thanks The the “A’ analyses. Wheat Industry Research Council and for financial asststance connected with amsted Experimental Station.
Mr G. Pruden’ for Royal Society. the Griffith University his visit to Roth-
REFERESCES Bremner J. Ll. (1965) Inorganic forms of nttro:en. In .Veritod.r qlSoi/ ._(no/rs~ IC. A. Black. Ed.). Vol. 1. pp. 1179-1237 .American Society of Agronomy. Madison.
Denitrification
at sub-optimal
Burford J. R. and Bremner J. M. (1975) Relationships between the denitrification capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Bio/og_v & Biochemistry 7. 389-394. Cho C. M, Sakdinan L. and Chang C. (1979) Denitrification capacity of three irrigated Alberta soils. Journ& of the Soil Science Society of America 43, 945-950. Craswell E. T. (1978) Some factors influencing denitritication and nitrogen immobilization in a clay soil. Soil Biology & Biochemistry 10. 241-235. Craswell E. T. and Martin A. E. (1975) Isotopic studies of the nitrogen balance in 3 cracking clay. I. Recovery of added nitrogen from soil and wheat in the glasshouse and gas Iysimeter. Australian Journal of Soil Research 13. 43-52. Fillery I. R. P. (1983) Biological denitrification. In Gaseous Loss of .Vitrogtn from Plunt-Soil S,utems (J. R. Freney and J. R. Simpson, Eds). Drcelopmrnts in Plunt und Soil Sciences. Vol. 9. pp. 3364. Martinus Sijhoff, The Hague. Firestone Xi. K. (1982) Biological Denitritication. In .Vitrogen in Agricultural Soils (F. J. Stevenson, Ed.). pp. 289-326. American Society of Agronomy, Madison. Gamble T. N.. Betlach M. R. and Tiedje J. M. (1977) Numerically dominant denitrifying bacteria from world soils. Applied & Environmental Microbiology 33. 926-939. Jacobson S. N. and Alexander M. (1980) Nitrate loss from soil in relation to temperature, carbon source and de-
temperatures
723
nitrifer populations. Soil Biology & Biochemistry 12, 501-505. Powlson D. S.. Pruden G. and Jenkinson D. S. (1984) Recovery of ‘5N-labrlled fertilizer by winter wheat. In The Sitrogen Requirements of Cereuls, XDAS Reference Book 365, 119. H.M.S.O.. London. Powlson D. S.. Pruden G., Johnston A. E. and Jenkinson D. S. (1986) The nitrogen cycle in the Broadbalk Wheat Experiment: recovery of “N-labelled fertilizer applied in spring and inputs of nitrogen from the atmosphere. Journai of Agricultural Science, Cumbridge 107,591-609. Pruden G.. Powlson D. S. and Jenkinson D. S. (1985) The measurement of “N in soil and plant material. Fertilizer Research 6, 205-218. Ryden J. C. (l983) Denitrilication loss from a grassland soil m the field receiving different rates of nitrogen as ammonium nitrate. /our& of Soii Science 34. 355-365. Safligna P. G.. Cogle A. L.. Strong W. M. and Waring S. A. (19S4) The effect ofearly application of “N fertilizer on recovery of nitrogen by wheat grown in a vertisol. In The Properties und Urili:arion of Cracking Clay Soils (J. W. McGarity, E. H. Hoult and H. B. So, Eds). Proceedings Symposium, Armidale, August 1981. Reciews in Rural Science 5, pp. 227-230. University of New England. Armidale. Australia. Stefanson R. C. and Greenland D. J. (1970) Measurement of nitrogen and nitrous oxide evolution from soil-plant systems using sealed growth chambers. Soil Science 109, 203-206.
0033-0'17 SS 53.00 -c 0.00 _ 1388 Pergamon Press plc
DENITRIFICATION AT SUB-OPTIMAL TEMPERATURES IN SOILS FROM DIFFERENT CLIMATIC ZONES D.
Soils
S. POWLSOS,
and Plan: Nutrition
P. G.
SAFFIGNA*
and
MOKIQLT
KRAGT-COTTAAR+
Department. Rothamsted Experimental Herts, AL5 2JQ. U.K.
(Accepred
I December
Station.
Harpcnden.
1987)
Summary-The effectof temperature on denitrification was compared in a temperate zone soil. a Paieudalf cropped to wheat in south-east England, and a sub-tropical soil, a Vertisol (Typic Pellustert) cropped to sorghum in Queensland, Australia. The 2 soils were incubated anaerobically in the presence of KNOt and of the added nitrate was measured glucose at 5, IO. 15, 20 or 25’C for either 2 d or 7 d. Immobilization in parallel incubations using K” NO!. After 2 d more nitrate was lost from the English soil than from the Australian at every temperature. After 7 d at IO C, all nitrate had been reduced in the English soil. 92oio denitrified and 8% immobilized, whilst in the Australian soil 91% of the nitrate still remained. Only at 2O.C or above was all of the nitrate reduced (89% denitrified. I I% immobilized) in the Australian soil. The results indicate that the denitrifying populations in each soil was adapted to its own environment. The dcnitrifiers in the temperate soil reduced nitrate at lower temperatures than did those in the sub-tropical soil. The occurrence of considerable denitrification at IO C in the temperate soil. and a sharp increase between 5 and IO-C, shows that dcnitrification could be a major cause of N loss in temperate areas in spring. when much N fertilizer is applied. Even though soil temperatures can fall to similar values in the subtropics in winter, much less denitrification would be expected.
ISTRODl’CTION Recent ticld experiments in the temperate zone indicate that denitrification can occur at fairly low tempcratures in soils that are at. or near, field capacity. “I\; balance cxpcriments in England, in which labelled fertilizer was applied in spring to winter wheat, indicated losses of up to 300/o in some years (Powlson rt ul.. 198-l. 1986). These losses were greater than would be expected from leaching alone and suggested that denitrification was occurring, even though soil temperatures were generally less than IO’C. Ryden (1983) measured the evolution of N,O from a grassland soil in southern England throughout the year and found that N,O evolution from moist soil began when the temperature exceeded about 8-C. In laboratory experiments with soils from a temperate region of the U.S.A.. some dsnitrification occurred at 7-C though much less than at IS’C (Jacobson and Alexandcr, 1980). In sub-tropical soils, however. denitrification seems less likely to occur even at soil temperatures above IO C. For example, in Queensland, Australia. SatIigna ef al. (1984) found no loss of “N-labclled fertilizer several weeks after application to a fallow soil at field capacity and at an ambient temperature of about IS-C, although when nitrogen was applied in a warmer period 35% was lost. Craswell (1978) observed no loss of “N-labelled nitrate from cores of Queensland soils held under waterlogged
conditions
at
IO-C.
‘Present address: School of Australian Environmental Studies, Griffith Unixersit). Brisbane, Queensland 41 I I. Australia. tPresent address: Department of Soil Science and Plant Nutrition. Agricultural University. De Dreyen 3. 6703 BC Wageningen. The Netherlands.
These observations suggest that the denitrifying population in temperate soils is better adapted to function at lower temperatures than that in tropical sdils. This possibility was investigated by comparing denitrification. over a range of temperatures, in a temperate and a sub-tropical soil incubated under identical anaerobic conditions in the presence of nitrate and glucose. Glucose was included so that temperature would be the dominant factor limiting denitrification.
.VATERIALS
AND METHODS
Soils
Details of the soils used are given in Table I. The English soil was a Paleudalf from the Broadbalk Continuous Wheat Experiment at Rothamsted Experiment Station (latitude 51-48’ N, longitude 0’21’ W). It was sampled @-IOcm in November 1983. The Australian soil was a Vertisol (Typic Pellustert) from an experiment on tillage and sorghum residue management near Biloela in central Queensland (latitude 24-21’ S. longitude 150-30’ E). The soil used (&lOcm) was a bulked sample from 4 plots under conventional cultivation without incorporation of residues. It was sampled in October 1983 and sent to England by air. Both soils were sieved < 2 mm and then stored moist, under aerobic conditions, for about 3 weeks at temperatures close to the field temperatures at the time of sampling: 5-C for the English soil and 25-C for the Australian soil. Incubation
procedure
and analyricul
methods
The anaerobic incubation procedure was based on that described by Burford and Bremner (1975). Triplicate portions of moist soil. containing 5 g oven-dry 71’)
D. S. POWLSOSe:
710
iile g -’ o\rn Cr&pin* Slsxl iirmUdl temperature Sfean annuBl temperature
df) soli)
5011 ( C) ;11r ( Cl
7. -2 Wheat -wo ZPI frrtrlrzer
I> Sorghum gtvcn no ferttliter
Y 2’
ND
89
(1000 R for IO min) and the soil residue extracted a further 4 times to remove all traces of inorganic 5. These additional extractions were each for 20 min with 70 ml 0.5 Y K:SO,. .After the tinal K$O, extraction and centrifugation the soil residue was dried at SO-C. Labelled N remaining in the soil was determined by Kjeldahl digestion and steam distillation followed by “N ‘% ratio analysis of the distilate using a VG ?f~icromass 602D mass spectrometer (Pruden er al.. 1955).
20. I
‘Lfsasured daily ;Lt ;t depth oi IOcm un&r
RESULTS
yrass at 09.00 h C.M.T.
soil, were incubated in sealed bottles (30 ml McCartney bottles) with 15 ml of a solution containing 2 mg N as KNO, and IO mg C as glucose (i.e. 400 pg N and 1000 pg C g - ’ oven-dry soil). They were incubated for either 2 or 7 d at 5. IO. IS, 20 or 25’C. After incubation, 5 ml of 4 M KCI were added to each bottle to give a final concentration of approx. I Y KCI. The bottles were shaken for I h and the filtered extracts analyscd for nitrate (including any nitrite) and ammonium by steam distillation with MgO and Dsvada’s alloy (Bremner, 1965). Zero time controls u-crc also extracted: recovery of nitrate was 96% in the English soil and 100% in the Australian soil. An additional replicate, containing ‘%-labclled KNO, (11 atom % excess) was included with each treatment in order to measure immobilization of added nitrate. After incubation. the “N-labellcd soils were cxtructed for I h with I M KCI, centrifuged
Afler
2 days
in!.
There were clear differences in the amounts of nitrate remaining in the :! soils following anaerobic incubation at different temperatures in the presence of glucose (Fig. I). After 2 d significantly more nitrate remained in the Australian than in the English soil at all temperatures above 5 C. After 7 d even more striking differences had emerged. In the English soil all nitrate had been lost after incubation at IO’C or above but, in the Australian soil, much nitrate remained after incubation at IO or IS’C. Only at 2O’C or above was all nitrate lost from the Australian soil. Although the losses of nitrate shown in Fig. I were mainly due to dcnitrification, some immobilization of the added N occurred. Figure 2 shows the percentage of the added labelled N that remained in the soils after extraction with I M KC1 and 0.5 M K2S0,. The amounts of N immobilized were fairly small: a maximum of 7.5% after ?. d and 15% after 7 d at the highest temperatures (Fig. 2). At every temperature
After
incubation
7 days
mcubalion
_
Australian
o--d
English I
Incubation
Fig. I. Effect of
temperature
of glucose.
on nitrate
(&here
no error
remaining
lemoeratuie
soil erfof
“C
in solIs durtng
bar is shown.
Standard
solI
anaerobic
mcubatlon
in the presence
SE KU less than size of symbol.)
Denitrificatmn A!:er
at sub-optimal
!erqxratures
2 days Incubation
After
7 :iays incuballon
Aas:ralian soil
?ercenta;e added
of
Englisn 531
lsN-labelled
nixale-N
immobilized
L
5
1
10
15
20
I
25 Incubation
Fig. 2. Effect of temperature
temperature
10
15
20
25
OC
on immobilization of “N-labelled nitrate-N incubation in the presence of glucose.
and time. slightly more N was immobilized in the English soil than in the Australian. WC assume that this retained N is predominantly in organic microbial assimilates. although it could include N that was reduced to ammonium and then held on nonexchangeable sites on clays. The percentage of the initial nitrate-?I lost by dcnitrification was calculated as: (decrcasc in total KCI-cxtractablc nitrate-N) less (amount of N immobilized as measured with ‘“N). After 2 d anaerobic incubation more nitrate had been denitrified in the English soil than in the Australian at each tcmpcraturc (Fig. 3). After 7d at 5 C. a 10% denitritication loss had occurred from the English soil Afler
1
5
in soils during
anaerobic
but none in the Australian. At 1O’C more than 90% of the initial nitrate in the English soil had been dcnitrificd, but the corresponding loss from the Australian soil was less than 100/o. Even at 15-C dcnitrification was not complete in the Australian soil within 7d. The small dccrcasc in the loss of nitrate due to denitrification in the English soil between 10 and 25-C (from 91 to 85%: Fig. 3) reflected an increase in immobilization of N over this temperature range (Fig. 2). At the three highest temperatures, about 3Oj~g ammonium-N g ’ soil were present after 7 d incubation. This could have been forrncd from nitrate by dissimilatory reduction; we did not check this with
2 days incubation
After 7 days incubation
oo-
iOO-
ao-
80-
60-
40 ~
c---r
Australian
M
English soil
soil
20
0I I
5
IO
15
20
25 Incubation
Fig. 2. Effect
of temper;iture
on don~trification
temperature
OC
in soils during glucose.
anaerobic
mcubation
in the presence of
7?2
D. S. Powtsos
“X as insufficient KCI extract was available. However. if dissimilatory reduction of nitrate did occur it uas a very minor cause of nitrate loss. If all of the ammonium present resuited from this process it would account for only 5% of the added nitrate. Much of this ammonium was almost certainly formed by anaerobic decomposition of soil organic N.
We interpret the results as indicating that the denitrifying population in the English soil was better adapted to reduce nitrate at low temperatures than was the population in the Australian soil. A possible alternative explanation is that glucose was decomposed more rapidly in the English soil because the organisms responsible for decomposition were better adapted to function at the lower temperatures. If this was so, the concentration of dissolved oxygen in the English soil would fail more rapidly than in the Australian soil, leading to an earlier onset of denitrification. We did not measure the rate of disappearance of glucose in the Z soils and so cannot cxcludc the possibility of a diffcrcncc. Slightly more S was immobilized in the English soil than in the Australian at each time and temperature (Fig. 2) and this might reflect a diffcrcncc in overall microbial activity. However. differences in the dcplction ol dissolved oxygen could have made only a minor contribution to observed ditTerence in the denitritication. The water in each incubation would have contained about 0.1 ml dissolved 02, less than 1% of the 0: rcquircd for complctc aerobic decomposition of the glucose. Thus. dissolved 0: would have been rapidly dcplctcd even if glucose decomposition was extremely slow. Furthermore, if there was ;1 significant lag in the onset of anacrobiosis in the Australian soil cornpatto the English, it would be expected to have caused the greatest difference in denitritication between the soils at the shorter incubation time: in fact the opposite was observed (Fig. I). The English soil may have contained a larger population of dcnitrihcrs than the Australian soil because it had received nitrogenous fertilizers for many years. If this was the major factor causing a ditTcrencc in dsnitrification between tht two soils at a given temperature. the expected result would bc a lag in the onset of denitrification in the Australian soil. Again, this would lcad to a greater difTerence between the soils at the shorter incubation time, the opposite of the observed result (Figs I and 3). Although there have &en many laboratory studiss on the cffcct of tcmpcrature on denitrificatton (see reviews by Fillery. 1983; Firestone, 1982). the experiments reported here appear to bs the first direct comparison of denitrihcation in soils from diffcrcnt climatic zones. Ho&ever the microbiological study of Gamble er al. (1977) indicated that differences such as those dcmonstratcd hcrc wcrc to be expected. They isolated dcntrifying bacteria from soils of different climatic zones and found that them were differences in the characteristics of the populations prsssnt. Of the 95 dcnitriticrs isolated from temperate soil samples (mean annual tcmpcraturc < 20 C). 68?b vvcre able to divide in a culture incuhatcd at 4 C. In
et nl
contrast, none of the 33 species isolated from tropical samples (mean annual temperature > 2OC) could divide at 4-C. Our results show that denitrification measurements made on soils in one climatic zone should only be extrapolated to conditions in other zones with caution. The existence of different denitrifying populations in soils developed under different climatic conditions may explain, at least in part, the differences in the temperature dependence of denitrification observed by workers in various parts of the world. For example. the reported minimum temperatures for the occurrence of denitrification in soil range from < 4-C in soils from Alberta. Canada, to IO-C in Queensland soils (Crasu-ell, 1978; Cho rr nl., 1979: Firestone, 1982). It may also explain differences that have been noted between soils within Crasuell (1978) found very little Australia. denitrification in a Queensland soil (sub-tropical) incubated under flooded conditions at ZO’C for 7d. By contrast, Stefanson and Greenland (1970) found considerable denitrification in a non-flooded soil from South Australia (Mediterranean climate) incubated at 10-C for the same period. Craswell and Martin (1975) discussed some possible reasons for such ditTerences: we suggest the major reason was that the denitrifying population in each soil was adapted to its own environment. At Rothamsted, mean soil temperatures for April, measured at 09.00 h G.hl.T. at a depth of IOcm under grass. arc in the range 5.8-7.5 C; higher temperatures are reached during the course of most days. Similar temperatures will occur in many temperate zone soils in spring and our results show that denitrification can occur at such temperatures, provided other conditions arc suitable. Figures I and 3 show that there was a sharp increase in denitrification over the temperature range 5-IOC. Thus, small changes in soil temperature due to weather, soil conditions’or crop cover, could have a major effect on the total quantity of N lost by denitrification. This is potentially serious in agricultural soils in the temperate zone which often contain much nitrate in spring following application of fertilizer N. A similar sharp increase In denitrification occurred in the subtropical soil. but over the higher temperature range of IO-20 C. For this reason preplant applications of nitrogen fertilizer for winter crops in sub-tropical areas should be made in autumn. when soil temperatures are lower, rather than in late summer. This the possibility of strategy should minimize denitrilication losses occurring.
Quernslan~ Department of Primary Industries for providinr the soil from Biloela and Dr D. S. Jenkinson for helpful d&ussion. We are Indebted to the late P.G.S. thanks The the “A’ analyses. Wheat Industry Research Council and for financial asststance connected with amsted Experimental Station.
Mr G. Pruden’ for Royal Society. the Griffith University his visit to Roth-
REFERESCES Bremner J. Ll. (1965) Inorganic forms of nttro:en. In .Veritod.r qlSoi/ ._(no/rs~ IC. A. Black. Ed.). Vol. 1. pp. 1179-1237 .American Society of Agronomy. Madison.
Denitrification
at sub-optimal
Burford J. R. and Bremner J. M. (1975) Relationships between the denitrification capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Bio/og_v & Biochemistry 7. 389-394. Cho C. M, Sakdinan L. and Chang C. (1979) Denitrification capacity of three irrigated Alberta soils. Journ& of the Soil Science Society of America 43, 945-950. Craswell E. T. (1978) Some factors influencing denitritication and nitrogen immobilization in a clay soil. Soil Biology & Biochemistry 10. 241-235. Craswell E. T. and Martin A. E. (1975) Isotopic studies of the nitrogen balance in 3 cracking clay. I. Recovery of added nitrogen from soil and wheat in the glasshouse and gas Iysimeter. Australian Journal of Soil Research 13. 43-52. Fillery I. R. P. (1983) Biological denitrification. In Gaseous Loss of .Vitrogtn from Plunt-Soil S,utems (J. R. Freney and J. R. Simpson, Eds). Drcelopmrnts in Plunt und Soil Sciences. Vol. 9. pp. 3364. Martinus Sijhoff, The Hague. Firestone Xi. K. (1982) Biological Denitritication. In .Vitrogen in Agricultural Soils (F. J. Stevenson, Ed.). pp. 289-326. American Society of Agronomy, Madison. Gamble T. N.. Betlach M. R. and Tiedje J. M. (1977) Numerically dominant denitrifying bacteria from world soils. Applied & Environmental Microbiology 33. 926-939. Jacobson S. N. and Alexander M. (1980) Nitrate loss from soil in relation to temperature, carbon source and de-
temperatures
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nitrifer populations. Soil Biology & Biochemistry 12, 501-505. Powlson D. S.. Pruden G. and Jenkinson D. S. (1984) Recovery of ‘5N-labrlled fertilizer by winter wheat. In The Sitrogen Requirements of Cereuls, XDAS Reference Book 365, 119. H.M.S.O.. London. Powlson D. S.. Pruden G., Johnston A. E. and Jenkinson D. S. (1986) The nitrogen cycle in the Broadbalk Wheat Experiment: recovery of “N-labelled fertilizer applied in spring and inputs of nitrogen from the atmosphere. Journai of Agricultural Science, Cumbridge 107,591-609. Pruden G.. Powlson D. S. and Jenkinson D. S. (1985) The measurement of “N in soil and plant material. Fertilizer Research 6, 205-218. Ryden J. C. (l983) Denitrilication loss from a grassland soil m the field receiving different rates of nitrogen as ammonium nitrate. /our& of Soii Science 34. 355-365. Safligna P. G.. Cogle A. L.. Strong W. M. and Waring S. A. (19S4) The effect ofearly application of “N fertilizer on recovery of nitrogen by wheat grown in a vertisol. In The Properties und Urili:arion of Cracking Clay Soils (J. W. McGarity, E. H. Hoult and H. B. So, Eds). Proceedings Symposium, Armidale, August 1981. Reciews in Rural Science 5, pp. 227-230. University of New England. Armidale. Australia. Stefanson R. C. and Greenland D. J. (1970) Measurement of nitrogen and nitrous oxide evolution from soil-plant systems using sealed growth chambers. Soil Science 109, 203-206.