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Immobilization and mineralization of nitrogen in soils incubated with pig slurry or ammonium sulphate
IMMOBILIZATION AND MINERALIZATION NITROGEN IN SOILS INCUBATED WITH SLURRY OR AMMONIUM SULPHATE T. H. Dcparlmcnt
FLOWERS*
and P. W.
ARNOLD
OT Soil Science. University of Newcastle upon Tyne. Tyne and Wear NEI 7RU, U.K.
(Accepfrd
IO Seprember
OF PIG
Newcastle
upon Tync.
1982)
Summary-Nitrogen mineralization and immobilization were investigated in two soils incubated with ammonium sulphatc or pig slurry over a range of temperatures and moisture contents. A reduction in the mineralization of soil organic N was observed in soils incubated with IOOpg NH:-N gg’ soil as ammonium sulphate at 30°C but not at lower temperatures. Addition of IOOpg NHf-N g-’ soil as pig slurry resulted in a period of nett immobilization lasting up to 30 days at 5°C. Although the length of the immobilization phase was shorter at higher temperatures the total N immobilized was similar. The subsequent rate of mineralization in slurry-treated soils was not significantly greater (P = 0.05) than in untreated soils. There was no evidence of any subsequent increased mineralization arising from the immobilized N or slurry organic N for up lo 175 days. The rate of immobilization was found to increase with increasing moisture content, though the period of nett immobilization was shorter, so that the amount of N immobilized was similar over a range of moisture contents from IO to 40%. Approximately 40”,,, of the NH:-N in the slurry was immobilized under the incubation conditions used.
INTRODUCTION
Changes in the methods of livestock rearing have led to the handling of animal wastes as slurry rather than the traditional farmyard manure. The majority of wastes are still spread on the land and due to the increasing costs of inorganic fertilizers more emphasis has been placed on the fertilizer value of wastes in recent years. They are, however, bulky heterogeneous materials, of variable composition and the ratio of the major plant nutrients, N, P and K may not be balanced in terms of crop requirements. For example, they must often be supplemented by additional inorganic N fertilizer. Particular attention is being given to the N content of animal slurries both because of the importance of crop responses to this element and the risks of nitrate pollution of surface and ground waters if excessive applications are used. Animal slurry can be considered as a mixture of faeces and urine, plus smaller amounts of other organic material such as wasted feed and bedding, with varying amounts of additional water from various sources depending on the conditions of collection. As voided by the animal the N in the faeces and urine is almost entirely in organic forms but urea and some labile organic compounds are rapidly converted to ammonium. There is considerable variation in both the composition and the proportion of N in the inorganic form. Usually, about half the N is present as ammonium, other inorganic forms generally being absent (Evans et al., 1978; Ministry of Agriculture, Fisheries and Food (M.A.F.F.), 1976). A large fraction of the total N is present as organic for& of varying resistance to microbial decomposition and it is important to measure the con‘Present address: Agricultural Chemistry, Chemistry Department, University of Glasgow, Glasgow Cl2 8QQ, U.K.
tribution of this fraction to the pool of plant available N in the first and subsequent years after application. Nitrogen immobilization and losses by denitrification may be encouraged by the presence of large amounts of readily-decomposible organic matter in the slurry. We have studied the mineralization and immobilization of pig slurry N in soil under laboratory incubation conditions. MATERIALS
AND METHODS
A single sample of pig slurry (Table I) was collected from under a veranda-type house on the University farm, at Cockle Park, Northumberland, thoroughly mixed and divided into small sub-samples which were deep frozen and stored at -20°C until required. Some details of the slurry composition are given in Table I. Soil samples (O-15 cm depth) were collected fresh from the field for each experiment. The soils were air-dried just sufficiently to permit sieving (< 2 mm) and for additions to be made without exceeding the desired moisture content for incubation. Two arable soils were used, a sandy loam (Rivington series) and a clay loam (Dunkeswick series) both from the University Farm, Cockle Park (Table 2). Soils were incubated in 350 ml polystyrene pots, closed with lids having a 2.5 cm dia hole to permit aeration. The equivalent of 50 g air-dried soil was added to each pot and the appropriate amendment solution added as required. Following thorough mixing of the soil, the moisture content was adjusted to the required level by the addition of deionized water. To reduce water losses during incubation, while providing adequate aeration, the pots were placed in polyethylene boxes in which a moist atmosphere could be maintained by pumping through a stream of moist air. The air stream was first washed by passing it through 5% HzSO,. then I M NaOH and then three
T. H. FLOWEKSand P. W. ARNOLD
330 Table
Table
I. Composition of the slurry sample used g I-’ in fresh slurry
Sand 2.CM.02mm (%) Silt 0.02-0.002 mm (:/,I Clay <0.002 mm (“J pH (5:l in 0.01 M CaCI,) Water holding capacity (g g Organic carbon (“{,) Total nitrogen (y,,) C:N ratio
7.70 84.5 71.2 30.9 4.23 2.45
PH Total solids Suspended solids Organic carbon Kjeldahl nitrogen Extractable NH;-N Extractable NOT-N Organic nitrogen Total copper Total zinc
0.00 1.78 0.064
water. incubation kO.5 c.
0.05 I
E~sp?riment
deionized water washes. The pots were incubated in a horizontal position so that the soil could be spread out in a layer of a maximum thickness 1 cm. Moisture losses were: /corrected by the addition of deionized 250
2. Some characteristics
’
of the soils
Rivington series
Dunkeswick
69.8
51.4
17.5 12.1 5.8 5.02 1.88 0.14
19.9 28.6
soil)
series
6.0 6.3 3.01 0.24 12.5
13.4
was at the stated temperatures
I
The two soils were treated with IOOng NH:-N g-’ as ammonium sutphate or in pig slurry. Untreated controls were also inchtded. Samples were incubated
la) 5°C
2w
4.50I
25 i
50 L
75 t
100 t
125 I
150 t
175 ,
0 1
25 I
50 t
75 ,
t
I
100
125
150
1
175
,
125
150
175
TIME (days!
25or
tc)l5'c
r
t 4,5*k---k ’ 50
75
t
!
I
J
100
125
150
175
(d)30"C
25
0
TIkIE (days) Fig. I, Eifect of tcmpcraturc and nitrogen additions on the mineralization of nitrogen m Rivmgton series soil. (+---•) No addition, (O--O) IOOpg NH:-N g--’ soil as ammonium sulphate. (A--A) 100 NH:-N g-’ soil as pig slurry. Symbols in Figs I. 3 and 4 represent the mean of three replicate
jig
incubations.
lrnn~obiiizatj~n of nitrogen from pig slurry
7.0 f
25
75
50
TIME
100
125
150
175
(days!
Fig. 2. Et%ecct of temperature and nitrogen additions on the mi~e~li~t~~~ of nitrogen in Dunkeswick series ~4. (+---e) No addition, {O----Of 100 yg NH;-N g-’ soil as ~mrno~j~rn sulphate @+-A) iOOpg NH:-N km’ soil as pig slurry. Symbols represent the mean or five replicate incubations.
at. 5, IO, 15 and 3O’C (Dunkeswick soil was not incubated at 5°C) and at a~proximateiy SO*i,water holding capacity (30’;: w/w moisture content for the Dunkeswick soil and 2O’j: w/w moisture content for the Rivington soil).
The two soils were treated with 250 pg NH:-N g-’ as ammonium suiphate or in pig slurry and incubzsted at 15°C over a range of moisture contents. Ammonium-N, nitrite-N and nitrate-N were extracted by shaking for 1 h with 0.5 M K,SQ, solution at a soil:solution ratio 1:20. Ammonium was determined using an automated phenol hypochlorite method {Brown, 1973). Nitrate and nitrite were determined using the automated procedure of Best (1976).
RESULTS
Slurry addition caused a rise in the pH of both soils while nitrification of added ammonium caused a fail ofpH (see Figs I and 2). The pH changes were slightly larger in the less well-buffered Rivington soil. The Dunkeswick soil showed an upward trend in pH during the incubation. The combined effect of the pH changes resulted in a lower pfT in the ammonjum sulphate treated soils compared with the control and slurry treated soils except in the Rivington soil at 30°C where nitrification was inhibited. The N accumulated as nitrate in both soils and for all treatments except the Rivington soil at 30°C where nitrification was inhibited sufbcientfy for ammonium to accumu-
332
T. H. FLOWERS Table
and P. W. ARNOLD
3. Mineralization rate constants (PgN g-’ Rivinaton and Dunkeswick soils Incubation IO
5 series Untreated Ammonium sulphate Slurry Dunkeswick series
soil day-‘)
temperature I5
30
for
Rivington
0.080~ 0. I I30 0.1 I Sa
0.188 0.265 0.373
0.480 0.686/J
ND ND ND
0.293~~ 0.296~~ 0.249~
0.3786 0.349hc 0.413/I
1.039d 0.931 1.16ld
Untreated Ammonium Slurry
0.712h
0.070a O.ll3a 0.118~
sulphate
Within each soil series for comparisons between temperatures and amendments, values with the same following letter are not significantly different (P = 0.05). ND-not determined.
late in addition to nitrate. Nitrite was not present at a concentration greater than 1 pg g-’ soil at any time. After the initial flush of mineralisation, the control and ammonium sulphate treatments showed a linear accumulation of inorganic N, with time, in both soils (see Figs 1 and 2). Both soils showed an initial decline in total inorganic N following slurry addition. The length of this phase was greatest at 5°C where it lasted approximately 30 days while at 30°C it was only apparent from a reduction in mineralization during the first 10 days. The total loss of inorganic N, estimated at the end of this phase and after allowing (0)
warer cantent
10%
r
300 r
(c) 20%
0
5
10
mineralization in the control soils, was approximately 28% of the applied slurry NH:-N. After this initial phase the increase in inorganic N was linear with time. Regression lines were fitted to the linear regions in order to calculate mineralization rate constants (see Table 3). The pattern of scatter of the data about the fitted lines suggests a systematic variation of inorganic N concentration between sampling dates. It was therefore not possible to perform an analysis to test for linearity and the regression lines were fitted to the means of the replicate N concentrations at each date.
for
water content
15
20
TIME
ldavrl
25
t b) I5 %
30%
IdI
30
35
40
0
5
water
content
water
10
content
15
20
TIME
ldavsl
25
30
35
40
Fig. 3. Effect of soil moisture content and addition of pig slurry on the changes in NH;-N. NOT-N and total inorganic nitrogen levels in Rivington series soil. (m-M) Total inorganic N in untreated soil, (m--&l) total inorganic N in soil treated with 25Opg NH;-N g-’ soil as ammonium sulphatc. (o---_O) total inorganic N in soil receiving 250 pg NH: -N g- ’ soil as pig slurry, (A--A) NH; -N in pig slurry treated soil. (0-O) NOi -N in pig slurry treated soil.
Immobilization of nitrogen from pig slurry
r
333
(bl 25%
water
content
td 140%
wafer
content
7 r” 250 1 c 200 K
350
r
lc) 35%
5
wafer
10
15
conrent
20
r
25
30
35
TIME ldayrl
TIME Idbvr)
Fig. 4. Effect of soil moisture content and addition of pig slurry on the changes in NH.++-N,NOT-N and total inorganic nitrogen levels in Dunkeswick series soil. (a---m) Total inorganic N in untreated soil, (~--a) total inorganic N in soil treated with 25Opg NH;-N g-’ soil as ammonium sulphate. (o--_o) total inorganic N in soil receiving 250 pg NH:-N g-’ soil as pig slurry, (A---A) NH:-N in pig slurry treated soil, (0-O) NOT-N in pig slurry treated soil. Experiment 2
pleted more rapidly at the higher moisture contents so that the total loss of inorganic N, allowing for the mineralization in the control soils, was similar irrespective of the moisture content (see Table 4). There were losses of inorganic N from the ammonium sulphate treated Rivington soil but these were much smaller than the losses in the slurry-treated soils.
In the second experiment the concentrations of inorganic N were measured more frequently to examine the decline in inorganic N following slurry addition in more detail. Figures 3 and 4 show ammonium, nitrate and total inorganic N for the two soils following treatment with 250 pg NH:-N g’ in pig slurry and incubation at a range of moisture contents. Also shown is the total inorganic N in the control and ammonium sulphate (250 pg NH:-N gg’ soil) treatments. In both soils at all moisture contents there was a marked decline in mineral N in the slurry treatments. The rate of decline was greatest in the initial stages of the incubation and increased with soil moisture content. However, the process was com-
DISCUSSION
It is generally accepted that only a part of the N present in soil organic matter is susceptible to decomposition. This has been termed potentially mineralizable N by Stanford and Smith (1972) and Campbell et ul. (1974) who suggested that the rate of mineral-
Table 4. Maximum immobilization of inorganic nitrogen in slurry treated soils incubated at a range of moisture contents Moisture content (%) IO 15 20 25 30 35 40 ‘Immobilization incomplete.
Maximum immobilization (% of applied NH,-N) Rivington series Dunkeswick series 41 38 41 44
30’ 31 39 36
314
T. H.
FLOWERS and
ization would be proportional to the concentration of potentially mineralizable N and fitted their data to tirst order reaction kinetics. An exponential decay rate has also been used frequently to describe the decomposition of added plant material in soil. Jenkinson (1977) fitted the results of a IO-yr study of the decomposition of plant material to a double exponential model which predicted that approximately 70% of the plant material decayed with a half life of 0.25 yr and the remainder with a half life of 8 yr. However, in our study the data for both control and amended soils (see Figs 1 and 2) were better fitted to a straight line relationship after the initial flush of mineralization, caused by disturbance of the soil, had subsided. Zero order rate constants calculated from the slopes of the fitted regression lines were used as the basis for comparing mineralization rates (see Table 4). Both soils showed the expected increase in mineralization rate with increase in temperature except that the Rivington soil showed no increase over the range 5-10°C for any of the treatments. Only in one instance was the rate of mineralization in the slurrytreated soil significantly greater than in the untreated control. However, compared with the ammonium sulphate treatment there is some evidence of a small contribution to mineralization by the organic fraction of slurry N. Although the contribution of slurry organic N to the pool of plant available N was too small to detect, the initial losses of inorganic N from the slurrytreated soils were large. In the second experiment ammonia volatilization losses were found to be negligible for both soils, even though transient increases of up to 1 pH unit were observed during the first week. This extremely low loss of ammonia was probably due to the slurry being thoroughly mixed with the soil. Nitrite did not accumulate at concentrations greater than I kg N gg’ soil at any stage of the incubation. Losses of inorganic N were observed over the full range of moisture contents and although the rate of loss was greatest at the highest moisture contents the total losses (see Table 4) were similar at all moisture levels. The loss was therefore unlikely to have been due to denitrification, particularly as the fastest rates of disappearance were greater than the corresponding nitrification rates, and can therefore be attributed to microbial immobilization. The rate of nitrate accumulation was constant throughout the early stages of incubation of Rivington soil (Fig. 3) and the reduction in total inorganic-N was due to an apparent loss of ammonium indicating that immobilization was mainly from ammonium. In Dunkeswick soil nitrate accumulation showed a lag phase suggesting immobilization of both ammonium and nitrate. Immobilization was marked in both soils, up to 40% of the applied ammonium N in the slurry being immobilized, and there was no clear evidence that either this N or the organic N initially present in the slurry was mineralized even after prolonged incubation. Many of the incubation studies using animal wastes reported in the literature are difficult to interpret because no distinction was made between organic and inorganic N initially present in the slurry.
P. W.
ARNOLD
Furthermore losses by ammonia volatilization and denitrification are usually not measured. Maass ef (I/. (1973) studied N and C mineralization in soils treated with 30-l 50 pg g-’ soil of total nitrogen as cattle, pig or poultry slurry and incubated at 25°C. Their results showed that the level of addition of slurry had no effect on the mineralization of N, but the results are difficult to interpret because there were losses of inorganic N in the early stages of the incubation. Cooper (1975) incubated two soils at 30°C and field capacity for 5 weeks to follow the nitrification of pig slurry NH:-N. Interpretation of his published figures to estimate changes in total inorganic N reveals losses of N in some experiments which may have been due to denitrification, ammonia volatilization or immobilization. Germon et (11.(1979) studied the mineralization of N in soils treated with pig slurry and incubated at field capacity and a range of temperatures. At 8 and 20°C some soil N was mineralized in the first week but none thereafter, while at 28°C there was mineralization throughout the 8 weeks of incubation. With additions of pig slurry there was no initial immobilization of N nor was there any additional mineralization that could be attributed to slurry organic N. Ammonia volatilization losses were found to be negligible and denitrification losses were unlikely under the incubation conditions used. McCalla et al. (1977) suggested that the organic fraction of animal waste consists mainly of two forms. (i) Proteins that have resisted animal digestion and are more or less combined with lignin or lignin-like substances. (ii) Dead and living microbial cells from the intestinal tract. The relative amounts of these two fractions are not known and may be expected to vary with the age and diet of the animal and possibly with storage. In particular, differences may be expected between the slurry from ruminant and non-ruminant animals. Very different rates of decomposition may be expected from the two fractions. Material that has resisted digestion by ruminant animals may be expected to be very resistant to further decomposition in the soil. Loynachan et al. (1976) reported that approximately 25% of the total N in swine liquid manure was resistant to mineralization under extensive aerobic digestion. Incubation studies show that the addition of pig or cattle slurry to soil can lead to an initial phase of N immobilization due to the decomposition of a readily-decomposable fraction of slurry organic matter. This may be dead microbial tissue but such material would be expected to have a narrow C:N ratio. There may be a period of net mineralization of N but several studies, including the present, have been unable to show a subsequent net mineralization that was greater than the control soil which could be attributed to mineralization of the slurry organic N or the immobilized N. This is not so surprising if a large fraction of the slurry organic N is extremely resistant to decomposition, but would appear to conflict with the evidence from some field experiments which suggest that slurries have a residual
Immobilization
of nitrogen
effect due to the mineralization of organic N in the years following application. This can be represented as a decay series, an approach which has been used by several workers to interpret field experimental data. Nitrogen in animal wastes is assumed to mineralize according to a decay series (0.75; 0.15; 0. IO; 0.05) representing the fraction of the total N of cattle slurry available in the first year and the fraction of the residual organic nitrogen made available by mineralization in the second and subsequent years (Pratt cf ul., 1976). However, experiments in Northern Ireland have found little residual elfect from pig or cattle slurry even following several years of applications to grassland and arable crops (McAllister, 1977). In attempts to model the decomposition of animal slurries and the associated N transformations, the effects of slurry additions on the mineralization and immobilization of N are of considerable importance. Sluijmans and Kolenbrander (1977) defined three fractions of slurry N, namely inorganic-N organic-N mineralizable in the year of application and resistant organic-N which mineralizes in subsequent years. Bhat et al. (1980) also defined three fractions, inorganic-N, mineralizable organic-N and nonmineralizable organic-N in their model and related the rate of mineralization to environmental conditions of pH, temperature and soil moisture content. Such approaches are based on the assumption that the inorganic N in slurry behaves like inorganic N fertilizers and that more N is mineralized as the slurry organic matter decomposes. In the absence of information on the C:N ratio of the different slurry fractions, however defined, it was assumed that these were the same as the overall C:N ratio and that the decomposition would result in mineralization of N. Our results clearly show that these assumptions are not valid under laboratory incubation conditions. The value of incubation studies in predicting what will happen under field conditions is, however, questionable. Further study is required of the decomposition of slurry in soil and of its effects on the immobilization and mineralization of N under both laboratory and field conditions. Ackrlo~c,/~,c/~rmnt/.s-We thank Mrs C. Camsell for technical assistance, This work was supported by grants from the Commission of the European Communities, Brussels and the Agricultural Research Council. REFERENCES Best E. K. (1976) An automated method for determtning nitrate-nitrogen in soil extracts. Quern.slund Journal o/ Agriculturul cmd Animul Scienccv 33, I6 l-l 66.
from pig slurry
335
Bhat K. K. S., Flowers T. H. and O’Callaghan J. R. (1980) A model for the simulation of the fate of nitrogen in fdrm wastes on land application. Journul of‘ Agriculfurul Science Cambridge 94, 1833193. Brown M. W. (1973) A highly sensitive automated technique for the determination of ammonium nitrogen. Journal 01 the Science of Food und Agriculture 24, 1119-1123. Campbell C. A., Stewart D. W.. Nicholaichuk W. and Viederbeck V. 0. (1974) Effects of growing season so11 temperature, moisture and NH,-N on soil nitrogen. Cunidiun Journul of‘ Science 54, 403412. Cooper J. E. (1975) Nitrification in soils incubated wtth nie slurry. Soil‘ Biology & Biochemi.v/r~ 7, I I Y- 124. ’ c Evans M. R., Hissett R.. Smith M. P. W. and Ellam D. F. (1978) Characteristics of slurry from fattening pigs. Agricul/ure
und Ent%wnnenr
4, 77-83.
Gcrmon J. C.. Giraud J. J.. Chaussod R. and Duthion C. (1979) Nitrogen mtncralisation and nitrilication of pig slurry added to soil in laboratory conditions. In Mudelling Nitrogen /ram Furm Wus/es (J. D. R. Gasser. Ed.). pp. 170-184. Apphed Science Publishers. London. Jenkinson D. S. (1977) Studies on the decomposition of plant material in soil. V.The effects of plant cover and soil type on the loss of carbon from ‘% labelled ryegrass decomposing under field conditions. Journal of Soil Science 28, 424434.
Loynachan T. E.. Bartholemew W. V. and Wollum A. G. (1976) Nitrogen transformations in aerated swine manure slurries. Journal o/ Enrironmentul Quuliry 5, 293-297.
G., Kaunat h. and Langecker W. (1973) Release of nitrogen and carbon from slurry under controlled conditions. Archias fur Acker-unt Pfunzenbuu und Bodenkunde
Maass
17, 645-659.
McAllister
of nutrients. In CEC Modena Seminar. Sept. 1976. EUR 5672 e. pp. 87-103. McCalla T. M., Peterson J. R. and Jue-Hing C. (1977) Properties of agricultural and municipal wastes. In Soils ,/i)r the Manugement qf OrganicWas/es und Wusle Waters (L. F. Elliot and F. J. Stevenson. Eds). pp. 1l-43. A.S.A.-C.S.S.A.-S.S.S.A., Madison. Ministry of Agriculture. Fisheries and Food (1976) Organic manures. Bulletin 210, HMSO. London. Pratt P. F., Davis S. and Sharpless R. G. (1976) A four year tield trial with animal manures. 2. Minerdlisation of nitrogen. Hilxurdiu 44, I l3- 125. Sluijmans C. M. J. and Kolenbrander G. J. (1977) The significance of ammal manure as a source of nitrogen in soils. Proceedings o/ Seminur on Soil Environment und Fcrri/i/!, Munu~emenr in lnrensicr Agriculture. Tokyo. pp. 40334 I I. Stanford G. and Smith S. J. (1972) Nitrogen minerahsatton potentials of soils. Soil &icnce Socie/,r of Amerrco Proc~cwlm~s 36, 4655472. Utilisation
J. S. V. (1977) Efficient recycling of Manure
by Lundspreading.
FLOWERS*
and P. W.
ARNOLD
OT Soil Science. University of Newcastle upon Tyne. Tyne and Wear NEI 7RU, U.K.
(Accepfrd
IO Seprember
OF PIG
Newcastle
upon Tync.
1982)
Summary-Nitrogen mineralization and immobilization were investigated in two soils incubated with ammonium sulphatc or pig slurry over a range of temperatures and moisture contents. A reduction in the mineralization of soil organic N was observed in soils incubated with IOOpg NH:-N gg’ soil as ammonium sulphate at 30°C but not at lower temperatures. Addition of IOOpg NHf-N g-’ soil as pig slurry resulted in a period of nett immobilization lasting up to 30 days at 5°C. Although the length of the immobilization phase was shorter at higher temperatures the total N immobilized was similar. The subsequent rate of mineralization in slurry-treated soils was not significantly greater (P = 0.05) than in untreated soils. There was no evidence of any subsequent increased mineralization arising from the immobilized N or slurry organic N for up lo 175 days. The rate of immobilization was found to increase with increasing moisture content, though the period of nett immobilization was shorter, so that the amount of N immobilized was similar over a range of moisture contents from IO to 40%. Approximately 40”,,, of the NH:-N in the slurry was immobilized under the incubation conditions used.
INTRODUCTION
Changes in the methods of livestock rearing have led to the handling of animal wastes as slurry rather than the traditional farmyard manure. The majority of wastes are still spread on the land and due to the increasing costs of inorganic fertilizers more emphasis has been placed on the fertilizer value of wastes in recent years. They are, however, bulky heterogeneous materials, of variable composition and the ratio of the major plant nutrients, N, P and K may not be balanced in terms of crop requirements. For example, they must often be supplemented by additional inorganic N fertilizer. Particular attention is being given to the N content of animal slurries both because of the importance of crop responses to this element and the risks of nitrate pollution of surface and ground waters if excessive applications are used. Animal slurry can be considered as a mixture of faeces and urine, plus smaller amounts of other organic material such as wasted feed and bedding, with varying amounts of additional water from various sources depending on the conditions of collection. As voided by the animal the N in the faeces and urine is almost entirely in organic forms but urea and some labile organic compounds are rapidly converted to ammonium. There is considerable variation in both the composition and the proportion of N in the inorganic form. Usually, about half the N is present as ammonium, other inorganic forms generally being absent (Evans et al., 1978; Ministry of Agriculture, Fisheries and Food (M.A.F.F.), 1976). A large fraction of the total N is present as organic for& of varying resistance to microbial decomposition and it is important to measure the con‘Present address: Agricultural Chemistry, Chemistry Department, University of Glasgow, Glasgow Cl2 8QQ, U.K.
tribution of this fraction to the pool of plant available N in the first and subsequent years after application. Nitrogen immobilization and losses by denitrification may be encouraged by the presence of large amounts of readily-decomposible organic matter in the slurry. We have studied the mineralization and immobilization of pig slurry N in soil under laboratory incubation conditions. MATERIALS
AND METHODS
A single sample of pig slurry (Table I) was collected from under a veranda-type house on the University farm, at Cockle Park, Northumberland, thoroughly mixed and divided into small sub-samples which were deep frozen and stored at -20°C until required. Some details of the slurry composition are given in Table I. Soil samples (O-15 cm depth) were collected fresh from the field for each experiment. The soils were air-dried just sufficiently to permit sieving (< 2 mm) and for additions to be made without exceeding the desired moisture content for incubation. Two arable soils were used, a sandy loam (Rivington series) and a clay loam (Dunkeswick series) both from the University Farm, Cockle Park (Table 2). Soils were incubated in 350 ml polystyrene pots, closed with lids having a 2.5 cm dia hole to permit aeration. The equivalent of 50 g air-dried soil was added to each pot and the appropriate amendment solution added as required. Following thorough mixing of the soil, the moisture content was adjusted to the required level by the addition of deionized water. To reduce water losses during incubation, while providing adequate aeration, the pots were placed in polyethylene boxes in which a moist atmosphere could be maintained by pumping through a stream of moist air. The air stream was first washed by passing it through 5% HzSO,. then I M NaOH and then three
T. H. FLOWEKSand P. W. ARNOLD
330 Table
Table
I. Composition of the slurry sample used g I-’ in fresh slurry
Sand 2.CM.02mm (%) Silt 0.02-0.002 mm (:/,I Clay <0.002 mm (“J pH (5:l in 0.01 M CaCI,) Water holding capacity (g g Organic carbon (“{,) Total nitrogen (y,,) C:N ratio
7.70 84.5 71.2 30.9 4.23 2.45
PH Total solids Suspended solids Organic carbon Kjeldahl nitrogen Extractable NH;-N Extractable NOT-N Organic nitrogen Total copper Total zinc
0.00 1.78 0.064
water. incubation kO.5 c.
0.05 I
E~sp?riment
deionized water washes. The pots were incubated in a horizontal position so that the soil could be spread out in a layer of a maximum thickness 1 cm. Moisture losses were: /corrected by the addition of deionized 250
2. Some characteristics
’
of the soils
Rivington series
Dunkeswick
69.8
51.4
17.5 12.1 5.8 5.02 1.88 0.14
19.9 28.6
soil)
series
6.0 6.3 3.01 0.24 12.5
13.4
was at the stated temperatures
I
The two soils were treated with IOOng NH:-N g-’ as ammonium sutphate or in pig slurry. Untreated controls were also inchtded. Samples were incubated
la) 5°C
2w
4.50I
25 i
50 L
75 t
100 t
125 I
150 t
175 ,
0 1
25 I
50 t
75 ,
t
I
100
125
150
1
175
,
125
150
175
TIME (days!
25or
tc)l5'c
r
t 4,5*k---k ’ 50
75
t
!
I
J
100
125
150
175
(d)30"C
25
0
TIkIE (days) Fig. I, Eifect of tcmpcraturc and nitrogen additions on the mineralization of nitrogen m Rivmgton series soil. (+---•) No addition, (O--O) IOOpg NH:-N g--’ soil as ammonium sulphate. (A--A) 100 NH:-N g-’ soil as pig slurry. Symbols in Figs I. 3 and 4 represent the mean of three replicate
jig
incubations.
lrnn~obiiizatj~n of nitrogen from pig slurry
7.0 f
25
75
50
TIME
100
125
150
175
(days!
Fig. 2. Et%ecct of temperature and nitrogen additions on the mi~e~li~t~~~ of nitrogen in Dunkeswick series ~4. (+---e) No addition, {O----Of 100 yg NH;-N g-’ soil as ~mrno~j~rn sulphate @+-A) iOOpg NH:-N km’ soil as pig slurry. Symbols represent the mean or five replicate incubations.
at. 5, IO, 15 and 3O’C (Dunkeswick soil was not incubated at 5°C) and at a~proximateiy SO*i,water holding capacity (30’;: w/w moisture content for the Dunkeswick soil and 2O’j: w/w moisture content for the Rivington soil).
The two soils were treated with 250 pg NH:-N g-’ as ammonium suiphate or in pig slurry and incubzsted at 15°C over a range of moisture contents. Ammonium-N, nitrite-N and nitrate-N were extracted by shaking for 1 h with 0.5 M K,SQ, solution at a soil:solution ratio 1:20. Ammonium was determined using an automated phenol hypochlorite method {Brown, 1973). Nitrate and nitrite were determined using the automated procedure of Best (1976).
RESULTS
Slurry addition caused a rise in the pH of both soils while nitrification of added ammonium caused a fail ofpH (see Figs I and 2). The pH changes were slightly larger in the less well-buffered Rivington soil. The Dunkeswick soil showed an upward trend in pH during the incubation. The combined effect of the pH changes resulted in a lower pfT in the ammonjum sulphate treated soils compared with the control and slurry treated soils except in the Rivington soil at 30°C where nitrification was inhibited. The N accumulated as nitrate in both soils and for all treatments except the Rivington soil at 30°C where nitrification was inhibited sufbcientfy for ammonium to accumu-
332
T. H. FLOWERS Table
and P. W. ARNOLD
3. Mineralization rate constants (PgN g-’ Rivinaton and Dunkeswick soils Incubation IO
5 series Untreated Ammonium sulphate Slurry Dunkeswick series
soil day-‘)
temperature I5
30
for
Rivington
0.080~ 0. I I30 0.1 I Sa
0.188 0.265 0.373
0.480 0.686/J
ND ND ND
0.293~~ 0.296~~ 0.249~
0.3786 0.349hc 0.413/I
1.039d 0.931 1.16ld
Untreated Ammonium Slurry
0.712h
0.070a O.ll3a 0.118~
sulphate
Within each soil series for comparisons between temperatures and amendments, values with the same following letter are not significantly different (P = 0.05). ND-not determined.
late in addition to nitrate. Nitrite was not present at a concentration greater than 1 pg g-’ soil at any time. After the initial flush of mineralisation, the control and ammonium sulphate treatments showed a linear accumulation of inorganic N, with time, in both soils (see Figs 1 and 2). Both soils showed an initial decline in total inorganic N following slurry addition. The length of this phase was greatest at 5°C where it lasted approximately 30 days while at 30°C it was only apparent from a reduction in mineralization during the first 10 days. The total loss of inorganic N, estimated at the end of this phase and after allowing (0)
warer cantent
10%
r
300 r
(c) 20%
0
5
10
mineralization in the control soils, was approximately 28% of the applied slurry NH:-N. After this initial phase the increase in inorganic N was linear with time. Regression lines were fitted to the linear regions in order to calculate mineralization rate constants (see Table 3). The pattern of scatter of the data about the fitted lines suggests a systematic variation of inorganic N concentration between sampling dates. It was therefore not possible to perform an analysis to test for linearity and the regression lines were fitted to the means of the replicate N concentrations at each date.
for
water content
15
20
TIME
ldavrl
25
t b) I5 %
30%
IdI
30
35
40
0
5
water
content
water
10
content
15
20
TIME
ldavsl
25
30
35
40
Fig. 3. Effect of soil moisture content and addition of pig slurry on the changes in NH;-N. NOT-N and total inorganic nitrogen levels in Rivington series soil. (m-M) Total inorganic N in untreated soil, (m--&l) total inorganic N in soil treated with 25Opg NH;-N g-’ soil as ammonium sulphatc. (o---_O) total inorganic N in soil receiving 250 pg NH: -N g- ’ soil as pig slurry, (A--A) NH; -N in pig slurry treated soil. (0-O) NOi -N in pig slurry treated soil.
Immobilization of nitrogen from pig slurry
r
333
(bl 25%
water
content
td 140%
wafer
content
7 r” 250 1 c 200 K
350
r
lc) 35%
5
wafer
10
15
conrent
20
r
25
30
35
TIME ldayrl
TIME Idbvr)
Fig. 4. Effect of soil moisture content and addition of pig slurry on the changes in NH.++-N,NOT-N and total inorganic nitrogen levels in Dunkeswick series soil. (a---m) Total inorganic N in untreated soil, (~--a) total inorganic N in soil treated with 25Opg NH;-N g-’ soil as ammonium sulphate. (o--_o) total inorganic N in soil receiving 250 pg NH:-N g-’ soil as pig slurry, (A---A) NH:-N in pig slurry treated soil, (0-O) NOT-N in pig slurry treated soil. Experiment 2
pleted more rapidly at the higher moisture contents so that the total loss of inorganic N, allowing for the mineralization in the control soils, was similar irrespective of the moisture content (see Table 4). There were losses of inorganic N from the ammonium sulphate treated Rivington soil but these were much smaller than the losses in the slurry-treated soils.
In the second experiment the concentrations of inorganic N were measured more frequently to examine the decline in inorganic N following slurry addition in more detail. Figures 3 and 4 show ammonium, nitrate and total inorganic N for the two soils following treatment with 250 pg NH:-N g’ in pig slurry and incubation at a range of moisture contents. Also shown is the total inorganic N in the control and ammonium sulphate (250 pg NH:-N gg’ soil) treatments. In both soils at all moisture contents there was a marked decline in mineral N in the slurry treatments. The rate of decline was greatest in the initial stages of the incubation and increased with soil moisture content. However, the process was com-
DISCUSSION
It is generally accepted that only a part of the N present in soil organic matter is susceptible to decomposition. This has been termed potentially mineralizable N by Stanford and Smith (1972) and Campbell et ul. (1974) who suggested that the rate of mineral-
Table 4. Maximum immobilization of inorganic nitrogen in slurry treated soils incubated at a range of moisture contents Moisture content (%) IO 15 20 25 30 35 40 ‘Immobilization incomplete.
Maximum immobilization (% of applied NH,-N) Rivington series Dunkeswick series 41 38 41 44
30’ 31 39 36
314
T. H.
FLOWERS and
ization would be proportional to the concentration of potentially mineralizable N and fitted their data to tirst order reaction kinetics. An exponential decay rate has also been used frequently to describe the decomposition of added plant material in soil. Jenkinson (1977) fitted the results of a IO-yr study of the decomposition of plant material to a double exponential model which predicted that approximately 70% of the plant material decayed with a half life of 0.25 yr and the remainder with a half life of 8 yr. However, in our study the data for both control and amended soils (see Figs 1 and 2) were better fitted to a straight line relationship after the initial flush of mineralization, caused by disturbance of the soil, had subsided. Zero order rate constants calculated from the slopes of the fitted regression lines were used as the basis for comparing mineralization rates (see Table 4). Both soils showed the expected increase in mineralization rate with increase in temperature except that the Rivington soil showed no increase over the range 5-10°C for any of the treatments. Only in one instance was the rate of mineralization in the slurrytreated soil significantly greater than in the untreated control. However, compared with the ammonium sulphate treatment there is some evidence of a small contribution to mineralization by the organic fraction of slurry N. Although the contribution of slurry organic N to the pool of plant available N was too small to detect, the initial losses of inorganic N from the slurrytreated soils were large. In the second experiment ammonia volatilization losses were found to be negligible for both soils, even though transient increases of up to 1 pH unit were observed during the first week. This extremely low loss of ammonia was probably due to the slurry being thoroughly mixed with the soil. Nitrite did not accumulate at concentrations greater than I kg N gg’ soil at any stage of the incubation. Losses of inorganic N were observed over the full range of moisture contents and although the rate of loss was greatest at the highest moisture contents the total losses (see Table 4) were similar at all moisture levels. The loss was therefore unlikely to have been due to denitrification, particularly as the fastest rates of disappearance were greater than the corresponding nitrification rates, and can therefore be attributed to microbial immobilization. The rate of nitrate accumulation was constant throughout the early stages of incubation of Rivington soil (Fig. 3) and the reduction in total inorganic-N was due to an apparent loss of ammonium indicating that immobilization was mainly from ammonium. In Dunkeswick soil nitrate accumulation showed a lag phase suggesting immobilization of both ammonium and nitrate. Immobilization was marked in both soils, up to 40% of the applied ammonium N in the slurry being immobilized, and there was no clear evidence that either this N or the organic N initially present in the slurry was mineralized even after prolonged incubation. Many of the incubation studies using animal wastes reported in the literature are difficult to interpret because no distinction was made between organic and inorganic N initially present in the slurry.
P. W.
ARNOLD
Furthermore losses by ammonia volatilization and denitrification are usually not measured. Maass ef (I/. (1973) studied N and C mineralization in soils treated with 30-l 50 pg g-’ soil of total nitrogen as cattle, pig or poultry slurry and incubated at 25°C. Their results showed that the level of addition of slurry had no effect on the mineralization of N, but the results are difficult to interpret because there were losses of inorganic N in the early stages of the incubation. Cooper (1975) incubated two soils at 30°C and field capacity for 5 weeks to follow the nitrification of pig slurry NH:-N. Interpretation of his published figures to estimate changes in total inorganic N reveals losses of N in some experiments which may have been due to denitrification, ammonia volatilization or immobilization. Germon et (11.(1979) studied the mineralization of N in soils treated with pig slurry and incubated at field capacity and a range of temperatures. At 8 and 20°C some soil N was mineralized in the first week but none thereafter, while at 28°C there was mineralization throughout the 8 weeks of incubation. With additions of pig slurry there was no initial immobilization of N nor was there any additional mineralization that could be attributed to slurry organic N. Ammonia volatilization losses were found to be negligible and denitrification losses were unlikely under the incubation conditions used. McCalla et al. (1977) suggested that the organic fraction of animal waste consists mainly of two forms. (i) Proteins that have resisted animal digestion and are more or less combined with lignin or lignin-like substances. (ii) Dead and living microbial cells from the intestinal tract. The relative amounts of these two fractions are not known and may be expected to vary with the age and diet of the animal and possibly with storage. In particular, differences may be expected between the slurry from ruminant and non-ruminant animals. Very different rates of decomposition may be expected from the two fractions. Material that has resisted digestion by ruminant animals may be expected to be very resistant to further decomposition in the soil. Loynachan et al. (1976) reported that approximately 25% of the total N in swine liquid manure was resistant to mineralization under extensive aerobic digestion. Incubation studies show that the addition of pig or cattle slurry to soil can lead to an initial phase of N immobilization due to the decomposition of a readily-decomposable fraction of slurry organic matter. This may be dead microbial tissue but such material would be expected to have a narrow C:N ratio. There may be a period of net mineralization of N but several studies, including the present, have been unable to show a subsequent net mineralization that was greater than the control soil which could be attributed to mineralization of the slurry organic N or the immobilized N. This is not so surprising if a large fraction of the slurry organic N is extremely resistant to decomposition, but would appear to conflict with the evidence from some field experiments which suggest that slurries have a residual
Immobilization
of nitrogen
effect due to the mineralization of organic N in the years following application. This can be represented as a decay series, an approach which has been used by several workers to interpret field experimental data. Nitrogen in animal wastes is assumed to mineralize according to a decay series (0.75; 0.15; 0. IO; 0.05) representing the fraction of the total N of cattle slurry available in the first year and the fraction of the residual organic nitrogen made available by mineralization in the second and subsequent years (Pratt cf ul., 1976). However, experiments in Northern Ireland have found little residual elfect from pig or cattle slurry even following several years of applications to grassland and arable crops (McAllister, 1977). In attempts to model the decomposition of animal slurries and the associated N transformations, the effects of slurry additions on the mineralization and immobilization of N are of considerable importance. Sluijmans and Kolenbrander (1977) defined three fractions of slurry N, namely inorganic-N organic-N mineralizable in the year of application and resistant organic-N which mineralizes in subsequent years. Bhat et al. (1980) also defined three fractions, inorganic-N, mineralizable organic-N and nonmineralizable organic-N in their model and related the rate of mineralization to environmental conditions of pH, temperature and soil moisture content. Such approaches are based on the assumption that the inorganic N in slurry behaves like inorganic N fertilizers and that more N is mineralized as the slurry organic matter decomposes. In the absence of information on the C:N ratio of the different slurry fractions, however defined, it was assumed that these were the same as the overall C:N ratio and that the decomposition would result in mineralization of N. Our results clearly show that these assumptions are not valid under laboratory incubation conditions. The value of incubation studies in predicting what will happen under field conditions is, however, questionable. Further study is required of the decomposition of slurry in soil and of its effects on the immobilization and mineralization of N under both laboratory and field conditions. Ackrlo~c,/~,c/~rmnt/.s-We thank Mrs C. Camsell for technical assistance, This work was supported by grants from the Commission of the European Communities, Brussels and the Agricultural Research Council. REFERENCES Best E. K. (1976) An automated method for determtning nitrate-nitrogen in soil extracts. Quern.slund Journal o/ Agriculturul cmd Animul Scienccv 33, I6 l-l 66.
from pig slurry
335
Bhat K. K. S., Flowers T. H. and O’Callaghan J. R. (1980) A model for the simulation of the fate of nitrogen in fdrm wastes on land application. Journul of‘ Agriculfurul Science Cambridge 94, 1833193. Brown M. W. (1973) A highly sensitive automated technique for the determination of ammonium nitrogen. Journal 01 the Science of Food und Agriculture 24, 1119-1123. Campbell C. A., Stewart D. W.. Nicholaichuk W. and Viederbeck V. 0. (1974) Effects of growing season so11 temperature, moisture and NH,-N on soil nitrogen. Cunidiun Journul of‘ Science 54, 403412. Cooper J. E. (1975) Nitrification in soils incubated wtth nie slurry. Soil‘ Biology & Biochemi.v/r~ 7, I I Y- 124. ’ c Evans M. R., Hissett R.. Smith M. P. W. and Ellam D. F. (1978) Characteristics of slurry from fattening pigs. Agricul/ure
und Ent%wnnenr
4, 77-83.
Gcrmon J. C.. Giraud J. J.. Chaussod R. and Duthion C. (1979) Nitrogen mtncralisation and nitrilication of pig slurry added to soil in laboratory conditions. In Mudelling Nitrogen /ram Furm Wus/es (J. D. R. Gasser. Ed.). pp. 170-184. Apphed Science Publishers. London. Jenkinson D. S. (1977) Studies on the decomposition of plant material in soil. V.The effects of plant cover and soil type on the loss of carbon from ‘% labelled ryegrass decomposing under field conditions. Journal of Soil Science 28, 424434.
Loynachan T. E.. Bartholemew W. V. and Wollum A. G. (1976) Nitrogen transformations in aerated swine manure slurries. Journal o/ Enrironmentul Quuliry 5, 293-297.
G., Kaunat h. and Langecker W. (1973) Release of nitrogen and carbon from slurry under controlled conditions. Archias fur Acker-unt Pfunzenbuu und Bodenkunde
Maass
17, 645-659.
McAllister
of nutrients. In CEC Modena Seminar. Sept. 1976. EUR 5672 e. pp. 87-103. McCalla T. M., Peterson J. R. and Jue-Hing C. (1977) Properties of agricultural and municipal wastes. In Soils ,/i)r the Manugement qf OrganicWas/es und Wusle Waters (L. F. Elliot and F. J. Stevenson. Eds). pp. 1l-43. A.S.A.-C.S.S.A.-S.S.S.A., Madison. Ministry of Agriculture. Fisheries and Food (1976) Organic manures. Bulletin 210, HMSO. London. Pratt P. F., Davis S. and Sharpless R. G. (1976) A four year tield trial with animal manures. 2. Minerdlisation of nitrogen. Hilxurdiu 44, I l3- 125. Sluijmans C. M. J. and Kolenbrander G. J. (1977) The significance of ammal manure as a source of nitrogen in soils. Proceedings o/ Seminur on Soil Environment und Fcrri/i/!, Munu~emenr in lnrensicr Agriculture. Tokyo. pp. 40334 I I. Stanford G. and Smith S. J. (1972) Nitrogen minerahsatton potentials of soils. Soil &icnce Socie/,r of Amerrco Proc~cwlm~s 36, 4655472. Utilisation
J. S. V. (1977) Efficient recycling of Manure
by Lundspreading.