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Nitrogen transformations in sludge amended soils
The Science o f the Total Environment, 37 (1984) 163--169 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
163
NITROGEN T R A N S F O R M A T I O N S IN SLUDGE AMENDED SOILS
G. B. REDDY, S. J. DUNN and S. M. ROSS
Department of Plant Science and Technology, North Carolina Agricultural and Technical State University, Greensboro, NC 27411 (U.S.A.) (Received May 5th 1983; accepted October 20th, 1983)
ABSTRACT A laboratory batch incubation study was conducted to evaluate the decomposition and N-transformation in the following soil: sludge mixtures representing, 0, 170 and 400 tons of sludge/ha. Soils selected for this study were Mecklenburg clay and Enon sandy loam of the Piedmont region in North Carolina. Soil~sludge mixtures were adjusted to pH 6.7 and 65--70% of water-holding capacity. Samples were taken at 0, 21, and 42 days of incubation to measure parameters. The rate of CO2 evoluted from the treatments was directly related to the amount of sludge in the soil and also to the N-mineralization. The amount of N-mineralized during the first 21 days of incubation was a function of the rate of dried sludge added to both soils. The greatest amount of N-mineralized was 141.0 and 109.5pg/g in the 4 0 0 t o n s / h a treatment by 21 days for Mecklenburg clay and Enon sandy loam soils, respectively. In both soils, the nitrification rate rapidly increased (P'~ 0.05) during the first 21 days and subsequently decreased by 42 days of incubation.
INTRODUCTION
The application of sewage sludge to agricultural land for crop production has gained considerable interest in recent years. Studies have shown that application of sludge can increase crop yields [2, 4, 5] indicating its efficacy as a fertilizer and soil conditioner. When applied to the soil, sludge will decompose [1, 9]. Miller [9] and Terry et al. [15] reported that the decomposition rate of the sludge was rapid during the first 28 days of incubation and declined to a slow rate and eventually remained constant. The rate of sludge decomposition is more dependent on sludge composition and incubation conditions than on soil properties [11]. Miller [9] found that sludge decomposition was much more limited in clay than in sandy soils. However, the degree of decomposition [16] and CO2 evolution [1 ] decrease with increasing sludge loading rates. It is important to determine the N-mineralization and transformation rates of any substrate applied to the soil for the release of nitrogen to increase crop yields. Hsieh et al. [6] found that the amount of N that accumulated as sewage sludge decomposed at 22°C was proportional to the rate
0048-9697/84/$03.00
© 1984 Elsevier Science Publishers B.V.
164 of sludge application when rates of digested sludge did not exceed 8% and rates of activated sludge did not exceed 4%. However, Stark and Clapp [13] found that sludge application rates were more important than the sludge type in affecting N-mineralization. Since the work on N-transformations in Piedmont soils with air dried anaerobically digested sludge was not fully investigated, this study was conducted to evaluate the effect of soil type, sludge application rates, and incubation periods on decomposition, N-mineralization, and nitrification.
METHODS Soil samples from the top 15 cm of a Mecklenburg clay soil (fine, mixed, thermic, Ultic Hapludalfs) and Enon sandy loam soil (fine, mixed, thermic, Ultic Hapludults) were collected, air dried and crushed to pass through a 2-mm sieve. Sewage sludge (anaerobically digested) was obtained from the drying beds of a sewage treatment plant (Winston-Salem, NC) and following collection, it was sun dried and ground to pass through a 1-mm sieve. The general characteristics of the soils and sludge are presented in Tables 1 and 2. The sludge and soils were stored at 4°C. Soil and sludge were mixed proportionately to obtain 15g samples of total treatments. The mixtures of soil--sludge were prepared, representing 0, 140, and 400 tons/ha sludge. The sludge application rates are based on the dry weight. Fifteen g soil--sludge mixtures of each treatment were mixed thoroughly in duplicate, adjusted to pH 6.7 with CaCO 3 or elemental sulfur, and placed in 20-ml screw-capped vials. Predetermined amounts of distilled water were added to the treatments, and uniform moisture levels of 65--70% water holding capacity were obtained. Screw caps were placed loosely on the vials to allow air into the samples, and samples were incubated at room temperature (25 + 2°C). At 0, 21, and 42 days of incubation, samples were taken, and NH~--N, NO~--N and NO~--N were determined in 2MKC1 extractions (shaking 1 g soil: sludge mixtures in 10ml 2MKC1 solution for 2 h and filtering through Whatman No. 40 filter paper). A Technicon autoanalyzer II was used to determine NH~--N by the EPA [3] methods using sodium salicylate/sodium nitroprusside and for NO~--N and NO~--N, hydrazine sulfate was used as a reducing agent. The Walkley--Black method [7] was used to determine organic carbon in soils and sludge. The pH was measured in a 1 : 1 (w/v) paste. Soil--sludge mixtures (100-g samples) of varying proportions of sludge were placed in wide mouth dilution milk bottles. The pH and water content were adjusted to the same as mentioned previously. A manifold system for continuous aeration with CO2-free air was used [12] and passed through the samples. CO 2 evolved was trapped in KOH, and the amount was determined after an addition of BaC12 by titrating with standard HC1 using phenolphthalein as indicator. Mineralized-N and nitrified-N were determined by using the following general equations:
165 TABLE 1 SELECTED PROPERTIES OF THE SOILS
Sand (%) Silt (%) Clay (%) Texture pH (1 : 1) Cation exchange capacity meq/100 g (O.D.) soil Organic carbon (%)
Mecklenburg
Enon
39.8 14.6 45.6 clay 7.1
75.1 17.9 7.0 sandy loam 6.35
16.9 2.39
8.4 5.45
N-mineralized = [Total inorganic-N (NH~ + NO~ + NO~) at final incubation period] - - [ t o t a l inorganic-N at zero incubation period]. Nitrified-N = [(NO~ + NO~)-- N at final incubation period] -[(NO~ + NO~) -- N at zero incubation period]. All the statistical analyses were done by using two-way analyses of variances.
RESULTS AND DISCUSSION
The CO2-evolution rate at different soil--sludge levels as a function of the incubation time is shown in Figs. 1 and 2. The rate of CO2-evolution was directly related to the a m o u n t of sludge added to soils. Decomposition was higher in the first 21 days than in the last 21 days of incubation in both soils. A similar trend was also reported by Terry et al. [15] by TABLE 2 ELEMENTAL COMPOSITION OF SEWAGE SLUDGE Element
Concentration (%)
Element
Concentration (Pg/g)
Total N P K Ca Mg
2.29 1.08 0.03 1.30 0.09
Total-Fe Mn Zn Cu Cd Ni Pb
8250 323 1020 496 14 200 210
Organic C pH ( 1 : 1 ) 4.9
46.2
166
,
~
0 t0ns/ha , 170
,
= 400
8.0
7-0
sludge
6.0 Mecklenburg clay "o
0 0
Enon sandy loam
5.0 4.0
30 0 0
2.0 1.0
21
42
0
21
42
TIME~ days
Fig. 1 and 2. CO2 e v o l u t i o n rate of soil--sludge m i x t u r e s as a f u n c t i o n of the incubation period.
using synthetically prepared sludge which was similar to anaerobically digested sludge. For treated groups, greater CO2-evolution occurred in Mecklenburg clay than in Enon sandy loam. The amount of N-mineralized during the first 21 days of incubation was a function of the rate of sludge addition in both soils (Table 3). The highest amounts of N-mineralized were 141.0 and 109.5pg/g soil: sludge treatment (400 tons/ha sludge) by 21 days for Mecklenburg clay and Enon sandy loam, TABLE 3 T H E R A T E O F M I N E R A L I Z E D - N IN S O I L - - S L U D G E M I X T U R E S U N D E R INCUBAT I O N (in ~g/g s o i l : s l u d g e ) Sludge rate (tons/ha)
Soil type
Days 0--21
0--42
5.1 39.0 141.0
--4.0 43.1 145.0
16.0 27.5 109.5
35.9 51.5 134.1
Mecklenburg clay 0 170 400 E n o n sandy loam 0 170 400
Days of incubation and sludge rates are significant at P ~ 0.05.
167 respectively. A similar trend was reported by Ryan et al. [10] working with anaerobically digested sewage sludge. In general, these values are lower than the liquid sludges added to the soil, and such observation is in agreement with the reports of Stewart et al. [14] and Lunt [8]. Mean daily mineralization rates during the first 21 days of incubation for the 0, 170, and 400 tons/ha groups were 0.24, 1.86, and 6.71 pg N/g; and 0.76, 1.30, and 5.20 pg N/g soils :sludge/day for Mecklenburg and Enon sandy loam soils, respectively. In control treatment of Mecklenburg clay, a negative mineralization was observed and this may be due to the higher immobilization. The data suggest that the release of available N would be m a x i m u m in the first 21 days. Mineralization was slightly higher in Mecklenburg clay than in Enon sandy loam soil, and this trend coincided with the CO2 evolution. The amounts of NH~--N and NO]--N at various incubation periods are shown in Fig. 3 for Mecklenburg clay and in Fig. 4 for Enon sandy loam soil. An increased level of NO3--N and decreased levels of NH~ in both soils indicate oxidation of NH~--N. In both sludge amended soils, nitrification rates increased rapidly during 0--21 days and decreased in the last 21 days of incubation. Nitrification at 42 days of incubation was greater in Mecklenburg clay ( 2 4 9 . 5 p g N / g soil: sludge) than in Enon sandy loam (196.6 pg N/g soil: sludge) soil amended with 400 tons/ha sludge (Fig. 5). Dried sewage sludge is easy to handle and involves less health risks. However, these results suggest that the a m o u n t of mineralization is less compared to liquid sludge. Decomposition, N-mineralization and nitrification was greater in the first 21 days than in the last 21 days.
400
- NH*4-N ..... NO'3-N • 0 tons/ha = 170 400
a. . . . . . . . . . . . . . .
"~ 500
/
/
zW~
/
/ / /
0
a: 200 I...Z
Q
/
/ /
.....
x
I00
0 7
2'1 T I M E , days
42
Fig. 3. Concentrations of NH4--N and NO3--N with days of incubation in Mecklenburg clay soil amended with sludge.
168 _ _ NH~4_N ..... N()i-N • 0 tons/ha
400
, 170
o
.
._-o
400
/ / o. . . . . / /
~- 300
/,
J
J
/ / UJ
/
/
jf
0
~: Z00 I-
0
~
.................
I00
°....
- .............
0 0
g 0
21 TIME ~ days
42
Fig. 4. Concentrations of NH4--N and NO3--N with days of incubation in Enon sandy loam amended with sludge. 0
u. ~-
300 •....
, Mecklenburg clay Enon sandy l o a m
i
i - ~ 200 ZO
jz
I00
0
0
170 TONS (dry) SLUDGE/ha
400
Fig. 5. Nitrification at different sludge rates under 42 days incubation. ACKNOWLEDGEMENTS
This work was supported by a grant from the U.S. Department of Agriculture, Science and Education Administration. We thank Mary Shanks for stenographic assistance.
REFERENCES 1 N . N . Agbim, B. R. Sabey and D. C. Markstrom, Land application of sewage sludge. V. carbon dioxide production as influenced by sewage sludge and wood waste mixtures, J. Environ. Qual., 6 (1977) 446--451.
169 2 J. D. Cunningham, D. R. Keeney and J. A. Ryan, Yield and metal composition of corn and rye grown on sewage sludge-amended soil, J. Environ. Qual., 4 (1975) 448--454. 3 Environmental Protection Agency, Methods of chemical analysis of water and wastes, Cincinnati, OH, 1971. 4 G, E. Ham and R. H. Dowdy, Soybean growth and composition as influenced by soil amendments of sewage sludge and heavy metals: Field studies, Agron. J., 70 (1978) 326--330. 5 T. D. Hinesly, R. L. Jones and E. L. Ziegler, Effects on corn by applications of heated anaerobically digested sludge, Compost Sci., 13, 4 (1972) 26--30. sludge, Compost Sci., 13, 4 (1972) 26--30. 6 Y . P . Hsieh, L. A. Douglas and H. L. Motto, Modeling sewage sludge decomposition in soil: 1. organic carbon transformations, J. Environ. Qual., 10 (1981) 54--58. 7 M. L. Jackson, Soil chemical analysis, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1958. 8 H . A . Lunt, Digested sewage sludge for soil improvement, Conn. (New Haven) Agric. Ext. Bull., 622 (1959). 9 R. H. Miller, Factors affecting the decomposition of an anaerobically digested sewage sludge in soil, J. Environ. Qual., 2 (1974) 356--358. 10 J. A. Ryan, D. R. Keeney and L. M. Walsh, Nitrogen transformations and availability of an anaerobically digested sewage sludge in soil, J. Environ. Qual., 2 ( 1 9 7 3 ) 4 8 9 492. 11 L. E. Sommers, D. W. Nelson and K. J. Yost, Variable nature of chemical composition of sewage sludges, J. Environ. Qual., 5 (1976) 3 0 3 - 3 0 6 . 12 B. P. Spalding, Enzymatic activities related to the decomposition of coniferous leaf litter, J. Soil Sci. Soc. Am., 41 (1977) 622--627. 13 S. A. Stark and C. E. Clapp, Residual nitrogen availability from soils treated with sewage sludge in a field experiment, J. Environ. Qual., 9 (1980) 5 0 5 - 5 1 2 . 14 N. E. Stewart, C. T. Corke, E. G. Beauchamp and L. B. Webber, Nitrification of sewage sludge using miscible displacement and perfusion techniques, Can. J. Soil Sci., 55 (1975) 467--472. 15 R. E. Terry, D. W. Nelson and L. E. Sommers, Decomposition of anaerobically digested sludge as affected by soil environmental conditions, J. Environ. Qual., 8 (1979) 3 4 2 - 3 4 7 . 16 C. F. Tester, L. J. Sikora, J. M. Taylor and J. F. Parr, Decomposition of sewage sludge compost in soil. 1. Carbon and nitrogen transformations, J. Environ. Qual., 6 (1977) 4 5 9 - 4 6 3 .
163
NITROGEN T R A N S F O R M A T I O N S IN SLUDGE AMENDED SOILS
G. B. REDDY, S. J. DUNN and S. M. ROSS
Department of Plant Science and Technology, North Carolina Agricultural and Technical State University, Greensboro, NC 27411 (U.S.A.) (Received May 5th 1983; accepted October 20th, 1983)
ABSTRACT A laboratory batch incubation study was conducted to evaluate the decomposition and N-transformation in the following soil: sludge mixtures representing, 0, 170 and 400 tons of sludge/ha. Soils selected for this study were Mecklenburg clay and Enon sandy loam of the Piedmont region in North Carolina. Soil~sludge mixtures were adjusted to pH 6.7 and 65--70% of water-holding capacity. Samples were taken at 0, 21, and 42 days of incubation to measure parameters. The rate of CO2 evoluted from the treatments was directly related to the amount of sludge in the soil and also to the N-mineralization. The amount of N-mineralized during the first 21 days of incubation was a function of the rate of dried sludge added to both soils. The greatest amount of N-mineralized was 141.0 and 109.5pg/g in the 4 0 0 t o n s / h a treatment by 21 days for Mecklenburg clay and Enon sandy loam soils, respectively. In both soils, the nitrification rate rapidly increased (P'~ 0.05) during the first 21 days and subsequently decreased by 42 days of incubation.
INTRODUCTION
The application of sewage sludge to agricultural land for crop production has gained considerable interest in recent years. Studies have shown that application of sludge can increase crop yields [2, 4, 5] indicating its efficacy as a fertilizer and soil conditioner. When applied to the soil, sludge will decompose [1, 9]. Miller [9] and Terry et al. [15] reported that the decomposition rate of the sludge was rapid during the first 28 days of incubation and declined to a slow rate and eventually remained constant. The rate of sludge decomposition is more dependent on sludge composition and incubation conditions than on soil properties [11]. Miller [9] found that sludge decomposition was much more limited in clay than in sandy soils. However, the degree of decomposition [16] and CO2 evolution [1 ] decrease with increasing sludge loading rates. It is important to determine the N-mineralization and transformation rates of any substrate applied to the soil for the release of nitrogen to increase crop yields. Hsieh et al. [6] found that the amount of N that accumulated as sewage sludge decomposed at 22°C was proportional to the rate
0048-9697/84/$03.00
© 1984 Elsevier Science Publishers B.V.
164 of sludge application when rates of digested sludge did not exceed 8% and rates of activated sludge did not exceed 4%. However, Stark and Clapp [13] found that sludge application rates were more important than the sludge type in affecting N-mineralization. Since the work on N-transformations in Piedmont soils with air dried anaerobically digested sludge was not fully investigated, this study was conducted to evaluate the effect of soil type, sludge application rates, and incubation periods on decomposition, N-mineralization, and nitrification.
METHODS Soil samples from the top 15 cm of a Mecklenburg clay soil (fine, mixed, thermic, Ultic Hapludalfs) and Enon sandy loam soil (fine, mixed, thermic, Ultic Hapludults) were collected, air dried and crushed to pass through a 2-mm sieve. Sewage sludge (anaerobically digested) was obtained from the drying beds of a sewage treatment plant (Winston-Salem, NC) and following collection, it was sun dried and ground to pass through a 1-mm sieve. The general characteristics of the soils and sludge are presented in Tables 1 and 2. The sludge and soils were stored at 4°C. Soil and sludge were mixed proportionately to obtain 15g samples of total treatments. The mixtures of soil--sludge were prepared, representing 0, 140, and 400 tons/ha sludge. The sludge application rates are based on the dry weight. Fifteen g soil--sludge mixtures of each treatment were mixed thoroughly in duplicate, adjusted to pH 6.7 with CaCO 3 or elemental sulfur, and placed in 20-ml screw-capped vials. Predetermined amounts of distilled water were added to the treatments, and uniform moisture levels of 65--70% water holding capacity were obtained. Screw caps were placed loosely on the vials to allow air into the samples, and samples were incubated at room temperature (25 + 2°C). At 0, 21, and 42 days of incubation, samples were taken, and NH~--N, NO~--N and NO~--N were determined in 2MKC1 extractions (shaking 1 g soil: sludge mixtures in 10ml 2MKC1 solution for 2 h and filtering through Whatman No. 40 filter paper). A Technicon autoanalyzer II was used to determine NH~--N by the EPA [3] methods using sodium salicylate/sodium nitroprusside and for NO~--N and NO~--N, hydrazine sulfate was used as a reducing agent. The Walkley--Black method [7] was used to determine organic carbon in soils and sludge. The pH was measured in a 1 : 1 (w/v) paste. Soil--sludge mixtures (100-g samples) of varying proportions of sludge were placed in wide mouth dilution milk bottles. The pH and water content were adjusted to the same as mentioned previously. A manifold system for continuous aeration with CO2-free air was used [12] and passed through the samples. CO 2 evolved was trapped in KOH, and the amount was determined after an addition of BaC12 by titrating with standard HC1 using phenolphthalein as indicator. Mineralized-N and nitrified-N were determined by using the following general equations:
165 TABLE 1 SELECTED PROPERTIES OF THE SOILS
Sand (%) Silt (%) Clay (%) Texture pH (1 : 1) Cation exchange capacity meq/100 g (O.D.) soil Organic carbon (%)
Mecklenburg
Enon
39.8 14.6 45.6 clay 7.1
75.1 17.9 7.0 sandy loam 6.35
16.9 2.39
8.4 5.45
N-mineralized = [Total inorganic-N (NH~ + NO~ + NO~) at final incubation period] - - [ t o t a l inorganic-N at zero incubation period]. Nitrified-N = [(NO~ + NO~)-- N at final incubation period] -[(NO~ + NO~) -- N at zero incubation period]. All the statistical analyses were done by using two-way analyses of variances.
RESULTS AND DISCUSSION
The CO2-evolution rate at different soil--sludge levels as a function of the incubation time is shown in Figs. 1 and 2. The rate of CO2-evolution was directly related to the a m o u n t of sludge added to soils. Decomposition was higher in the first 21 days than in the last 21 days of incubation in both soils. A similar trend was also reported by Terry et al. [15] by TABLE 2 ELEMENTAL COMPOSITION OF SEWAGE SLUDGE Element
Concentration (%)
Element
Concentration (Pg/g)
Total N P K Ca Mg
2.29 1.08 0.03 1.30 0.09
Total-Fe Mn Zn Cu Cd Ni Pb
8250 323 1020 496 14 200 210
Organic C pH ( 1 : 1 ) 4.9
46.2
166
,
~
0 t0ns/ha , 170
,
= 400
8.0
7-0
sludge
6.0 Mecklenburg clay "o
0 0
Enon sandy loam
5.0 4.0
30 0 0
2.0 1.0
21
42
0
21
42
TIME~ days
Fig. 1 and 2. CO2 e v o l u t i o n rate of soil--sludge m i x t u r e s as a f u n c t i o n of the incubation period.
using synthetically prepared sludge which was similar to anaerobically digested sludge. For treated groups, greater CO2-evolution occurred in Mecklenburg clay than in Enon sandy loam. The amount of N-mineralized during the first 21 days of incubation was a function of the rate of sludge addition in both soils (Table 3). The highest amounts of N-mineralized were 141.0 and 109.5pg/g soil: sludge treatment (400 tons/ha sludge) by 21 days for Mecklenburg clay and Enon sandy loam, TABLE 3 T H E R A T E O F M I N E R A L I Z E D - N IN S O I L - - S L U D G E M I X T U R E S U N D E R INCUBAT I O N (in ~g/g s o i l : s l u d g e ) Sludge rate (tons/ha)
Soil type
Days 0--21
0--42
5.1 39.0 141.0
--4.0 43.1 145.0
16.0 27.5 109.5
35.9 51.5 134.1
Mecklenburg clay 0 170 400 E n o n sandy loam 0 170 400
Days of incubation and sludge rates are significant at P ~ 0.05.
167 respectively. A similar trend was reported by Ryan et al. [10] working with anaerobically digested sewage sludge. In general, these values are lower than the liquid sludges added to the soil, and such observation is in agreement with the reports of Stewart et al. [14] and Lunt [8]. Mean daily mineralization rates during the first 21 days of incubation for the 0, 170, and 400 tons/ha groups were 0.24, 1.86, and 6.71 pg N/g; and 0.76, 1.30, and 5.20 pg N/g soils :sludge/day for Mecklenburg and Enon sandy loam soils, respectively. In control treatment of Mecklenburg clay, a negative mineralization was observed and this may be due to the higher immobilization. The data suggest that the release of available N would be m a x i m u m in the first 21 days. Mineralization was slightly higher in Mecklenburg clay than in Enon sandy loam soil, and this trend coincided with the CO2 evolution. The amounts of NH~--N and NO]--N at various incubation periods are shown in Fig. 3 for Mecklenburg clay and in Fig. 4 for Enon sandy loam soil. An increased level of NO3--N and decreased levels of NH~ in both soils indicate oxidation of NH~--N. In both sludge amended soils, nitrification rates increased rapidly during 0--21 days and decreased in the last 21 days of incubation. Nitrification at 42 days of incubation was greater in Mecklenburg clay ( 2 4 9 . 5 p g N / g soil: sludge) than in Enon sandy loam (196.6 pg N/g soil: sludge) soil amended with 400 tons/ha sludge (Fig. 5). Dried sewage sludge is easy to handle and involves less health risks. However, these results suggest that the a m o u n t of mineralization is less compared to liquid sludge. Decomposition, N-mineralization and nitrification was greater in the first 21 days than in the last 21 days.
400
- NH*4-N ..... NO'3-N • 0 tons/ha = 170 400
a. . . . . . . . . . . . . . .
"~ 500
/
/
zW~
/
/ / /
0
a: 200 I...Z
Q
/
/ /
.....
x
I00
0 7
2'1 T I M E , days
42
Fig. 3. Concentrations of NH4--N and NO3--N with days of incubation in Mecklenburg clay soil amended with sludge.
168 _ _ NH~4_N ..... N()i-N • 0 tons/ha
400
, 170
o
.
._-o
400
/ / o. . . . . / /
~- 300
/,
J
J
/ / UJ
/
/
jf
0
~: Z00 I-
0
~
.................
I00
°....
- .............
0 0
g 0
21 TIME ~ days
42
Fig. 4. Concentrations of NH4--N and NO3--N with days of incubation in Enon sandy loam amended with sludge. 0
u. ~-
300 •....
, Mecklenburg clay Enon sandy l o a m
i
i - ~ 200 ZO
jz
I00
0
0
170 TONS (dry) SLUDGE/ha
400
Fig. 5. Nitrification at different sludge rates under 42 days incubation. ACKNOWLEDGEMENTS
This work was supported by a grant from the U.S. Department of Agriculture, Science and Education Administration. We thank Mary Shanks for stenographic assistance.
REFERENCES 1 N . N . Agbim, B. R. Sabey and D. C. Markstrom, Land application of sewage sludge. V. carbon dioxide production as influenced by sewage sludge and wood waste mixtures, J. Environ. Qual., 6 (1977) 446--451.
169 2 J. D. Cunningham, D. R. Keeney and J. A. Ryan, Yield and metal composition of corn and rye grown on sewage sludge-amended soil, J. Environ. Qual., 4 (1975) 448--454. 3 Environmental Protection Agency, Methods of chemical analysis of water and wastes, Cincinnati, OH, 1971. 4 G, E. Ham and R. H. Dowdy, Soybean growth and composition as influenced by soil amendments of sewage sludge and heavy metals: Field studies, Agron. J., 70 (1978) 326--330. 5 T. D. Hinesly, R. L. Jones and E. L. Ziegler, Effects on corn by applications of heated anaerobically digested sludge, Compost Sci., 13, 4 (1972) 26--30. sludge, Compost Sci., 13, 4 (1972) 26--30. 6 Y . P . Hsieh, L. A. Douglas and H. L. Motto, Modeling sewage sludge decomposition in soil: 1. organic carbon transformations, J. Environ. Qual., 10 (1981) 54--58. 7 M. L. Jackson, Soil chemical analysis, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1958. 8 H . A . Lunt, Digested sewage sludge for soil improvement, Conn. (New Haven) Agric. Ext. Bull., 622 (1959). 9 R. H. Miller, Factors affecting the decomposition of an anaerobically digested sewage sludge in soil, J. Environ. Qual., 2 (1974) 356--358. 10 J. A. Ryan, D. R. Keeney and L. M. Walsh, Nitrogen transformations and availability of an anaerobically digested sewage sludge in soil, J. Environ. Qual., 2 ( 1 9 7 3 ) 4 8 9 492. 11 L. E. Sommers, D. W. Nelson and K. J. Yost, Variable nature of chemical composition of sewage sludges, J. Environ. Qual., 5 (1976) 3 0 3 - 3 0 6 . 12 B. P. Spalding, Enzymatic activities related to the decomposition of coniferous leaf litter, J. Soil Sci. Soc. Am., 41 (1977) 622--627. 13 S. A. Stark and C. E. Clapp, Residual nitrogen availability from soils treated with sewage sludge in a field experiment, J. Environ. Qual., 9 (1980) 5 0 5 - 5 1 2 . 14 N. E. Stewart, C. T. Corke, E. G. Beauchamp and L. B. Webber, Nitrification of sewage sludge using miscible displacement and perfusion techniques, Can. J. Soil Sci., 55 (1975) 467--472. 15 R. E. Terry, D. W. Nelson and L. E. Sommers, Decomposition of anaerobically digested sludge as affected by soil environmental conditions, J. Environ. Qual., 8 (1979) 3 4 2 - 3 4 7 . 16 C. F. Tester, L. J. Sikora, J. M. Taylor and J. F. Parr, Decomposition of sewage sludge compost in soil. 1. Carbon and nitrogen transformations, J. Environ. Qual., 6 (1977) 4 5 9 - 4 6 3 .