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Role of soil and residue microorganisms in determining the extent of residue decomposition in soil
Soil Biol. Biochrm. Vol. 20. No. Pnmd in Great Bnlam
6. pp.
915-919.
1988
0038-0717,88
S3.00
+ 0.00
PergamonPressplc
ROLE OF SOIL AND RESIDUE MICROORGANISMS DETERMINING THE EXTENT OF RESIDUE DECOMPOSITION IN SOIL
IN
C. F. TESTER Bldg 318,
Room
108, Soil-Microbial
Systems Laboratory, Agricultural Beltsville, MD 20705, U.S.A. (Accepted
5 May
Research
Service,
USDA,
1988)
Summary-The effects of residue (wheat straw or sewage-sludge compost) incorporation in soil and the relative contribution of microorganisms in the residues, or in the soil to decomposition of the added residue, (COr production) was evaluated in an incubation experiment. All residues and soils were adjusted to 33 kPa moisture tension and maintained at 25°C under a constant flow of CO,-free air for 72 days. Residue decomposition was determined by monitoring CO, evolution from the treatments. Mixing an aged sewage-sludge compost (IO%, 224 Mg ha-‘) with soil stimulated decomposition of the compost l.64-fold when compared with any of the localized placements, and indicated that the indigenous soil microorganisms were the major contributors to the transformations of this mature compost. Wheat straw was populated with organisms capable of decomposing readily-available substrates in the straw during the first stage of the decomposition, whereas it appeared that soil organisms contributed to an acceleration of straw decomposition during the final stages. After 65 days approx. 30% of the added wheat straw C had been evolved as CO,. Soil basidiomycetes doubled the extent of decomposition when the indigenous decomposers in wheat were inactivated by y-irradiation. Model equations are presented for residue decomposition relative to time.
hlATERlALS
INTRODUCTION
An understanding of the factors controlling residue decomposition is prerequisite to the design of conservation tillage systems that will ensure sustainable and profitable agriculture. Some agricultural regions produce an abundance of crop residues, far in excess of that required to control erosion and maintain good soil tilth. In other regions, however, particularly in the lower rainfall areas, with droughty soils and with some crops, insufficient amounts of residues are produced for adequate soil protection. Where limited quantities of residue exist, management methods must be devised to achieve maximum effectiveness of residues for conservation purposes. Where crop residues are produced in excessive amounts, they tend to accumulate because of slow rates of decomposition, causing difficulties with tillage and planting (Parr and Papendick, 1978b). Reduced or minimum tillage systems tend to leave much of the crop residue at the soil surface, compared with conventional tillage. Most knowledge of residue decomposition is based on soil incorporation, whereas little is known about the interplay of factors (e.g. moisture, temperature, particle size of the residue, chemical composition of the residue, C:N ratio, and the indigenous microflora of the residue) that govern the rate and extent of residue decomposition at the soil surface (Parr and Papendick, 1978a). My objectives were to investigate the interactions between the residue microbiota and the soil microbiota and how these affect the decomposition process of residues placed on the soil surface or mixed in the soil. In addition of wheat straw, an aged sewage sludge compost was included as an amendment for comparison purposes. 915
AND METHODS
The soil used was an Evesboro loamy-sand subsoil (Typic Quartzisamments) collected from a coniferous site, air-dried and sieved ( < 2 mm). This soil has been used to evaluate residue decomposition (Tester et al., 1977; Tester and Parr, 1983) and to determine crop response to residue-amended soils (Sikora et al., 1980, 1982; Tester et al., 1982). This soil was also collected from a fescue meadow and exhibited the same physical and chemical properties as from the coniferous site. Lime was added to the soil in the control flasks to yield a pH equivalent to that of the residue-amended soils. The compost used was produced by the Beltsville aerated pile method (Willson et al., 1980) from a mixture of woodchips and a high-limed, undigested sewage-sludge (volume ratio of woodchips:sludge of 2.5:1). The cured compost was passed through a 6-mm Sweco screen to remove large woodchips and stored for 5 yr at 5’C before use (the same compost as used by Tester and Parr, 1983). Mature wheat (Triticum aestivum L. “Potomac”) was harvested, air-dried, and the seed were removed, before the straw was chopped to 2 mm. Some properties of the soil, compost and wheat straw are shown in Table I. All experimental substrates and soils [soil, compost, wheat straw, a mixture of soil and compost (IO%,, 224 Mg ha-‘), or a mixture of soil and wheat straw (5%, I I2 Mg ha-‘)] were adjusted to 33 kPa moisture tension, as defined by the pressure-plate method of Richards (1965). They were equilibrated for 24 h at 5’C before portions were irradiated at 3 Mrad with 6oCo at the Goddard Space Flight
C.
916
TJblc
I
Properties
F.
of the Evcsboro
TESTER
loamy
sand. compost
and wheat
strw
33 kPa Total Materul Ebesboro
sod
Total
N
C-10-N
3.60
stmw
0.103
tension
(%H>O)
35
7.7
15.15
I2
60.0
459.0
8.64
53
210.0
Center. All irradiated samples were stored at 5C for a minimum of 30 days before use. Treatments. where indicated, were aseptically inoculated with either Chaeromium glohosum Kunze ex Fries or Chaetomium cellulol_vticum Chahal and Hawksworth at approx. IO’ propagules per treatment mixture. Either 90 g soil, IO g compost, 5 g wheat straw or 100 g samples of each treatment mixture were placed into individual incubation flasks (6 x l2cm). The treatments were replicated three times and placed randomly on a manifold system with temperature controlled at 25°C. The incoming air was passed through 4 N NaOH and then through distilled water to maintain the 33 kPa moisture tension (Tester and Parr, 1983). The effluent gas from each treatment was passed through a tube containing 2 N NaOH for adsorption of evolved CO,. Empty control flasks were included to determine background levels of co:. The extent of residue decomposition was assessed by CO2 evolution. A specific conductance method was used for CO2 determinations, which utilized a Row-cell connected to a Beckman RA-5 Solu-meter with a Fisher digital concentration computer and printer (Tester and Parr, 1983). Assay temperature was controlled within 0.02 C with a Forma Scientific CH/P control system. The coctlicients of variation of the analytical technique were below 0.2%; thus, cxtrcmcly small effects of treatment were detectable. Total organic C was determined on the soil, residues and appropriate mixtures by dry combustion using a Lcco C analyzer and total Kjeldahl-N was dctcrmincd by block digestion and Technicon AutoAnalyzer II methods (Schuman rt al., 1973). The trcatmcnt mixtures were extracted with H,O or 0.1 bt Ba(OH)2 and the soluble organic C measured
Table
moisture
ratio
181.0
Compost Wheat
C
fmeg~‘drywU
2. Symbol
codes for
with a Technicon TOC analyzer (Industrial method 455-76W/A or 535-781 M). Total organic C and total Kjeldahl-N were performed on dry, ground mixtures (CO.5 mm). Treatments and their symbol codes are presented in Table 2. All localized placements of the residues (either placement on the soil surface, horizontal band placement between soil layers or vertical band surrounded by soil) resulted in essentially the same degree of decomposition of the residue during the incubation. As there were no significant differences between the different placements, they are represented as either LCS or LWS in the tables and figures. RESULTS AND DISCUSSION
Mixing aged sewage-sludge compost with soil (CS) stimulated compost decomposition 64% as compared with the localized placements (LCS) (Fig. I). This finding was opposite to that observed by Parr and Papendick (1978a) for raw sewage sludge. The raw sewage sludge contained a significantly higher indigenous microbial population than did the soil, whereas the aged compost used in my study had been stored for 5 yr at 5’C and, thus, represented a relatively stable organic matter with few viable microorganisms. This same compost, when fresh (i.e. immediately after curing), decomposed three times more quickly than the aged compost in the same soil (Tester and Parr, 1983). Decomposition of the compost was reduced 60% when mixed with irradiated soil (C-IS) as compared with nonirradiated soil (CS) (Fig. I), supporting this explanation. The indigenous soil microorganisms were apparently the major contributors to the transformations of this mature compost when mixed with soil. Tester rt nl. (1977) concluded that a less mature compost alone con-
the specific
materials
IrrJdiared
and
treatments
Inoculated’
Svmbol
Soil
-
-
S
Composl
-
-
C
Wheat
-
Compost Wheat
and soil. and soil.
cs
-
and soil. mixed
Compost
and soil. mtxed
Compost
and soil. mixed and soil.
-
mixed
mixed
Compost
Whrut
W ws -
+
mined
KS
+
C. crlluloi.rlicum
KS-C
+
C. glohosm
ICS-G
+
IWS
Wheat
and soil. mired
+
C. cellulo(vricum
IWS-c
Wheat
and soil. mixed
+ -
C. glohosm -
IWS-G
-
-
Compost Wheat
and soil loulized and soil localized
placement’ placement
LCS LWS
Wheat
and soil.
mixed
Wheat
-
IW-s
When!
and soil.
mixed
Soil only
-
W-IS
Soil
-
C-IS
Compost
and soil. mixed
‘Inoculatrd ‘Either
aseptically
placement
band
plaxment
atier
irradiation
on soil surface. surrounded
only
treatment.
horizonaral
by soil.
only
band
placement
between
soil layers.
or vertical
917
Role of soil and residue microorganisms
or, 03
I
10
20
30 41 4Qk6 TIME (DAYS)
6572 TtME (DAYS)
Fig. I. Cumulative CO2 evolved during the decomposition of IOg of sewage-sludge compost mixed with 90g of Evesboro soil. CS = uniform mixture of compost soil. LCS = localized placement of the compost. C-IS = uniform mixture of compost and irradiated soil.
Fig. 3. Cumulative CO, evolved during the decomposition of 5 g of wheat straw alone (W) and 5 g of irradiated wheat straw mixed uniformly with 95 g of Evesboro soil (IW-S). The irradiated wheat straw-soil mixture (IWS) is shown for reference.
tained sufficient viable microbes to elicit decomposition comparable to that of a compost-soil mixture (i.e. compost decomposition in compost mixtures with sterile sand was comparable to that in compost-soil mixtures). Mixing wheat straw with soil (WS) had no effect on decomposition for the first 15 days as compared with that for the straw alone (W) (Fig. 2). indicating that the straw was populated with organisms capable of decomposing the readily-available substrates, as postulatcd by Simpson et al. (1986). This differed from the results of Stott et al. (1986, Table I, p. 580). who reported that, under optimum conditions, the time required to reach a given degree of decomposition for surface applied straw was one-half that required for incorporated straw. However, in my study, soil organisms apparently contributed to an acceleration of decomposition during the final stage (slopes after 48 days). This observation was supported when irradiated wheat straw was mixed with soil (IW-S) (Fig. 3). in that the extent of decomposition was similar to
that for wheat straw alone for the first 30 days. Thereafter there was an increase in the rate of decomposition which resulted in a 2-fold increase by day 65. This increased rate of straw decomposition was concomitant with the development of soil basidiomycetes (identified by P. D. Millner but not to specific species in present study; unfeasible within the scope of the investigation), which probably contributed to the transformations of the more resistant substrates (Kirk et al., 1980). These soil basidiomycetes were apparently less competitive in the presence of the indigenous wheat straw microbes during the early period of this incubation. The extent of decomposition of wheat straw (Table 3) was comparable to that reported by El-Shakweer el al. (1977). Martin et (11. (1980) Pal and Broadbent
Table
3. Toul
CO,
decomposition
of
evolw~d
the
25 C during
and
indicated
percentage
treatments
at
72 dabs of incubation’
co: evolved Material
2.8
s (IM)g)
‘; 5
5 L t-
1.6
% :
32.0
7a
0.1
S5b
0.8
l2le
1.X
99c
I.?
I4lSh
16.8
I577i
18.7
Corrected
l-2
g0.a E
2.7
26621
ICS ICS-c 112-G IWS IWS-c IWS-G
2
8.9
I aof
w (5 g)
Y
(%)
I17e
c (106)
2.4
Decomposition
(w)
0.4
O,l 03
I
10
20
I
I
30 41 4956 TIME (DAYS)
I
6572
LCS
IIOd
1.7
cs
23lg
3.5
LWS
I899j
22.6
WS
2449k
29.1
IW-S
4946n
58.8
W-IS
3293m
See Table ‘Means the
2 for symbol
of three sane
‘Reported rected evolved
39. I
codes for the mztterial.
replications
letters
(P c 0.001).
Fig. 2. Cumulative CO: evolved during the decomposition of Sg of wheat straw alone (W) and Sg of wheat straw uniformly mixed with 9.5 g of Evesboro soil (WS).
for soil’
are
All
mixtures
quantities
of CO:
for
that were
different 100 g.
evolved
the appropriate
from
do not share
significantly
the untreated
amount soil.
were corof CO:
918
C. F. -
(1975). Paul (1984) Stott er al. (1986) and Zunino et 01. (1982) with the exception of the IW-S treatment, where the soil microorganisms were not in competition with indigenous straw microorganisms. No viable microorganisms were detected in any of the treatments after they had been y-irradiated. Because the samples were stored for a minimum of 30 days after irradiation, all radiolytic decomposition of organic matter should have ceased (Cawse and Mableson, 1971). However, the activities of residual enzymes were not determined, and they possibly contributed to some of the evolved CO, in these treatments (Cawse, 1975; Cawse and Mableson. 1971; Jackson er al., 1967; McLaren, 1969; Powlson and Jenkinson, 1976; Skujins. 1978). The amounts of oresumed nonmicrobial evolved CO, were verv low (Fig. 3 and Table 3), 1% for iriadiated wheat straw-soil mixture (IWS) and 0.1% for irradiated compost-soil mixture (Ids). Inocula of both C. globosum and C. cellulol_vticum survived and proliferated in the irradiated compost-soil and wheat straw-soil mixtures. C. globown adapted more rapidly to the substrates present and evolved 33 and 58% of the C02. respectively, in the irradiated compost-soil and wheat straw-soil mixtures as evolved from the nonirradiated mixtures with their indigenous microbes (Table 3, compare ICS-G less ICS with CS + S and IWS-G less IWS with WS + S). C. cellulol_vticum relcased approx. 14 and 51% of the CO,. respectively, from the irradiated compost-soil and wheat straw-soil mixtures as from their nonirradiated mixtures. The greater proliferation and activity by C. globosum was confirmed by viable propagule counts of both fungi at the end of the incubation (compare I2 x 105cfu g-’ of mixture for C. globosum with 9 x IO”cfu g-’ of mixture for C. cellulolyticum). Transformations of plant residues in soil are not usually described by simple kinetic reaction equations. The complex nature of the more refractile substrates, the multitude of microorganisms and microfauna that elicit the transformations, spatial compartmentalization, and wide variances in the environ-
‘\
CPCOe-kf
\’
I
k=0.00679day-1
\ \.
fl;yo.+a-~\, i
03
10
20
i0 41 i9i66SiS TIME (DAYS)
Fig. 5. Decomposition stages and first-order rate constants for wheat straw mixed uniformly with Evesboro soil.
mental conditions are major contributors to the usual variable nature of measured responses. However. numerous deterministic model equations have been presented, which describe the decomposition of different organic materials in soil (Cheshire et al., 1979; Christensen, 1985; Douglas et al., 1980; Lynch, 1979; Moorhead el al., 1987; Pal and Broadbent, 1975; Paul, 1984; Tester and Parr, 1983; Van Veen and Paul, 1981). The decomposition of the aged sewage-sludge compost in soil can best be described by a power function (with high correlation coefficient) (Fig. 4). The slope agreed well with those reported by Pal and Broadbent (1975) and Tester and Parr (1983). The kinetics of the decomposition of the straw are more complex, as the straw contained soluble carbohydrates as well as cellulose, hemicellulose and lignin. First-order equations describing different stages of decomposition of specific substrates have been described by Moorhead et al. (1987). Paul (1984) and Reddy et al. (1980). The integrated rate equation used to describe the particular stages of wheat straw decomposition in my study was expressed as C = C,e-”
360 /’
/’
320
)/
6oJ
F12= .999 P
/ CO,= 26.36
t-‘6174
40?
03 i0 20 30 41 49 j, TIME(DAYS)
where: C, C k I
/“tcs
/ i
1
05 72
Fig. 4. Comparison of goodness of fit for theoretical power function (-) with actual data points (M) for the decomposition of compost mixed uniformly with Evesboro soil.
= = = =
C concentration at t = 0, C concentration at a specific time, first-order decomposition rate constant, time (days).
A comparison of the derived curves and actual data are given in Fig. 5 for wheat straw uniformly mixed with soil (WS). The rate constants for the cellulose and hemicellulose (0.00679 day-‘) and lignin (O.O0356day-‘) fractions of this mature wheat straw were almost identical to those reported by Moorhead ef al. (1987) for a low N-content water hyacinth decomposing in soil. In conclusion, the results of my study suggest that localized placement of residues retards residue decomposition, which may result in a higher steadystate amount of soil organic matter. This is consistent with the known build-up of organic matter with conservation-tillage systems.
919
Role of soil and residue microorganisms Acknowkdgemena-I thank R. Anthony for expert technical assistance, Drs P. D. Millner and M. E. Simpson for the inocula and identifications, and Larry Bromery of NASA for providing the irradiation facilities.
tems (W. R. Gschwald, Ed.). pp. 245-248. American Society of Agronomy Special Publication No. 31, Madison. Paul E. A. (I 984) Dynamics of organic matter in soils. Plant
REFERENCES
Powlson D. S. and Jenkinson D. D. (1976) The effects of biocidal treatments on metabolism in soil--II. Gamma irradiation, autoclaving, air-drying and fumigation. Soil
and Soil 76, 275-285.
Cawse P. A. (1975) Microbiology and biochemistry of irradiated soils. In Soil Biochemistry (E. A. Paul and A. D. McLaren, Eds), pp. 213-267. Marcel-Dekker, New York. Cawse P. A. and Mableson K. M. (1971) The effect of gamma-radiation on the release of carbon dioxide from fresh soil. Communications in Soil Science & Plunt Analvses 2, 42 143 I. Cheshire M. V., Sparling G. P. and lnkson R. H. E. (1979) The decomposition of straw in soil. In Straw Deccr) and its Eflecr on Disposul and Utilixtion (E. Grossbard, Ed.), pp. 65-71. Wiley, New York. Christensen 8. T. (1985) Wheat and barley straw decomposition under field conditions: effect of soil type and plant cover on weight loss, nitrogen and potassium content. Soil Biology & Biochemistry 17, 691-697. Douglas C. L. Jr, Allmaras R. R., Rasmussen P. E.. Ramig R. E. and Roager R. E. Jr (1980) Wheat straw composition and placement effects on decomposition in dryland agriculture of the Pacific northwest. Soil Science Society of Americu Proceedings
44, 833-837.
El-Shakweer M. H. A., Gomah A. M.. Barakat M. A. and Abdel-Ghaffar A. S. (1977) ElTects of salts on decomposition of plant residues. In Soil Organic Mutter Studies, Vol. 1. pp. 205213. International Atomic Energy Agency, Vienna. Jackson N. E.. Corey J. C., Frederick L. R. and Picken J. C. Jr (1967) Gamma irradiation and the microbial population of soils at two water contents. Soil Science Society of America Proceedings
31. 491494.
Kirk T. K.. Higuchi T. and Chang H. (1980) Lignin Biodegradation: Microbiology, Chemistry and Potentiul Applicufions. Vol. II. CRC Press. Boca Raton.
Lynch J. M. (1979) Straw residues as substrates for growth and product formation by soil microorganisms. In Straw Decoy und its Eflect on Disposal und Utilixtion (E. Grossbard. Ed.), pp. 47-56. Wiley, New York. Martin J. P., Haider K. and Kassim G. (1980) Biodegradation and stabilization after two years of specific crop, lignin, and polysaccharide carbons in soils. Soil Science Society of America Journal 44, 1250-1255. McLaren A. D. (1969) Radiation as a technique in soil biology and biochemistry. Soil Biology & Biochemistry 1. 63-73.
Moorhead K. K., Graetz D. A. and Reddy K. R. (1987) Decomposition of fresh and anaerobically digested plant biomass in soil. Journal of Environmental Quality 16, 25-28.
Pal D. and Broadbent F. E. (1975) Kinetics of rice straw decomposition in soils. Journul of Environmental Quulity 4. 256-60.
Parr J. F. and Papendick R. I. (1978a) Factors affecting the decomposition of crop residues by microorganisms. In Croo Residue Munugement Systems (W. R. Oschwald, Ed.), pp. 101-129. American Society of Agronomy Special Publication No. 31. Madison. Parr J. F. and Papendick R. I. (1978b) Crop residue management systems: new perspective for soil, water, and energy conservation. In Crop Residue Management Sys-
Biology 8 Biochemistry
8, 179-18.
Reddy K. R., Khaleel R. and Overcash M. R. (1980) Carbon transformations in the land areas receiving organic wastes in relation to non-point source pollution: a conceptual model. Journal of Environmental Quality 9, 434-442.
Richards L. A. (1965) Physical condition of water in soil. In Methouk of Soil Anulysis Part I, Agronomy 9(C. A. Black Ed.), pp. 134-137. American Society of Agronomy, Madison. Schuman G. E., Stanley M. A. and Knudsen D. (1973) Automated total nitrogen analysis of soil and plant samples. Soil Science Society of America Proceedings 37, 48048
I.
Sikora L. J., Tester C. F., Taylor J. M. and Parr J. F. (1980) Fescue yield response to sewage sludge compost amendments. Agronomy Journal 72, 79-84. Sikora L. J., Tester C. F., Taylor J. M. and Parr J. F. (1982) Phosphorus uptake by fescue from soils amended with sewage sludge compost. Agronomy Journul74, 27-33. Simpson M. E., Marsh P. B. and Sikora L. J. (1986) Fungal associations on field-weathered plants and their role in residue decomposition. American Journal of Ahernufive Agriculture
I, 89-94.
Skujins J. J. (1978) History of abiontic soil enzyme research. In Soil Enzymes (R. G. Burns, Ed.), pp. 149. Academic Press. London. Stott D. E., Elliott L. F., Papendick R. I. and Campbell G. S. (1986) Low temperature or low water potential effects on the microbial decomposition of wheat residue. Soil Biology & Biochemistry
18, 577-582.
Tester C. F. and Parr J. F. (1983) Decomposition of sewage sludge compost in soil: IV. Effect of indigenous salinity. Journal of Environmental
Ouulitv
12. 123-126.
Tester C. F, Sikora L. J.. figOr J.-M. and Parr J. F. (1977) Decomposition of sewage sludge compost in soil: I. Carbon and nitrogen transformations. Journal of Environmental Quality 6, 459463.
Tester C. F., Sikora L. J., Taylor J. M. and Parr J. F. (1982) Nitrogen utilization by tall fescue from sewage sludge compost amended soils. Agronomy Journal 74, 1013-1018.
Van Veen J. A. and Paul E. A. (1981) Organic carbon dynamics in grassland soils. I. Background information and computer simulation. Canadian Journul of Soil Science 61, 185-201.
Willson G. B., Parr J. F., Epstein E., Marsh P. B.. Chanev R. L., Colacicco D.. Burge W. D., Sikora L. J.. Tester C. F. and Hornick S. 8. (1980) Munuul for Cornposting Sewuge Sludge by the Beltsville Aerated Pile Method. Joint Publication by -USDA/US. Environmental Protection Agency. EPA-600/9-60-002. U.S. Government Printing Office, Washington. Zunino H., Boric-F., Aquilera S., Martin J. P. and Haider K. (1982) Decomposition of “C-labeled alucose. olant and’ microbial products and phenols in ;olcanic’ ashderived soils of Chile. Soil Biology & Biochemistry 14, 3743.
6. pp.
915-919.
1988
0038-0717,88
S3.00
+ 0.00
PergamonPressplc
ROLE OF SOIL AND RESIDUE MICROORGANISMS DETERMINING THE EXTENT OF RESIDUE DECOMPOSITION IN SOIL
IN
C. F. TESTER Bldg 318,
Room
108, Soil-Microbial
Systems Laboratory, Agricultural Beltsville, MD 20705, U.S.A. (Accepted
5 May
Research
Service,
USDA,
1988)
Summary-The effects of residue (wheat straw or sewage-sludge compost) incorporation in soil and the relative contribution of microorganisms in the residues, or in the soil to decomposition of the added residue, (COr production) was evaluated in an incubation experiment. All residues and soils were adjusted to 33 kPa moisture tension and maintained at 25°C under a constant flow of CO,-free air for 72 days. Residue decomposition was determined by monitoring CO, evolution from the treatments. Mixing an aged sewage-sludge compost (IO%, 224 Mg ha-‘) with soil stimulated decomposition of the compost l.64-fold when compared with any of the localized placements, and indicated that the indigenous soil microorganisms were the major contributors to the transformations of this mature compost. Wheat straw was populated with organisms capable of decomposing readily-available substrates in the straw during the first stage of the decomposition, whereas it appeared that soil organisms contributed to an acceleration of straw decomposition during the final stages. After 65 days approx. 30% of the added wheat straw C had been evolved as CO,. Soil basidiomycetes doubled the extent of decomposition when the indigenous decomposers in wheat were inactivated by y-irradiation. Model equations are presented for residue decomposition relative to time.
hlATERlALS
INTRODUCTION
An understanding of the factors controlling residue decomposition is prerequisite to the design of conservation tillage systems that will ensure sustainable and profitable agriculture. Some agricultural regions produce an abundance of crop residues, far in excess of that required to control erosion and maintain good soil tilth. In other regions, however, particularly in the lower rainfall areas, with droughty soils and with some crops, insufficient amounts of residues are produced for adequate soil protection. Where limited quantities of residue exist, management methods must be devised to achieve maximum effectiveness of residues for conservation purposes. Where crop residues are produced in excessive amounts, they tend to accumulate because of slow rates of decomposition, causing difficulties with tillage and planting (Parr and Papendick, 1978b). Reduced or minimum tillage systems tend to leave much of the crop residue at the soil surface, compared with conventional tillage. Most knowledge of residue decomposition is based on soil incorporation, whereas little is known about the interplay of factors (e.g. moisture, temperature, particle size of the residue, chemical composition of the residue, C:N ratio, and the indigenous microflora of the residue) that govern the rate and extent of residue decomposition at the soil surface (Parr and Papendick, 1978a). My objectives were to investigate the interactions between the residue microbiota and the soil microbiota and how these affect the decomposition process of residues placed on the soil surface or mixed in the soil. In addition of wheat straw, an aged sewage sludge compost was included as an amendment for comparison purposes. 915
AND METHODS
The soil used was an Evesboro loamy-sand subsoil (Typic Quartzisamments) collected from a coniferous site, air-dried and sieved ( < 2 mm). This soil has been used to evaluate residue decomposition (Tester et al., 1977; Tester and Parr, 1983) and to determine crop response to residue-amended soils (Sikora et al., 1980, 1982; Tester et al., 1982). This soil was also collected from a fescue meadow and exhibited the same physical and chemical properties as from the coniferous site. Lime was added to the soil in the control flasks to yield a pH equivalent to that of the residue-amended soils. The compost used was produced by the Beltsville aerated pile method (Willson et al., 1980) from a mixture of woodchips and a high-limed, undigested sewage-sludge (volume ratio of woodchips:sludge of 2.5:1). The cured compost was passed through a 6-mm Sweco screen to remove large woodchips and stored for 5 yr at 5’C before use (the same compost as used by Tester and Parr, 1983). Mature wheat (Triticum aestivum L. “Potomac”) was harvested, air-dried, and the seed were removed, before the straw was chopped to 2 mm. Some properties of the soil, compost and wheat straw are shown in Table I. All experimental substrates and soils [soil, compost, wheat straw, a mixture of soil and compost (IO%,, 224 Mg ha-‘), or a mixture of soil and wheat straw (5%, I I2 Mg ha-‘)] were adjusted to 33 kPa moisture tension, as defined by the pressure-plate method of Richards (1965). They were equilibrated for 24 h at 5’C before portions were irradiated at 3 Mrad with 6oCo at the Goddard Space Flight
C.
916
TJblc
I
Properties
F.
of the Evcsboro
TESTER
loamy
sand. compost
and wheat
strw
33 kPa Total Materul Ebesboro
sod
Total
N
C-10-N
3.60
stmw
0.103
tension
(%H>O)
35
7.7
15.15
I2
60.0
459.0
8.64
53
210.0
Center. All irradiated samples were stored at 5C for a minimum of 30 days before use. Treatments. where indicated, were aseptically inoculated with either Chaeromium glohosum Kunze ex Fries or Chaetomium cellulol_vticum Chahal and Hawksworth at approx. IO’ propagules per treatment mixture. Either 90 g soil, IO g compost, 5 g wheat straw or 100 g samples of each treatment mixture were placed into individual incubation flasks (6 x l2cm). The treatments were replicated three times and placed randomly on a manifold system with temperature controlled at 25°C. The incoming air was passed through 4 N NaOH and then through distilled water to maintain the 33 kPa moisture tension (Tester and Parr, 1983). The effluent gas from each treatment was passed through a tube containing 2 N NaOH for adsorption of evolved CO,. Empty control flasks were included to determine background levels of co:. The extent of residue decomposition was assessed by CO2 evolution. A specific conductance method was used for CO2 determinations, which utilized a Row-cell connected to a Beckman RA-5 Solu-meter with a Fisher digital concentration computer and printer (Tester and Parr, 1983). Assay temperature was controlled within 0.02 C with a Forma Scientific CH/P control system. The coctlicients of variation of the analytical technique were below 0.2%; thus, cxtrcmcly small effects of treatment were detectable. Total organic C was determined on the soil, residues and appropriate mixtures by dry combustion using a Lcco C analyzer and total Kjeldahl-N was dctcrmincd by block digestion and Technicon AutoAnalyzer II methods (Schuman rt al., 1973). The trcatmcnt mixtures were extracted with H,O or 0.1 bt Ba(OH)2 and the soluble organic C measured
Table
moisture
ratio
181.0
Compost Wheat
C
fmeg~‘drywU
2. Symbol
codes for
with a Technicon TOC analyzer (Industrial method 455-76W/A or 535-781 M). Total organic C and total Kjeldahl-N were performed on dry, ground mixtures (CO.5 mm). Treatments and their symbol codes are presented in Table 2. All localized placements of the residues (either placement on the soil surface, horizontal band placement between soil layers or vertical band surrounded by soil) resulted in essentially the same degree of decomposition of the residue during the incubation. As there were no significant differences between the different placements, they are represented as either LCS or LWS in the tables and figures. RESULTS AND DISCUSSION
Mixing aged sewage-sludge compost with soil (CS) stimulated compost decomposition 64% as compared with the localized placements (LCS) (Fig. I). This finding was opposite to that observed by Parr and Papendick (1978a) for raw sewage sludge. The raw sewage sludge contained a significantly higher indigenous microbial population than did the soil, whereas the aged compost used in my study had been stored for 5 yr at 5’C and, thus, represented a relatively stable organic matter with few viable microorganisms. This same compost, when fresh (i.e. immediately after curing), decomposed three times more quickly than the aged compost in the same soil (Tester and Parr, 1983). Decomposition of the compost was reduced 60% when mixed with irradiated soil (C-IS) as compared with nonirradiated soil (CS) (Fig. I), supporting this explanation. The indigenous soil microorganisms were apparently the major contributors to the transformations of this mature compost when mixed with soil. Tester rt nl. (1977) concluded that a less mature compost alone con-
the specific
materials
IrrJdiared
and
treatments
Inoculated’
Svmbol
Soil
-
-
S
Composl
-
-
C
Wheat
-
Compost Wheat
and soil. and soil.
cs
-
and soil. mixed
Compost
and soil. mtxed
Compost
and soil. mixed and soil.
-
mixed
mixed
Compost
Whrut
W ws -
+
mined
KS
+
C. crlluloi.rlicum
KS-C
+
C. glohosm
ICS-G
+
IWS
Wheat
and soil. mired
+
C. cellulo(vricum
IWS-c
Wheat
and soil. mixed
+ -
C. glohosm -
IWS-G
-
-
Compost Wheat
and soil loulized and soil localized
placement’ placement
LCS LWS
Wheat
and soil.
mixed
Wheat
-
IW-s
When!
and soil.
mixed
Soil only
-
W-IS
Soil
-
C-IS
Compost
and soil. mixed
‘Inoculatrd ‘Either
aseptically
placement
band
plaxment
atier
irradiation
on soil surface. surrounded
only
treatment.
horizonaral
by soil.
only
band
placement
between
soil layers.
or vertical
917
Role of soil and residue microorganisms
or, 03
I
10
20
30 41 4Qk6 TIME (DAYS)
6572 TtME (DAYS)
Fig. I. Cumulative CO2 evolved during the decomposition of IOg of sewage-sludge compost mixed with 90g of Evesboro soil. CS = uniform mixture of compost soil. LCS = localized placement of the compost. C-IS = uniform mixture of compost and irradiated soil.
Fig. 3. Cumulative CO, evolved during the decomposition of 5 g of wheat straw alone (W) and 5 g of irradiated wheat straw mixed uniformly with 95 g of Evesboro soil (IW-S). The irradiated wheat straw-soil mixture (IWS) is shown for reference.
tained sufficient viable microbes to elicit decomposition comparable to that of a compost-soil mixture (i.e. compost decomposition in compost mixtures with sterile sand was comparable to that in compost-soil mixtures). Mixing wheat straw with soil (WS) had no effect on decomposition for the first 15 days as compared with that for the straw alone (W) (Fig. 2). indicating that the straw was populated with organisms capable of decomposing the readily-available substrates, as postulatcd by Simpson et al. (1986). This differed from the results of Stott et al. (1986, Table I, p. 580). who reported that, under optimum conditions, the time required to reach a given degree of decomposition for surface applied straw was one-half that required for incorporated straw. However, in my study, soil organisms apparently contributed to an acceleration of decomposition during the final stage (slopes after 48 days). This observation was supported when irradiated wheat straw was mixed with soil (IW-S) (Fig. 3). in that the extent of decomposition was similar to
that for wheat straw alone for the first 30 days. Thereafter there was an increase in the rate of decomposition which resulted in a 2-fold increase by day 65. This increased rate of straw decomposition was concomitant with the development of soil basidiomycetes (identified by P. D. Millner but not to specific species in present study; unfeasible within the scope of the investigation), which probably contributed to the transformations of the more resistant substrates (Kirk et al., 1980). These soil basidiomycetes were apparently less competitive in the presence of the indigenous wheat straw microbes during the early period of this incubation. The extent of decomposition of wheat straw (Table 3) was comparable to that reported by El-Shakweer el al. (1977). Martin et (11. (1980) Pal and Broadbent
Table
3. Toul
CO,
decomposition
of
evolw~d
the
25 C during
and
indicated
percentage
treatments
at
72 dabs of incubation’
co: evolved Material
2.8
s (IM)g)
‘; 5
5 L t-
1.6
% :
32.0
7a
0.1
S5b
0.8
l2le
1.X
99c
I.?
I4lSh
16.8
I577i
18.7
Corrected
l-2
g0.a E
2.7
26621
ICS ICS-c 112-G IWS IWS-c IWS-G
2
8.9
I aof
w (5 g)
Y
(%)
I17e
c (106)
2.4
Decomposition
(w)
0.4
O,l 03
I
10
20
I
I
30 41 4956 TIME (DAYS)
I
6572
LCS
IIOd
1.7
cs
23lg
3.5
LWS
I899j
22.6
WS
2449k
29.1
IW-S
4946n
58.8
W-IS
3293m
See Table ‘Means the
2 for symbol
of three sane
‘Reported rected evolved
39. I
codes for the mztterial.
replications
letters
(P c 0.001).
Fig. 2. Cumulative CO: evolved during the decomposition of Sg of wheat straw alone (W) and Sg of wheat straw uniformly mixed with 9.5 g of Evesboro soil (WS).
for soil’
are
All
mixtures
quantities
of CO:
for
that were
different 100 g.
evolved
the appropriate
from
do not share
significantly
the untreated
amount soil.
were corof CO:
918
C. F. -
(1975). Paul (1984) Stott er al. (1986) and Zunino et 01. (1982) with the exception of the IW-S treatment, where the soil microorganisms were not in competition with indigenous straw microorganisms. No viable microorganisms were detected in any of the treatments after they had been y-irradiated. Because the samples were stored for a minimum of 30 days after irradiation, all radiolytic decomposition of organic matter should have ceased (Cawse and Mableson, 1971). However, the activities of residual enzymes were not determined, and they possibly contributed to some of the evolved CO, in these treatments (Cawse, 1975; Cawse and Mableson. 1971; Jackson er al., 1967; McLaren, 1969; Powlson and Jenkinson, 1976; Skujins. 1978). The amounts of oresumed nonmicrobial evolved CO, were verv low (Fig. 3 and Table 3), 1% for iriadiated wheat straw-soil mixture (IWS) and 0.1% for irradiated compost-soil mixture (Ids). Inocula of both C. globosum and C. cellulol_vticum survived and proliferated in the irradiated compost-soil and wheat straw-soil mixtures. C. globown adapted more rapidly to the substrates present and evolved 33 and 58% of the C02. respectively, in the irradiated compost-soil and wheat straw-soil mixtures as evolved from the nonirradiated mixtures with their indigenous microbes (Table 3, compare ICS-G less ICS with CS + S and IWS-G less IWS with WS + S). C. cellulol_vticum relcased approx. 14 and 51% of the CO,. respectively, from the irradiated compost-soil and wheat straw-soil mixtures as from their nonirradiated mixtures. The greater proliferation and activity by C. globosum was confirmed by viable propagule counts of both fungi at the end of the incubation (compare I2 x 105cfu g-’ of mixture for C. globosum with 9 x IO”cfu g-’ of mixture for C. cellulolyticum). Transformations of plant residues in soil are not usually described by simple kinetic reaction equations. The complex nature of the more refractile substrates, the multitude of microorganisms and microfauna that elicit the transformations, spatial compartmentalization, and wide variances in the environ-
‘\
CPCOe-kf
\’
I
k=0.00679day-1
\ \.
fl;yo.+a-~\, i
03
10
20
i0 41 i9i66SiS TIME (DAYS)
Fig. 5. Decomposition stages and first-order rate constants for wheat straw mixed uniformly with Evesboro soil.
mental conditions are major contributors to the usual variable nature of measured responses. However. numerous deterministic model equations have been presented, which describe the decomposition of different organic materials in soil (Cheshire et al., 1979; Christensen, 1985; Douglas et al., 1980; Lynch, 1979; Moorhead el al., 1987; Pal and Broadbent, 1975; Paul, 1984; Tester and Parr, 1983; Van Veen and Paul, 1981). The decomposition of the aged sewage-sludge compost in soil can best be described by a power function (with high correlation coefficient) (Fig. 4). The slope agreed well with those reported by Pal and Broadbent (1975) and Tester and Parr (1983). The kinetics of the decomposition of the straw are more complex, as the straw contained soluble carbohydrates as well as cellulose, hemicellulose and lignin. First-order equations describing different stages of decomposition of specific substrates have been described by Moorhead et al. (1987). Paul (1984) and Reddy et al. (1980). The integrated rate equation used to describe the particular stages of wheat straw decomposition in my study was expressed as C = C,e-”
360 /’
/’
320
)/
6oJ
F12= .999 P
/ CO,= 26.36
t-‘6174
40?
03 i0 20 30 41 49 j, TIME(DAYS)
where: C, C k I
/“tcs
/ i
1
05 72
Fig. 4. Comparison of goodness of fit for theoretical power function (-) with actual data points (M) for the decomposition of compost mixed uniformly with Evesboro soil.
= = = =
C concentration at t = 0, C concentration at a specific time, first-order decomposition rate constant, time (days).
A comparison of the derived curves and actual data are given in Fig. 5 for wheat straw uniformly mixed with soil (WS). The rate constants for the cellulose and hemicellulose (0.00679 day-‘) and lignin (O.O0356day-‘) fractions of this mature wheat straw were almost identical to those reported by Moorhead ef al. (1987) for a low N-content water hyacinth decomposing in soil. In conclusion, the results of my study suggest that localized placement of residues retards residue decomposition, which may result in a higher steadystate amount of soil organic matter. This is consistent with the known build-up of organic matter with conservation-tillage systems.
919
Role of soil and residue microorganisms Acknowkdgemena-I thank R. Anthony for expert technical assistance, Drs P. D. Millner and M. E. Simpson for the inocula and identifications, and Larry Bromery of NASA for providing the irradiation facilities.
tems (W. R. Gschwald, Ed.). pp. 245-248. American Society of Agronomy Special Publication No. 31, Madison. Paul E. A. (I 984) Dynamics of organic matter in soils. Plant
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