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IV. Organic Residue Effects on Soil Organic Matter

IV. Organic Residue Effects on Soil Organic Matter

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106



PAUL E. RASMUSSEN AND HAROLD P. COLLINS

20



A



18



0



Y

\



0



v



z

0



16



0



0



5



14



0

LT



0

12



10



CARBON

CHECK

ALFALFA



0 CORNSTALKS



NITROGEN (x 0.1)

SAWDUST

OAT STRAW



0BROMEGRASS



FIG.1. The influence of different types of residue on organic C and N in the top 15 cm of

an Iowa soil. (From Larson et al.. 1972. Reproduced from Agronomy Journal, 64(2), MarchApril 1972, pp. 204-208, by permission of the American Society of Agronomy, Inc.)



Tenney and Waksman (1929) initially suggested that the rate and nature of

residue decomposition depended upon the chemical composition of the

plant material. The most important residue constituents were the amount

and nature of cold-water-soluble C, the abundance of cellulose and hemicellulose, the N content, and the amount of lignin.

Decomposition of plant material occurs in several steps involving both

chemical and physical transformations. In general, water-soluble C fractions (sugars, organic acids, and proteins and part of the nonstructural

carbohydrates) are degraded first (Reber and Schara, 1971; Knapp et al.,

1983), followed by structural polysaccharides (cellulose and hemicellulose) (Harper and Lynch, 1981), and then lignin, which decomposes at a

much slower rate (Herman et al., 1977; Collins et al., 1990).

Tracer techniques make it possible to follow the fate of residues during

decomposition. In a classical study, Jenkinson (1965)followed the decomposition of buried 14C-labeledryegrass (Lolium spp.) straw over a 10-year

period. After one year, approximately 33% of the original C remained in



107



SOIL ORGANIC MATTER IN SEMIARID REGIONS

Table 111

Effect of Type of Residue Added on the Change in Organic C and N in Sand

and Clay Soil, 1945-1965, Ottawa, Canada"

~



Change, 1945-1965 (%)

Rideau clay'



Uplands sand"



Residue addedb



C



N



C



N



None

Straw

Alfalfa

Deciduous leaves

Peat

Muck

Manure

Rye (green manure)



- I8



- 14



-1



+3

+1

-8

-8

+ 33

+7

- I3



- 10

+ 28

+ 10

+46

+80

+ I35

+ 59



- 24

+7

+5

+4

+9

+68

+35

-9



a



-1



0



+ 27



+ 51

+8

- I6



+ 19



Data from Sowden and Atkinson (1968) and Sowden (1968).

Residues added annually at 11.1 t/ha.

Soil C and N content in 1945 was 29.1 and 2.38 g/kg, respectively.

Soil C and N content in 1945 was 13.7 and 1.29 g/kg, respectively.



the soil, of which one third was associated with soil microbial biomass.

About 20% of the I4C label was found in the soil microbial biomass after 4

years and 12% of the initial labeled C remained in the soil after 10 years

(Jenkinson, 1977). Voroney et uf. (1989) found that 20-30% of added C

became stabilized in the soil organic matter after 10 years. Other researchers have shown similar results, although the rate of decomposition varied

due to abiotic conditions (Shields and Paul, 1973; Nyhan, 1975).

The level of organic matter in soil depends on both the amount and

chemical composition of the material added. Stabilization of soil organic

matter is more a function of microbial product recalcitrance than of the

initial residue composition (Voroney et af., 1989). Soil organic matter

decomposition averages from 2 to 5% a year, with turnover of newly

formed humus more rapid than that of old humus. Soil organic matter can

be partitioned into old and young fractions, as shown by radiocarbon

dating (Table IV).

Most laboratory incubations indicate that from 60 to 75% of crop residue

C is evolved as C 0 2after one year in soil (Martin and Stott, 1983). Most of

the remaining C becomes stabilized in new soil humus, with 5-15% incorporated into soil microbial biomass. Stott et uf. (1983) found that the

majority of polysaccharide and protein C in wheat straw became associ-



108



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



Table IV

Radiocarbon Ages of Soil Organic Matter

Reference

Decalcified soil

Hydrolysate

Residue from hydrolysis

Humus

Unfractionated

Fulvic acid

Humic Acid

Humus

Virgin Oxbow soil

Cultivated Oxbow (15-yr)

Cultivated Oxbow (60-yr)



1,450

515

2,560

1,240

870

495

1,235

1,140

250 f 65

295 f 75

710 C 60



Jenkinson and Rayner (1977)



Campbell et al. (1967)



Martel and Paul (1974)



ated with the fulvic acid fraction of new soil humus. From 36 to 54% of the

C derived from wheat straw lignin was found in the humic acid fraction.

Corn (Zea mays) residue underwent a similar trend of incorporation into

new humus fractions.

Lignin and other phenolic compounds are the most resistant to microbial

degradation and probably serve as the primary source of material found in

highly stable fractions of soil organic matter. Immature plant tissue contains a low percentage of lignin and generally has limited capability to

increase stable organic matter components even though it may increase

labile N fractions.

2 . Rate of Addition



Several studies show that organic C and N in soil respond linearly to

increasing rates of residue addition (Table V). Horner et al. (1960) summarized some of the early research in the Pacific Northwest, U.S.,which

found linear increases in soil C and N with rates of residue application from

0 to 3.5 t/ha. Results were similar for studies in 240- and 564-mm rainfall

zones. Oveson (1966) and Rasmussen et al. (1980) reported on changes in

soil C and N in a long-term wheat-fallow experiment in eastern Oregon

that included straw, pea (Pisurn satiuurn) vine, and manure residues. Soil

C and N continued to decrease with time for all residue additions except

manure (Fig. 2). The rate of decrease was related to the level but not type

of residue returned to the soil (Fig. 3).



SOIL ORGANIC MATTER IN SEMIARID REGIONS



109



Table V

The Influence of Rate of Residue Addition on Organic C and N in Soil"

Location

and time

of study



Crop

rotationb



Soil

depth

(cm)



Organic C



Organic N



b



U



a

~



Culbertson, MT

1964-1972

Lind, WA

1923-1946

Pendleton, OR

1931-1966

Pullman, WA

1922-1952

1922-1952

Shenandoah, IA

1953- 1966



b



Reference'



~~~~~



W-F



I5



+56



0.83



-11.9



0.046



1



W-F



15



-86



0.12



-



-



2



W-F



30



-389



0.17



-32.7



0.019



3d



W-F



30

30



-406

-117



0.21

0.15



-45.5

-18.8



0.022

0.011



2

2



15



-366



0.14



-



4



w-w

c-c



~



~



~~~~~



a As defined by the regression equation Y = a + bX, where Y = change in soil C or N (kg/ha/yr) and

X = carbon input (kg C/ha/yr).

W, wheat; F, fallow; C, corn.

' 1. Black (1973); 2, Homer et al. (1960); 3, Rasmussen el al. (1980); 4, Larson et al. (1972).

Data modified slightly by additional measurements.



\

\



-



w 22 tiha MANURE, NO BURNING

A 2.2 t/ha PEA VINES, NO BURNING



* 45 kg "ha,



-



NO BURNING



0 0 kg N/ha, NO BURNING

0 0 kg N/ha, FALL BURNING



I



I



1



1881



1901



1921



I



1941

YEAR



I



I



1961



1981



I



FIG.2. The effect of management practices on the long-term change in organic C in the

top 30 cm of a Haploxeroll soil in Oregon. (From Rasmussen er a / . , 1989.)



110



PAUL E. RASMUSSEN AND HAROLD P. COLLINS

L



0



*



100



al



x



>

.f

y"



0



-



-100



-



-200



-



BURN



STRAW



PEA VINES

0 MANURE

A



v



w

(3



Z



a



I

0 -300 Z

0



m-400



**.

'



I



5



Y = 0.18OX



-



460



'



I



Rz = 0.66

I



'



I



'



'



"



'



'



'



FIG.3. The effect of the rate of carbon input on organic C change in a Haploxeroll soil in

Oregon. Study conducted from 1967 to 1986.



Black (1973) applied straw-mulch rates of 0, 1.68, 3.36, and 6.73 t/ha

biennially to a spring wheat-fallow system in Montana for 8 years and

measured organic C and N at the beginning and end of the study. Different

combinations of N and P fertilizer rates were included. Residue addition

increased soil organic C and N levels linearly, with 7040% of the applied

C retained in the organic fraction at the end of 8 years. This percentage is

much higher than reported in other studies. The difference may be partially

attributable to a difference in residue burial. Residue was incorporated in

other studies but remained on the soil surface in the Montana study. About

50% of straw had not yet decomposed by seeding, some 12 months after

application. Thus a new equilibrium may not have been attained during the

lifetime of this experiment. Sampling procedures do not indicate if coarse

organic matter was removed before C analysis. Allmaras et al. (1988)

reported that 23-46% of wheat residue did not pass a 0.5-mm sieve after 12

months of burial in soil.

The Montana study (Black, 1973) indicated that neither N nor P application altered the rate of organic C or N retention in soil, although both

increased grain yield and residue production. Residue rates were established by complete removal and subsequent return of specific amounts,

which eliminated any effect of N or P on residue input. As in other studies,

carbon control of biological reactions was indicated by the failure of

fertilizer to affect C or N content in soil. In a humid region (Iowa), Larson

et al. (1972)returned different rates of cornstalks to continuous corn for 13

years and measured organic C and N in soil. Both C and N increased

linearly with increasing rate of residue.

From the studies listed in Table V, it is possible to calculate the amount



SOIL ORGANIC MATTER IN SEMIARID REGIONS



111



of C and N retained in the system and the amount of residue need to

prevent a decline in organic C or N in soil. The regression equation Y = a +

bX can be developed, where Y = the change in C or N divided by number

of years of study (in kg/ha/yr) and X = annual C input from residue

(kg/ha/yr). Except for the study by Black (19731, the slope of the regression line ranged between 0.14 and 0.21, suggesting limited variation in

the proportion of organic residue that is ultimately retained in soil organic

matter under a variety of climatic conditions. Data from Washington and

Oregon (Table V) suggest that the percentage of retention may increase

with increasing precipitation and decrease with intensity of cropping. Less

residue was required to prevent further decline in organic C and N under

annual cropping than under wheat-fallow rotation, even though the rate of

C retention appeared to be slightly lower.

In the studies listed in Table V, the maximum rate of residue applied

exceeded the average residue production for that area. Linearity indicates

that soil is capable of sequestering from 10 to 25% of the C supplied as

processing waste, feedlot manure, or crop residue. Rates of feedlot wastes

from 30-90 t/ha applied to soil have shown only limited decrease in the

fraction converted into organic matter (Somrnerfeldt et al., 1988).

The negative intercepts in Table V indicate that loss of organic matter

will continue in many of the present cropping systems without adequate

residue return to soil. The amount of residue required to prevent further

loss can be estimated by dividing the absolute value of intercept by the



Table Vl

Increase in Organic C in Soil with Residue Addition (1947-1954)and

Subsequent Decrease after Termination of Residue Input (1954-1WO)'~6

~~



~



Rate of change

g C/kglyr



Material

added'

~~



Rotted manure

Fresh manure

Straw

Green manure

None (NPK)

a



1947- 1954



1954-1970



+ 1S O



-0.51

-0.41

-0.36

-0.21

-0.09



Time required to

return to original

level"

(yr)



~~



+ 1.06

+0.69

+0.43

0



Adapted from Sauerbeck (1982).

Silt loam on loess soil, 865-mm precipitation zone, Germany.

Residue added at 20 t DM/ha/yr for 6 years.

Original level was 13.0 g Cfkg.



20.6

18.0

13.3

14.3



112



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



slope and multiplying by 2.38 to convert organic C to total residue. Residue

returns of 1.7, 5.4, and 4.6 t/ha/yr were required in wheat-fallow regions

of the Pacific Northwest, U.S. receiving 240,416, and 564 mm of precipitation. Much lower residue return was needed each year if cropped annually (1.9 t) rather than to a crop-fallow rotation (4.6 t). The Iowa data

projected that a residue return of 6 t/ha/yr was required to prevent further

organic matter loss for continuous corn in a humid climate.

While residue input can increase organic matter content, continued

input must be sustained. Large applications of residue for 6 years in

Germany increased soil organic C substantially (Table VI). But when

residue addition was discontinued, organic C returned to its original level

within 13 to 2 1 years. It appears from this study that very little of the added

C was incorporated into relatively stable C fractions in soil.

3 . Residue Burning



Luebs (1962), Oveson (1966), and Rasmussen et al. (1980) in the United

States and Dormaar et al. (1979) and Biederbeck et al. (1980) in Canada

addressed the long-term effects of residue burning on cereal grain yield and

soil nutrient content. Most of the early work involved burning for less than

20 years and did not find any reduction in grain yield or soil organic matter

content. More recent studies (Biederbeck et al., 1980; Rasmussen et al.,

1989) have shown accelerated C loss and lower microbial activity in soil

where straw has been burned for over 20 years. The failure of burning to

increase C loss from soil when 50-70% of the residue C is volatilized

probably results because much of the charred residue left after the burn is

not biologically active. It is likely that burning is changing the quality

rather than the quantity of organic matter in soil. To our knowledge, no

one has determined if there is a change in the equivalent-age of any of the

organic fractions as a result of repeated burning of crop residue.

4 . Residue Removal



Crop residues are used for fuel or animal feed in many areas of the

world. Residue removal decreases C input into soil and thus inherently

lowers organic matter level. Specific studies to determine the effects of

residue removal have seldom been conducted. The effects of residue

removal on soil organic matter can be estimated from the residue-rate

studies of Horner er al. (1960), Larson et al. (1972), and Black (1973). Few

semiarid regions have a productivity level that permits substantial residue

removal without accelerating soil organic matter depletion. Present pro-



SOIL ORGANIC MATTER IN SEMIARID REGIONS



113



duction ranges from 1 t/ha in arid soils with low organic matter content to 6

t/ha in subhumid soils with high organic matter levels. Current input in

most areas is presently only about 80% of the amount needed to prevent

further loss of organic matter. Residue removal is more feasible in humid

regions where productivity is higher (Larson ef al., 1978).



B. ANIMALMANURE

The value of animal manure in relation to crop residue is difficult to

evaluate. Comparisons of C inputs from manures and crop residues are not

always possible because the dry matter and C content of manure was not

determined. Most mature air-dry plant residue contains about 900 g dry

matterjkg and 420 g Cikg of residue. Manure, on the other hand, usually

contains significantly more water and the organic C content is normally

much less than 420 g/kg. In general, manure contains about 500 g dry

matterlkg and 150 g C and 11 g N/kg of dry material. Yearly variation and

the 1 1-year average C and N content of manure applied in two long-term

studies is shown in Table VII. Nitrogen content is especially variable, with

a coefficient of variation over 50%.



Table VII



Dry Matter (DM), C, and N Content of Manure Applied at Two Locations for 11 Consecutive Y e a d

Consecutive

year



Pendleton. Oregon, U.S.A.



Lethbridge, Alberta, Canadab



DM



C



N



DM



C



N



11



540

568

367

434

437

544

480

817

470

468

468



59.9

52.3

69.4

75.5

86.1

62.0

66.2

31.9

49.8

78.6

96.2



4.91

3.80

4.66

4.82

5.59

4.79

5.28

3.43

4.65

6.22

6.45



517

492

52 I

659

658

495

512

574

55 I

476

653



99.8

119.6

86.0

110.1

111.2

99.5

104.4

75.8

29.2

36.7

136.5



9.10

9.64

10.32

9.16

11.78

9.70

7.83

9.36

3.86

4.33

12.02



Average

Std. error of mean

CV(%)



508

35

23



66.1

5.4

27



4.96

0.86

58



555

21

13



91.7

10.0

36



8.83

2.50

94



1

2

3

4

5

6

7

8

9

10



Manure obtained from same source at each location. Values are given in g i k g (moist weight basis).

Sommerfeldt et al. (1988).



114



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



In most studies, the effect of manure on soil organic matter exceeded

that of plant residue. Organic C and N levels in soil in many areas were

higher with manure application than with straw or legume addition (Haas

et al., 1957; Horner et al., 1960; Sowden, 1968; De Haan, 1977;Sauerbeck,

1982). It is not possible to determine if C input from manure was the same

as that of other residues. In two long-term experiments where the nutrient

content of manure was measured (Table VII), there were wide yearly

differences in dry matter, C , and N content even though the manure was

always obtained from the same source in each study. Variability would be

expected to be even greater if different sources and different storage

methods were included.

Using average values for dry matter, C , and N content to calculate

manure inputs when it is not reported is tenuous at best. Where the C

content of manure has been determined, the effect of manure on organic C

and N appears to be the same as that of legume and straw residues (Fig. 3).

But if the Pendleton values for manure are used to estimate C input from

manure in the Pullman and Lind studies (Horner et a!., 1960), the manure

data does not fit the regression line obtained with straw addition. Manure

appears to have greater effect, suggesting that either the calculated input is

incorrect or the materials were not equally beneficial.



C . GREENMANURE

Green manures have generally been less effective than crop residues or

animal manures for stabilizing or increasing soil organic matter levels

(Haas et al., 1957; De Haan, 1977; Power, 1990). The primary function of

green manure is to sequester biologically fixed N in sufficient quantity to

meet the requirement of the following crop, As such, vegetation is normally incorporated into soil before it is mature, and as a result, does not

have the same chemical composition (i.e., lignin) that is found in mature

plants. Lignin content has a large influence on the stabilization of C and N

in recalcitrant soil fractions (De Haan, 1977).



V. ORGANIC MATTER AND MICROBIAL BIOMASS

Soil organic matter exerts a positive influence on the soil biomass, which

mediates processes of soil organic matter turnover (McGill et al., 19861,

and nutrient cycling and soil aggregation (Aspiras et af., I97 1; Molope et

af., 1987).There is a dramatic decrease in microbial biomass as well as soil



SOIL ORGANIC MATTER IN SEMIARID REGIONS



115



organic matter when a virgin soil is cultivated (Martel and Paul, 1974;

Houghton er al., 1983). Factors that affect crop production ultimately

affect the size and activity of the soil microbial biomass (Biederbeck et al.,

1984).

Net primary production of native grasslands is generally greater than

that of agroecosystems, with the majority of fixed C accumulating in

belowground biomass (Kucera et al., 1967; Allison, 1973; Anderson and

Coleman, 1985). Typically, perennial grassland soils contain about twice

the amount of root biomass as agricultural soils (Yegorov and Dyuryagina,

1973; Sims and Singh, 1978; Sala et al., 1988), and therefore maintain

higher soil organic matter contents. The amount of root biomass is important since it supplies a significant quantity of available C to the microbial

biomass (Lynch and Panting, 1980; McGill et al., 1986). In most cases, the

microbial biomass of grassland soils is about twice that of cultivated soils.

Microbial biomass C ranges from 100 to 600 mg C/kg soil when cropped

to cereals (Insam et al., 1989; Anderson and Domsch, 1989) and can

exceed 1500 mg C/kg soil under native grassland or managed grass pasture

(Ross et al., 1980). Extended cultivation usually reduces soil biomass,

which influences organic matter cycling. Reductions in microbial biomass

result from lower C inputs and environmental stresses created by management. These stresses include increased soil acidity from ammoniacal fertilizer use, soil erosion that decreases C, N , and other essential nutrients,

and increased soil density, which reduces aeration and water availability.

It is generally assumed that microbial biomass C and activity measurements are correlated with soil organic C because soil biomass depends on

the quantity of degradable C sources present in soil (Adams and Laughlin,

1981). Anderson and Domsch (1989) evaluated this relationship over a

range of soils and Insam et al. (1989) evaluated over different climatic

zones. Both found a correlation but the relationship was complicated by a

number of integrative factors, especially in drier regions. Anderson and

Domsch (1989) surveyed 134 plots located on 25 experimental sites within

a narrow temperate climatic zone of central Europe and found a high

correlation between soil microbial biomass C (CM)and soil organic C (CO)

(Fig. 4a). Microbial biomass C averaged 2.3 and 2.9% of organic C for

monocultures and crop rotations, respectively. The difference was primarily due to the type and amount of organic C input; soil physical properties (i.e., texture) were of minor consequence. They suggested that monocultures and rotations were comparable with respect to steady state

conditions, but that no universal equilibrium constant could describe the

increase in microbial C per unit of soil C between cropping systems.

Regression analyses indicated that higher CM: CO ratios were characteristic of soils with regular crop rotation (Fig. 4a). Fallow in the rotation

reduced soil organic matter and subsequently influenced the CM: COratio.



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