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II. Tillage Effects on Soil Organic Matter

II. Tillage Effects on Soil Organic Matter

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100



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



that involve “delayed tillage.” Delayed tillage usually occurs in cerealfallow rotations where tillage is delayed following harvest until late fall or

the following spring. Thus, delayed tillage does not always reduce the

number of operations, but simply moves tillage from the summer when soil

temperature is high to the fall or spring when it is much lower. Summer

tillage can stimulate oxidation rates substantially, especially in those areas

where summer rain is sufficient to moisten the soil following harvest.

In the Great Plains, the loss of soil N following initial cultivation was

considerably greater with row crops than with small grains (Haas et al.,

1957). Increased loss was attributed to both less surface protection from

rainfall and more tillage to control weeds. Greater loss of soil N with

plowing rather than subsoiling or listing occurred at Archer, Wyoming, but

not at Hays or Garden City, Kansas, or at Lawton, Oklahoma. In general,

spring plowing was less detrimental than fall plowing, and delaying spring

plowing further reduced N loss. Fall and early spring plowing often increased the number of secondary tillage operations to control weeds. Soil

organic matter in a wheat-fallow system in Texas after 36 years was 27%

higher with delayed tillage compared to tillage immediately following

wheat harvest (Unger, 1982).

C . CONSERVATION

TILLAGE



Conservation tillage is described as noninversion tillage that leaves a

significant fraction of crop residue on or only shallowly incorporated into

the soil to control erosion, reduce energy use, and conserve soil and water

(Unger and McCalla, 1980). Stubble-mulch, ecofallow, no-till, directdrilling, and trashy-fallow are all forms of conservation tillage. Tillage for

cereal grains is usually performed with unidirectional disks or sweeps that

undercut the residue without substantial burial.

Many studies have shown that conservation tillage increases organic C

and N in the top 5-15 cm of soil compared to conventional methods of

tillage (Table 11). The rate of increase is biased to some extent by the

sampling depth. In general, the increase averages from 1 to 2%/yr for both

C and N , in the upper 15 cm of soil. The range for C in Table I1 is -0.1 to

7.3%/yr, and the range for N is 0.1 to 5.1%/yr. Below the upper few cm,

the amount of C and N has been either equal or less than that in conventional tillage (Doran, 1980). Thus, the net change in the soil profile is not as

positive as it might seem, even though the amount near the surface is much

greater. Increased levels of C and N near the surface are attributed to

delayed residue decomposition, slower oxidation of soil C, reduced erosion, or any combination of these factors (Pam and Papendick, 1978;



101



SOIL ORGANIC MATTER IN SEMIARID REGIONS



Table I1

Effect of Conservation Tillage on Organic C and N in Soil

Location

and

soil



Soil

depth

(cm)



Length

of study

(yr)



Tillage

system“



412

412



10

10



10

10



30

30

30



5



345

307

389



15

15



698



S. Africa

Haploxeralf

Haploxeralf

Germany

“Podsol”

“Podsol”

”Podsol”

Australia

Western

Psamment

Alfisol

Alfisol

Queensland

Pellustert

Canada

Saskatchewan

Chernozem

United States

N . Dakota

Haploboroll

Haploboroll

Argiboroll

Kansas

Haplustoll

Nebraska

Haplustoll



C



N



Reference”



TT

NT



5.6

7.3



3.4

5.1



1

1



6



NT

NT

NT



3.2

2.4

I .3



1.4

I .6

1.3



2

2

2



NT

NT

NT



1.6

0.7

1.4



-



I5



9

9

9



-



3

3

3



10



6



NT



I .2



1.3



4



I5



6



NT



6.7



2.8



5



45

45

45



25

25

25



SM

SM

SM



I .u

-0.1

0.5



1.3

0. I

0.4



6

6

6



15



I1



NT



0.7



0.6



7



446

446



9

10



15

15



NT

NT



2.8

1.2



2.4



1 .o



n

n



416



15



44



SM



0.3



0.4



9



560



5



10



NT



I .9



2.0



10



2.2

-0.1

7.3



1.7

0. I

5. I



375

375

375



Oregon

Haploxeroll

Washington

Haploxeroll

Mean

Minimum

Maximum

~



~~



Increase

(%lyr)



Annual

precipitation

(mm)



5



-



~



TT, tine-till; NT. no-till; SM, stubble-mulch.

I . Agenbag and Maree (1989); 2. Fleige and Bauerner (1974); 3, White (1990); 4. Saffigna ei t i / .

(1989); 5, Campbell et a / . (1989); 6, Bauer and Black (1981); 7. Havlin et a/. (1990); 8. Doran (1980);

9, Rasmussen and Rohde (1988); 10, Granatstein ef al. (1987).



102



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



Doran, 1980). There is very little evidence that organic C or N moves

substantially from the zone where it is placed if it is stabilized into the

humus fraction of soil.



111. FERTILIZER EFFECTS ON SOIL ORGANIC MATTER

A. NITROGEN

I . Influence on Vegetative Production



Nitrogen addition often has an effect on the amount of C and N incorporated into soil organic matter, but its effect in semiarid environments is not

as pronounced as in humid environments. Biological systems are carbon

controlled and N affects soil organic matter mainly through its influence on

residue production. The primary effect of fertilizer N is to increase vegetative production and the amount of organic C available for recycling back

into the soil system. Nitrogen is limiting for maximum vegetative production in vast areas of the world (Stevenson, 1986). Supplying additional N

increases both growth rate and efficiency of water use of most grasses. In

practically every instance, this results in higher vegetative production.

Higher vegetative production does not always translate to higher grain

yield of cultivated crops, however, because of the tendency for higher

water use to intensify drought stress during seed formation.

Even native grasslands in many semiarid regions are inherently N deficient, and N fertilization will increase production (Rogler and Lorenz,

1974). Introduced grasses generally outyield native grasses, and often are

more responsive to N application (Power, 1980). Thus, cultivated crops

and introduced grasses may have higher aboveground production capability than the original native grasslands. However, the total of net biomass is

probably higher in native grasses because of their much larger reservoir of

belowground crown and root biomass (Kucera et al., 1967; Sims and

Singh, 1978).

2. Retention of Applied Nitrogen



Agricultural experiments suffer from “the nitrogen enigma”; complete

recovery of N in plant and soil is seldom attained and the fate of the

unrecovered portion cannot be determined. The recovery of fertilizer N by

crops is seldom over 50% and often as low as 20% (Gilliam et al., 1985).



SOIL ORGANIC MATTER IN SEMIARID REGIONS



103



Recovery of N from manure application is also low, rangingfrom 40 to 86%

(Bouldin et al., 1984). The unrecovered portion is of concern because of its

potential to pollute ground and surface water. Leaching and denitrification

are usually blamed because retention of N in soil organic matter is not

easily determined. Incorporation of N into the organic fraction of soil is

important because it reduces the movement of soluble N out of the root

zone. The amount of N retained in the organic N reservoir can only be

determined with properly designed N isotope experiments.

Nitrogen applied to cultivated land in excess of crop removal may be

incorporated into the soil organic fraction, remain in inorganic form, or

leach below the root zone. Long-term studies in Oregon (Rasmussen and

Rohde, 1988) indicated that 18%of the N applied to a wheat-fallow system

was incorporated into the organic fraction. The amount of N that is leached

depends on the amount and intensity of rainfall, and time of N application

in relation to crop need. The tendency of N to leach below the root zone is

very low in soils with a calcareous horizon in the profile (Stevenson, 1986).

Nitrogen applied to grassland in excess of crop need in these soils tends to

accumulate as inorganic N with only partial incorporation in the organic

fraction (Sneva, 1977; Power, 1983).

3 . Inorganic versus Organic Sources



Inorganic N sources are generally commercially manufactured fertilizer. The sources are either of ammonium or nitrate origin. Nitrate materials comprised a majority of fertilizer applied prior to 1940, but have since

declined dramatically as synthetic ammonia manufacture became more

economical. The form of inorganic fertilizer used (nitrate or ammonium)

has seldom affected crop yield or inorganic N transformations in soil,

except where long-term addition has changed soil pH, Ammonium-based

fertilizers are acid forming and sustained use can lower soil pH to levels

detrimental to plant growth (Mahler et al., 1985; Rasmussen and Rohde,

1989). Fertilizer use is not the only contributor to soil acidity; some

semiarid Australian soils in long-term wheat-legume-pasture rotations

(ley farming) have become acid because of mineralization of biologically

fixed N (Haynes, 1983). The effect of increasing acidity on microbial

populations, N mineralization, and the rate of turnover of easily decomposable and resistant organic matter is not well defined for semiarid regions, although its effect has been studied in humid regions.

Organic sources of N are usually green manure and animal manure.

Organic waste from processing plants represents a minor source, primarily

because of transportation problems. Green manures can be a legume, a



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