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CHAPTER 3. LONG-TERM IMPACTS OF TILLAGE, FERTILIZER, AND CROP RESIDUE ON SOIL ORGANIC MATTER IN TEMPERATE SEMIARID REGIONS

CHAPTER 3. LONG-TERM IMPACTS OF TILLAGE, FERTILIZER, AND CROP RESIDUE ON SOIL ORGANIC MATTER IN TEMPERATE SEMIARID REGIONS

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94



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



I. INTRODUCTION

A. BENEFICIAL

EFFECTSOF ORGANIC

MATTERIN



SOIL



Organic matter in soil has been of concern for decades because it has a

pronounced beneficial effect on soil management and crop productivity

(Allison, 1973). In recent years, soil organic matter has received additional

attention because of its potential to sequester carbon emanating from

atmospheric Cot increases. Organic matter also has a strong influence on

the persistence and degradation of pesticides and organic wastes in soil,

yet full appreciation of this effect remains largely ignored in today’s agricultural sector. Presently, increasing awareness and concern that environmental quality is deteriorating has fostered renewed interest in improving

soil and water management. It is therefore appropriate that we review past

progress towards enhancing the level and quality of organic matter in

soil.

Allison (1973) listed the important contributions of organic matter to

soil: (1) it is the major natural source of inorganic nutrients and microbial

energy, (2) it serves as an ion exchange material and a chelating agent to

hold water and nutrients in available form, (3) it promotes soil aggregation

and root development, and (4) it improves water infiltration and water-use

efficiency. A productive soil that is easy to till is identified as having “good

tilth.” An appreciable amount of organic matter is usually a prime prerequisite for good tilth, especially in soils with high sand or clay content. Good

tilth has no defined limits (Karlen et al., 1990), but is readily recognized by

everyone from weekend gardeners to biological scientists.



B. DETERMINATION

OF ORGANIC

MATTER

Most organic matter values are derived from organic carbon (organic C )

values because the quantitative determination of organic matter has high

variability and questionable accuracy (Nelson and Sommers, 1982). A

conversion factor of 1.724 is used to convert organic C to organic matter,

even though it is generally recognized that the value can range from 1.6 to

3.3 (Jackson, 1958; Nelson and Sommers, 1982). Organic C analysis is

reasonably accurate. Values from wet digestion with acid-dichromate and

heat (modified Walkley-Black) correlate fairly well with results from dry

combustion (Tabatabai and Bremner, 1970; Kalembasa and Jenkinson,

1973; Sheldrick, 1986).



SOIL ORGANIC MATTER IN SEMIARID REGIONS



95



Organic C in soil is generally highly correlated with organic nitrogen

(organic N) (Unger, 1968). Organic N comprises more than 99% of the total

N present in soil in the absence of substantial N03-N accumulation (Allison, 1973). Analysis of total N by wet digestion (Kjeldahl) or dry combustion (Dumas) has been highly accurate since the 1880s (Bremner and

Mulvaney, 1982). Accuracy and precision of Kjeldahl digestion has not

changed substantially during the progression from macro- to microdigestion techniques. Modem combustion analyzers now have the capability to reliably determine both C and N simultaneously.

Organic C values will be used whenever possible in this report, and

references to soil organic matter content will be restricted to generalized

statements. Unless otherwise noted, it is assumed that ( 1 ) organic N =

total N, and (2) % organic matter = (% organic C) (1.724). Organic matter

can be further separated into various fractions. The most commonly defined fractions are shown in Table I.



C. FACTORS

INFLUENCING

SOILORGANIC

MATTERCONTENT

The amount of organic matter in mineral soil can vary from less than

10 g/kg (1%) in coarse-textured sands to more than 50 gikg (5%) in fertile

prairie grasslands. The level of organic matter in soil is influenced by

climate, topography, parent material, vegetation and organisms, and time

(Jenny, 1941; Allison, 1973; Stevenson, 1986). Jenny (1941) arranged the

order of importance of these factors as climate > vegetation > topography

= parent material > age. All of the factors are partially interactive. For

example, higher rainfall (climate) generally results in greater biomass

production (vegetation), greater weathering and higher clay content in soil

(parent material), and perhaps modified relief (topography). Organic matter in soil reaches a stable equilibrium when all factors with the exception

of time change very little. The time needed for organic matter to reach a

stable level in uncultivated soil can range from less than 100 to over 2,000

years, depending upon climate conditions (Stevenson, 1986).

It is possible to make some general statements about organic matter

levels in virgin grasslands soils based on the pioneering work of Jenny

(1941).

1 . Grasslands soils have higher organic matter content than forest soils.

2. Organic matter content increases with increasing precipitation and

decreases with increasing temperature.

3. Fine-textured soils have higher organic matter content than coarsetextured soils.



96



PAUL E. RASMUSSEN AND HAROLD P. COLLINS

Table 1

Definitions of Organic Matter Fractions in Soil”

Term



Organic residues

Soil organic matter



Humus

Soil biomass

Humic substances



Nonhumic

substances

Humin

Humic acid

Fulvic acid



Definition

Undecayed plant and animal tissues and their partial

decomposition products

Total of the organic compounds in soil exclusive of undecayed

plant and animal tissues, their “partial decomposition”

products, and the soil biomass

Same as organic matter

Organic matter present as live microbial tissue

A series of relatively high molecular-weight, brown-to-blackcolored substances formed by secondary synthesis reactions.

The term is a generic name to describe the colored material

obtained on the basis of solubility characteristics. These

materials are distinctive to the soil (or sediment)

environment in that they are dissimilar to the biopolymers of

microorganisms and higher plants (including lignin)

Compounds belonging to known classes of biochemistry (e.g.,

amino acids). Humus probably contains most, if not all, of

the biochemical compounds synthesized by living organisms

The alkali-insoluble fraction of soil organic matter or humus

The dark-colored material that can be extracted from soil by

various reagents and that is insoluble in dilute acid

The colored material that remains in solution after removal of

humic acid by acidification



From Stevenson ( 1982). Reproduced from “HumusChemistry: Genesis, Composition,

Reactions” by permission of John Wiley & Sons, Inc. Copyright 01982 by John Wiley

and Sons, Inc.



4 . Naturally moist and poorly drained soiis have higher organic matter



than well-drained soils.

5. Soils in lowlands have higher organic matter than soils on upland

positions.

All of the above are affected when virgin land is cultivated. A much larger

portion of biomass is removed for use as feed or fuel. Crop selection, crop

rotation, and residue utilization influence the amount of biomass cycling in

the ecosystem. Accelerated wind and water erosion may significantly

modify the soil surface and alter organic matter accumulation, soil texture,

and water-holding capability. Soil aeration may be changed by drainage,

tillage pan development, or ripping of indurated layers. Tillage to control

weeds or prepare a seedbed increases soil disturbance and accelerates

organic matter oxidation.



SOIL ORGANIC MATTER IN SEMIARID REGIONS



97



D. TEMPERATE

SEMIARID

REGIONS

This article will primarily discuss organic matter relations in temperate

zones with semiarid climate.Soi1 organic matter is of greater concern in

semiarid regions because of its unusually large impact on water conservation, nutrient availability, and stabilization of yield. Temperate semiarid

regions, in this article, will comprise those areas between 30 and 60 degrees latitude that were grassland in their virgin state, and if cultivated,

have been cropped primarily to cereal grains. This encompasses the grasslands of mid-continent North America and Eurasia, and includes portions

of Argentina, South Africa, Australia, and the northwestern United States.

Native vegetation consists primarily of short- and medium-height grasses,

with a limited shrub component. In Europe and Eurasia, this region is

sometimes referred to the “The Steppes.” Drought stress occurs most

years, and affects the level of production. The upper limit of annual

precipitation is about 500 mm where winter precipitation dominates and

750 mm where summer rainfall dominates. Most of the soils are classified

as Mollisols under the U.S. system, and were formerly included in the

Chestnut and Chernozem great soil groups.

Changes in soil properties in semiarid regions will frequently be compared with changes in more humid environments where numerous studies

have been conducted. Comparisons will be restricted to studies in humid

regions that involved grasslands or that were cropped primarily to cereal

grains.

E. EFFECTSOF CULTIVATION

OF GRASSLANDS

Virgin grassland soils traditionally lose organic matter rapidly after

they are first cultivated (Allison, 1973; Mann, 1985). Organic matter is high

in undisturbed soil because little native vegetation is removed, erosion is

negligible, and oxidation is at a minimum. Root and crown tissue production is much greater for native grasses than for cultivated crops, and

comprises a higher proportion of net primary productivity (Sims and

Singh, 1978; Salaer al., 1988). Withcultivation, a substantial portionof dry

matter production is removed for food or forage, wind and water erosion

may increase, and frequent cultivation degrades soil aggregates and accelerates oxidation of easily decomposable root and crown tissue.

The loss of organic matter with cultivation is usually exponential, declining rapidly during the first 10-20 years, then more slowly, and finally

approaching a new equilibrium in 50-60 years (Jenny, 1941; Haas Pt a l . ,

1957; Campbell, 1978). Some early investigators believed that a minimum



98



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



existed, below which soil organic matter content could not go. We now

know this is not the case. The organic matter level depends on the rate of

residue addition in relation to the rate of residue decomposition and soil

erosion. Soil organic matter will continue to change as long as any of the

controlling factors continue to change. New equilibrium levels will be

highly dependent on farming practices, especially those involving crop

residue utilization, crop rotation, and tillage.



F. EVALUATING

CHANGES

I N ORGANIC

MATTERCONTENT

A N D QUALITY

Soil organic matter usually changes only slowly with time following a

change in land use or management. Differences are difficult to measure

against the large background of soil organic matter until sufficient years

have elapsed for the differences to be larger than analytical variability. As

a result, this requires long-term experiments with annotated history, or

paired conditions that have been in place for 20-30 years. The time requirement can be shortened by determining changes in microbial biomass

in relation to the change in organic C (Saffigna et al., 1989), but caution

must be exercised since the microbial biomass : organic C ratio is influenced by crop management (Granatstein et al., 1987; Anderson and

Domisch, 1989). Small changes in organic C or N can often be elucidated

through the use of 14C and 15N isotopes. This aids substantially in interpreting long-term changes, and vastly improves the estimates of nutrient

cycling in different organic matter fractions.

Accurate assessment of results from long-term experiments requires

close scrutiny to determine their validity. An evaluation of change over

time may not be possible if soil C or N was not determined at the beginning

of the experiment. If C and N have been determined periodically over

time, values may require adjustment for different methods of analysis.

Analytical results from historical samples may not be valid if samples have

undergone change during storage. Specific methods of analysis are not

always identified in published reports, which can lead to erroneous assumptions of what components were measured. The bulk density of soil

(weighthnit volume) has not been determined in many instances, even

though it has changed significantly over time and affects the amount of C or

N remaining. This is especially true when virgin and cultivated sites are

being compared, since cultivation can increase soil bulk density 30% or

more.

In spite of the potential problems associated with long-term experiments, they remain the primary method to identify organic matter changes

over time. Carbon dating and isotope discrimination techniques are invalu-



SOIL ORGANIC MATTER IN SEMIARID REGIONS



99



able in defining nutrient fluxes and half-lives, but they enhance, not

replace, valid long-term data.



II. TILLAGE EFFECTS ON SOIL ORGANIC MATTER

A. FREQUENCY

OF FALLOW

Increasing the frequency of fallowing generally increases the loss of

organic matter from soil. The loss is usually greater in higher rainfall

zones. Ridley and Hedlin (1968) reported 49% less organic matter in a

black lacustrine soil after 37 years when fallowed every other year rather

than cropped annually. Haas et al. (1957) reported that cropland in grainfallow rotation lost more N than did annually cropped land at 13 of 14

locations throughout the midwestern U.S. The average loss after 30-43

years of cultivation was 24% with continuous small grain versus 29% with

alternating grain-fallow. Dormaar and Pittman (1980) reported organic C

levels of 19.6, 15.6, and 14. I g/kg in the top 13 cm of soil after 64 years of

cropping a dark brown Chernozem soil in Canada to wheat (Triticum

aestiuum L.)-wheat, wheat-wheat-fallow, and wheat-fallow rotation,

respectively. This was a decrease of 21 and 28% in organic matter content

when soil was fallowed 33 and 50% of the time, respectively, rather than

cropped annually. Soil in west Texas, U.S. cropped to wheat-fallow

contained 14% less organic matter in the top 8 cm of soil after 36 years than

did soil cropped to continuous wheat (Unger, 1982). Biederbeck et al.

(1984) and Insam e f al. (1989), in their reviews on cropping practices and

frequency of fallow, reported similar losses of organic matter in a wide

range of Canadian soils.

Loss of C and N is a combination of increased oxidation due to more

cultivation, lower residue return to soil, and, frequently, increased wind

and water erosion. Frequency of cultivation can be estimated from established farming practices and residue input calculated from crop yield with

reasonable accuracy if 10 or more years are involved. But, isolation of

erosion effects is difficult because accurate soil losses are not easily calculated from climatic and agronomic data.



B. INTENSITY



OF



TILLAGE



Appropriate data specifically comparing the long-term effects of tillage

intensity on depletion of soil organic matter are not readily available. Most

often reported are comparisons of disking versus plowing and experiments



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;



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CHAPTER 3. LONG-TERM IMPACTS OF TILLAGE, FERTILIZER, AND CROP RESIDUE ON SOIL ORGANIC MATTER IN TEMPERATE SEMIARID REGIONS

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