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III. Fertilizer Effects on Soil Organic Matter

III. Fertilizer Effects on Soil Organic Matter

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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



grass, or a grass-legume combination. The legumes are most often clovers

and the grasses cereal grains. Green manures are usually incorporated into

soil before they reach seed formation. Animal manure in semiarid regions

is primarily from cattle or sheep. Because confined-feeding operations are

localized and farm size quite large, there is limited use of manure in the

United States and Canada.

There is little difference between inorganic and organic N sources for

supplying N to plants if sufficient time is allowed for mineralization to

N03-Nprior to crop need. Organic materials contain many other nutrients

while inorganic N sources usually do not. The major problem with organic

N sources is the uncertainty in the amount and availability of N supplied.

The actual amount of dry matter and N in animal or green manure at the

time of incorporation is seldom determined, although materials may contain up to 70% water. The average water content of manure is about 50%,

that of air-dry forage 10-15%, and that of green manure 50-75%. Actual

nutrient input from organic materials is much more variable than that of

inorganic materials such as lime or fertilizer. This variability may influence

short-term reaction rates, but should have little effect on long-term nutrient turnover in soil.






The effect of nutrients other than N on organic matter in soil is much less

pronounced. Their availability, to some degree, is derived from the mineral fraction, thus they are less likely to be affected by a change in organic

matter level or biological reaction rate. Organic P constitutes from 15 to

80% of the total P, and organic S from 50 to 70% of the total S in soil

(Allison, 1973). The organic fraction of K, Ca, Mg, and many micronutnents constitutes a much lower proportion of the total in soil. Deficiencies

of these nutrients occur so infrequently in grassland soils that their influence on vegetative production and nutrient cycling is limited.

Phosphorus deficiency occurs in parts of the Great Plains (Read et al.,

1977; Power, 1983; Nuttall et al., 1986). Even native grassland soils are

sometimes P deficient (Power, 1983). Phosphorus fertilization may increase dry matter yield, contributing to increased organic matter in fine or

coarse textured soil. Extensive organic matter loss may lower available P

in soil. In Canada, an organic matter loss of 35% was accompanied by a

12% decrease in available P concentration (Tiessen et af., 1982). All P loss

was generally accounted for by the decrease in the organic fraction.

Sulfur is the most likely element other than N to have an influence on



organic matter in soil since it is derived primarily from the organic fraction

and is required in direct proportion to N for protein synthesis. But S

deficiency rarely occurs in the Great Plains, and only infrequently in

Canada and the western United States (Beaton and Soper, 1986; Rasmussen and Kresge, 1986). Sulfur deficiency is also found in some Australian soils (Blair and Nicolson, 1975). Increases in vegetative production

from applied S are usually much less than those obtained with N application. The overall potential for S to affect organic C and N in soil is therefore

not large.




1 . Type of Residue

It now appears that residue input plays an important role in setting a new

organic matter equilibrium level in soil. The effect of crop residue on soil

organic matter content is highly related to the amount and only weakly

related to the type of residue applied. Larson et al. (1972) found that

alfalfa, cornstalks, oat straw, sawdust, and bromegrass produced similar

increases in organic C in a Hapludoll in Iowa (Fig. 1). The influence of

residue type on organic N was similar but more variable, with sawdust

providing significantly less N retention than the other materials. Horner e?

al. (1960) also found little difference between effects of wheat straw and

alfalfa hay applied at rates from 0 to 1.63 t/ha to a Pachic Argixeroll in

Washington. Sowden (1968) and Sowden and Atkinson (1968) found little

differences between the effects of straw, alfalfa, and deciduous leaves

incorporated into soil for 20 years in Canada (Table 111). All prevented

further decline in organic C and N in a clay soil and increased levels in a

sandy soil. Peat, muck, and manure increased C and N more than did

straw, alfalfa, or tree leaves. Sauerbeck (1982) in Germany also concluded

that different types of crop residue had similar effects on soil organic


Residue decomposition is a fundamental factor in organic matter stabilization, since degradation products are incorporated into various soil organic matter pools. Biological decomposition of plant materials is influenced by temperature, moisture, aeration, pH, C, N, lignin content,

particle size, and degree of burial in soil (Parr and Papendick, 1978).



























NITROGEN (x 0.1)




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

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