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III. Fertilizer Effects on Soil Organic Matter
SOIL ORGANIC MATTER IN SEMIARID REGIONS
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
PAUL E. RASMUSSEN AND HAROLD P. COLLINS
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
SOIL ORGANIC MATTER IN SEMIARID REGIONS
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
IV. ORGANIC RESIDUE EFFECTS ON SOIL
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).
PAUL E. RASMUSSEN AND HAROLD P. COLLINS
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