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II. Tillage Effects on Soil Organic Matter
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
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;
SOIL ORGANIC MATTER IN SEMIARID REGIONS
Effect of Conservation Tillage on Organic C and N in Soil
N . Dakota
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).
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
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
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
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