Soil
Carbon and C Sequestration Under Different Cropping
and Tillage Practices in the Pacific Northwest
David
Bezdicek and Mary Fauci, Crop and Soil Sciences, WSU, Pullman
Steve Albrecht and Katherine Skirvin, USDA-ARS, Pendleton, OR
Introduction:
Concern for loss of soil organic carbon (SOC) through tillage and erosion
and concern on global warming has stimulated interest in direct seeding
(DS) in the Pacific Northwest. The issue of global warming has raised
awareness of the delicate balance between fixation of atmospheric CO2
and respiration from terrestrial and aquatic sources. Tillage of the North
American Prairies raised the flux of terrestrial C to the atmosphere (Wilson,
1978; Allmaras et al., 2000) with the concomitant loss of soil organic
carbon (Reicosky, et al. 1997; Huggins et al.,1998; Janzen, et al., 1998;
Rasmussen et al.,1998). Increased intensity of tillage, fallow, and burning
of residue contribute to the loss of SOC, although a portion of the CO2
can be from trapped gasses already in the soil (Reicosky et al., 1997).
Numerous studies
have shown that higher SOC levels accrue under no till as compared to
conventional primary and secondary tillage. Hendrix et al. (1998) reported
that intensive cultivation decreased SOC 40% compared to 18 % under NT
in Georgia over 16 years. In Texas, Franzluebbers et al., (1994) showed
that SOC and mineralizable C were 33 to 125 % higher under NT than conventional
till (CT) at 0 to 50 cm in depth. Under Canadian conditions, Janzen et
al, (1998) reported significant gains of 3 Mg C ha-1 in a decade under
reduced tillage. Loss of SOC in Oregon was attributed to tillage intensity,
residue burning, and the absence of residue input and biological oxidation
during fallow (Collins et al., 1992; Rasmussen et al., 1998).
Although soil organic matter (SOM) or SOC are common measures of organic
matter accumulation in soil, some fractions of SOM may be more sensitive
to changes in management and may provide a better indication of management
that favors sequestration of atmospheric C. The objective of this study
was to identify and measure fractions of SOC that provide a better indication
of tillage and crop practices favoring accumulation of C.
Approach:
Soils with different tillage and cropping histories were collected for
C analyses from a WA grower field in DS for 25 years and from long-term
plots at the Pendleton Agricultural Experiment Station Center. The following
areas were studied.
Washington Treatments:
Direct Seed (WA25DS):
Long-term DS field of 25 years of winter wheat-lentil and occasional years
of winter wheat-barley. The only tillage was a light harrow after lentil
planting to level the ground for harvest.
Direct Seed (WA01DS): First year of DS
Direct Seed (WA03DS): Third-year of DS.
Oregon Treatments:
Rotations at Pendleton briefly discussed below have been described
in detail by Rasmussen and Smiley (1994).
Conventionally
Tilled Wheat-Pea (ORCTWP): The experiment was established in 1963,
using a winter wheat rotation with four tillage treatments and four replications.
The treatment sampled employs conventional tillage of fall plowing.
Conventionally
Tilled Winter Wheat (ORCTWW): Established in 1931, cropped annually
to winter wheat, the site was modified in 1982 to include spring wheat
and spring barley. The experiment was moldboard plowed, received both
chemical and mechanical weed control.
Grass Pasture
(ORGP). Maintained as permanent pasture since 1931, it was grazed
until 1985 but has not been grazed since. It serves as a baseline from
which to measure changes in cultivated cropping systems.
Direct Seed Wheat
Fallow (ORDSWF): Started in 1982, it was cropped annually (wheat)
from 1982 to 1988, then converted to NT winter wheat/summer fallow. Wheat
stubble is flailed and left on the field.
Conventionally
Tilled Wheat Fallow (ORCTWF): Established n 1931, as a winter wheat-fallow
rotation. The winter wheat residue remains over winter, burned in the
spring, then moldboard plowed.
Soil sampling
and analysis: In spring 1998, soil samples were taken at three depths,
0-2, 2-4, and 4-10 inches and analyzed for the following: 1) total SOC
or the C obtained from a traditional soil test; particulate organic matter
(POM) C; and light fraction (LF) C. The POM C is a sand-sized fraction
and larger that includes partially decomposed residue and roots that is
believed to build up under DS and under grasses and releases nutrients
slowly over time. The LF is a low density organic matter fraction that
floats on the surface of a particular liquid. It has some of the same
nutrient release characteristics as POM C.
Results and Discussion:
Higher SOC was found at most depths at the WA than OR site (Fig. 1).
The higher rainfall and greater dry matter production in WA would explain
these differences. In WA, total accumulation of SOC was about 28% higher
over all depths after 25 years of DS compared to the first season in DS.
The distribution of soil C with depth for WA25DS resembled ORGP and to
a lesser extent ORDSWF. However, the longer period of DS in WA resulted
in an apparent increase in soil SOC at all depths. After three years in
DS there was a noticeable increase in soil C at the surface.
In OR, SOC is reflected both from differences in crop rotation and tillage.
Surface SOC was the highest for ORGP, with elevated surface values noted
for ORDSWF, and to a lesser extent for ORCTWP. The SOC generally decreased
with increased tillage or fallow. For ORCTWF, SOC was lowest and uniform
over depth, reflecting the reduced surface residue after burning, the
mixing of soil due to tillage, and oxidation of residue during fallow.
Under ORCTWW, SOC was higher, but again the same at all depths due to
mixing of residues with tillage. Positive effects on SOC from continuous
cropping and negative effects from fallow have been reported by Collins
et al. (1992) in OR, Larney et al. (1997) in Alberta, and Campbell et
al. (1998) in Saskatchewan. Rasmussen et al. (1989) reported that SOC
loss may be more intense in the Pacific Northwest because fallowing maintains
higher soil moisture favoring microbial decomposition during the summer
months when soil conditions are normally dry.
The greatest stratification
of residues was noted for ORDSWF and ORGP, where residues were maintained
near or at the soil surface. The ORGP consistently maintained high levels
of soil C at the two upper depths. These trends on the positive impact
of intensive cropping on SOC have been reported by Franzluebbers et al.
(1994) in TX.
Surface POM C followed
the same general trends as for SOC, although the differences were amplified
for WA25DS and ORGP, and to a lesser extent for ORDSWF (Fig 2.). The POM
C at WA was highest at the soil surface in proportion to the period of
time in NT. Substantial shifts in POM C were noted after three years of
NT in WA. In IL, Wander et al (1998) in IL reported increases of 70% POM
C in the surface 0-5 cm after 10 years of NT, whereas Bowman et al. (1999)
reported a doubling of POM C in the surface 0-5 cm after four years in
CO. The POM C at the 4-8 inch depth changed very little after 25 years
of DS in WA. While these depths receive little residue from the surface,
root derived POM C is considered more stable than above ground residues
and may be responsible for maintaining the levels of POM C at lower depths
(Gale and Cambardella, 2000). The lowest POM C values for the surface
and intermediate depths were noted for ORCTWF, which were consistent with
SOC.
The LF followed similar
trends as observed for POM C except these values are slightly higher (Fig
3). However, differences by depth for the LF within each treatment appeared
to be more dramatic than for either SOC or POM C. For example, the LF
estimated greater differences by depth after only three years of DS in
WA compared to the POM C fraction. Thus using the LF, substantial stratification
of soil C in the LF can be observed after only three years of DS. Substantial
increases in the LF was noted in the soil surface under native sod and
DS compared to CT in CO (Six et al. (1998) and under zero till in Alberta,
(Larney et al.,1997). The same trends are observed for WA25 DS, ORGP,
and ORDSWF. Even distribution with depth of POM C and LF C was noted for
the ORCTWW rotation, although numerically the values for the POM C fraction
were roughly 60% of the LF.
Total C sequestered
under DS is shown in Table 1. After 25 years of DS in WA, a total of 4151
lbs. C per acre was sequestered to a depth of 8 inches. After only three
years of DS, 1761 lbs. C per acre was sequestered.
Conclusions: Both tillage and cropping influence the sequestration
of C. Highest SOC was found after 70 years of grass-pasture in OR and
25 years of DS in WA in contrast to the lowest values for 68 years of
conventional wheat-fallow in OR. POM C was concentrated at the soil surface
under OR grass pasture and 25 years of DS in WA and represented 33-35
% of total surface SOC. The LF and to a lesser extent POM C, showed the
greatest differences between cropping systems and with depth under DS.
After only three years of DS in WA, the LF increased 180% and the POM
fraction 29% in the surface two inches, suggesting that changes in soils
C take place very rapidly under DS.
Figure 1. SOC at three depths for the WA and OR rotations. Error bar is
standard error of the mean, n = 4.
Figure 2. POM C at
three depths for the WA and OR rotations. Error bar is standard error
of the mean, n = 4.
Figure 3. Light fraction
of soil at three depths for the WA and OR rotations.
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