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2001 STEEP III Progress ReportTITLE: Impact
of direct seeding on crop water use efficiency, soil physical and microbial INVESTIGATORS:
David Bezdicek, WSU; Steve Albrecht, ARS, Pendleton; Mary Fauci, PROJECT OBJECTIVES: 1) Determine crop water use efficiency, seed zone temperature, soil profile winter water storage, and N use efficiency under direct seed (DS) and conventional systems. 2) Evaluate the transitional soil physical and biological interactions under DS systems and the influence from different crop rotations. 3) Evaluate the quantity and quality of SOM changes under DS systems and the significance in sequestering atmospheric carbon dioxide. KEY WORDS: direct seed, soil quality, carbon sequestration, crop rotation STATEMENT OF PROBLEM:
ZONE OF INTEREST: High, intermediate, and low rainfall zones. ABSTRACT OF RESEARCH
FINDINGS: RESULTS AND INTERPRETATION: Research sites for our studies were conducted both in the high (Pullman) and intermediate rainfall (Pendleton) zones. The Pullman site consists of comparing SOC and various organic matter fractions at 3 depths after zero, 3 (WA03DS), and 25 years (WA25DS) of DS. At the Pendleton station, five crop rotations with different tillage practices were compared: ORGP- 65 years of grass pasture; ORCTWW- 65 years conventionally tilled, wheat-wheat; ORCTWP- conventionally tilled, wheat pea rotation; ORCTWF- 68 years conventional till, wheat-fallow; and ORDSWF-16 years DS, wheat-chem fallow. Highest SOC was found at the soil surface under DS due to the lack of tillage and redistribution of residues. With tillage, SOC is more evenly distributed with depth. Surface SOC was highest and deceased in order for WA25DS = ORGP > WA03DS > ORCTWW, ORCTWP and ORDSWF > ORCTWF. The ORCTWF rotation was the lowest in SOC because fallowing and tillage encourage residue decomposition and loss of SOC. We also looked at the distribution of (POM) C and the light fraction (LF) C because these fractions are more sensitive to changes in residue input and tillage than soil organic matter or SOC. POM C, which is organic matter sand size and larger, was more sensitive to changes in management than SOC and it increased more rapidly under DS and GP than other rotations. The POM C at WA was highest at the soil surface in proportion to the period of time in DS. Substantial shifts in surface POM C were noted after only three years of DS in WA. See last year's STEEP report. This POM fraction is an intermediate nutrient pool with high microbial life and activity and one that contributes positively to the soil physical condition under DS. Since this pool builds up slowly under DS, this may explain the transitional period noted for early DS. Once this pool of organic matter is built up, nutrient release would increase with time. Conversely, if these soils are tilled, this POM C fraction is lost very rapidly as was observed when our native prairies were tilled in the early 1900's. Research in Alberta and British Columbia suggests that under zero tillage, POM C is of higher quality and more mineralizable (higher release rate) than other C pools. The relation of SOC with POM C is a direct and linear pattern, but different for the WA and OR soils (Fig. 1). In other words as tillage and cropping systems encourage build up of soil organic matter, the POM C fraction increases as well, but differently for WA and OR soils. Another organic matter
fraction is the LF, low-density organic matter that floats on the surface
of a particular liquid. It has some of the same nutrient release characteristics
as POM C. Numerically, the LF values are slightly higher that for POM
C (see last years STEEP report), although the trends are nearly identical
(Fig 2). 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 three years of DS in WA compared
to the POM C fraction. Thus using the LF, substantial stratification and
build up of soil C can be observed in as few as three years of DS. Therefore,
changes in soil organic matter as the LF and POM C fractions do occur
rapidly under DS. This change would not be detected using the traditional
soil organic matter test. Note the very high surface LF at the soil surface
for WA25DS and ORGP.
Another way of characterizing fractions of soil organic matter is the active fraction or that amount of C released with acid treatment. The stable fraction is what survives this treatment. Table 1 shows the active, and stable fractions by depth for the seven treatments. Higher active soil C ranged from 18 g C kg-1 soil for ORGP and 16.5 g C kg-1 soil for WA25DS at 0-5 cm depth to a low of 5 g C kg-1 soil for ORCTWF. In general, higher values were observed for WA than OR treatments reflecting the higher SOC for WA soils. Aside from the ORGP treatment, the differences in active C for all other OR treatments did not differ greatly and were more subtle with depth than either SOC or POM C. Stable C was the highest for WA25DS and ORGP at the surface 0-5 cm and was slightly more pronounced with depth than active C for some treatments. For example, stable C was essentially the same with depth for ORDSWF, but the active fraction was highest at the surface and dropped off markedly with depth. When stable soil C is expressed on a SOC basis, all soils ranged from 45 to 61%, with increases generally noted with increasing depth (Table 1). This suggests that the lack of residue replenishment and the humification processes maintain greater proportions of stable C lower in the soil profile. Due to continual deposition of fresh residues from undisturbed treatments, more surface C is available for microbial decomposition and less susceptible to humification processes with clay that occur deeper in the soil profile.
Given the recent emphasis of C sequestration and C credits, we calculated the total amount of C in the soil profile to a depth of 20 cm (8 inches) for the WA soils (Table 2). After 25 years of DS, 4,648 kg ha-1 of C (4,150 lbs C) was sequestered to a depth of 20 cm. After 2 years of DS, 2,002 kg ha-1 of C (1788 lbs C) was sequestered. It is likely that greater quantities were sequestered since the sampling depth only went down to 20 cm (8 inches).
INTERACTION (COOPERATION) WITH OTHER SCIENTISTS CONDUCTING RELATED ACTIVITY: We work with Dennis Roe, NRCS, Colfax on the Northwest Crop Project. David Huggins, ARS, Pullman has collaborated on the water use efficiency, seed zone temperature, water storage, N use efficiency and crop modeling portion of this research. Stewart Wuest and Dale Wilkins, ARS, Pendleton, are involved with the earthworms and soil C modeling, respectively. Steve Albrecht, USDA-ARS, Pendleton, is cooperating with us on the carbon research. PUBLICATIONS AND PRESENTATIONS:
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