Carbon Concentration in Sediment

Surface runoff from the watersheds was automatically measured with H-flumes and sampled with Coshocton wheels (Brakensiek et al., 1979) that were modified to continuously deliver a proportional sample of runoff water and suspended sediment during each runoff event. Sediment was occasionally deposited in the flume floor and flume approach. This sediment was also collected.

Sediment losses occurred almost every year from each of six small watersheds (Table 18.1) during the 15-year study period, but there was sufficient sediment for C analyses

Table 18.1 Tillage Treatments and Selected Landscape and Soil Characteristics of Six Watersheds

Watershed Area Average Slope Dominant

Table 18.1 Tillage Treatments and Selected Landscape and Soil Characteristics of Six Watersheds

#

Tillage

(ha)

(%)

Soila

WS 113

No tillb

0.59

11

Coshocton SiL

WS 118

No tillb

0.79

10

Coshocton SiL

WS 109

Chisel plowb

0.68

13

Rayne SiL

WS 123

Chisel plowb

0.55

7

Keene SiL

WS 115

Paraplow/diskc

0.65

7

Coshocton SiL

WS 127

Paraplow/diskc

0.68

9

Coshocton SiL

a Rayne: fine-loamy, mixed, mesic Typic Hapludult. Keene: fine-silty, mixed, mesic Aquic Hapludalf. Coshocton: fine-loamy, mixed, mesic Aquultic Hapludalf. SiL: sil loam. Soils were residual and formed from sandstone and shale bedrock. They are moderately well-drained and well-drained silt loams. For more detail on soils, see Edwards et al. (1993) and Kelley et al. (1975). b Two-year corn/soybean-rye (Zea mays LJGlycine max (L.) Merr. - Secale cereale L.) rotation for 15 years beginning in 1984 (Edwards et al., 1993). Rye was sown in the soybeans as a winter cover crop. c Two-year corn/soybean-rye rotation for 6 years beginning in 1984. In 1990, they were placed in a 3-year, reduced chemical input rotation (corn/soybean/wheat [Triticum aestivum L.]-clover [Trifolium pretense L.]).

Source: From Shipitalo, M.J., and W.M. Edwards. 1998. Soil Tillage Res., 46:1-12. With permission.

Table 18.2 Weighted Average C Concentration on Sediments Collected with the Coshocton Wheel (g/kg)ab

No

Chisel

Paraplow

Disk

Tillage

Plow

(1984-1989)

(1990-1998)

Corn years

29.6a

21.3a

18.9a

21.9a

Soybean years

24.1b

20.2a

20.5a

21.4a

Wheat/clover years

NA

NA

NA

29.0b

Overalld

26.1x

20.7y

20.3y

22.0xy

a Annual cycle was May through April.

b Means within a column followed by the same letter (a, b) are not significantly different at the 0.05 level. Statistically significant differences were determined by general linear model procedures (SAS Institute, 1985).

c Not available: wheat/clover only grown in the disk treatment. d Means in this row followed by the same letter (x, y) are not significantly different at the 0.05 level. Source: From Owens, L.B., R.W. Malone, D.L. Hothem, G.C. Starr, and R. Lal. 2002a. Soil Tillage Res., 67:65-73. With permission.

a Annual cycle was May through April.

b Means within a column followed by the same letter (a, b) are not significantly different at the 0.05 level. Statistically significant differences were determined by general linear model procedures (SAS Institute, 1985).

c Not available: wheat/clover only grown in the disk treatment. d Means in this row followed by the same letter (x, y) are not significantly different at the 0.05 level. Source: From Owens, L.B., R.W. Malone, D.L. Hothem, G.C. Starr, and R. Lal. 2002a. Soil Tillage Res., 67:65-73. With permission.

(determined by a dry-combustion method) in less than half of the watershed years. Average C concentration in sediments passing through the H-flume (wheel sediment) usually did not differ significantly among crops within the rotations (Table 18.2). Although sediment C concentration from years in soybeans was usually lower than from years in corn, this difference was significant only in the no-till watersheds. Moreover, the weighted average of C concentration in sediment under no-till was significantly greater than the C concentration in sediments from the other tillage practices (Table 18.2). The higher C concentration in the no-till sediments reflects the higher C concentration in the topsoil of the no-till watersheds than in the tilled watersheds.

Wheel sediments were enriched with C compared with flume floor sediments. Flume floor and flume approach sediments, however, only accounted for approximately 18% of the total sediment leaving the watersheds. The weighted-average C concentration in these sediments was 22.5 g/kg for no-till watersheds, and 17 g/kg for the chisel-plow watershed. Wheel sediments were also enriched compared with the surface soil. The enrichment ratio (ER) for sediments from the no-till and chisel-plow watersheds to the 0- to 2.5-cm soil layer was 1.5. The lowest ER (1.2) was for sediments from the paraplow/disk watersheds. In the disked watersheds, rills developed and allowed some erosion to occur from depths greater than 2.5 cm, where the C concentrations were lower. Thus, the overall C concentration in the sediments was probably lower than if the sediments had come only from the 0- to 2.5-cm layer (Owens et al., 2002a).

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