Carbon Loss In Subsurface Flow

Seven monolith lysimeters, each having a surface area of 8.1 m2 and a depth of 2.4 m, were used to study TOC moving through the soil profile under a corn/soybean-rye rotation

Table 18.3 Average Annual Wheel and Total (Wheel Plus Flume Floor) Sediment and C Transport by Crop and Tillage Practice

Wheel Sediment (kg/ha) Total Sediment (kg/ha)

Table 18.3 Average Annual Wheel and Total (Wheel Plus Flume Floor) Sediment and C Transport by Crop and Tillage Practice

Wheel Sediment (kg/ha) Total Sediment (kg/ha)

Management

Sediment

C

Sediment

C

No Till (15 years)

Corn

236

± 245

6.9

± 7.2

262

± 271

7.5

± 7.8

Soybean

682

± 789

17.4

± 19.0

800

± 850

20.0

± 20.2

Overall

459

± 617

12.1

± 15.1

531

± 686

13.8

± 16.6

Chisel

-Plow

(15 years)

Corn

974

± 2141

17.4

± 34.0

1258

± 2954

22.0

± 46.6

Soybean

356

± 584

7.2

± 10.1

399

± 640

8.0

± 11.2

Overall

665

± 1573

12.3

± 25.2

828

± 2180

15.0

± 34.6

Paraplow

(6 years)

Corn

273

± 416

5.5

± 8.4

317

± 463

6.4

± 9.2

Soybean

737

± 830

16.3

± 21.0

883

± 937

19.1

± 23.0

Overall

505

± 672

10.9

± 16.2

600

± 791

12.7

± 18.7

Disk (9

years)

Corn

302

± 444

6.8

± 9.7

348

± 539

7.8

t 11.8

Soybean

2193

± 2043

43.9

± 36.2

2956

± 2778

60.1

± 49.6

Wheat/clover

127

± 128

3.5

± 3.7

151

± 151

4.1

± 4.3

Overall

874

± 1529

18.1

± 28.4

1152

± 2076

24.0

± 39.0

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.

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.

with a chisel-plow tillage treatment. Developed springs were used to study TOC movement through well-drained and moderately well-drained, residual silt loam soils under pasture. Low TOC concentrations were found in both studies, with annual averages ranging between 0.5 and 3.2 mg/L (Table 18.4). These values are similar to the levels of DOC reported in New England streams and tributaries (David et al., 1992; Likens et al., 1977). With the exception of stormflow situations, much of the stream flow would be from subsurface return flow. Therefore, the water would be "filtered" much

Table 18.4 Flow-Weighted Average3 Annual Total Organic Carbon Concentrations and Flux in Subsurface Flow

Standard Coefficient of Minimum Maximum Mean Deviation Variation (%)

Flow-Weighted Concentrations (mg/L)

Corn/soybean-rye rotations

Y102 ABCb

0.5

3.2

1.3

0.6

44

Y103 ABCDb

0.5

3.0

1.4

0.5

37

All lysimetersb

0.5

3.2

1.3

0.5

40

Rotationally grazed pastures

B Areac

1.0

3.0

1.6

0.5

30

D-E Aread

1.0

3.1

2.1

0.6

26

All watersheds

1.0

3.1

1.9

0.6

31

Total Organic Carbon Flux (kg/ha)

Corn/soybean-rye

rotations

Y102 ABCb

1.6

12.4

4.6

2.4

52

Y103 ABCDb

1.5

11.9

4.5

2.4

52

All lysimetersb

1.5

12.4

4.5

2.4

52

Rotationally grazed pastures

B Areac

1.5

11.9

4.4

2.1

47

D-E Aread

1.2

14.9

5.0

3.4

67

All watersheds

1.2

4.9

4.7

2.8

60

a Average for 10 years.

b Constructed without disturbing the soil profile and underlying fractured shale bedrock (Harrold and Dreibelbis, 1958). c Forage was fertilized orchardgrass (Dactylis glomerata L.). d Forage was legumes, principally alfalfa (Medicago sativa L.) mixed with orchardgrass.

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 Average for 10 years.

b Constructed without disturbing the soil profile and underlying fractured shale bedrock (Harrold and Dreibelbis, 1958). c Forage was fertilized orchardgrass (Dactylis glomerata L.). d Forage was legumes, principally alfalfa (Medicago sativa L.) mixed with orchardgrass.

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.

like the lysimeter percolate and water from springflow developments.

There were no TOC concentration trends during the 10-year study period with either the lysimeter percolate or springflow. This indicates that there were no differences in TOC concentrations based on the management practices of corn/soybean-rye rotations and rotationally grazed pastures. The TOC flux closely followed the quantities of percolate and springflow, indicating that the amount of subsurface water movement was much more important in determining TOC flux than TOC concentration (Owens et al., 2002b).

Average annual TOC concentrations in lysimeter percolate had greater coefficients of variation (Table 18.4) than TOC concentrations in springflow, even though the mean concentrations by management practice were within a single standard deviation. The greater variation with the lysimeters probably was influenced by the shorter flow paths, which would respond more quickly to climate and treatment variations. Coefficients of variation for the average annual TOC flux were greater than for concentrations.

The seasonal relationships between management practices and TOC concentrations and TOC transport were consistent with the observations for annual comparisons. There were some monthly TOC concentration variations in the subsurface flow from the corn/soybean-rye rotations and from the rotationally grazed pastures, but there were no seasonal variations. Coefficients of variations of the average monthly TOC concentrations for the lysimeters and watersheds (Table 18.5) were similar to the respective coefficients of variations for the annual averages. Plots of the average monthly flow and TOC transport show great similarity. This further indicates that the amount of flow is much more dominant in determining TOC transport than is TOC concentration.

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