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Soil TC, TN, and MBC results are shown in Table 2. Differences in TC occurred between soil depths and it was higher in the marsh area than in the pond area. Contents of TN and MBC were also higher in the marsh area but differences were not as large as with TC. In general, averaged across depths, TC content accumulated over 12 years was 21 Mg ha−1 (i.e., 1.75 Mg ha−1 yr−1) in the marsh area and 12.1 Mg ha−1 (i.e., 1 Mg ha−1 yr−1) in the pond area. TN contents in the marsh and pond areas were 2.0 Mg ha−1 and 1.7 Mg ha−1, respectively, while the concentrations of MBC were 579.8 mg kg−1 and 445.7 mg kg−1. The MBC was less than 2% of the TC in both wetland areas. Bernal & Mitsch (2008) reported a C accumulation of 2.42 Mg ha−1 yr−1 in created riverine wetlands. Also, Mitsch et al. (2012) reported a stable C accumulation of 2.19 and 2.69 Mg C ha−1 yr−1 for planted and non-planted wetlands, respectively. These values are consistent with ours, which were 1.75 Mg ha−1 yr−1 in marsh area and 1 Mg ha−1 yr−1 in the pond area, respectively, based on accumulations per year.

Table 2

TC, TN, and MBC in two soil depths within marsh and pond areas after 12 years of swine wastewater treatment in a constructed wetland cell

 Marsh area
Pond area
Soil property0–6 cm6–12 cm0–6 cm6–12 cm
TC, Mg ha−1 25.1 (3.1) 16.8 (0.4) 14.5 (6.2) 9.7 (2.7) 
TN, Mg ha−1 2.5 (0.2) 1.6 (0.1) 1.8 (0.4) 1.6 (0.1) 
MBC, mg kg−1 606.9 (35.9) 552.8 (32.9) 476.2 (13.1) 415.2 (3.6) 
 Marsh area
Pond area
Soil property0–6 cm6–12 cm0–6 cm6–12 cm
TC, Mg ha−1 25.1 (3.1) 16.8 (0.4) 14.5 (6.2) 9.7 (2.7) 
TN, Mg ha−1 2.5 (0.2) 1.6 (0.1) 1.8 (0.4) 1.6 (0.1) 
MBC, mg kg−1 606.9 (35.9) 552.8 (32.9) 476.2 (13.1) 415.2 (3.6) 

*Numbers in parentheses are standard errors.

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