Organic carbon storage and dynamics in croplands and terrestrial deposits as influenced by subsurface tile drainage

Soil erosion contributes to the removal of soil organic carbon (SOC) from cultivated soils and its entrapment in terrestrial depressions. The fate of SOC entrapped in terrestrial deposits is largely unknown, but there has been speculation that such entrapment could lead to C sequestration, thereby playing a role in global C cycling. It has been hypothesized that the fate of eroded C in these deposits depends on SOC quality (bioavailability) and the environmental conditions at the depositional site. The SOC storage and dynamics were studied in cultivated, forested, and deposited soils at sites with and without subsurface tile-drainage. Microbial biomass carbon (MBC), readily mineralizable C (MinC), and basal soil respiration (BSR) rates were used as SOC quality indicators and were assessed in field-moist soil aggregates of four sizes: 2-3 mm, 1-2 mm, 0.5-1 mm and <0.5 mm. Soil organic carbon inventory (mass of C over the depth sampled) was significantly different (P < 0.01) among the land uses and was in the order: deposit > forest > cropland. It was also significantly (P < 0.1) different between the two deposits, amounting to 14.6 and 17.1 kg C m in the tile-drained and undrained deposits, respectively. Over that same depth, the total SOC pool in the forest soil was 9.5 kg C m. Overall, the various aggregate sizes did not differ significantly in terms of their total SOC content, but the SOC quality indicators (MBC, MinC, and BSR) were generally higher in the larger than in the smaller aggregates. These indices were also higher in the forest and depression sites than in the croplands. The data indicated that cultivation and erosion resulted in depletion of both total and labile SOC, but the labile pools were depleted at rates 1.5 to 3 times faster. Conversely, there was an enrichment in both clay (1.4 to 2 times) and SOC (1.3 to 1.6 times) in the depression areas, indicating removal of fine particle-associated SOC from the cultivated fields and its entrapment in the deposits. However, the levels of labile C (MBC, MinC) in the entrapped materials were 20 to 46% lower than would be anticipated based on their total C contents. These reductions suggest that, compared with the forest and cropland, a relatively greater proportion of the C retained in the deposits is in the slow and passive pools, and that distribution is favorable to sequestration of C in these landscape positions.