Sediment carbon concentration and transport from small watersheds under various conservation tillage practices

Carbon sequestration by soils is viewed as a process that can reduce CO2 emission and its potential impacts on global climate change. Therefore, impacts of various agricultural management practices on carbon (C) release/sequestration need to be assessed. The objective of this study was to measure C concentrations and transport in sediments lost with various tillage practices on small watersheds. Corn-soybean/rye (Zea mays L.-Glycine max (L.) Merr./Secale cereale L.) rotations with no-till, chisel-plow, and paraplow were studied on small watersheds (0.55-0.79 ha). Disk tillage preceding the corn and soybean crops of a corn-soybean-wheat (Triticum aestivum L.)/clover (Trifolium pratense L.) rotation was also studied. Each small watershed was instrumented with a 60 cm H-flume mounted on a concrete approach, and a Coshocton wheel for collecting a proportional sample of water and sediment. Samples of sediment deposited in the flume approach and in runoff were collected during a 15-year period and analyzed for total C concentration. Weighted averages of C in the sediment that passed through the flumes during the treatment periods did not differ significantly among tillage treatments, although no-till had the highest C (30 g kg-1) and disk had the lowest C (23 g kg-1) in the last 9 years of the study period. Weighted averages of C concentration in the flume floor sediments were slightly lower (21-23 g kg-1). For comparison, weighted C concentration in sediment that passed through flumes from small fertilized, pastured watersheds ranged from 52 to 72 g kg-1. Average annual sediment loss was 532, 828, and 1152 kg ha-1 for no-till, chisel-plow, and disk, respectively. Annual average transport of C via sediment was 13.8, 15.0, 12.7, and 24.0 kg ha-1 for no-till, chisel-plow, paraplow, and disk, respectively. Although tillage practices may reduce C transport in sediment by lowering concentrations, a greater factor for reducing C movement is reducing sediment movement. This information will be useful to policy makers and others who need to put definitive values on land management practices in terms of C sequestration/release.