Effects of Tillage and Forest Succession on DOC and Total Soil Carbon: A Full Profile Perspective.

The Horseshoe Bend tillage experiment is a long-term investigation of no-tillage (NT) and conventional tillage (CT) agroecosystems begun in 1978 in the piedmont of Georgia, USA.  In this work, we sought to compare the NT and CT treatments to adjacent forest succession (FS) plots in terms of soil carbon contents and dissolved organic carbon (DOC) dynamics with a full-profile perspective.  We measured total soil C in seven depth classes from 0-2 m, DOC in litter leachate and in soil solution at 15 and 100 cm depth, and quantified soil aggregate stability in the upper 30 cm.  Soil C concentration differs significantly only in the 0-5 cm layer: FS > NT > CT with decreasing aggregate stability from FS to CT.  Both the NT and FS soil profiles have carbon contents of 60 Mg ha^-1, which is 20% greater than the CT profile.  Approximately 40% of this additional C content is a result of small mean differences below 15 cm.  Mean DOC concentrations at 15 cm were consistently 2-3 times greater under FS compared to NT and CT from November to March.  During this leaching period, the flux estimate of DOC at 15 cm under FS was only 20 kg C ha^-1.   At 100 cm depth, mean DOC levels were consistently 1-2 mg L^-1 for all treatments.  This suggests that DOC is likely sorbed or decomposed at greater rates between 15 and 100 cm under FS.  Current measurements of total alkalinity in water samples should help identify which process is responsible for this reduction of DOC concentration with depth under FS.  Currently, treatment effects on soil C contents are only clearly evident in the surface horizon.  Although DOC fluxes may be greater under FS at 15 cm, this has not had a statistically significant effect on C concentrations at greater depth.