The objective of this research was to understand how land cover and topography act, independently or together, as determinants of soil carbon and nitrogen storage over a complex terrain. Such information could help to direct land management for the purpose of carbon sequestration. Soils were sampled under different land covers and at different topographic positions on the mostly forested 14,000 ha Oak Ridge Reservation in Tennessee, USA. Most of the soil carbon stock, to a 40‐cm soil depth, was found to reside in the surface 20 cm of mineral soil. Surface soil carbon and nitrogen stocks were partitioned into particulate (≥53 μm) and mineral‐associated organic matter (<53 μm). Generally, soils under pasture had greater nitrogen availability, greater carbon and nitrogen stocks, and lower C:N ratios than soils under transitional vegetation and forests. The effects of topography were usually secondary to those of land cover. Because of greater soil carbon stocks, and greater allocation of soil carbon to mineral‐associated organic matter (a long‐term pool), we conclude that soil carbon sequestration, but not necessarily total ecosystem carbon storage, is greater under pastures than under forests. The implications of landscape‐level variation in soil carbon and nitrogen for carbon sequestration are discussed at several different levels: (1) nitrogen limitations to soil carbon storage; (2) controls on soil carbon turnover as a result of litter chemistry and soil carbon partitioning; (3) residual effects of past land use history; and (4) statistical limitations to the quantification of soil carbon stocks.

We present results on changes in soil properties following land use change over an approximately 55-year period at Fort Benning, Georgia, U.S.A. Soil cores were taken at 129 locations that were categorized as reforested (field/ bare ground in 1944 and forest in 1999), disturbed (field/ bare ground in 1944 and 1999), or reference forests (forest in 1944 and 1999). Soil disturbance included historic agriculture (pre-1944) and military training (post-1944). Density in mineral soils exhibited a historic land use legacy effect (reference < reforested < disturbed). Rates of change in bulk density decreased with depth and estimated total times to reach reference forest levels ranged from 83 (0-10 cm) to 165 (30-40 cm) years. A land use legacy effect on C stock was apparent in the O-horizon and in 30-to 40-cm soil increment (reference > reforested > disturbed). Soil C stock in all other increments and in particulate organic matter was affected by disturbance; however, no legacy was apparent (reference = reforested > disturbed). For the entire soil profile (O-horizon to 40 cm), rate of C accrual was 28 g m 22 yr 21 (1.5%/yr). Nitrogen stocks were affected by disturbance in the O-horizon and 0-to 10-cm increment; however, no legacy effect was detected (reference = reforested > disturbed). Nitrogen accumulated at 0.56 g m 22 yr 21 (0.6%/yr) for the entire soil profile. At Fort Benning, soil C and N stocks of reforested stands were similar to those of reference forested stands after approximately 55 years. However, soil bulk density was greater on reforested stands than reference forest stands at 55 years and may require an additional century to reach reference levels.

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