Effects of Heavy, Tracked-Vehicle Disturbance on Forest Soil Properties at Fort Benning, Georgia

The purpose of this report is to describe the effects of heavy, tracked-vehicle disturbance on various measures of soil quality in training compartment K-11 at Fort Benning, Georgia. Predisturbance soil sampling in April and October of 2002 indicated statistically significant differences in soil properties between upland and riparian sites. Soil density was less at riparian sites, but riparian soils had significantly greater C and N concentrations and stocks than upland soils. Most of the C stock in riparian soils was associated with mineral-associated organic matter (i.e., the silt + clay fraction physically separated from whole mineral soil). Topographic differences in soil N availability were highly dependent on the time of sampling. Riparian soils had higher concentrations of extractable inorganic N than upland soils and also exhibited significantly greater soil N availability during the spring sampling. The disturbance experiment was performed in May 2003 by driving a D7 bulldozer through the mixed pine/hardwood forest. Post-disturbance sampling was limited to upland sites because training with heavy, tracked vehicles at Fort Benning is generally confined to upland soils. Soil sampling approximately one month after the experiment indicated that effects of the bulldozer were limited primarily to the forest floor (O-horizon) and the surface (0-10 cm) mineral soil. O-horizon dry mass and C stocks were significantly reduced, relative to undisturbed sites, and there was an indication of reduced mineral soil C stocks in the disturbance zone. Differences in the surface (0-10 cm) mineral soil also indicated a significant increase in soil density as a result of disturbance by the bulldozer. Although there was some tendency for greater soil N availability in disturbed soils, the changes were not significantly different from undisturbed controls. It is expected that repeated soil disturbance over time, which will normally occur in a military training area, would simply intensify the changes in soil properties that were measured following a one-time soil disturbance at the K-11 training compartment. The experiment was also useful for identifying soil measurements that are particularly sensitive to disturbance and therefore can be used successfully as indicators of a change in soil properties as a result of heavy, tracked-vehicle traffic at Fort Benning. Measurements related to total O-horizon mass and C concentrations or stocks exhibited changes that ranged from {approx}25 to 75% following the one-time disturbance. Changes in surface (0-10 cm) mineral soil density or measures of surface soil C and N following the disturbance were less remarkable and ranged from {approx}15 to 45% (relative to undisturbed controls). Soil N availability (measured as initial extractable soil N or N production in laboratory incubations) was the least sensitive and the least useful indicator for detecting a change in soil quality. Collectively, the results suggest that the best indicators of a change in soil quality will be found at the soil surface because there were no statistically significant effects of bulldozer disturbance at soil depths below 10 cm