Overwinter changes to near-surface bulk density, penetration resistance and infiltration rates in compacted soil

Abstract

Previous studies at Yakima Training Center (YTC), in Washington State, suggest freeze-thaw (FT) cycles can ameliorate soil compacted by tracked military vehicles [J. Terramechanics 38 (2001) 133]. However, we know little about the short-term effects of soil freezing over a single winter. We measured bulk density (BD), soil penetration resistance (SPR), and steady-state runoff rates in soil newly tracked by an Abrams tank and in uncompacted soil, before and after a single winter at YTC. We similarly measured BD, SPR and saturated hydraulic conductivity (k_fs) in simulated tank tracks at another site near Lind Washington. Average BD was significantly greater in tank ruts at YTC and in simulated tracks at the Lind site than in uncompacted soil soon after tracking and did not change significantly during the winter of 1997–1998. Measurements of SPR were strongly influenced by soil moisture. When soil was moist or tracks were newly formed, SPR was significantly higher in tank ruts than in uncompacted soil from the surface to a depth of about 10–15 cm. The greatest average SPR in compacted soil was observed between 4 and 6 cm depth. We observed less difference in SPR between tank ruts and uncompacted soil near-surface at YTC as the time after trafficking increased. We observed highest SPR ratios (compacted rut:undisturbed) in fresh tracks near the surface, with lower ratios associated with increasing track age or soil depth, indicating that some recovery had occurred at YTC near-surface. However, we did not observe a similar over-winter change in SPR profiles at the Lind site. Rainfall simulator data from YTC showed higher steady-state runoff rates in tank ruts than over uncompacted soil both before and after winter. However, more time was required to reach steady-state flow in tank ruts and the proportion of runoff was slightly lower in May 1998 than in August 1997. At the Lind site, k_fs was lower in newly compacted soil than in one-year old compacted soil or uncompacted soil. Our data suggest that indices of water infiltration such as steady-state runoff rates or k_fs, are more sensitive indicators of soil recovery after compaction than are BD or SPR.

Introduction

Military training with tracked vehicles can disturb vegetation, form ruts, and compact soil, which can impact runoff and soil erosion on military lands such as Yakima Training Center (YTC) located in Washington State. Vegetation is directly impacted by maneuvers [15], [20], [28] and indirectly affected through changes in soil nutrient availability, soil physical characteristics, and patterns of soil moisture storage [5], [7], [14], [35]. Tracking can destroy individual plants and alter community composition [2], [3], [4], [19], [31], or influence larger-scale patterns in the landscape [21] especially when combined with grazing, drought or fire.

Ruts can concentrate surface water flow, depending on orientation, slope, soil characteristics, and landscape position [11], [33]. The geometry of hillslope channels, such as rills or ruts, is important because it influences the velocity and, thus, erosivity of water flowing in them [10], [12]. Compacted soil in ruts affects erosion by changing the stability and size distribution of soil aggregates, and increasing soil bulk density and penetration resistance [12], [16], [30]. Small increases in soil bulk density can result in disproportionately large decreases in infiltration rates that increase the potential for runoff [22]. Rut geometry and the degree of compaction are influenced by vehicle factors and site factors [14], [20], [34]. Vehicle factors include contact area, surface pressure, weight, track slip, track design, vehicle speed, and driving pattern and the frequency and season of tank traffic. Site factors include soil characteristics such as texture, moisture, depth, and topography; plant characteristics, such as species composition, coverage, and growth stage; and climatic conditions, such as precipitation and temperature.

Once formed, compacted ruts are affected by environmental factors such as freeze-thaw cycles and wetting and drying [5], [27], [30]. Although the effects of soil compaction by tanks can persist for decades in some desert soils [23], data collected during 1996 in the shrub-steppe at the YTC demonstrate that significant changes (smoothing) in tank rut geometry can occur during a single year [16], [17]. The data also suggest that soil is less compacted by tanks at the soil surface than deeper in the profile or, alternatively, that surface compaction does not persist. Less compaction may occur at the soil surface if water content is lower or if soil texture differs from that deeper in the profile. Alternatively, compacted soil near the surface may be more strongly affected by forces such as wind, and wetting-drying and freeze-thaw cycles that fluctuate with higher frequency and amplitude at the soil surface than deeper in the soil profile. Variation in the degree of compaction throughout the soil profile has important implications for potential erosion and prediction because surface conditions do not necessarily represent the underlying soil.

This research is part of collaboration between the US Army Engineer Research and Development Center's Cold Regions Research and Engineering Laboratory and the United States Department of Agriculture, Agricultural Research Service to determine soil freeze-thaw effects on hydraulic geometry, soil strength, infiltration, runoff erosivity, and soil erodibility of vehicular ruts and natural rills. The objectives of this work were to study the short-term dynamics of change in soil compacted by tracked military vehicles. We measured indicators of compaction, such as soil bulk density, penetration resistance, steady-state runoff, and saturated hydraulic conductivity in soil soon after track formation and again after winter, to record amelioration of compaction over time. Changes in soil compaction are important to rut-flow hydraulics and erosion, and they can be readily measured by military land managers.