Soil disturbance and hydrologic response at the National Training Center, Ft. Irwin, California

Military training exercises at the National Training Center (NTC), Ft. Irwin, California have led to the degradation of large areas of the Mojave Desert. Soil physical properties in areas subjected to low and high disturbance at interspace and plant (Larrea tridentata) mound microsites were quantitatively compared to undisturbed surfaces of both well-developed soils formed on Pleistocene age alluvial deposits and weakly developed soils formed on Holocene age alluvial deposits. Results indicate that geomorphically young and old soils differ in hydraulic and pedogenic properties with and without anthropogenic disturbance. Multi-tension infiltrometers showed that young coarser textured surfaces had saturated conductivities (Ksat) that were nearly twice those of older surfaces. Mound microsites were more sand-rich than interspace locations although Ksat was not significantly different. After both low and high disturbance by tracket vehicles, bulk density was significantly increased from 1.54 to 1.62 and 1.80 g cm−3, respectively. Similarly, penetration resistance increased following low and high disturbance from 1.32 to 1.47 and 3.90 kg cm−2, respectively. Although low disturbance increased Ksat, high disturbance resulted in significant reductions in both Ksat (2.4 to 1.3 cm h−1 (p<0.05)) and mean Gardnerʹs α (0.20 to 0.10 (p<0.001)) at all sites. The resulting decrease in α is indicative of a loss in macroporosity from mixing and compaction of the soil matrix. In general, young interspaces were less sensitive to disturbance than older well-developed interspace soils. Although results showed that Ksat on these older surfaces was not dramatically reduced despite large losses of fines and increased bulk densities, the overall morphology of the soil was dramatically impacted. Disking or ripping are common methods to alleviate soil strength prior to restoration activities. However, results indicate that these practices may result in further destruction of the soil structure.