THE EFFECTS OF TOPSOIL REAPPLICATION ON VEGETATION REESTABLISHMENT. FINAL REPORT

Topsoils were harvested and stockpiled during road construction in northern California. After five months in storage, topsoils were reapplied to barren fill slopes (1.5:1 to 1:1). Early rains caused slumping and gullying on many of the plots. The remainder of the plots were used to compare plant growth on topsoiled and nontopsoiled plots and to compare soil nutrient content and biological activity of stockpiled topsoil with fresh topsoil which had not been stockpiled. A parallel greenhouse experiment allowed more detailed comparison of soil properties. Topsoil reapplication improved plant growth by 250% after three years compared to fill slopes which had no topsoil, but had equivalent application of all other nutrients, erosion control and seed materials. Topsoiled plots were predominantly covered by grasses, while the fill slopes were covered by clovers. Topsoil was not degraded in the stockpiling process. Indicators of plant growth, soil nutrient content, mycorrhizal infection and microbial biomass showed no decrease compared to treatments using fresh topsoil rather than stockpiled topsoil. Topsoil fraction had to exceed 20% of soil volume before significant improvements in plant growth and soil characteristics occurred. Higher rates are recommended in more severe environments. Moderate fertilization improved plant growth without decreasing mycorrhizal root production. Mineralizable nitrogen was shown to be predominantly derived from soil microbes. Topsoil stockpiling and reapplication are strongly recommended for improved regeneration of plant-soil systems. Stockpiling had little or no negative impact on topsoil quality.