Managing stockpiled topsoil: The biophysical recovery characteristics of topsoil salvaged from a native coastal sage habitat

Topsoil is an essential component to the structure and functioning of terrestrial ecological systems. Thus, stockpiling and reapplying mined topsoil has long been recognized as a useful tool in restoring land degraded through construction and mining activities. The soil biota change dramatically during topsoil mining operations and the physico-chemical properties are known to deteriorate over time. The structural stability of macro aggregates and bulk density are two important soil physical properties due to their respective influence on soil erodibility, water infiltration, carbon sequestration, and nutrient cycling. In a two-year field study using permanent reference plots and salvaged topsoil from a native coastal sage/scrub habitat in San Mateo county California, aggregate stability declined and bulk density increased under test conditions. Live and sterile forms of a commercially available soil inoculant containing arbuscular mycorrhizal fungi (AMF) and a soil erosion seed mix were set as levels within a two-way restoration experiment consisting of 20 stockpiles (each 6 m³) of sandy loam topsoil. One year after treatment implementation, aggregate stability remained in a degraded state. Significant effects of AMF inoculation were observed for total plant productivity in all plots receiving live AMF, but these positive growth effects did not improve the water stability of macro aggregates, which are known to be influenced by AMF/root interactions. Soil bulk density, however, was lower in the live seed or live AMF treatments, where total productivity had increased. These variable responses suggest topsoil can be managed to improve its physical condition during storage, but soil recovery is likely controlled by multiple processes.