Low-Temperature Biochar Affects an Eroded Calcareous Soil.

Previous research showed little benefit from using a high temperature, high pH biochar for improving the fertility of eroded calcareous soils. We thus explored the potential of an activated, low-temperature, low pH biochar to improve their fertility status. In a microcosm study conducted at 20oC, increasing amounts of a steam-activated, low temperature (350oC), low pH (5.8) switchgrass biochar (0, 1, 2, or 10% by weight) were mixed into an exposed calcareous soil (pH 8.3). Microcosms were bought to field capacity twice per week with reverse osmosis water, and then destructively sampled at 1, 2, 3, 4, 5, and 6 months. Soil pH, NO3-N, Olsen-extractable P, plant-available nutrients (as measured by DTPA), total C and N, organic C, and substrate induced CO2 respiration were measured. Increasing biochar application rate caused a decrease in soil pH, NO3-N concentration, and DTPA-extractable Cu, alongside increases in total C and N content, organic C content, Olsen-extractable P, substrate induced respiration, and plant-available Fe, Mn, Ni, and Zn content. The reduction in soil NO3-N concentration, coupled with the increase in organic C content and an increase in the C/N ratio, suggested that N immobilization was occurring. The increase in DTPA-extractable Zn suggests that this biochar may help overcome Zn deficiencies in exposed calcareous subsoil, but the material has the potential to cause plant Cu deficiencies. Over time, soil pH and NO3-N concentration generally decreased, while the organic C content remained elevated. When added to exposed calcareous subsoil, steam-activated, low temperature switchgrass biochar application may improve the short-term micronutrient status and can increase the soil C content; yet excessive biochar applications may cause reductions in soil NO3-N and plant-available Cu.