Topsoil (TS) shortage often limits the successful reclamation of older mined sites. We evaluated the effectiveness of one-time application of biochar or oxidized lignite (humalite) alone or in combination with a mix of conventional organic materials to reconstruct functioning TS using subsoil (SS) as a substrate. Biochar or humalite carbon (C) represented a stable form of C, whereas C from a mix of sawdust, wheat straw, and alfalfa (labile organic mix (LOM)) represented the labile C fraction. The amount and composition of organic amendment mix were determined so that organic C levels of reconstructed TS would be equivalent to that of the native TS in the long-term. Three SS substrates differing in texture (clay, loam, and sand) and organic C levels were used in the study. We used field pea (Pisum sativum L.) and barley (Hordeum vulgare L.) as test crops in rotation in four sequential greenhouse studies. All treatments except the control SS and TS received supplemental fertilizer nutrients. Plant biomass yield and tissue concentrations were evaluated at the end of each study, whereas soil nutrient levels were assessed at the end of Study II and Study IV. Labile organic mix amendment alone was superior in biomass production relative to any of the other treatments at the early stages of the study. Cumulative biomass yield of SS amended with either biochar or humalite in the presence of LOM was statistically identical for clay and sand soils. These values were also statistically indistinguishable from the fertilized native TS control treatment for the clay soil but not for the sand soil. Humalite application at a high rate (76 g/kg soil) increased soil CEC, decreased soil pH and P concentration, and increased both soil and plant tissue B concentrations. Our data show that a functioning TS can be reconstructed using either biochar or humalite in the presence of LOM and adequate supplemental fertilizers particularly N and P. Detailed characterization of organic amendments is recommended to avoid undesirable effects emanating from their use. Field-based long-term studies are needed to confirm the longevity of benefits of using these amendments.
