Vertical distribution of organic amendment influences the rooting depth of revegetation species on barren, subgrade serpentine substrate

Abstract

Barren, subgrade serpentine substrates are difficult to revegetate due to N, P, and K deficiencies, low Ca:Mg molar ratios, potentially high levels of heavy metals including Ni, Cr, and Co, low organic matter, low CEC, and poor water holding capacity. Several large, bare roadcuts exist in the North Coast Ranges along the west coast of California, USA. Substrate was collected from a large roadcut that has remained barren for more than a decade despite conventional surface amendment with NPK fertilizer and seeding with a mix including nonserpentine grasses and herbs. Although serpentine plant communities have been studied globally in great detail, very little information exists on how to effectively restore these communities after drastic disturbance (removal of topsoil and loss of biological activity). This rhizotron study examined how surface layering and mixing yard waste compost into serpentine substrate affected biomass production, rooting distribution, and tolerance of native, as well as invasive grass species that grow on serpentine. Roots of less serpentine-tolerant species became necrotic upon contact with the serpentine substrate in the layered compost application treatment. The roots of highly serpentine-tolerant species, however, displayed a superior tolerance to the serpentine substrate and were able to grow through its entire depth. Mixing the compost into the serpentine substrate significantly improved the Ca content of the growing medium, allowing the less serpentine-tolerant species to root through the entire depth. Although the compost surface layer treatment promoted growth of the highly serpentine-tolerant species in the serpentine substrate, mixing the compost amendment into the substrate would also permit additional desirable revegetation species to become established.