Land rehabilitation under pasture on volcanic parent materials - Changes in soil microbial biomass and C-metabolism and N-metabolism

Abstract

Changes in soil biochemical properties during land restoration after mining in a high-rainfall area in the Coromandel Range, North Island, New Zealand, were studied over 3 years in mown, fertilized, grass-legume pastures developed on weathered andesite overburden spread to 60 cm depth over waste rock, and covered with 0 or 15 cm of a silt loam topsoil (a Typic Hapludand). Herbage yields were 53-57% of an adjacent pasture control after 1 year, and 63-69% after 3 years. Total C and N, and microbial C, at 0-7.5 and 7.5-15 cm depths changed little during the trial, but microbial N increased. The fumigation-extraction procedure was routinely employed for estimating microbial biomass C; problems were found with the substrate-induced respiration and fumigation-incubation methods. Ratios of 0.5 M K2SO4-extractable C to total C, and CO2 production to total C or microbial C (qCO2), were highest throughout in the reconstructed soils. Aerobic net N mineralization increased during the trial, but continuing low rates of mineralization and a predominance of N-fixing legumes in the herbage indicated a deficiency of available N in the reconstructed soils. Nitrification occurred readily in all but 7.5-15 cm depth samples of the no-topsoil treatment, where physical conditions were probably adverse. Invertase activity increased appreciably in 0-7.5 cm depth samples of the topsoil treatment, but remained well below control soil values. Overall, the biochemical results are consistent with agronomic data in showing that the use of some topsoil in rehabilitation at a site such as this is desirable.