LAND REHABILITATION UNDER PASTURE ON VOLCANIC PARENT MATERIALS - CHANGES IN SOIL MICROBIAL BIOMASS AND C-METABOLISM AND N-METABOLISM

Changes in soil biochemical properties during land restoration after m ining in a high-rainfall area in the Coromandel Range, North Island, N ew Zealand, were studied over 3 years in mown, fertilized, grass-legum e 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 tops oil (a Typic Hapludand). Herbage yields were 53-57% of an adjacent pas ture control after 1 year, and 63-69% after 3 years. Total C and N, an d 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 w ere found with the substrate-induced respiration and fumigation-incuba tion 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 i n the reconstructed soils. Aerobic net N mineralization increased duri ng the trial, but continuing low rates of mineralization and a predomi nance of N-fixing legumes in the herbage indicated a deficiency of ava ilable N in the reconstructed soils. Nitrification occurred readily in all but 7.5-15 cm depth samples of the no-topsoil treatment, where ph ysical conditions were probably adverse. Invertase activity increased appreciably in 0-7.5 cm depth samples of the topsoil treatment, but re mained well below control soil values. Overall, the biochemical result s are consistent with agronomic data in showing that the use of some t opsoil in rehabilitation at a site such as this is desirable.