Soil carbon storage and turnover in temperate forests and grasslands - A New Zealand perspective

Carbon storage and turnover times of total soil C (up to I m depth) and topsoil C (to 23 cm depth of mineral soil) were compared at six sites in native tussock grasslands and three in old-growth forests in New Zealand. Annual C inputs to the topsoils were also determined. Most of the sites approached steady state, and covered a wide range of climoedaphic environments. Total soil C ranged from 44 to 268 t ha(-1) (to 1 m depth), and was highest in a Spodosol beneath a mixed podocarp-hardwood forest. Turnover times of total soil C, estimated from the radiocarbon content of soil organic matter, mostly ranged from 30 to 125 years. Anomolously slow turnover times for two grassland sites and one forest site were ascribed partly to an accumulation of Al-humus, and partly to intermittent periods of waterlogging. Annual inputs of C to the upper part of the soil profile averaged 7 and 3 t ha(-1) for the forest and grassland soils, respectively. Turnover times for decomposable C [(total C - inert C)/annual C input] in the topsoil ranged from 11 to 40 years, and were closely related to mean annual temperatures. Application of the approaches described for assessing soil C storage and turnover in a wider range of climates and vegetation types now seems warranted.