CARBON STORAGE AND TURNOVER, AND RESPIRATORY ACTIVITY, IN THE LITTER AND SOIL OF AN OLD-GROWTH SOUTHERN BEECH (NOTHOFAGUS) FOREST

Soil and litter carbon pools and turnover, and their relationship to f orest floor respiratory activity, were estimated in a lowland old-grow th beech (Nothofagus) forest in New Zealand. Although the soils varied spatially over the site, their morphological, chemical and some physi cal properties were characteristic of spodosols. Two profiles represen ting the range of soils were analysed by horizon for: C, C-14 and a nu mber of chemical and physical properties. C in the fine litter and min eral soil, in the most representative soil at the site, was ca 3.0 and 15.8 kg m-2 respectively. About 0.65 kg CO2-C m-2 was respired annual ly from the forest floor, based on a simple exponential model relating CO2 efflux measured by a chamber technique, and soil temperature valu es. Temperature mainly controlled CO2 production by litter and soil be cause the site was well supplied with rainfall throughout the year. An nual transfers of C from the litter to the atmosphere and the soil, an d mean residence times for C in the litter (ca 12 yr) and soils were e stimated from the distribution of C-14 using a 'bomb' radiocarbon mode l. A major source of soil C was root turnover, based on fine litterfal l measurements and modelling of total C input. Root turnover and woody debris together represented 64% of the total C input of ca 0.8 kg m-2 yr-1 to the soil. Live root respiration, estimated experimentally and from a relationship between total root C allocation and litterfall, w as ca 23% of the forest floor respiration. An imbalance of ca 0.35 kg C m-2 was observed in the annual soil C cycle, assuming steady-state c onditions. Errors in estimating C fluxes from a combination of direct measurements and models, and underestimation of forest floor respirati on, were probably responsible for this imbalance. Modelled mean reside nce times for soil C ranged from 76 to 207 yr, and resulted from widel y different amounts of 'inert organic matter' being present in the upp er 23 cm of the soils. A long history of tree overturn is the most lik ely cause of this concentration of recalcitrant C near the soil surfac e, and may explain the longer C storage time of soils developed under beech in New Zealand compared to those developed under native grasslan d.