Mass balance of cadmium in two contrasting oak forest ecosystems

The mass balance of cadmium in forest ecosystems was parameterized. Soil pH is the main variable controlling retention of Cd in the soil and, hence, determines whether Cd is leached from the system or not. However the extent to which root uptake and biomass accumulation of Cd, or the return of Cd to the soil as internal cycling, influences forest Cd balances is unknown. Also unknown is whether these fluxes might counteract Cd leaching from forest soils. The objective of this study was to compare the Cd mass balance of two contrasting oak forest ecosystems, one grown on an acid sandy soil and one on a near-neutral loamy soil. The oak forest ecosystem grown on the acid sandy soil was a source of Cd with an input flux from deposition of 64 microg Cd m(-2) yr(-1), which was only 30% of the output flux with seepage water (175 microg Cd m(-2) yr(-1)). The oak forest ecosystem on the loamy soil acted as a sink for Cd, with an input flux (92 microg Cd m(-2) yr(-1)) 8.4 times higher than the output flux (11 microg Cd m(-2) yr(-1)). Biomass accumulation was 46% and 74% of root uptake on the sandy and the loamy soil, respectively, indicating that biomass accumulation, if harvested, will reduce the net return to the soil and hence the potential amount of Cd prone for leaching.