XYLEM SAP AS A PATHWAY FOR TOTAL MERCURY AND METHYLMERCURY TRANSPORT FROM SOILS TO TREE CANOPY IN THE BOREAL FOREST

Conifer needles are an important link in the cycling of Total Mercury (THg) and Methylmercury (MeHg) in the boreal ecosystem due to the high THg and MeHg concentrations in litterfall. Translocation within the t ree of Hg from soils to the crown canopy has been assumed to be a mino r source of the Hg in litterfall. This paper, however, is the first to present direct observations of THg/MeHg transport from the soil via x ylem sap. Xylem sap concentrations of THg and MeHg were measured in sa p drained from different levels along the boles of freshly cut 100 yea r old Norway spruce (Picea abies) and Scots pine (Pinus sylvestris). T he trees came from a mixed stand growing on podzolized till soils at t he Svartberget Forest Research Station in N. Sweden. Soil solution con centrations of THg and MeHg at different levels in the soil profile we re measured for comparison. Concentrations of THg in xylem sap ranged from 10-15 ng L-1 in both the Scots pine and Norway spruce. Concentrat ions of MeHg varied from 0.03 ng L-1 to 0.16 ng L-1, with higher value s in Scots pine than Norway spruce. If these concentrations are repres entative of the transport from soils to needles in xylem sap at this s ite, then only 3% of the MeHg in litterfall (0.12 mg ha(-1) yr(-1)) an d 11% of the THg (26 mg ha(-1) yr(-1)) can originate via this pathway. The upward transport via xylem sap is larger relative to the open fie ld inputs (84% of THg and 17% of MeHg). Comparison of soil solution an d xylem sap THg/MeHg suggested some degree of THg exclusion during wat er uptake in Scots pine and Norway spruce, but MeHg exclusion only in Norway spruce.