ENVIRONMENTAL AND SUBSTRATE CONTROLS OVER CARBON AND NITROGEN MINERALIZATION IN NORTHERN WETLANDS

Northern wetlands may be a potential carbon source to the atmosphere u pon global warming, particularly with regard to methane. However, rece nt conclusions have largely been based on short-term field measurement s. We incubated three wetland soils representing a range of substrate quality for 80 wk in the laboratory under both aerobic and anaerobic c onditions at 15 degrees and 30 degrees C. The soils were obtained from a Scirpus-Carex-dominated meadow in an abandoned beaver pond and from the surface and at 1 m depth of a spruce (Picea)-Sphagnum bog in Voya geurs National Park, Minnesota. Substrate quality was assessed by frac tionation of carbon compounds and summarized using principal component s analysis. Nitrogen and carbon mineralization, the partitioning of ca rbon between carbon dioxide and methane, pH, and Eh were measured peri odically over the course of the incubation. The responses of nitrogen mineralization, carbon mineralization, and trace gas partitioning to b oth temperature and aeration depended strongly on the substrate qualit y of the soils. Sedge meadow soil had the highest nitrogen and carbon mineralization rates and methane production under anaerobic conditions , and carbon mineralization under aerobic conditions, but the surface peats had the highest nitrogen mineralization rates under aerobic cond itions. Methanogenesis was highest in the sedge soil but less sensitiv e to temperature than in the peats. A double exponential model showed that most of the variation in nitrogen and carbon mineralization among the soils and treatments was accounted for by differences in the size and kinetics of a relatively small labile pool. The kinetics of this pool were more sensitive to changes in temperature and aeration than t hat of the larger recalcitrant pool. Principal components analysis sep arated the soils on the basis of labile and recalcitrant carbon fracti ons. Total C and N mineralization correlated positively with the facto r representing labile elements, while methanogenesis also showed a neg ative correlation with the factor representing recalcitrant elements. Estimates of atmospheric feedbacks from northern wetlands upon climati c change must account for extreme local variation in substrate quality and wetland type; global projections based on extrapolations from a f ew field measurements do not account for this local variation and may be in error.