Soil nitrogen, microbial biomass, and respiration along an arctic toposequence

To investigate the interactions among mineral N, C availability, microbial biomass, and respiration in arctic soils, we sampled soils five times during a growing season from a toposequence on a slope in northern Alaska. The toposequence consisted of six vegetative types from the ridge top to the stream bank: lichen heath, dry cassiope, moist carer (Carex spp.), water track, tussock tundra (intertussock), and riparian. The spatial distribution and temporal variation of soil mineral N, microbial biomass, soil C availability, and C turnover were soil type dependent. During the growing season, the concentration of soil NH4+-N decreased in tussock tundra soils but increased in lichen heath soils. Soil C availability at all locations was the highest at the beginning of the growing season and declined thereafter. The C availability index (CAI) and the potential C turnover rate increased as soils became wetter. Tussock-forming tundra soil was generally colder than other sites and had high C/N ratios, low amounts of mineral N, and a low potential C turnover index, and therefore, was the least biologically active type. In contrast, water track was the most biologically active site in the sequence and had the highest C and N availability, the highest potential C turnover index, and the highest microbial biomass C and N. The mosaic of diverse plant communities and soil types that comprise arctic landscapes necessitates that accurate estimates of large-scale C or N budget can only be made by integration of all types of plant communities and soils.