Douglas-fir forest soils colonized by ectomycorrhizal mats. I. Seasonal variation in nitrogen chemistry and nitrogen cycle transformation rates

Monthly samples of ectomycorrhizal mat soils from a maturing Douglas-fir forest and adjacent nonmat soils werecollected and analyzed for respiration, acetylene reduction activity, denitrification rates, extractable ammonium, nitrogenmineralization, microbial biomass, temperature, pH, percent moisture, total phosphate, nitrogen, and carbon. Seasonalpatterns suggested complex interactions among the host tree, ectomycorrhizal fungus, and the mat microbial commu-nity as influenced by seasonal changes in moisture, temperature, and light availability. The most dramatic changesin rates were found during moisture-temperature transition periods in the spring and fall. Respiration within the matcommunity was highest during the period when tree growth is normally the greatest (in the spring and fall). In addition,there was a major respiration peak observed in the winter that we hypothesize was caused by the utilization of labilecarbon by microheterotrophs. Differences were also observed between mat and nonmat soils in respiration rates, microbialbiomass carbon, acetylene reduction activity, and levels of mineralizable nitrogen, which were all generally higher inthe mat soils, and pH and denitrification rates, which were generally lower in nonmat soils. There is also evidencethat suggests that nitrogen is very tightly coupled within the mat communities.