DOUGLAS-FIR FOREST SOILS COLONIZED BY ECTOMYCORRHIZAL MATS .1. SEASONAL-VARIATION IN NITROGEN CHEMISTRY AND NITROGEN-CYCLE TRANSFORMATION RATES

Monthly samples of ectomycorrhizal mat soils from a maturing Pseudotsuga menziesii forest and adjacent nonmat soils were collected and analyzed for respiration, acetylene reduction activity, denitrification rates, extractable ammonium, N mineralization, microbial biomass, temperature, pH, percent moisture, total phosphate, N and C. Seasonal patterns suggested complex interactions among the host tree, ectomycorrhizal fungus and the mat microbial community as influenced by seasonal changes in moisture, temperature and light availability. Respiration within the mat community was highest during the period when tree growth is normally the greatest (in spring and fall). There was a major respiration peak in winter possibly caused by utilization of labile carbon by microheterotrophs. Differences were also observed between mat and nonmat soils in respiration rates, microbial biomass carbon, acetylene reduction activity, and levels of mineralizable nitrogen, which were all generally higher in the mat soils, and pH and denitrification rates, which were generally lower in nonmat soils. Nitrogen is probably very tightly coupled within the mat communities. -from Authors