Biologic activity in two western Oregon Douglas-fir stands : a research link to management

Graduation date: 1996Information about forest substrate respiration, nitrogenase activity and\ud mineralizable nitrogen may be incorporated into carbon and nitrogen budgets that comprise an important element of forest management planning. In this study, substrate respiration, nitrogenase activity and mineralizable nitrogen were measured in two western Oregon Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands within litter, well decayed logs and mineral soils. To examine the potential effect of living conifer roots on substrate respiration, nitrogenase and mineralizable nitrogen, the perimeters of one half the plots were trenched. After two years, there was no significant difference between plot types in respiration, nitrogenase activity or mineralizable nitrogen rates at both sites. The 75-80 year old lower elevation stand located in the Coast Range had greater levels of all three variables than the higher elevation western Cascade 450 year old stand in the H. J. Andrews Forest. At both sites, litter usually yielded greater rates of respiration, nitrogenase activity and mineralizable nitrogen than either decayed logs or mineral soils. Greater rates occurred in decayed logs than in mineral soils; rates at 4 cm in these substrates were significantly higher than rates at 20 cm. Strongest correlations were found between litter volumetric moisture content,\ud temperature in litter, and respiration and/or nitrogenase activity. In general, positive correlations existed between substrate moisture content and respiration; substrate moisture content and nitrogenase activity were often significantly and positively correlated. Both respiration and nitrogenase activity correlated negatively with temperature. Regardless of substrate, few significant correlations existed between\ud substrate moisture or temperature and mineralizable nitrogen. This study confirmed\ud that substrate respiration, nitrogenase activity and mineralizable nitrogen in western Oregon may be controlled more by substrate moisture content than substrate\ud temperature and that litter and down woody debris play an important role in determining potential nitrogen reservoirs within forests