Effect of temperature on humus respiration rate and nitrogen mineralization: Implications for global climate change

Respiration and nitrogen mineralization rates of humus samples from 7 Scots pine stands located along a climatic transect across the European continent from the Pyrenees (42 degrees 40') to northern Sweden (66 degrees 08') were measured for 14 weeks under laboratory conditions at temperatures from 5 degrees C to 25 degrees C. The average Q(10) values for the respiration rate ranged from about 1.0 at the highest temperature to more than 5 at 10 degrees C to 15 degrees C in the northernmost samples. In samples from more northern sites, respiration rates remained approximately constant during the whole incubation period; in the southern end of the transect, rates decreased over time. Respiration rate was positively correlated with incubation temperature, soil pH and C:N ratio, and negatively with soil total N. Regressions using all these variables explained approximately 71% of the total variability in the respiration rate. There was no clear relation between the nitrogen mineralization rate and incubation temperature. Below 15 degrees C the N-mineralization rate did not respond to increasing temperature; tit higher temperatures, significant increases were found for samples from some sites. A regression model including incubation temperature, pH, N-tot and C:N explained 73% of the total variability in N mineralization. The estimated increase in annual soil respiration rates due to predicted global warming at the high latitudes of the Northern Hemisphere ranged from approximately 0.07 x 10(15) to 0.13 x 10(15) g CO2 at 2 degrees C and 4 degrees C temperature increase scenarios, respectively. Both values are greater than the current annual net carbon storage in northern forests, suggesting a switch of these ecosystems from net sinks to net sources of carbon with global warming.