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

Abstract

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°40′) to northern Sweden (66°08′) were measured for 14 weeks under laboratory conditions at temperatures from 5 °C to 25 °C. The average Q10 values for the respiration rate ranged from about 1.0 at the highest temperature to more than 5 at 10 °C to 15 °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 °C the N-mineralization rate did not respond to increasing temperature; at higher temperatures, significant increases were found for samples from some sites. A regression model including incubation temperature, pH, Ntot 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 × 1015 to 0.13 × 1015 g CO2 at 2 °C and 4 °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.