Increased understanding of nutrient immobilization in soil organic matter is critical for predicting the carbon sink strength of forest ecosystems over the next 100 years

• Published in: Tree Physiology
• Main Authors: McMurtrie, Ross E., Medlyn, Belinda E., Dewar, Roderick C.
• Format: Text
• Language: English
• Published: Oxford University Press 2001
• Subjects: Original Articles; Norway; Northern Sweden
• Online Access: http://treephys.oxfordjournals.org/cgi/content/short/21/12-13/831; https://doi.org/10.1093/treephys/21.12-13.831

The terrestrial biosphere is currently thought to be a significant sink for atmospheric carbon (C). However, the future course of this sink under rising [CO 2 ] and temperature is uncertain. Some contrasting possibilities that have been suggested are: that the sink is currently increasing through CO 2 fertilization of plant growth but will decline over the next few decades because of CO 2 saturation and soil nutrient constraints; that the sink will continue to increase over the next century because rising temperature will stimulate the release of plant-available soil nitrogen (N) through increased soil decomposition; that, alternatively, the sink will not be sustained because the additional soil N released will be immobilized in the soil rather than taken up by plants; or that the sink will soon become negative because loss of soil C through temperature stimulation of soil respiration will override any CO 2 or temperature stimulation of plant growth. Soil N immobilization is thus a key process; however, it remains poorly understood. In this paper we use a forest ecosystem model of plant–soil C and N dynamics to gauge the importance of this uncertainty for predictions of the future C sink of forests under rising [CO 2 ] and temperature. We characterize soil N immobilization by the degree of variability of soil N:C ratios assumed in the model. We show that the modeled C sink of a stand of Norway spruce ( Picea abies (L.) Karst.) in northern Sweden is highly sensitive to this assumption. Under increasing temperature, the model predicts a strong C sink when soil N:C is inflexible, but a greatly reduced C sink when soil N:C is allowed to vary. In complete contrast, increasing atmospheric [CO 2 ] leads to a much stronger C sink when soil N:C is variable. When both temperature and [CO 2 ] increase, the C sink strength is relatively insensitive to variability in soil N:C; significantly, however, with inflexible soil N:C the C sink is primarily a temperature response whereas with variable soil N:C, it is a combined ...