Identification of decomposition thresholds between -10 and 10 °C, and potential mechanisms

Historically low temperatures at northern latitudes have caused large soil carbon (C) stocks to accumulate. This organic matter is increasingly vulnerable to decomposition with rapid Arctic warming. Arctic winters are warming faster than summers, lengthening shoulder seasons and increasing overwinter soil microbial activity. Microbes are more sensitive to warming in the winter, as physical restrictions on decomposition at low temperature are alleviated. To make more accurate C-flux predictions, identifying temperature thresholds for decomposition and microbial metabolism and their underlying mechanisms is critical. Recent evidence indicates that winter soil respiration can become C limited with warmer winter temperatures. This suggests that a predictive understanding of soil respiration requires knowledge of antecedent soil temperatures and C availability. To determine how microbial substrate use is affected by both substrate availability/size and temperature at low temperatures that reflect winter to shoulder season transitions, we manipulated these variables in a series of lab experiments.

Arctic soils were incubated at temperatures below freezing where microbial processes are known to be limited by low temperatures and ice but still active; and just above freezing where some microbial reactions can be temperature limited, but all ice is thawed. Various sized cellulose polymers were added to the soil in equal amounts. During the incubations respiration was measured and soils were harvested at the end to quantify microbial biomass, nutrients, enzyme activity and C chemistry. To identify molecular controls on low temperature decomposition, we analyzed changes in C-chemistry via FTICR and GC-MS.

Our results indicate temperature limitations to cellulose decay begin just below 10 °C and the , the limit for larger polymers appears to be between -2 and -6 °C, and the low temperature limit for decomposition of small cellulose polymers is around -10 °C. This indicates that arctic soil decomposition will become increasingly significant during the shortening and warming winters and lengthening shoulder seasons, suggesting that substrate availability become an increasingly important control on respiration rates during these times.