High Arctic Dry Heath CO2 Exchange During the Early Cold Season

Climate change may alter the terrestrial ecosystem carbon balance in the Arctic, and previous studies have emphasized the importance of cold season gas exchange when considering the annual carbon balance. Here, we examined gross ecosystem production (GEP), ecosystem respiration (R (eco)) and net ecosystem exchange (NEE) during autumn at a high arctic dry open heath, over a period where air temperatures decreased from +9.8 to -16.5A degrees C. GEP declined by 95-100% during autumn but GEP significantly different from 0 was measured on October 8 despite sub-zero temperatures. R (eco) declined by 90% and dominated NEE throughout the study as the ecosystem on all measurement days was a source of atmospheric CO2. We estimated net September carbon losses (NEE) to be 17 g CO2 m(-2), emphasizing the importance of autumn in relation to annual carbon budgets. The study site has been subjected to 14 summers of water addition, and occasional pulses of nitrogen (N) addition in a fully factorial design. N addition enhanced GEP up to 17-fold during September, although there was no effect in October when GEP was very low. Summer water addition decreased autumn R (eco) by up to 25%. Both N amendment and water addition decreased carbon loss, that is, increased NEE; N amendment increased NEE on all dates by 13-64% whereas water addition increased NEE by 20-54% late in September and onward, demonstrating the importance of nutrient and water availability on carbon balance in high arctic tundra, also during the autumn freeze-in.