Freezing-thawing cycles affect organic matter decomposition in periglacial maritime Antarctic soils

Antarctic King George Island is the fastest-warming area in the Southern Hemisphere. Organic matter inputs are scarce in this area, as they are derived from lichens, mosses, avian faeces, and minor inputs from two vascular plant species, Deschampsia antarctica E. Desv. and Colobanthus quitensis (Kunth) Bartl. Here, we examined the effects of freezing and thawing (FT) cycles on the priming effect (PE). We hypothesised that soil microorganisms preferentially use freeze-preserved soil organic carbon (SOC) exposed after thawing as an important energy source, resulting in intense PE. Two soils with contrasting clay contents were characterised by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and incubated with and without C-13-glucose for 21 d. CO2 and (CO2)-C-13 were recorded from soil (i) without FT, (ii) one FT, and (iii) three FT cycles (- 18/12 degrees C). SOC exhibited low aromaticity stretching at 920 cm(-1) and 1650 cm(-1). Glucose-derived CO2 was maximal (26 +/- 2.2 mg g(-1) C) in the control soil without FT and decreased to 8.6 +/- 0.1 mg g(-1) C after three cycles. Glucose induced an intensely positive PE, 41-64% of basal respiration for a single FT cycle and 72-76% for no cycles. However, after three FT cycles, there was null or negative PE (- 9.5-0.4%). On average, the SOC content after net C balance increased with freezing frequency from 103 +/- 14 to 212 +/- 7. mg C kg(-1) in low clay forming soil and from 129 +/- 14 to 156 +/- 2 mg C kg(-1) in high clay forming soil and declined with increasing PE in both soils (R-2 = 0.87, p < 0.01). Diminution in freezing frequency because of global warming will increase the positive PE, affecting the C sequestration of incipient SOC formation in maritime Antarctic soils. [GRAPHICS]