Responses of dissolved organic carbon to freeze-thaw cycles associated with the changes in microbial activity and soil structure

Abstract. Arctic warming accelerates snowmelt, exposing soil surfaces with shallow or no snow cover to freeze-thaw cycles (FTCs) more frequently in early spring and the late autumn. FTCs influence Arctic soil C dynamics by increasing or decreasing the amount of dissolved organic carbon (DOC); however, mechanisms-based explanation of DOC changes considering other soil biogeochemical properties is limited in previous research. To understand the effects of FTCs on Arctic soil responses, we designed microcosms with surface organic soils from Alaska and investigated several soil biogeochemical changes under seven-successive temperature fluctuations of freezing at -9.0±0.3 °C and thawing at 6.2±0.3 °C for 12 h each. Our study found that FTCs significantly changed the following soil variables: soil respiration, DOC and total dissolved nitrogen (TDN) contents, two DOC quality indices, micro-aggregate distribution, and small-sized mesopore volume. Multivariate statistical analyses supported that the FTCs improved soil structure and functions which led to facilitated DOC decomposition by soil microbes, and changes in DOC quantity and quality by FTCs. This study showed that FTCs affected DOC characteristics without negatively impacting soil microbial respiration activity, as soil microbes had previously adapted to temperature fluctuations in the Arctic. Soil micro-aggregation enhanced by FTCs and the subsequent increase in soil respiration and small-sized pore volume could promote DOC decomposition, eventually decreasing the DOC content in the soil solution. This study provides a mechanism-based interpretation of how FTCs alter DOC characteristics in Arctic soil by incorporating its structural changes and microbial responses, ultimately improving our understanding of Arctic soil C dynamics.