Repeated freeze–thaw cycles increase extractable, but not total, carbon and nitrogen in a Maine coniferous soil

Northeastern North America has been experiencing warmer winters with reduced snow accumulation, with more frequent winter freeze–thaw cycles. We conducted a laboratory experiment to investigate how increased frequency of freeze–thaw cycles (FTC) would alter soil C and N availability. Organic (O) and mineral (B) horizon soils were collected from a coniferous forest in Maine, processed to exclude roots, and then frozen in the laboratory (−10 °C) with one (FTC-1), two (FTC-2), or six (FTC-6) thaw periods (+5 °C). Soils were analyzed for extractable ammonium (NH₄-N), water extractable organic carbon (WEOC), carbon dioxide flux (respiration), and total C and N. Extractable NH₄-N increased following FTC (all levels), for both horizons. While WEOC concentrations did not change for FTC vs. control, the WEOC in O horizons had a lower SUVA₂₅₄ in FTC soils compared to control, indicating a stronger microbial influence (i.e., microbial cell lysis) in these soils after FTC. Respiration in O horizon soils decreased post-incubation and did not differ between FTC and Control soils. In the B horizon, however, FTC soils showed greater respiration than Control soils, suggesting that the newly available nutrients may have stimulated microbial activity. In contrast to these results, total C and N remained unaltered by FTC, presumably because the FTC disturbances represented mostly a translocation of C and N from one pool into another, and losses due to respiration were too small to significantly influence the large TC and TN pools. The effect of FTC on NH₄-N did not change with FTC frequency, suggesting that a single FTC is sufficient to alter both C and N availability and/or quality, and that additional FTC may not have a significant further effect. This study provides fresh insights on how organic and mineral horizon soils might respond to increased freeze–thaw frequency in winter.