Impact of freeze-thaw on the contributions of AOA and AOB to N-flush induced nitrification in meadow soils

Abstract

In mid latitude regions of North America, many soils are subjected to freeze-thaw conditions (Fr–Th) during the late winter and early spring months. Fr–Th events are usually accompanied by flushes of C and N mineralization, yet, the concomitant response of nitrification has not been consistently characterized. A laboratory experiment was conducted on three soils that differ in their annual history of exposure to freeze-thaw events, and where both ammonia oxidizing bacteria (AOB) and archaea (AOA) are capable of contributing to nitrification activity at 10 and 16 °C under NH4+ replete conditions. Emergence and rates of N mineralization/nitrification were compared between soils frozen at −20 °C, with non-frozen soils stored at 4 °C. In all soils N mineralization commenced immediately (<1 d) in the Fr–Th soil treatments. Rates and/or amounts of mineralizable N released during the flush differed between soils, but rates for each soil were the same at 10 and 16 °C. Only ammonia oxidizing archaea (AOA)-driven nitrification emerged immediately (<1 d) from the Fr–Th treatment of the two soils with a history of freezing; these rates were higher at 16 than at 10 °C, and were not NH4+ limited. No AOB activity was detected in these soils for up to 3 d. AOB contribute the vast majority (88–92%) of nitrification under NH4+ replete conditions in the soil that rarely freezes. In this soil AOA did not contribute to nitrification in the Fr–Th treatment, and the emergence of AOB activity was delayed for 3 d at 10 °C unless supplemental NH4+ was provided, but commenced immediately at 16 °C with or without supplemental NH4+; however, rates of nitrification at 10 and 16 °C did not differ. The dominance of AOA activity immediately after Fr–Th in soils with a history of freezing suggests that AOA handle the transition during the freeze-down phase, and/or recover during the freeze-thaw phase with less constraint of NH4+ availability and temperature, than the AOB.