Modification of the composition of dissolved nitrogen forms, nitrogen transformation processes, and diversity of bacterial communities by freeze-thaw events in temperate soils

Freeze-thaw (FT) events can influence the soil nitrogen (N) availability, although their effect on the diversity of bacterial communities during these processes remains uncertain. To test our hypothesis that close relationships exist between dissolved N pools (including NO3-, NH4+, NO3- + NH4+, and dissolved organic N [DON]), N transformation processes (including N mineralization, N immobilization, nitrification, and denitrification), and bacterial communities in response to FT events, we varied the temperature amplitude of these events by using freezing temperatures of -15 degrees C, - 9 degrees C, and - 3 degrees C and thawing temperatures of 2 degrees C and 5 degrees C in microcosms incubated in a temperate agricultural soil. All the pools of total dissolved inorganic N (DIN: i.e., NO3- + NH4+) decreased in concentration as the amplitude of freezing and thawing temperatures increased, whereas the opposite trend was observed for dissolved organic N (DON). Changes in the concentration of all the dissolved N forms examined were highly linearly correlated with N transformation processes. Partial correlation networks highlighted the transformation between the dissolved N forms during N mineralization and the direct immobilization of either NH4+ or DON by microorganisms in response to FT events. In addition, the total dissolved N content was primarily driven by the DON pool, thus suggesting a primary role for protease activity in response to FT. The Mental and Procrustean tests indicated that the N pools examined and the transformation processes were significantly correlated with bacterial community diversity. This linkage could be significantly weakened by warmer freezing temperatures, in which the orders Cytophagales and Sphingobacteriales may play an important role.