Moisture-Dependent Partitioning and Asynchronous Responses of Inorganic Nitrogen During Freeze–Thaw Incubation of Northeast China Black Soils

Freeze–thaw cycles may alter soil inorganic nitrogen (N), yet endpoints may overlook multi-stage responses. We examined paddy- and maize-field black soils from Northeast China at initial wet-basis moisture levels (10%, 20%, 30%, and 40%) and freeze–thaw stages (FT0, FT1, FT3, FT6, FT9, FT12, and FT15), comparing cycle-duration and freezing-temperature regimes. Nitrate-N, ammonium-N, and KCl-extractable inorganic N (DIN) were assessed using endpoint changes, nitrate fractions, cycle-weighted mean log response ratios (lnRR), and sampled maximum stages. Nitrate fractions increased from FT0 to FT15 in seven of eight soil–moisture combinations. In paddy soil at 30% moisture, endpoint nitrate increases ranged from 32.0% to 41.3% across stage assignments, while nitrate and DIN integrated responses remained positive. In maize soil, DIN increased by 5.6% at 20% moisture despite a negative mean lnRR (−0.027); nitrate fell by 69.1% at FT15 under 40% moisture, with integrated responses sensitive to the final observation and integration interval. Relative to −20 °C, −10 °C produced positive mean nitrate ΔlnRR across all paddy-soil moisture treatments but negative values across all maize-soil treatments. Sampled maximum stages differed among N forms. Without time-matched unfrozen controls, changes from FT0 may also reflect incubation and rewetting. Joint analysis informs thaw-period monitoring by characterizing concentration trajectories.