Shifting soil N regimes over a 70-year chronosequence of wetland reclamation and restoration

Shifting nitrogen (N) cycles are fundamental to the ecosystem functioning during wetland reclamation and restoration. Our study investigated N transformation across a 70-year chronosequence in Northeast China, encompassing natural wetland, cultivated paddy conversed from wetland for approximately 20, 30, 40, 50, and 60 years, and restoration from cultivated cropland for 3, 5, and 10 years. There was a 73.2 % decrease in soil total nitrogen (TN) in the first 40 years of reclamation, with a modest subsequent recovery. After one decade of restoration, soil N content accumulated and increased by 34.7 % from its level after 60-year reclamation. The inorganic N concentrations along the chronosequence echoed this pattern. Mineralization primarily drove increased nitrous oxide (N2O) production potential during the early (20-30 years) and later (40-60 years) reclamation phases. Conversely, the net nitrification rate significantly impacted N2O production during restoration, with a coefficient of 0.43 (p < 0.05). In pristine and restored wetlands, nitrification positively correlated with nutrient content, including total carbon (TC) and TN. These dynamic shifts in N cycling, particularly regarding N2O emissions during the wetland-cropland transition, highlighted the need for tailored management strategies at different reclamation and restoration phases. Our detailed assessment of N fluxes and pool sizes throughout the chronosequence enriches the understanding of N dynamics and could be used as a benchmark for validating ecosystem models for simulating long-term N dynamics during land use change.