Effects of nitrogen additions on greenhouse gas fluxes during continuous freeze-thaw cycles in a cold temperate forest

Abstract

Abstract Either nitrogen (N) deposition or soil freeze-thaw cycles (FTCs) induce a pulse of greenhouse gas (GHG) in the cold temperate zone caused by changes in soil carbon (C) and N substrates turnover. However, the synthetic effects between N additions and FTCs to GHG fluxes have received little attention in experiments, particularly in the boreal forest. We conducted a laboratory incubation using intact soil cores from Rhododendron dauricum-Larix dahurica to investigate GHG fluxes that responded to the combined effects. We separated the soil samples into seven groups—none, low, medium, and high sodium nitrate addition, and low, medium, and high ammonium chloride addition—and exposed each group to continuous FTCs conditions. The results demonstrated that N2O and CO2 emissions were eventually stimulated by FTCs, while CH4 uptake was inhibited by FTCs, but they manifested themselves differently under different N addition treatments. All treatments resulted in substantial increases in N2O emissions compared to the control. However, the soil respiration rate increased significantly only with medium sodium nitrate addition, and high N level additions inhibited CH4 uptake regardless of N form. These findings demonstrate that FTCs and N additions, including various N forms and levels, have a considerable effect on GHG emissions in temperate forest ecosystems, which is necessary to predict future feedback effects of GHG emissions on climate change.