Effect of nitrogen addition on N ₂ O emission from soils during freeze–thaw periods

Abstract Increased nitrogen (N) availability resulting from N addition may modify N turnover and nitrous oxide (N 2 O) emissions in regions experiencing freeze–thaw cycles, where relatively high N 2 O emissions from soils are frequently reported during freeze–thaw periods. However, the response of soil N 2 O emissions during freeze–thaw periods to N addition remains unclear. In this study, we compiled 675 paired observational cases from 36 peer-reviewed articles to perform a meta-analysis assessing the effects of N addition on N 2 O emissions during freeze–thaw periods and to identify the associated key factors. Our results indicated that N additions increased the available soil N pool, thereby enhancing N 2 O emissions ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mover> <mml:mrow> <mml:mi>ln</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>R</mml:mi> <mml:mi>′</mml:mi> </mml:msup> </mml:mrow> <mml:mo accent="true">―</mml:mo> </mml:mover> </mml:mrow> </mml:math> was 0.795 with a range of 0.7–0.89 at a 95% confidence interval) primarily associated with denitrification processes during freeze–thaw periods. N additions during the previous fall had the most significant impact on increasing N 2 O emissions during the subsequent freeze–thaw periods. The application of manure and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> fertilizer has been shown to significantly enhance N 2 O emissions compared to other fertilizer types. The rate of N addition directly contributed positively to N 2 O emissions during freeze–thaw periods. Altitude was identified as the primary factor influencing N 2 O emissions in response to N addition. Along the altitudinal gradient, the increase in N 2 O emissions resulting from N addition was more pronounced at lower altitudes (&lt;3000 m) compared to middle and high altitudes (&gt;3000 m). Our meta-analysis elucidates the stimulatory effects of N addition on N 2 O emissions during freeze–thaw periods, identifying denitrification and soil N availability as primary drivers. Crucially, our findings indicate that projected increases in anthropogenic N inputs, when coupled with the climate-induced intensification of freeze–thaw cycles and snow cover reductions, could substantially amplify non-growing season N 2 O pulses. This synergistic interaction constitutes a critical, yet potentially underestimated, positive feedback mechanism in global N 2 O budgets and climate projections.