Spring thaw pulses decrease annual N2O emissions reductions by nitrification inhibitors from a seasonally frozen cropland

Abstract

Nitrification inhibitors (NIs) have been widely demonstrated to reduce soil nitrous oxide (N2O) and nitric oxide (NO) emissions from nitrogen (N)-fertilized croplands. However, few studies have evaluated the efficacy of NIs during the cold nongrowing season, especially the spring thaw – a period that may dominate the annual emissions. We conducted an experiment to examine the effects of three types of NIs (DCD, DMPP, and nitrapyrin) on soil N2O and NO emissions from a seasonally frozen cropland in northeast China in two experimental years with contrasting snowfall. In 2017–2018, DCD significantly decreased growing season N2O emissions by 54%, DMPP achieved a non-significant reduction of 32% but nitrapyrin had no inhibition effect. In the winter following this growing season, there was high snowfall and large pulses of N2O fluxes during spring thaw were observed across all treatments totaling 1.1–4.2 kg N ha−1 and contributing 54–76% of annual emissions. NIs did not significantly affect the spring thaw emissions, which partly offset the decreased emissions during growing season, leading to an overall neutral effects of DCD and DMPP on N2O emissions when accounting for a complete year. In 2018–2019, there was less snowfall and spring thaw N2O emissions totaled just 0.32–0.64 kg N ha−1 and contributed to only 13–40% of annual emissions. All three types of NIs significantly decreased the annual N2O emissions by 37–55%. Yet, this annual reduction was lower than that during the growing season (56–66%) due to the inefficacy of NIs to abate the spring thaw emissions. In particular, nitrapyrin significantly increased the spring thaw N2O emissions by 81% probably owing to higher residual fertilizer-N and carbon availibility. In contrast to N2O, NO fluxes during spring thaw were negligible in all years, and NIs application significantly decreased the annual NO emissions by 68–86% (except for nitrapyrin in 2017–2018). Low NO/N2O ratios and positive relationships of N2O fluxes with soil moisture and nitrate suggested that freeze–thaw induced N2O production mainly from denitrification, which might be stimulated by soil moisture from melting snow. Overall, our results highlight that changes in winter weather conditions, and particularly snow dynamics, could create large uncertainties in assessing the efficacy of soil N-oxide emissions mitigation strategies under climate change.