Nitrite accumulation and nitrogen gas production increase with decreasing temperature in urea-amended soils: Experiments and modeling

Nitrite (NO₂ ⁻) accumulation and associated production of nitric oxide (NO) and nitrous oxide (N₂O) gases in soils amended with nitrogen (N) fertilizers are well documented, but there remains a poor understanding of their regulation and variation among soil types. We examined responses to urea inputs in two soils at five temperatures from 5 to 30 °C and developed a process-driven model to describe the dynamics. A microcosm system was used to measure ammonia gas (NH₃), ammonium (NH₄ ⁺), NO₂ ⁻, nitrate (NO₃ ⁻), NO, N₂O and pH over 12 weeks. Unexpectedly, NO₂ ⁻, NO and N₂O production tended to increase as soil temperature declined in both soils. The maximum NO₂ ⁻ concentration, or compensation point (CP), differed by soil type but the time required to reach CP decreased exponentially with increasing temperature in both soils. A two-step nitrification model (’2SN’) accounted for interactions of ammonia-oxidation (AmO), nitrite oxidation (NiO), urea hydrolysis, NH₄ ⁺ sorption, N gas production and pH dynamics. Both steps of nitrification (AmO and NiO) were modeled using NH₃ inhibition kinetics. The model adequately simulated the observed dynamics and temperature responses and showed that increased uncoupling of AmO and NiO at colder temperatures resulted from their differential temperature responses. The dynamics observed here may be important following high-rate and banded N fertilizer applications and in ruminant urine patches. The results may help explain elevated N₂O emissions observed under cold temperatures. The 2SN model can account for interactions among multiple processes and may be useful for studying the effects of management practices and climate factors, including climate change scenarios, on soil N cycling.