Lime and soil moisture effects on nitrous oxide emissions from a urine patch

Abstract

Liming has been mooted as a mitigation option for lowering soil N₂O emissions. This study investigated the effect of soil pH and soil water content on N₂O and N₂ emissions following the addition of synthetic urine (500 kg N ha⁻¹) containing ¹⁵N-labeled urea-N. Soil pH treatments ranged from 4.7 to 7.2 with either saturated or field capacity soil. Dinitrogen and N₂O fluxes were measured from soil cores kept on water tension tables for 85 d following urine-N addition. Soil inorganic N transformations were also monitored over time by destructively sampling soil cores on five occasions over the 85 d. At field capacity, soil pH affected the N₂O fluxes with the lowest cumulative N₂O fluxes at soil pH 5.9. Nitrous oxide losses ranged from <0.1 to 0.4% of ¹⁵N applied in the field capacity treatment but this increased to be 0.4 to 1.7% of the ¹⁵N applied in the saturated treatment. Dinitrogen fluxes were low (<23 ng N₂–N cm⁻² h⁻¹) at field capacity but exceeded 4000 ng N₂–N cm⁻² h⁻¹ under saturated conditions. Cumulative N₂ fluxes increased with increasing soil pH in the saturated soil. The flux ratio of N₂O-N/(N₂O-N + N₂–N) remained high (0.68–0.71) under the field capacity treatment but decreased with time from 0.64 to 0.16 in the saturated treatment. This study suggests that while the use of soil liming has merit for lowering N₂O emissions from urine patches where soils are at field capacity, the resulting NO₃–N will be susceptible to enhanced rates of N₂O and N₂ loss if the soils are wetted up beyond field capacity.