Can liming mitigate N2O fluxes from a urine-amended soil?

Abstract

Soil pH affects N₂O production mechanisms. The ratio of N₂O to N₂ may decrease with increasing soil pH during denitrification. Two laboratory experiments were performed to examine the effect of liming on N₂O emissions following synthetic urine applications over 42 and 60 days. A silt loam soil at field capacity (water-filled pore space of 57% after treatment application) was adjusted with Ca(OH)₂ to produce soils ranging from pH 4.7 to 7.7. The first experiment employed 4 treatments: a control, KNO₃ (500 kg N/ha), and synthetic urine at 500 kg N/ha and 1000 kg N/ha. A second experiment used ¹⁵N labelled urea in synthetic urine at 500 kg N/ha. The main effect of lime was to promote nitrification, which markedly affected N₂O fluxes. After 60 days, the 500 kg N/ha synthetic urine treatment limed to pH 6.1 produced more N₂O (0.82% of N applied) than any other soil pH. No optimum soil pH was found for the synthetic urine treatment at 1000 kg N/ha with nitrification incomplete after 60 days. N₂O production via nitrifiers dominated the N₂O production pathway with large residual NO₃^– pools. Denitrification was not enhanced since no ¹⁵N labelled N₂ was detected. However, before any final conclusions can be drawn about the efficacy of liming as a mitigation tool it is vital that the effect of liming on possible denitrification mechanisms and products also be assessed, following nitrification and formation of a nitrate pool. Application of synthetic urine also initiated a priming effect with more carbon evolved as CO₂ than was applied.