Emissions of Nitrous Oxide, Ammonia, and Carbon Dioxide from a Cambisol at Two Contrasting Soil Water Regimes and Urea Granular Sizes

Abstract

The superiority of mixing and deep placement of prilled urea (PU) or urea supergranules (USG) over surface‐broadcast application for reducing nitrogen (N) loss from lowland rice is well established. In upland agricultural systems, rainfall and/or the application and loss of irrigation water from soil systems may regulate urea N transformations and gaseous losses, depending on the method of fertilizer application and the particle size. To develop further insights into these processes, experiments were carried out in a silt loam soil mixed with PU or amended with point‐placed USG at a depth of 7.5 cm. Two soil water regimes were used: around field capacity (AFC) with low evaporative conditions (depletion: 77 to 69% water‐filled pore space, WFPS) and below field capacity (BFC) with high evaporative conditions following two irrigations (depletion: 70 to 55% WFPS). The nitrous oxide (N₂O) emission was greater at AFC than at BFC, where nitrification was more rapid. The N₂O peaks appeared mostly after the disappearance of nitrite (NO₂ ⁻), presumably dominated by nitrifier and/or chemodenitrification and the degree of emissions probably depended on the stability period and the reduction of NO₂ ⁻ induced by the soil water regimes. The relative N₂O losses from the added N were small (⩽0.20%) for all treatments after 21 days. The point at which 50% of its emissions (t_½) occurred was delayed up to 6 days longer than found from the application of PU. The differences between PU and USG application were likely linked with the concentrations of ammonium (NH₄ ⁺), NO₂ ⁻, and pH. These high concentrations continued longer at AFC than at BFC and were limited to a distance of <5.0 cm from the application zone. Similarly, the relative losses of the added N ranged from 0.19 to 0.56% at AFC and 0.08 to 0.37% at BFC, the highest being with USG application. Based on the areas receiving equal N, the N₂O and ammonia (NH₃) emissions from USG differed marginally with PU. Carbon dioxide (CO₂) release was higher at AFC than BFC, in which the USG application probably limited microbial respiration preferentially to methane oxidation. A correlation study showed that the N₂O flux was best explained together with CO₂, nitrate (NO₃ ⁻), NO₂ ⁻, and WFPS (R ²  =  0.67***). This indicates the influence of both auto‐ and heterotrophic microbial activities toward N₂O emission, with soil water being an important regulatory factor.

Keywords:

• Cambisol
• NH₃ volatilization
• N₂O emissions
• soil water regimes
• urea size

ACKNOWLEDGMENTS

We gratefully acknowledge the Alexander von Humboldt Foundation, Germany, for awarding a research fellowship to the senior author to carry out this research. We appreciate the assistance of Dr. Andreas Weber, Mr. Reinhold Manhart, and Mr. Jürgen Plass in conducting these experiments and of Ms. Christine Haas and Ms. Claudia Schütz for providing analytical support in the laboratory.