Gaseous nitrogen emission from soil aggregates as affected by clay mineralogy and repeated urine applications

Urine-treated soils make a significant contribution to gaseous N losses to the atmosphere. Our goal was to investigate the influence of clay type and content on ammonia (NH3) and nitrous oxide (N2O) emissions from urine under different wetting-drying soil conditions and to relate these results to urine-N transformation processes in soil. Three types of silt loam soils and synthetic sand-clay aggregates with three different clay-dominated materials (kaolinite, montmorillonite and vermiculite) were used in this laboratory study. Bulk soil, 4-4.75 mm and 9.5-11.2 mm aggregates were incubated with synthetic urine at 50% and 75% saturation under aerobic conditions. Repeated urine application affected the properties of the aggregates depending on the type of clay present. Greater clay content increased aggregate stability and reduced NH3 volatilization. The variation in clay ammonium (NH4+) fixation capacities was reflected in NH3 volatilization as well as in the onset of N2O emissions, occurring first from kaolinite-dominated and last from vermiculite-dominated soils. Nitrous oxide production was greater in aggregates than in bulk soil, a difference that consistently increased with repeated urine applications for kaolinitic and vermiculitic treatments. A dual-peak N2O emission pattern was found, with the second maximum increasing with the number of urine applications. Emission of N-15-labeled N-2 was found at 75% saturation in kaolinite and vermiculite-dominated samples. Anaerobic conditions were less pronounced with montmorillonite-dominated samples because shrink-swell action caused aggregate breakage.