Urea treatment decouples intrinsic pH control over N₂O emissions in soils

Abstract

Soil N₂O emission potential is commonly investigated under idealized denitrifying conditions (e.g. nitrate-N supplied and anaerobic soil), with pH commonly identified as a major determinant of N₂O emission potential. However, under urine patch conditions in grazed pastures soils a more complex series of abiotic and biotic factors may influence emissions due to the complex N transformations that occur following urea hydrolysis. These transformations may decouple native and/or expected controls of N₂O emissions encountered under classic denitrifying conditions. Here, we tracked O₂, CO₂, NO, N₂O and N₂ emissions from urine amended soils (i.e. simulating a urine patch) to determine putative controls of N₂O emissions within 13 different pasture soils from northern (Ireland) and southern hemispheres (New Zealand). Incubations were performed under aerobic conditions±artificial urine (13.3 mg N vial¯¹) equivalent to field ruminant urine deposition rates of 1000 kg N ha¯¹. Results revealed that pH was not an important regulator of the emission ratio (N₂O /(NO + N₂O + N₂)) in urine amended soils. Within urine affected soils, a new set of variables emerged as regulators of N₂O emissions, likely due to the unique environment created within this system. We show that urine results in decoupling of the initial soil pH control of the emission ratio allowing other regulators such as nitrite to dominate. In addition, we observed that the emission ratio of N₂O increased linearly with the rate of N- gas loss (NO + N₂O + N₂ μmol N h¯¹), O₂ consumption was positively associated with ammonia oxidising bacteria (AOB) and that the production of NO and N₂O were also enhanced under urine conditions.