Influence of soil bulk density and matric potential on microbial dynamics, inorganic N transformations, N2O and N2 fluxes following urea deposition

Abstract Transformation of ruminant urine-nitrogen (N) can contribute to negative environmental outcomes such as nitrate leaching which leads to eutrophication of waterways and production of nitrous oxide (N 2 O), a greenhouse gas. Although abiotic factors influencing urine-N processing have been well studied, detailed studies of the soil microbial community dynamics following urine application are required to improve mitigation strategies for reducing harmful N fluxes from urine deposition. A factorial laboratory experiment using packed silt-loam soil cores with two levels each of urea (±), soil matric potential ( ψ  −6.0 or −0.2 kPa) and soil bulk density ( ρ b 1.1 or 1.5 g cm −3 ) was performed to study the interaction of urea and soil physical conditions on both soil inorganic N transformations and the abundance of ammonia-oxidizers and denitrifiers. Soil ψ and ρ b treatments had an immediate impact on soil pH, dissolved organic carbon, inorganic N pools and emissions of N 2 O and N 2 following urea deposition, and microorganisms carrying the nosZ gene were present in lower numbers in the most aerobic soil (−6.0 kPa and 1.1 g cm −3 ) from day 7. In all treatments, both bacterial amoA and denitrifier nirS , nirK and nosZ gene copy numbers increased within 1 day following urea application. Dynamics in the NH 4 + concentrations were significantly correlated with cumulative changes in the abundance of the ammonia-oxidizers, but no relation was found between cumulative changes in the denitrifier nirS , nirK and nosZ gene copy numbers and the dynamics in soil inorganic N, N 2 O or N 2 emissions. Throughout most of the study period the specific soil conditions, induced by the ψ and ρ b treatments, determined nitrifier and denitrifier activity rather than the size of the microbial communities involved. However, by day 35 soil ψ and ρ b treatments exerted large treatment effects on bacterial amoA , nirS and nirK gene copy numbers. Thus, although nitrate concentrations and N 2 O emissions following urea deposition were determined by the soil ψ and ρ b conditions in the short-term, our results indicate that changes in the population sizes of denitrifiers and AOB in ruminant urine patches may influence environmental N fluxes in the long-term.