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

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 (N2O), 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 (ρb 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 ρb treatments had an immediate impact on soil pH, dissolved organic carbon, inorganic N pools and emissions of N2O and N2 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 NH4+ 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, N2O or N2 emissions. Throughout most of the study period the specific soil conditions, induced by the ψ and ρb treatments, determined nitrifier and denitrifier activity rather than the size of the microbial communities involved. However, by day 35 soil ψ and ρb treatments exerted large treatment effects on bacterial amoA, nirS and nirK gene copy numbers. Thus, although nitrate concentrations and N2O emissions following urea deposition were determined by the soil ψ and ρb 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.