Biochar does not affect soil N-transformations or microbial community structure under ruminant urine patches but does alter relative proportions of nitrogen cycling bacteria

•Biochar amendment did not affect the microbial community structure.•Biochar presence increased the relative abundance of 50% of the detected bacteria.•Biochar presence aided nitrogen cycling bacteria after urine addition.•Nitrogen cycling genes changed in a way consistent with microbe abundance changes.•Biochar amendment did not reduce N2O flux under the conditions tested.

Nitrogen (N) cycling, especially denitrification, can be significantly altered when biochar is used as a soil conditioner. These alterations in N-cycling have been attributed to a combination of physicochemical change, alterations in microbial community ecology and pervading climatic conditions. This study investigated seasonal bacterial community change over two years in combination with a short-term winter study of N-transformations under bovine urine patches. A silt-loam pastoral soil in Canterbury, New Zealand was amended with either 0, 15 or 30 t ha−1 of Pinus radiata biochar (pyrolysed at ∼450 °C) and bovine urine was added to patches within the 0 and 30 t ha−1 biochar amended plots (designated as 0 U and 30 U treatments, where U indicates ‘urine’).No discernible differences in bacterial community structure were observed during the two year study or the short term N-transformation study when comparing non-amended and biochar-amended soil. Differences in bacterial community structure were only evident when comparing seasons, with data pertaining to each season from successive years clustering together. During the short-term N-transformation study, bacterial communities formed 3 distinct clusters corresponding to elevated levels of urine derived NH4+-N (days 0–10), increases in NO3−-N and N2O (days 10–22) and a decline in NO3−-N and N2O (day 20 onward). Biochar amendment did increase the relative abundance of up to 50% of individual operational taxonomic units (OTUs or ‘species’), including key nitrite oxidisers and nitrate reducers. Biochar amendment did not affect the concentrations of inorganic-N compounds.The nirS (nitrite reductase) gene became elevated in the 30 U treatment relative to the 0 U treatment ∼10 days after the initial urine application. The nosZ (nitrous oxide reductase) gene became elevated in the 30 U plots during the latter part of the experiment.Conclusions:•Biochar did not have a significant impact on the microbial community structure in pastoral soil over the course of two years.•The relative proportion of nitrifiers and denitrifiers increased in biochar amended soils subjected to large influxes of urine derived N.•Differences in N-transformation dynamics in the presence of biochar during the winter months were not statistically significant.