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.