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Assessing microbial contributions to N2O impacts following biochar additions

Lin, Xiurong, Spokas, Kurt A., Venterea, Rodney T., Zhang, Renduo, Baker, John M., Feyereisen, Gary W.
Agronomy 2014 v.4 no.4 pp. 478-496
agricultural soils, biochar, carbon dioxide, carbon dioxide production, chemical inhibitors, denitrification, forest soils, forests, gas emissions, hulls, macadamia nuts, nitrates, nitrification, nitrites, nitrogen, nitrous oxide, prairie soils, sand, soil amendments, soil bacteria, soil fungi
Varying degrees of soil nitrous oxide (N2O) mitigation have been observed following biochar applications. However, the mechanisms of biochar influence on soil and microbial N2O production have not been fully elucidated. The aim of this study was to examine the impact of a biochar addition on soil N2O production on 3 different soil types and to assess the relative contributions of bacteria and fungi to this alteration. Laboratory incubation experiments were conducted using soils from agricultural field, forest, prairie, and a sterilized sand. These 4 soils possessed a range of N2O production potentials. Soils were amended with a fast-pyrolysis macadamia nut shell biochar (10% w/w). Selective chemical inhibitors were used to distinguish the relative contributions of fungal and bacterial groups to N2O production/suppression in each soil type following biochar addition. Overall, suppressed production of N2O was initially observed between the agricultural and prairie soils. On the other hand, the forest soil had higher net N2O production following biochar addition. Incidentally, there was a small quantity of N2O (4.0 ng-N/g soil/day) produced by the sterilized sand over the 90 day incubation. After subtracting the biochar + sterilized sand production from the forest soil+biochar, N2O production following biochar addition was consequently lower than the control forest soil. In terms of the microbial contributions, there were no significant differences in N2O production between the microbial inhibitor treatments across the three soils, despite differences in the CO2 production rates. Therefore, the response in the N2O production could not be directly attributed to a particular microbial group (fungi or bacteria). From the incubations in the absence of soil microbial populations, we did observe the abitoic production of N2O and N2, which was coupled with the loss of nitrate/nitrite. The results presented here suggest the presence of abiotic production and consumption routes for nitrogen species in biochar amended soils, which for the forest soil-biochar combinations were more important than the alterations in microbial nitrification/denitrification functional dynamics.