Use of microdialysis to assess short-term soil soluble N dynamics with biochar additions

To date there has been little effort to assess fine-scale soil solution nitrogen (N) dynamics over time and space in response to biochar application within days. Herein we applied microdialysis to investigate the spatial and temporal dynamics of free amino acids (AA), ammonium (NH4+), and nitrate (NO3-) following biochar application to a column containing sandy loam forest soil. Wood biochar (diameter <= 5 mm) was applied either at surface or mixed through the entire soil column. Biochar stimulated localized soluble N diffusive fluxes, and the vertical distribution of AA and NH4+ hotspots gradually matched the distribution of biochar particles in the soil matrix over time. Increases in soil AA and NH4+ concentrations were more homogeneous along soil profile when biochar was mixed through the soil core and were more concentrated at the surface when biochar was surface applied. Increases in NO3- concentrations were only observed at surface soil layers following biochar addition regardless of the application strategy and generally exhibited a high degree of variation over the course of the experiment. Microdialysis sampling allowed for quantification of 'hotspots' of soil soluble N in association with the charosphere across time and space.