Prior studies of wood biochar application to forest soils have exhibited increased nitrification and mineralization rates at the plot scale; however, there have been few studies that effectively evaluate fine-scale soil soluble nitrogen (N) dynamics within the “charosphere” (soil immediately surrounding biochar). Microdialysis probes were originally developed for use in neuroscience, but more recently have been applied to soil science research. The small probes cause minimal disturbance to soil structure and the passive diffusion of soil solutes across a semi-permeable membrane allows dialysate to be collected over time without re-taking samples, allowing “real-time” evaluation of fine scale nutrient dynamics. Therefore, microdialysis has the potential to provide higher spatial and temporal resolution for understanding short-term solution N chemistry in the charosphere. Herein we applied microdialysis to investigate the spatial and temporal dynamics of free amino acids, ammonium (NH4+), and nitrate (NO3-) following biochar application to a column containing sandy loam forest soil over a 16 day period. To examine the spatial distribution of soil N hotspots in response to biochar over time, wood biochar (diameter ≤ 5 mm) was either applied as surface dressing or mixed through the entire soil column.
Results/Conclusions
Biochar stimulated localized soluble N diffusive fluxes, and the vertical distribution of amino acids and NH4+ hotspots gradually matched the distribution of biochar particles in the soil matrix over time. Increases in soil amino acids 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. Overall, the use of microdialysis to the assess biochar effects on soil solution N demonstrated: (1) Addition of wood biochar to a high C:N ratio, low NO3- forest soil increased the localized diffusive fluxes of amino acids, NH4+, and NO3-; (2) The spatial distribution of N hotspots along the soil profile dynamically responded to the biochar application strategy during a 16 day period. Microdialysis sampling allowed for quantification of hotspots of soil soluble N in association with the “charosphere” across time and space.