Short Term Effects of Biochar On Air, Water and Particle Transport Characteristics Along a Natural Bulk Density Gradient.

Biochar, pyrolysed organic matter, is a stable form of carbon with strong potential to mitigate climate change through carbon sequestration, to improve soil fertility, and control chemical leaching in soils. Despite increased application of biochar in agricultural fields, soil texture and structure effects on biochar amendment have not been well-recognized. In this study, we examined the short-term effects of biochar on gas and water transport as well as particle and phosphorous leaching in differently-structured intact soil samples. The soil (sandy loam) was sampled from an experimental field in Risoe (Denmark) along a natural bulk density gradient. The field consists of 4 control plots and 4 biochar-amended plots with birch wood biochar (20 tons/ha). Bulk soil and 40 undisturbed soil cores (10 cm x 8cm) were retrieved from top layer at each plot 7 months after biochar amendment. Air permeability was measured at in-situ condition followed by a series of leaching experiments (10 mm h^-1). The leachate was analyzed for tritium (applied as a tracer), particles and phosphorus. Results of tracer experiments showed that matrix flow dominated in soils with low bulk density (< 1.52 gcm^-3) whereas preferential flow dominated in soils with high bulk density (> 1.57 gcm^-3), independent of biochar amendment. The 5% tracer arrival time  and air permeability both exhibited strong negative correlations with bulk density. Further, leached phosphorous showed a strong positive correlation with particles leached in low-dense soils but not in high-dense soil (likely due to limited colloid-P contact during preferential flow). Biochar amendment reduced P leaching and air permeability in high-dense soil while effects were not marked in low-dense soils. Overall, the short-term effects of biochar amendment on gas, water, and particle transport processes seemed associated primarily with reduced density and improved pore-network connectivity in originally high-density soil, thereby reducing likelihood of preferential flow through the top soil.