Biochar Addition Rate Mediates Greenhouse Gas Flux of Four Soils - the Importance of Long-Term Data.

Biochar addition to soil has been proposed as a method of soil C sequestration with added benefits of improving nutrient retention and decreasing greenhouse gas flux. However, research to date has documented a net positive, neutral, or negative effect on CO₂, N₂O, and CH₄ emissions depending on the biochar feedstock, pyrolysis temperature, as well as soil type.  Our objective was to determine the effect of biochar addition rates (0, 1, 5, 10, and 20% by weight) on trace gas emission from four soils varying from a clay to a sand on during a long-term incubation. To trace CO₂ source, we added C₃ biochar (pyrolyzed at 550C) to C₄ soils and measured CO₂, N₂O, and CH₄ over a 1.5 yr incubation.  Biochar addition initially suppressed CO₂, but after 3-4 months, biochar-derived CO₂ increased in accordance with biochar addition rate. In all soils, N₂O emissions were suppressed, although the response to addition rate varied from 20% to 94% reduction depending on soil type.