Investigating the contribution of the coarse fraction to total pyrogenic carbon stocks in forest soil

Pyrogenic carbon (PyC) quantification methods based on visual identification typically measure only the PyC particles >0.5 mm, whereas oxidative methods are usually conducted on the <2 mm soil fraction. To reconcile the discrepancy between these two methods, we used the weak nitric acid digestion method to quantify the PyC mass in the <2 mm soil, and in the charcoal particles >2 mm, in a California dry mixed conifer forest spatially heterogeneous in fire severity. We found that charcoal particles >2 mm represented 5-10% of mineral soil PyC in recently burned areas (three years post-fire in our study) and 3% in the unburnt areas (>100 years since the last fire). We, therefore, stress the importance of considering the size fraction of the charcoal quantified.