Black Carbon in Old Growth and Thinned Ponderosa Pine Forest Soils in Northern Arizona.

Black carbon (BC) from wildfire and prescribed burns is ubiquitous in forest soils.  The importance of BC in the global carbon cycle has become widely recognized recently. Black carbon is produced primarily from incomplete biomass consumption during smoldering or glowing combustion.  When retained in a soil matrix, soil charcoal is considered part of a super-persistent C pool with residence times longer than 1,000 yr., which is far longer than most labile or humic soil organic matter. This is a heterogeneous material with uncertain accumulation, storage, and decomposition patterns  No data are available for BC quantities for legacy (old-growth forest)  and none for managed forests following prescribed or wildfire burns in the Southwest. In order to better understand accumulation and persistence of black carbon in Southwestern coniferous forest, we sampled soils at three sites in ponderosa pine (Pinus ponderosa) forests near Flagstaff, AZ at 2,030m elevation.  Two of the sites were replicated, 20 year old cut and burn thinning treatments while the third site is an old-growth stand.  At all sites we sampled mineral soil (0-10 cm) randomly, compositing three soil samples per sample point.  These samples were air dried and sieved within 24 hours.  Black C was determined on all soil samples using high performance liquid chromatography (HPLC) equipped with a photodiode array detector to quantify benzene polycarboxylic acids (BPCA).  Both soil %C and total BPCA-C (g/kg dry mass) are not significantly different between the thinned forest soils and the old growth soils.  Similarly, the relative abundances of B5CA (p < 0.06) and B4CA (p < 0.37) are not significantly different.  However, the relative abundance of BP6CA (p < 0.01) is significantly less in the old growth forest soil than the cut and burn, thinned forests.  This is important for understanding long term carbon storage in forest soils.