Modelling the long-term response to positive and negative priming of soil organic carbon by black carbon

Observed increases in the mineralization rate of labile organic carbon (LOC) in the presence of black carbon (BC) have led to speculation that corresponding decreases in non-pyrogenic (i.e. non-BC) soil organic carbon (npSOC) could significantly reduce or negate the C sequestration benefit of BC in soils. Here we show that the potential effect of an increased LOC decomposition rate on long-term npSOC stocks is negligible, even when using assumptions that would favour large losses, potentially causing no more than 3-4 % loss of npSOC over 100 years if 50 % of above-ground crop residues were converted to BC annually. Conversely, if the BC-stimulated enhanced stabilization of npSOC that has been observed in laboratory trials is extrapolated to the long-term, it would greatly increase soil carbon stocks by 30-60 %. Annual additions of BC derived from crop residues would increase total SOC (including both BC and npSOC) by an amount five times greater than the potential increase from enhanced stabilization and an order of magnitude greater than losses of npSOC caused by annual removals of biomass to provide BC feedstock.