Stability of black carbon in soils across a climatic gradient

The recalcitrant properties of black carbon (BC) grant it to be a significant pool of stable organic C (OC) in soils. Up to now, however, the longevity of BC under different climates is still unclear. In this study, we used BC samples from historical charcoal blast furnace sites to examine the stability of BC across a climatic gradient of mean annual temperatures (MAT) from 3.9 to 17.2 degrees C. The results showed that OC concentration and OC storage in the BC-containing soils at a soil depth of 0-0.2 m were 9.0 and 4.7 times higher than those in adjacent soils, respectively. Organic C in the BC-containing soils was more stable, with a significantly lower amount of the labile OC fraction (4.4 mg g(-1) OC versus 27.5 mg g(-1) OC) and longer half-life of the recalcitrant OC fraction (59 years versus 9 years) than the adjacent soils determined by incubation experiments. The stability of BC was primarily due to its inherently recalcitrant chemical composition as suggested by short-term incubation and solid state C-13 nuclear magnetic resonance spectra of isolated BC particles. A significant negative relationship between OC storage and MAT further indicated that OC storage was decreased with warmer climate. However, the lack of a relationship between MAT and BC mineralization suggested that the stability of the remaining BC was similar between sites with very different MAT. Despite the fact that warming or cooling result in immediate consequences for BC stocks, it may have little impact on the stability of remaining BC over the period studied.