Biodegradability of organic matter in fire-affected mineral soils of Southern Spain

Incorporated into the soil, naturally formed pyrogenic organic matter (PyOM) is considered as highly recalcitrant, but direct estimation of PyOM decomposition rates are scarce. With this aim in mind, we subjected organic matter (OM) of fire-affected and unaffected soils to biochemical degradation under laboratory conditions and monitored CO2 production over a period of seven months. The soils derived from the Sierra de Aznalcóllar, Southern Spain, and were sampled 4 weeks and 5 years after a severe fire. Virtual fractionation of the solid-state 13C nuclear magnetic resonance (NMR) spectra of the fire-affected soils into fire-unaffected soil organic matter (SOM) and PyOM yielded charcoal C contributions of 30 to 50% to the total organic C (Corg) of the sample. Fitting the respiration data with a double exponential decay model revealed a fast carbon flush during the first three weeks of the experiment. Solid-state 13C NMR spectroscopy evidenced the contribution of aromatic moieties of the PyOM to this initial carbon release and to the biosynthesis of new microbial biomass. Considering the loss of microbiologically easily available fresh litter by wildfires, this relatively labile PyOM fraction may contribute to a fast recovery of a fire-affected site. The input of PyOM resulted in an increase of the mean residence time (MRT) of the slow OM pool of the soil by a factor of 3–4 to approximately 40 years. Assuming that under field conditions, the microbial activity corresponds to approximately 10% of the value observed under optimal laboratory conditions, MRTs of 500–600 years were estimated for the slow PyOM pool. The fact that these times are only 5–6 times longer than those calculated for fire-unaffected SOM rises doubts about the presumed big influence of PyOM as an additional C-sink in soils. On the other hand, although being small the difference in turnover rates is evident and has some major implication with respect to long-term alteration of the chemical composition of OM in fire-affected soils. In case of a reduced input of fresh litter, the preferential degradation of fire-unaffected SOM yields in a selective preservation of PyOM. To what extent this can alter soil properties, has still to be elucidated. In cultivated soils rarely affected by fire or with low charcoal input after burning of harvest, the impact of PyOM accumulation may be of minor importance. On the other hand, for soils regularly amended with high amounts of biochar or subjected to frequent natural or prescribed burnings, it may be an important factor.