Oxidized charcoal residues as essential constituent of soil organic matter composition in subsoils in fire-affected environment.

Abstract

Natural and prescribed vegetation fires lead to the input of considerable amounts of charcoal into soils. This amount will even increase by the application of biochar as a tool to improve soil fertility. Whereas it is well accepted that an immediate effect of charcoal input represents the enhancement of the aromaticity of the soil organic matter (SOM) in particular of the topsoils, our knowledge about the long-term impact of this material on the humification processes is still scarce. Therefore, the present review intends to outline this issue by complementing present knowledge with results of our recent studies. Analyzing the organic matter along profiles of typical soils in Brazil which were frequently exposed to fire since decades and even centuries revealed the presence of charcoal residues down to the C horizons. Interestingly, lower charcoal contributions were found in the topsoils than in the deeper horizons. Comparable results were obtained for fire-affected soil of the Mediterranean region. Most tentatively, charcoal was efficiently oxidized and biodegraded at the surface turning it into a more humus-like substance. However, as revealed by solidstate NMR spectroscopy, some of the degradation products must have been transported into deeper soil regions where they were selectively preserved. Possibly, the oxygen depletion in subsoils or the interaction of oxidized PyOM with the mineral phase has increased its biochemical recalcitrance resulting in a preferential degradation of SOM derived from fire-unaffected sources. Our data clearly show that frequent charcoal addition can have a higher long-term impact on SOM of deeper soil horizons than commonly assumed. It may even represent an essential factor for defining the properties of such subsoil. Considering further that oxidized charcoal residues may also leach into the aquifer, a further evaluation of the impact of such residues on the groundwater is urgently needed.