Effect of wildfire on soil organic matter in an Andisol of a southern Chilean temperate rain forest

Old-growth forests of southern Chile represent an important reserve of temperate forests worldwide. The precipitation chemistry in Chile still reflects the time of pre-industrial conditions. Thus, the productivity of the ecosystem mainly depends on the internal cycling of soil organic matter (SOM). However, catastrophic events, such as wildfires can initiate important changes in the physical and biogeochemical properties of SOM and therefore on the ecosystem habitat. We studied Araucaria-Nothofagus forests in the Andean mountains of Chile (38° S, 71° W) three years after they were affected by a huge wildfire in February 2002. The aims of this investigation were: (1) to analyze the effect of fire on the quality of SOM at different soil depths and (2) the changes in carbon content of an incomplete combustion of vegetal material after wildfire, refractory to decomposition, referred to as black carbon (BC). We used a combination of C and N analyses and SOM fractionation. The chemical and elemental composition of the soil samples was studied by solid state C CPMAS NMR spectroscopy and BC determination. We found an increase in nitrification rates after fire, and a wider aromatic C to alkyl C ratio of the incorporated char, leading to an increase in BC quantity. Our results point out that wildfire induces changes in the chemical composition of the SOM. A clear relationship between isotopic SOM signature and C and N cycling processes could be established. This indicates that C and N and NMR techniques can be used as proxy to assess the fire impact on the SOM dynamics.