Long-term effects of wildfires on the biochemical quality of soil organic matter: A study on Mediterranean shrublands

Wildfires affect soil organic matter (SOM), mainly resulting in losses of the most labile fractions (in particular carbohydrates), an increased abundance of recalcitrant fractions and, specifically, an increase in SOM aromaticity. Most of these effects have been studied in laboratory experiments: under field conditions, post-fire recovery of the vegetation must be taken into account, for it results in new and fresh inputs of organic matter to the soil. Thus the short-term effects of wildfires on SOM biochemical characteristics could be of little relevance in the medium or long term. We tested this hypothesis in the Valencia region (E Spain), in very healthy shrublands, never cropped, and which have been diversely affected by wildfires in recent decades (either 0, 1 or 2 wildfires). The study aimed at a quantitative description of SOM recalcitrance. Soil samples (uppermost 5 cm) were submitted to a four-step chemical fractionation, consisting of an extraction with dilute K2SO4 solution, and two consecutive hydrolyses, first with H2SO4 2.5 M and next with 72% H2SO4, further diluted down to 1 M. The unhydrolysed residue was taken as the recalcitrant fraction, and submitted to a further chemical attack with H2O2, to obtain the refractory fraction. Wildfires dropped both total OC and N in soil, and increased the fraction of total C and (somewhat less clearly), the total N found in both recalcitrant and refractory fractions. Carbohydrates accounted for about 20% of total OC, irrespective of the number of fires. Carbohydrates are selectively lost as a consequence of thermal shock during wildfires; thus their recovery in burned soils is an indicator of the post-fire restoration of the C cycle. The effects of wildfires are shown in the relationship between several parameters (cellulose to total carbohydrates ratio, aromatic extractable C, etc.) and total OC in the horizon, which suffer shifts as a consequence of the first wildfire, and disappear after the second. Overall, our dataset show that in burned plots the biochemistry of SOM has apparently recovered many features of the unburned plots, but a long-term effect of wildfires is still detectable in the relative abundance of recalcitrant or refractory forms. We did not detect increased aromaticity either in the overall SOM (by 13C NMR analysis) or in the extractable or hydrolysable fractions (either by absorption at 280 nm or by analysis of phenolic compounds), a fact that suggests that the increased SOM recalcitrance results from an increased abundance of condensed forms, not necessarily linked to an increase in aromatic C.