Effect of decomposing post-fire coarse woody debris on soil fertility and nutrient availability in a Mediterranean ecosystem

Abstract

Post-fire coarse woody debris can represent a valuable nutrient reservoir for a regenerating ecosystem, helping to prevent soil fertility losses after a wildfire. However, there is scarce information on its effect on soil nutrient cycling and availability. We established three study sites along an altitudinal gradient in a burnt pine forest (SE Spain). At each site we determined: (1) decomposition rates and nutrient dynamics in charred logs left on the ground, 2 and 4 years after the fire, and (2) available nutrients in the soil and in the microbial fraction below charred logs and in bare soil areas. Despite the relatively slow decay rates in this Mediterranean climate (ca. 10 % of dry weight lost after 4 years), N and P were progressively released by logs, accounting for ca. 40 and 65 % of the initial content respectively after 4 years. This implies that the total aboveground biomass of the burnt forest released around 20 kg ha⁻¹ of N and 2 kg ha⁻¹ of P during this period. The presence of post fire coarse woody debris consistently increased soil organic matter by around 18 %, total C and N by 42 and 26 %, respectively, dissolved organic C and N by 47 %, available inorganic P by 68 %, and microbial biomass and nitrogen by some 36 and 48 %, respectively. By contrast, soil bulk density decreased by ca. 18 % under logs compared to bare areas. Thus, the fire-killed wood was useful in the recovery of soil fertility and nutrient availability. Leaving the post-fire woody debris on site can enhance the biogeochemical sustainability, microbiological processes and soil ecological functioning. The detrimental effect of post-fire salvage logging on soil fertility should be therefore considered when making management decisions.

Appendix: Estimation of the initial dry weight of the wood discs to calculate the dry weight lost by the charred wood over time

To estimate the initial dry weight of the discs collected after two (2008) and four (2010) years of decomposition, we used a regression model constructed with the volume and the dry weight of the wood discs initially collected in 2006. Previously, we checked that diameter had no effect on the initial wood density for any of the sites (P > 0.05 for all sites), so the diameter did not have to be included as an independent variable in the model. Initial wood density differed among sites (Table 6) despite the absence of differences in the rest of variables, so a regression model was fitted separately for each site. Further, the dry weight of a wood disc must be zero when its volume is zero. For this, the intercept of the regression line was forced to be zero for each regression model. Nonetheless, once the models with intercept were fitted, the H₀ that the intercept was zero was tested in all of them, and the H₀ could not be rejected in most of the cases. The resulting regression equations for each site are:

• W _d  = 0.7304172*V _d for site 1;

• W _d  = 0.7348488*V _d for site 2;

• W _d  = 0.7237018*V _d for site 3;

where W _d and V _d are the dry weight and the volume of the initial wood discs.As the external fragmentation of the log was negligible over the study period, we can assume that the volume of the wood discs (V _d ) remained constant during these initial stages of wood decomposition. Thus, the volume of the wood discs of 2008 and 2010 (V _d ) was introduced in the constructed regression model to estimate their initial dry weight (W _d ).

Table 6 Summary of the variables measured in the wood discs collected in 2006 which were used in the regression models and results of testing the differences between the experimental sites