Soil properties, phosphorus fractions and sorption after wildfire in north-central Portugal

This study examined soil physico-chemical properties, P fractions, and P sorption-desorption at two depths (0-3 and 3-6 cm) after a wildfire in a Pinus pinaster stand in north-central Portugal. Depending on fire severity, soils from three sites were considered for this study: long-unburned (US), low crown consumption (LCC) and high crown consumption (HCC). The fraction of fines (0-3 and 3-6 cm), total Mn concentration (0-3 and 3-6 cm), and Fe oxyhydroxide content (0-3 cm) were significantly larger for burned (LCC and HCC) than for unburned soils (US). On the contrary, the total organic content (TOC; 0-3 cm) of US was significantly larger than that of LCC and HCC. Organic P showed the same pattern as TOC, while no significant differences were determined for the rest of the P fractions. With respect to P sorption, faster kinetics and larger capacity onto LCC and HCC soil were respectively related to their higher percentage of fines, and to their higher concentrations of Fe and Mn oxyhydroxides, as compared to US. Meanwhile, desorption of P pointed to less tight binding of P on both LCC and HCC soil than on US soil. Finally, only the Fe oxyhydroxide content at 0-3 cm depth, and both the fraction of fines and the P sorption velocity at 3-6 cm depth, were sensitive to fire severity. It was concluded that wildfire had significant short term effects on soil P fractions and P sorption-desorption, which were highly related to direct effects on soil physico-chemical properties, and which may have relevant consequences at an ecosystem level.