Effect of severe wildfire on soil phosphorus fractions and adsorption in a cold temperate coniferous forest after 5 years

Forest fires are a primary driver of biogeochemical processes in ecosystems and affect the soil nutrient balance by altering the distribution of organic matter and associated nutrients as well as the composition and availability of elemental nutrients. We investigated the changes in soils' phosphorus (P) fractions and the adsorption characteristics of potassium dihydrogen orthophosphate (orthoP) and myo-inositol hexakisphosphate (IHP) at two depths (0-5 and 5-10 cm) in soils from two sites differing in fire severity: long-unburned and heavily burned. Five years after the fire, there were increases in the total P and extractable inorganic P (H2O-P, NaHCO3-Pi, NaOH-Pi, D.HCl-Pi and C.HCl-Pi) contents. Conversely, there was a decrease in organic P (NaHCO3-Po, NaOH-Po and C.HCl-Po). The adsorption of soil for orthoP at a depth of 0-5 cm was dependent on the amount of P supplied by the soil; at low concentrations, heavily burned soil showed a lower adsorption than long-unburned soil. However, at high concentrations, the adsorption of heavily burned soil was significantly larger than that of long-unburned soil. With respect to IHP, the adsorption of long-unburned soil was also more significant at depths of 0-5 cm. The adsorption of both orthoP and IHP of heavily burned soil was significantly higher than that of long-unburned soil at a depth of 5-10 cm. Fire also decreased the binding degree (k1) of soil-P and the solid phase at depths of 0-5 and 5-10 cm. The increase in P adsorption capacity outweighed the addition of P from ash, resulting in less leaching in heavily burned areas and minimizing P loss. Overall, forest fires significantly affect the adsorption characteristics and P fractions of soil, specifically orthoP and IHP, during the initial stage of postfire recovery. These effects are closely related to the direct influences of iron oxide, soil organic matter and aluminium oxide in the soil and may even produce relatively long-term effects on the P recycling process of the entire ecosystem.