Changes in net N mineralization rates and soil N and P pools in a pine forest wildfire chronosequence

The concern that climate change may increase fire frequency and intensity has recently heightened the interest in the effects of wildfires on ecosystem functioning. Although short-term fire effects on forest soils are well known, less information can be found on the long-term effects of wildfires on soil fertility. Our objective was to study the 17-year effect of wildfires on forest net mineralization rates and extractable inorganic nitrogen (N) and phosphorus (P) concentrations. We hypothesize that (1) burned forest stands should exhibit lower net mineralization rates than unburned ones; (2) these differences would be greatest during the growing season; (3) differences between soil variables might also be observed among plots from different years since the last fire; and (4) due to fire-resistant geochemical processes controlling P availability, this nutrient should recover faster than N. We used a wildfire chronosequence of natural and unmanaged Pinus canariensis forests in La Palma Island (Canary Islands). Soil samples were collected during winter and spring at 22 burned and unburned plots. We found significantly higher values for net N mineralization and extractable N pools in unburned plots. These differences were higher for the winter sampling date than for the spring sampling date. Unlike extractable N and N mineralization rates, extractable P levels of burned plots exhibited a gradual recovery over time after an initial decrease. These results demonstrate that P. canariensis forest soils showed low resilience after wildfires, especially for N, and that this disturbance might induce long-term changes in ecosystem functioning.