Fire enhances phosphorus availability in topsoils depending on binding properties

Fire can have large effects on ecosystems, with phosphorus being highly important in this regard, especially when considering productivity in burned or adjacent ecosystems after fire. Unfortunately, existing data pose contradictions and methodological challenges to assessing P availability. Here, the impact of fire on topsoil available P was estimated using three different types of organic soil layers (raw humus from spruce, humus from beech, and peat) and two fire regimes (obtaining black carbon and ash). Our findings suggest a strong fire impact on P availability, by enhancing available P during burning to black carbon (~10-fold), and to ash (~2- to 7.5-fold) dependent on calcite content. Fire impact on P availability is on a similar order of magnitude as mineral weathering and annual P cycling/uptake in/by ecosystems. Furthermore, the proportion of available P in relation to total P depends on the origin of the organic soil layers. When related to the remaining mass after burning, as is commonly done, P availability can be overestimated by up to 1400%. Results from this study provide a realistic magnitude of shifts in P release and P availability by fire events of different intensity for widely abundant ecosystem conditions. Taking the importance of P for ecosystem processes into account, these results are highly relevant, because fires for fuel reduction are a common application, and for temperate and boreal ecosystems even moderate climate change scenarios predict an increasing appearance of fires in these regions.