Frequent fire alters soil total phosphorus but does not affect phosphorus availability in a montane grassland

Phosphorus (P) is often a limiting nutrient in highly weathered soils. Fire is a major driver of nutrient redistribution and can temporarily increase the pool of plant-available P in P-limited ecosystems. Yet, the long-term effects of frequent fire on soil P in montane grasslands remain poorly understood. We investigated how fire regime influences soil P pools using data from a long-term fire experiment in the South African Drakensberg. Total soil P, moderately labile organic and inorganic P and plant-available P were measured across five prescribed fire regimes varying in frequency (annual, biennial or infrequent) and season of burn (autumn or spring). We hypothesised that frequent fire would not alter total P in the topsoil, but expected it would increase inorganic P and plant-available P. Infrequent and biennial burns had little effect on total P; however, total P was significantly higher under annual spring burns than the other treatments, particularly the infrequent burns and annual or biennial autumn burns. In contrast, plant-available P did not respond to any fire treatment. Frequent spring burns generally increased organic P relative to inorganic P, indicating a shift in the composition of soil P pools with fire frequency and season. Overall, despite changes in topsoil total and organic P, plant-available P remained constrained, reflecting a bottleneck in the P cycle likely driven by the high P-retention capacity of these acidic Andosols. These findings highlight the complex and sometimes counterintuitive effects of fire on nutrient dynamics in montane grasslands.

The online version contains supplementary material available at 10.1007/s10533-025-01304-w.