Pyromineralization of soil phosphorus in a South African savanna

Abstract

Savannas are shaped by drought, herbivory, nutrient limitation, and fire. We assessed the interactions between two of these factors—nutrient limitation and fire—across a savanna landscape in Kruger National Park, South Africa, by quantifying increases in plant-available forms of soil phosphorus (P) following experimental or simulated burns. Plant-available pools of P were defined for granitic surface soils subjected to four experimental fire regimes (no fire, triennial fire in the wet season, triennial fire in the dry season, and annual fire in the dry season). To provide context for these results, we also fractionated surface soils from a granitic catena after simulating burning of these soils in a furnace. Burned soils showed comparable pyromineralization rates, with the experimental burn plot soils averaging 0.49 ± 0.04 g labile P m − 2 y − 1 and catena soils averaging 0.63 ± 0.12 g labile P m − 2 y − 1 . Only soils from subplots burned triennially during the wet summer season with moderate fire intensities (∼ 1.1 MW m − 1 ) showed significant increases in labile P relative to control soils. Soils from other burned subplots with greater fire intensities showed smaller gains in labile P, suggesting pyromineralization rates may peak at intermediate fire intensities. We estimated ash contributed up to 33% of pyromineralized P. For catena soils, simulated burning led to significant increases in pyromineralized P for the relatively P-rich footslope soils and smaller increases for sandy crest and midslope soils. These pyromineralization P fluxes are of the same order of magnitude as plant P demand estimated using foliar P levels, and about one-half microbial mineralization rates. In P-limited ecosystems where chemical weathering rates are slow, moderate-intensity fires could play a critical biogeochemical role in the supply of labile P.