HR: 14:25h
AN: B13E-03    [Abstracts]
TI: Pyromineralization of soil phosphorus in savanna ecosystems
AU: * Hartshorn, A
EM: soildoc@gmail.com
AF: University of California, Department of Geography, Santa Barbara, CA 93106-4060,
AU: Coetsee, C
EM: ccoetsee@gmail.com
AF: University of Cape Town, Department of Botany, Cape Town, 1, South Africa
AU: Chadwick, O
EM: oac@geog.ucsb.edu
AF: University of California, Department of Geography, Santa Barbara, CA 93106-4060,
AB: The weathering of rock supplies phosphorus (P) to ecosystems. Phosphorus limitation of ecosystems can be severe in thicker or older soils, where soil production rates from rock and therefore release of P is slower than in thinner or younger soils. Limitation may be especially pronounced in drier ecosystems that are experiencing increasing N deposition. Our savanna field sites in Kruger National Park, South Africa meet all three of these criteria: soil residence times average 250 ky, the climate is semiarid, and N inputs average 20 kg ha-1 y-1. Not all soil P is plant-available, and because our field sites experience occasional fires, our objectives were to quantify the importance of pyromineralization of soil P, the transfer by fire of soil P from recalcitrant to labile (HCO3- extractable) pools. We quantified these soil P pools using a modified Hedley scheme (an array of chemical extractants). Three sets of soils were fractionated: 1. soils from 10 profiles along an intensively studied hillslope, bracketing a pronounced structural and functional ecotone; 2. surface soils from these 10 profiles after a simulated burn; and 3. surface soils from the Shabeni Experimental Plots, where 4 fire treatments have been maintained for decades: no fire, annual fire in the dry season, triennial fire in the dry season, and triennial fire in the wet season. Total P for hillslope soils ranged from 45 to 135 g m-2 (to 50 cm depth) and from 8 to 15 g m-2 (to 5 cm depth). Total soil P was lowest in midslope soils, where upslope sandy soils dominated by broad-leafed vegetation shift abruptly to downslope clayey soils with fine-leafed vegetation. Simulated fire for the hillslope soils reduced total P slightly, but boosted labile P by 1.7 g m-2 (to 5 cm), representing 17% of total P in the surface 5 cm. This pyromineralization effect was not uniform across the hillslope: downslope soils gained about 50% more labile P than midslope soils with simulated burning. With a fire return interval of 4 years, pyromineralization produces a flux of P into plant-available pools of +0.4 g m-2 y-1, a flux that approximates plant uptake of labile P based on foliar P. We observed the same general patterns for soils burnt triennially in the wet season. There, the difference in labile P was greatest between treatment and control plots (10 ug g-1 or 0.9 g m-2 to 5 cm depth), producing a flux of labile P of +0.3 g m-2 y-1. Soils from the other two fire treatments contained less labile P than the control plot, with the most pronounced deficit (-0.25 g m-2 to 5 cm) for the plots burnt triennially in the dry season. Infrequent, high severity fires may prevent the incorporation of biomass P into soils, eliminating the opportunity for pyromineralization of recalcitrant P. These fire-mediated transfers from recalcitrant to labile P pools are nearly 1000 times greater than our estimated flux from rock to soil of total P (0.0008 g P2O5 m-2 y-1). These patterns suggest that pyromineralization represents a significant input of labile P to these P-limited ecosystems, where low chemical weathering rates lead to intense cycling of rock-derived nutrients.
DE: 0400 BIOGEOSCIENCES
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting