All phosphorus (P) in terrestrial ecosystems ultimately comes from apatite weathering, during which, P speciation, hence availability, changes with time. Phosphorus speciation evolution with time in humid ecosystems can be well described by the Walker and Syer (WS) model, but whether its speciation evolves in similar way in arid and semi-arid ecosystems remains uncertain. Here, we used P K-edge XANES spectroscopy and 31P NMR spectroscopy to directly characterize both inorganic (Pi) and organic P (Po) species in surface and C horizon soils of the San Francisco Volcanic Field soil chronosequence (4 ages, 1 ky – 3000 ky) in Arizona, as well as aeolian dust deposit at the site. Results show that with increasing age, total surface soil P concentration decreased quickly while the fraction of each Po species (diester, monoester, DNA, phosphonates and phytic acid) increased and then decreased, consistent with the WS model. Pi (Fe- and Ca-P) dominates the soil speciation at each age, with Fe-P accounting for a large fraction (&gt; 50%), even for the soil of 1 ky with a low weathering degree (CIA = 59). However, neither the Ca-P nor Fe-P fractions show any trends with soil ages. P speciation of the C horizon soils contain 27 – 43% of Fe-P with the rest mainly as Ca-P. The dust samples contain a large fraction of Po (30 – 60%), with relatively low Fe-P (13 – 22%) and Ca-P (12 – 22%). The high Fe-P than expected in 1 ky soil suggest that some Fe-bearing parent materials may undergo relatively fast weathering to form Fe oxides. This may also contribute to the observed inconsistence of surface Pi speciation evolution patterns with the WS model. Given its low Fe-P content, dust input may not be the reason for the observed inconsistence. Sequential chemical extraction analysis is ongoing and will provide complementary evidence.

Phosphorus speciation evolution during pedogenesis in a semi-arid environment

251st National Meeting (2016)

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