PHOSPHORUS CYCLING AND SOIL P-FRACTIONS IN DOUGLAS-FIR AND RED ALDER STANDS

Nitrogen-fixing species can dramatically increase soil acidity and org anic matter content, and potentially alter biogeochemical P dynamics. We compared ecosystem P cycling in adjacent stands of N-2-fixing red a lder (Alnus rubra Bong.) and non-fixing Douglas-fir (Pseudotsuga menzi esii France) in order to determine whether P-cycling rates within stan ds were related to soil P forms as measured by sequential P fractionat ion. Above-ground annual P uptake was 61% greater in the red alder sta nd, although soil available P, as measured by Bray (NH4F-HCl) extracti on, was only 10% of that found in the Douglas-fir stand. Total ecosyst em P in the alder stand was only 69% of that found in the Douglas-fir stand, and could indicate a prestablishment difference between stands. However, the percentage of total soil P released by Bray or NaOH extr action was also lower in the alder stand, which suggests that differen ces in total P alone did not control the patterns observed in P fracti ons. Concentrations of inorganic P sorbed to Fe and Al minerals and co ntained in Fe minerals and apatite were greater under Douglas-fir, whi le organic P was slightly greater under red alder. While fluxes of P i n litterfall, uptake and resorption were 94, 60 and 292% higher in the alder stand, soil extractable fractions meant to represent available P were lower under alder. Static measures of available P do not appear to adequately reflect P supply, and the development of techniques to assess P turnover is needed to better understand cycling and plant ava ilability of P. (C) 1998 Elsevier Science B.V.