EFFECTS OF DINITROGEN-FIXING TREES ON PHOSPHORUS BIOGEOCHEMICAL CYCLING IN CONTRASTING FORESTS

Phosphorus availability in soils is controlled by both geochemical and biological reactions. Dinitrogen-fixing trees differ from non-N-2-fixing trees in litter production and litter chemistry, and these differences may substantially alter soil P biogeochemistry. We examined the effects of two N-2-fixing tree species on labile P pools and potential P transformation rates and correlated these variables with soil acid phosphatase activity. Experimental treatments include stands of N-2-fixing red alder (Alnus rubra Bong.), conifers, and a mixture of both in Oregon, and plantations of N-2-fixing Albizia falcataria (L.) Fosberg, Eucalyptus saligna Sm., and a mixture of both in Hawaii. Data from Oregon indicated that red alder increased labile organic P (P-o) levels and soil phosphatase activity. Soil labile P-o concentrations in the red alder and mixed stands were twice those of the conifer stand. Labile inorganic P (P-i) pools and net P solubilization rates in soils under the mixed stand were two- to fivefold greater than in pure stands, suggesting that interactions between alder and conifers, rather than alder's effect, triggered changes in P geochemical reactions. In Hawaii, labile P-o concentrations in both pure A. falcataria and mixed plantations were higher than in the pure E. saligna plantation. Soil acid phosphatase activity in the pure A. falcataria plantation was twice as high as that in the pure E. saligna plantation. However, labile P-i pools and P transformation rates did not differ among the three Hawaiian plantations. Our results suggest that N-2-fixing trees increase soil labile P-o levels and soil phosphatase activities whereas their interactions with non-N-2-fixing trees vary with species and may result in altering both geochemical and biological P transformations.