PHOSPHORUS AVAILABILITY IN ACID ORGANIC SOILS OF THE LOWER NORTH-CAROLINA COASTAL-PLAIN

A natural landscape gradient of P-deficient pocosin (evergreen shrub bog) and bay forest ecosystems was used to test three hypotheses concerning P availability and P reabsorption efficiency: (1) that variations in vegetation height and aboveground biomass among even-aged pocosin stands are related to variations in soil P availability; (2) that microbial processes control P availability in these soils; and (3) that the proportion of P reabsorbed from senescing foliage in native plants is inversely related to soil P availability. The natural gradient from short pocosin to bay forest was found to represent a gradient of increasing P availability, as indicated by increases in: (1) total P in surface (0-15 cm) soils, from 19.6 to 118.3 kg/ha; (2) seasonal and annual PO43- supply to in situ anion-exchange resins; and (3) P concentrations in native plant foliage. Increases in P availability were accompanied by a significant decline in the N:P ratios of both plant leaves and surface soils. Microbial processes strongly influenced the supply of available P. Soil microorganisms immobilized up to 90% of added (PO43-)-P-32 during laboratory incubations, but when microbial uptake was inhibited, as much as 90% of added (PO43-)-P-32 remained in the available pool. A decline in the ratio of microbial: total P in surface soils, from 56.3 to 37.4%, and an increase in the ratio of net: gross P mineralization (estimated by isotope dilution), suggested that as P availability increased, microbial control decreased. Native plants reabsorbed both P and N efficiently from senescing foliage, reaching maxima of 86.6 and 80.5%, respectively; natural variations in soil P availability did not affect the proportion of either nutrient reabsorbed. In general, however, plants tended to reabsorb greater proportions of foliar P than N. Natural variations in P availability, the factors controlling P availability in these soils, and the potential for differential nutrient reabsorption by native plants have important implications for the cycling and accumulation of P and N in pocosin and bay forest soils. Landscape-level processes may have contributed to the observed variations in vegetation and P availability along this natural gradient.