CANCELLED - Resource partitioning of soil organic phosphorus: Investigations from a tropical montane forest

Background/Question/Methods

One of the major limitations of resource-niche theory to explain plant species diversity and distribution is the paucity of recognized resources. Plant productivity over much of the globe is limited by the soil nutrients nitrogen and/or phosphorus. Recent investigations in grassland and arctic tundra ecosystems indicate that plant species can specialize to take up different forms of soil nitrogen. I hypothesized that a similar phenomena occurs in the tropics with soil phosphorus (P). Soil P exists in several different organic forms, including monoesters (glucose-phosphate), inositols (phytic acid), and diesters (DNA, RNA). As soils age, the proportion of the total soil P pool in inorganic and organic monoester forms decreases, while the proportion in inositol and diester forms increases. Concomitantly, arbuscular mycorrhizal (AM) plants are associated with young soils, while ectomycorrhizal (EM) and nonmycorrhizal (NM) plants are associated with older soils. Therefore, I predicted that AM plants should be specialized to take up inorganic and organic monoester-P, while EM and NM plants should be specialized to take up organic diester and inositol-P. To test this, I grew seedlings of AM (Mollinedia darensis, Podocarpus olieofolius), EM (Oreomunnia mexicana), and NM (Roupala montana) tree species in a hydroponic growth medium containing exclusively either inorganic, monoester, diester, or inositol-P. After a growth period of 4 months, I harvested my plants and calculated relative growth rate (RGR) and foliar and whole-plant P content. Differences in mean RGR between treatments of the same species were analyzed using ANOVA and least significant difference (p<0.05) in SAS. Species were compared by evaluating the significance of the species*treatment interaction when the growth in each organic-P treatment was compared to growth in the inorganic phosphate treatment.

Results/Conclusions

My results indicate that growth in the mycorrhizal tree species is high in inorganic-P and organic monoester-P and low in organic inositol and diester-P treatments. The NM tree species of the family Proteaceae exhibit significantly greater growth when limited to inositol phosphate (p=0.0004) and nearly-significantly greater growth when limited to diester phosphate (p=0.058) than the mycorrhizal tree species. I found no significant differences between AM and EM species’ growth response to the different phosphorus treatments. This indicates that the potential exists for partitioning of soil P between mycorrhizal and NM plants, but not between AM and EM plants. Measurements of the foliar and whole-plant P-content (a surrogate for P-uptake) are currently in progress.