Interactions between nitrogen, phosphorus, and molybdenum in forest soils and cyanobacterial lichen in the Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/fn107141h

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  • Molybdenum is an essential component of biogeochemical cycling, most notably as a component of the nitrogenase enzyme used in biological nitrogen (N) fixation. While the important role of phosphorus (P) in limiting N fixation in ecosystems has been well documented, occurrence and prevalence of molybdenum (Mo) limitation is largely unknown. In the Oregon Coast Range, of the Pacific Northwest, USA the primary successional symbiotic N-fixer red alder has left a legacy of elevated soil N, even in forests that are now dominated by Douglas-fir. Many sites with red alder legacy have moved beyond N-limitation to N-saturation, which can change soil Mo chemistry and availability through soil acidification. As Coast Range forests mature to old-growth, their source of ecosystem N shifts from primary successional N-fixers like red alder, to asymbiotic N-fixers in the soil and to epiphytic cyanobacterial lichens. Our study examines how Mo, P and N interact across the Oregon Coast Range in soil, forest floor litter, and Douglas-fir foliage, and in the cyanolichen Lobaria pulmonaria. To investigate how nutrient limitation may affect cyanolichen growth rates, we fertilized a tripartite cyanobacterial lichen (Lobaria pulmonaria) and a green algal lichen (Usnea longissima) with the macronutrient P and micronutrients Mo and vanadium (V), and grew treated lichens in the field for one year in western Oregon. At this site, lichen growth did not differ across treatments, despite a previous demonstration of P-limitation in L. pulmonaria at a nearby location. The lack of treatment effect cannot be explained by differences in N deposition or changes in thallus N content. Instead, we propose that local differences in P availability and monthly precipitation during the peak growing season may cause the same species to exhibit variable responses to P fertilization. We conclude that neither P nor Mo or V limits the growth of either cyanolichens or chlorolichens at this study site. Furthermore, these results demonstrate relatively high concentrations of all three nutrients P, Mo, and V in untreated lichens. Our findings point to the need for a more comprehensive understanding of how cyanolichens are affected by landscape-level variation in available P. To determine how Mo and P vary across soils with different bedrock lithology, we conducted a detailed assessment of soil Mo geochemical fractions and total soil and foliar Mo and P concentrations across two N-induced pH gradients in the Oregon Coast Range. We determined that extractable Mo in soil was not controlled by adsorption on iron and manganese oxides, as previous research has suggested, but rather by adsorption to soil organic matter. Organically-bound Mo concentrations were ~30 times larger than reducible Fe-Mn oxide bound Mo. Total Mo mobility (measured as τMo[subscript Nb]) showed Mo enrichment in the top 10 cm of soil on both bedrock types, suggesting low Mo loss through dissolution and leaching relative to adsorption to organic matter. Across a decreasing pH gradient, driven by increasing soil C and N, total exchangeable and organically-bound Mo concentrations increased. However, base saturation and aluminum saturation were better predictor variables for extractable Mo than soil pH, %N or %C. Molybdenum concentrations in forest floor were on average, 5.4 times higher than concentrations in Douglas-fir foliage, possibly due to a combination of increased Mo concentrations during foliar abscission, mixing of the mineral soil and litter layer via bioturbation, and by adsorption of Mo from rainwater. This mixing obscures the signal of any internal Mo recycling that may be occurring in Douglas-fir ecosystems. No soil extractable Mo pools correlated with foliar Mo concentrations, suggesting either active plant regulation of Mo uptake, or poor fidelity of extractable pools to bioavailable Mo. At the same time, the strong influence of organic matter on Mo distribution and retention in soils likely plays an important role in the bioavailability of Mo to soil biota.
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