Year of Award

2023

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Systems Ecology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Cory C. Cleveland

Commitee Members

Thomas H. DeLuca, James J. Elser, Ylva Lekberg, Benjamin P. Colman

Publisher

University of Montana

Abstract

Many Earth System Models (ESMs) predict a plant fertilization effect associated with elevated atmospheric carbon dioxide (CO2) concentrations that may increase the capacity of the biosphere to store carbon, stabilizing or even reducing atmospheric CO2 concentrations. This could plausibly mitigate global climate change. However, this effect is only possible in the absence of other limiting factors, such as the availability of nutrients like phosphorus (P). Critical uncertainties remain in our understanding of the soil biogeochemical controls of terrestrial P availability including: the response of soil P recycling rates to global climate warming and extent to which occluded soil P is biologically available in tropical forest ecosystems.

Accelerated recycling of soil P via increased efficiency of the activities of the enzymes that catalyze the hydrolysis of available phosphate from organic P compounds may increase P availability to support increased plant productivity. While this assumption is sound given our understanding of extracellular enzyme activities, it has not been extensively tested across terrestrial ecosystem types. I predicted that rates of P recycling would increase across terrestrial ecosystem types as a function of the temperature sensitivity of extracellular phosphatase enzymes. To test this prediction, in Chapter 1, I quantified the response of estimated P mineralization rates and potential extracellular phosphatase activities by subjecting soils from six terrestrial ecosystem types to a temperature gradient, quantifying the response of both processes, and modelling the relationship between these responses. While both processes were accelerated by warming, the rates of this acceleration did not covary across ecosystem types.

The least soluble form of occluded P, residual P, is the predominant form of soil P in many tropical forest ecosystems, which otherwise have low soil available P concentrations. If residual P can be mobilized by biota, this would expand the pool of P available to support tropical forest growth and regeneration. Here, in Chapter 2, I quantified changes in soil P fractions across a tropical secondary successional forest chronosequence. Using these data to model P use over the course of secondary succession at both sampling depths, I observed a significant reduction in subsoil residual P concentrations consistent with the previously identified pattern of biomass accumulation in this ecosystem. This is inconsistent with the unavailability of residual P to biota. Considering this, I tested the feasibility of a possible mechanism for the solubilization of residual P, the exudation of low weight organic acids by treating subsoil samples from this secondary successional chronosequence with three such organic acids: citric, oxalic, and malic acids. While I identified an increase in resin P concentrations, I did not identify a measurable decrease in the residual P pool, indicating that further investigation of this mechanism in regenerating tropical forests is warranted.

Chapter 3 is a reflection on the production of Chapters 1 and 2. In this chapter, I contextualized this work in terms of ecosystem functioning, connected Chapters 1 and 2, and reflected on the ways in which completing a Ph.D. changed how I see the world.

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