Date of Award

Spring 2000

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

Supervisor: William H McDowell

Abstract

The purpose of this research was to quantify the major sources of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils and ascertain mechanisms for their production and export to surface waters. To quantify the source of DOC we made use of the on-going litter manipulation study (DIRT) at Harvard Forest, Massachusetts. The organic horizon supplies 74% of DOC to bulk soil solution, 12% is supplied by leaf litter, and 13% from root exudate and decay. In plots with no inputs, DON concentrations were 9% higher than the control plots. When either roots or litter were excluded, DON concentrations increased by 17% and 12% respectively. Both DON and DOC concentrations were significantly related to fungal biomass (R2 = 0.99 and 0.90; p < 0.01).

We investigated the mechanisms of DOC and DON production and their relationship with CO2 and soil C:N ratio and the effect of chronic carbon and nitrogen manipulation on these relationships. DOC was significantly related to soil respiration in the hardwood plots (R2 = 0.61; p < 0.05), chronic carbon and nitrogen manipulation did not affect this relationship. In the coniferous control plots, the relationship between DOC and soil respiration was strong and significant (R2 = 0.93 p < 0.05) but nitrogen fertilization affected the relationship. DOC was significantly related to soil C:N among forest type and treatment suggesting that the overall mechanisms of DOC production are unaffected by either carbon or nitrogen manipulation.

We examined the effect of cold and warm temperature on the relationships between DOC, DON soil respiration and soil C:N in a laboratory controlled study. Temperature had no significant effect on the relationships between DOC and soil C:N, DOC and DON, DON and soil C:N but a significant temperature effect was apparent between both DOC and DON and soil respiration. We used mean biome soil C:N ratio and mean biome DOC export to derive an empirical model (R2 = 0.99 p < 0.001). The model predicted DOC export from contrasting forest types to within 4.5% of their observed exports. We estimated global annual riverine DOC export to be between 0.41--0.48 Pg yr-1.

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