The organic nature and atmosphere-climate dependency of nitrogen loss from forest watershed ecosystems

Abstract

In this dissertation I describe how coupled internal cycling and external forcing from the atmosphere and climate can regulate the dynamics of nitrogen (N) loss from forest watersheds. I address three major gaps in our understanding of the global N cycle: 1) the role of dissolved organic N (DON) in internal N cycling in low-N ecosystems; 2) The influence of atmospheric pollution on DON production and loss from forests; and 3) the inherent climate sensitivity of forest N cycling and loss. In chapter 2, I present the results of a study of DON spiraling that showed enormous capacity for stream microorganisms to immobilize and transform organic nutrients. Although most DON in surface waters is highly refractory products of SOM dissolution, this study revealed very tight internal cycling of DON at the sediment interface and suggested significant production of DON in the hyporheic zone. Most remarkably, this DON was not expressed in stream waters, supporting the idea that watershed DON losses would have been higher in the absence of pronounced benthic demand. The experiments also suggested that coupled dynamics between DOC and DON spiraling may be altered under conditions of elevated N supply.

Chapter 3 challenges the idea that soil organic matter (SOM) and its dissolved products are stoichiometrically static as N pools accumulate. Using a broad geographic survey of forest streams, I show that DON losses increase as a consequence of N pollution and that this occurs through a disproportionate enrichment of N on dissolved organic matter rather than alteration of soil and dissolved carbon dynamics. These results have implications for N limitation in forests and aquatic systems. In particular, DOC: DON ratios of DOM draining N-saturated forests were strikingly low suggesting possible increases in DOM bioavailability with increasing N supply.

Chapter 4 provides insight into how local forest nutrient cycles may be organized by synchronous global-scale climate-atmosphere dynamics. This study of long term (30 yr) hydro-chemistry from reference forest watersheds provides an integrated example of the overall climate sensitivity of N cycling and underscores the importance of complex synergies between simultaneous vectors of global change. Results from this study argue that the combined influence of N pollution and warming are likely to have pronounced long-term effects on ecosystems globally.