Ecosystem recovery from acid precipitation: Carbon and nitrogen cycling in the soil-stream continuum

Abstract

While the problem of acid precipitation has been greatly reduced across the developed world, acid-impaired ecosystems still bear important legacies of historic acid deposition, and these ecosystems are only slowly beginning to recover. Early evidence suggests that this recovery may have multiple unexpected, substantial impacts on ecosystem carbon (C) and nitrogen (N) cycling, including large reductions in soil organic C and N pools and greatly increased export of dissolved organic C to aquatic ecosystems. These losses of soil C and N, driven by ecosystem recovery from acid precipitation, have the potential to dwarf any gains in biomass pools due to enhanced forest growth. The mechanisms driving these changes remain poorly understood, however. In this work, I ask the following questions: a) How does ecosystem recovery from acid precipitation alter soil C and N cycling, and b) what are the consequences of these changes for ecosystem N retention?A watershed acid remediation experiment, performed at Hubbard Brook, White Mountains National Forest, New Hampshire, offers an opportunity to study now processes of ecosystem recovery from acid precipitation that will take decades to centuries to occur naturally. In this experiment, researchers applied calcium (Ca) silicate to a watershed to restore soil Ca fertility and raise soil pH. This treatment caused a rapid, substantial decrease in soil organic C, and greatly increased streamwater export of inorganic nitrogen. My dissertation examines my research questions through the lens of this experiment. In the first part of my dissertation (Chapter 2), I examined how changes to soil pH and Ca fertility, individually and together, drive changes in the solubility and respiration of soil C, and how vegetation mediates these effects. I examined this topic through a soil mesocosm experiment in which I modified soil pH and soil Ca status individually and in combination. I found that there was a strong interactive effect between increases in soil pH and the presence of vegetation. Increasing soil pH increased soil respiration and soil C solubility, but only in the presence of sugar maple (Acer saccharum) roots. I also found that Ca fertilization stimulated plant growth, but had no effect on soil C dynamics. This suggests that vegetation and soil microbiota may respond to different aspects of ecosystem recovery from acid precipitation, with vegetation sensitive to changes in soil Ca content and microbiota sensitive to changes in soil pH. In the second part of my dissertation (Chapter 3), I performed a field-based study to examine how the acid remediation treatment altered soil N cycling. I found that the acid remediation treatment increased both gross N mineralization and N uptake in the leaf litter, causing an accelerated N cycle. This accelerated N cycling resulted in larger inorganic N pools throughout the soil profile. These observed changes in N cycling contrast with earlier studies showing no treatment effect on N cycling rates, and this lag may be coupled to changes in plant community structure. In the last part of my dissertation (Chapter 4), I examined how these changes in soil N cycling resulted in changes to ecosystem N export, using a combination of re-analysis of long term data sets and intensive stream monitoring. I found that, as a result of the Ca treatment, flushing of inorganic N during storms became a substantially more important mechanism of ecosystem N loss. I found evidence that the flushing of N during stormflow was the result of the mobilization of distal, hydrologically disconnected pools of soil N. This suggests that changes in forest floor N cycling, observed in Chapter 3, were responsible for this response. Finally, I found that in-stream uptake of N was significantly enhanced during baseflow conditions in the Ca-enriched watershed, reducing N export during baseflow. This dissertation adds new mechanistic insight into the drivers by which ecosystem recovery from acid precipitation will affect C and N cycling. It demonstrates that, while acid precipitation has abated in the developed world, its legacy effects will remain with us for a long while. The changes to ecosystem C and N cycling studied here are concerning from the perspectives of climate change and loading of nutrients to aquatic ecosystems. This suggests the need for further work that couples vegetation dynamics, changes to geochemical properties of soils, and watershed hydrology.