Browsing by Subject "Nitrogen"

Rivers interact with their valleys from headwaters to mouth, but nowhere as dynamically as in their floodplains. Rivers deliver water, sediments, and solutes onto the floodplain land surface, and the land in turn supplies solutes, leaves, and woody debris to the channel. These reciprocal exchanges maintain both aquatic and terrestrial biodiversity and productivity. In this dissertation I examine river-floodplain exchanges on the well-studied Nyack Floodplain, a dynamic, gravel-bedded floodplain along the Middle Fork Flathead River in the mountains of northwest Montana. I quantify exchanges at multiple timescales, from moments to centuries, to better understand how connectivity between aquatic and terrestrial habitats shapes their ecology.

I first address connectivity in the context of a long-standing question in ecosystem ecology: What determines the rate of ecosystem development during primary succession? Rivers have an immediate effect on floodplains when scouring floods remove vegetation and nutrients such as nitrogen (N) and leave only barren soils, but they might also affect the ensuing primary succession through the gradual delivery of N and other materials to floodplain soils. I quantify N inputs to successional floodplain forest soils of the Nyack Floodplain and find that sediment deposition by river flood water is the dominant source of N to soils, with lesser contributions from dissolved N in the river, biological N fixation, and atmospheric deposition. I also synthesize published rates of soil N accumulation in floodplain and non-floodplain primary-successional systems around the world, and I find that western floodplains often accumulate soil N faster than non-floodplain primary successional systems. My results collectively point to the importance of riverine N inputs in accelerating ecosystem development during floodplain primary succession.

I next investigate the role of river-floodplain exchanges in shaping the spatial distribution of a suite of soil properties. Even after flood waters have receded, dissolved N, carbon (C), and moisture could be delivered from the river to floodplain soils via belowground water flow. Alternatively, C inputs and N withdrawals by floodplain vegetation might be a dominant influence on soil properties. To test these hypotheses, I excavated and sampled soil pits from the soil surface to the water table (50-270 cm) under forests, meadows, and gravel bars of the Nyack Floodplain. Near-surface soils had C and N pools and N flux rates that varied predictably with vegetation cover, but soil properties below ~50 cm reflected influence by neither vegetation cover nor aquifer delivery. Instead, soil properties at these depths appear to relate to soil texture, which in turn is structured by the river's erosional and depositional activities. This finding suggests the revised hypothesis that soil properties in gravel-bedded alluvial floodplains may depend more on the decadal-scale geomorphic influences of floods than on short-term vertical interactions with floodplain vegetation or aquifer water.

Lastly, I explore the potential sources of organic C to the diverse and active community of aquatic organisms in the floodplain aquifer, where the lack of light prohibits in-situ organic C production by photosynthesis. I quantify floodplain carbon pools and the fluxes of organic carbon connecting the aquifer, river, and overlying forest. Spring flood waters infiltrating the soil are responsible for the largest dissolved carbon flux into the aquifer, while very large floods are essential for the other major C input, the burial of woody carbon in the aquifer. These findings emphasize the importance of a dynamic river hydrograph - in particular, annual floods and extreme annual floods - in delivering organic C to the aquifer community.

Overall, this dissertation draws our attention not just to the current exchanges of C, N, water, and sediment but to the episodic nature of those exchanges. To fully understand floodplain ecosystems, we have to consider not just present-day interactions but also the legacies of past floods and their roles in delivering solutes, eroding forests, depositing sediments, and physically shaping the floodplain environment.

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.

Root carbon exudation is a critical element of the soil carbon cycle, and how both environmental conditions and plant traits influence exudation remains uncertain. I studied relationships between environmental conditions, plant traits, and carbon exudation in four herbaceous plant species: Asclepias incarnata, Microstegium vimineum, Panicum virgatum, and Scirpus cyperinus. Mature individuals were given short-term factorial light and N treatments, and exudates were collected from 8-hour carbon-free hydroponic incubations. I measured size traits (biomass, leaf area, root length, and root volume), photosynthesis (leaf-level and whole-plant), and tissue N traits (root, stem, and leaf percent N and C:N ratio). Neither light nor N treatments affected exudation, while exudation varied with species and traits. Species alone substantially explained mass-specific exudation (estimated R2 = 0.38). Size strongly predicted both total and mass-specific exudation, interacting with species (estimated R2 = 0.52 and 0.48, respectively). Generally, larger individuals exuded more overall but less per unit mass, although larger M. vimineum plants exuded more per unit mass. Whole-plant photosynthetic rate was weakly related to total exudation (estimated R2 = 0.17), and tissue N concentration moderately predicted mass-specific exudation (estimated R2 = 0.23). Other researchers have found that high light and low nitrogen availability stimulate exudation; my results indicate that this relationship is not straightforward. Plant traits, however, significantly explained variation in exudation, including some variation across species, supporting trait-based analyses of plant species' effects on ecosystem processes.

Species invasions are more prevalent than ever before. While the addition of a species can dramatically change critical ecosystem processes, factors that mediate the direction and magnitude of those impacts have received less attention. A better understanding of the factors that mediate invasion impacts on ecosystem functioning is needed in order to target which exotic species will be most harmful and which systems are most vulnerable. The role of invasion on nitrogen (N) cycling is particularly important since N cycling controls ecosystem services that provision human health, e.g. nutrient retention and water quality.

We conducted a meta-analysis and in-depth studies focused on the invasive grass species, Microstegium vimineum, to better understand how (i) plant characteristics, (ii) invader abundance and neighbor identity, and (iii) environmental conditions mediate the impacts of invasion on N pools and fluxes. The results of our global meta-analysis support the concept that invasive species and reference community traits such as leaf %N and leaf C:N are useful for understanding invasion impacts on soil N cycling, but that trait dissimilarities between invaded and reference communities are most informative. Regarding the in-depth studies of Microstegium, we did not find evidence to suggest that invasion increases net nitrification as other studies have shown. Instead, we found that an interaction between its abundance and the neighboring plant identify were important for determining soil nitrate concentrations and net nitrification rates in the greenhouse. In field, we found that variability in environmental conditions mediated the impact of Microstegium invasion on soil N pools and fluxes, primarily net ammonification, between sites through direct, indirect, and interactive pathways. Notably, we detected a scenario in which forest openness has a negative direct effect and indirect positive effect on ammonification in sites with high soil moisture and organic matter. Collectively, our findings suggest that dissimilarity in plant community traits, neighbor identity, and environmental conditions can be important drivers of invasion impacts on ecosystem N cycling and should be considered when evaluating the ecosystem impacts of invasive species across heterogeneous landscapes.

Supercritical water oxidation (SCWO) had been investigated as an advanced technology for the removal of inert and stable organics found in wide range of wastes. Ammonia and nitrous oxide are confirmed in outlet of SCWO system treating municipal sludge. In this study, a mathematical model was established to simulate nitrogen reaction, in order to explore the kinetics of ammonia reaction and reduce the nitrous oxide generation. This developed mathematical model was trained by data from Duke Sanitation Solution group where a pilot-scale supercritical water oxidation facility is invested to treat municipal sludge. The final model was validated by practical data obtained from this facility, and give instruction on SCWO operation.