Jason A. Lynch

Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States

Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge. The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species.

Forest fire and climate change in western North America: insights from sediment charcoal records

Millennial-scale records of forest fire provide important baseline information for ecosystem management, especially in regions with too few recent fires to describe the historical range of variability. Charcoal records from lake sediments and soil profiles are well suited for reconstructing the incidence of past fire and its relationship to changing climate and vegetation. We highlight several records from western North America and their relevance in reconstructing historical forest dynamics, fire-climate relationships, and feedbacks between vegetation and fire under climate change. Climatic effects on fire regimes are evident in many regions, but comparisons of paleo-fire records sometimes show a lack of synchrony, indicating that local factors substantially affect fire occurrence, even over long periods. Furthermore, the specific impacts of vegetation change on fire regimes differ among regions with different vegetation histories. By documenting the effects on fire patterns of major changes in climate and vegetation, paleo-fire records can be used to test the mechanistic models required for the prediction of future variations in fire.

Ecological effects of nitrogen and sulfur air pollution in the US: what do we know?

Four decades after the passage of the US Clean Air Act, air-quality standards are set to protect ecosystems from damage caused by gas-phase nitrogen (N) and sulfur (S) compounds, but not from the deposition of these air pollutants to land and water. Here, we synthesize recent scientific literature on the ecological effects of N and S air pollution in the US. Deposition of N and S is the main driver of ecosystem acidification and contributes to nutrient enrichment in many natural systems. Although surface-water acidification has decreased in the US since 1990, it remains a problem in many regions. Perturbations to ecosystems caused by the nutrient effects of N deposition continue to emerge, although gas-phase concentrations are generally not high enough to cause phytotoxicity. In all, there is overwhelming evidence of a broad range of damaging effects to ecosystems in the US under current air-quality conditions.

Decreased atmospheric sulfur deposition across the southeastern U.S.: When will watersheds release stored sulfate?

Emissions of sulfur dioxide (SO2) to the atmosphere lead to atmospheric deposition of sulfate (SO4(2-)), which is the dominant strong acid anion causing acidification of surface waters and soils in the eastern United States. Since passage of the Clean Air Act and its Amendments, atmospheric deposition of SO2 in this region has declined by over 80%, but few corresponding decreases in streamwater SO4(2-) concentrations have been observed in unglaciated watersheds. We calculated SO4(2-) mass balances for 27 forested, unglaciated watersheds from Pennsylvania to Georgia, by using total atmospheric deposition (wet plus dry) as input. Many of these watersheds still retain SO4(2-), unlike their counterparts in the northeastern U.S. and southern Canada. Our analysis showed that many of these watersheds should convert from retaining to releasing SO4(2-) over the next two decades. The specific years when the watersheds crossover from retaining to releasing SO4(2-) correspond to a general geographical pattern of later net watershed release from north to south. The single most important variable that explained the crossover year was the runoff ratio, defined as the ratio of annual mean stream discharge to precipitation. Percent clay content and mean soil depth were secondary factors in predicting crossover year. The conversion of watersheds from net SO4(2-) retention to release anticipates more widespread reductions in streamwater SO4(2-) concentrations in this region.

LAGOS-NE: a multi-scaled geospatial and temporal database of lake ecological context and water quality for thousands of US lakes

Abstract Understanding the factors that affect water quality and the ecological services provided by freshwater ecosystems is an urgent global environmental issue. Predicting how water quality will respond to global changes not only requires water quality data, but also information about the ecological context of individual water bodies across broad spatial extents. Because lake water quality is usually sampled in limited geographic regions, often for limited time periods, assessing the environmental controls of water quality requires compilation of many data sets across broad regions and across time into an integrated database. LAGOS-NE accomplishes this goal for lakes in the northeastern-most 17 US states. LAGOS-NE contains data for 51 101 lakes and reservoirs larger than 4 ha in 17 lake-rich US states. The database includes 3 data modules for: lake location and physical characteristics for all lakes; ecological context (i.e., the land use, geologic, climatic, and hydrologic setting of lakes) for all lakes; and in situ measurements of lake water quality for a subset of the lakes from the past 3 decades for approximately 2600–12 000 lakes depending on the variable. The database contains approximately 150 000 measures of total phosphorus, 200 000 measures of chlorophyll, and 900 000 measures of Secchi depth. The water quality data were compiled from 87 lake water quality data sets from federal, state, tribal, and non-profit agencies, university researchers, and citizen scientists. This database is one of the largest and most comprehensive databases of its type because it includes both in situ measurements and ecological context data. Because ecological context can be used to study a variety of other questions about lakes, streams, and wetlands, this database can also be used as the foundation for other studies of freshwaters at broad spatial and ecological scales.

Effects and Empirical Critical Loads of Nitrogen for Ecoregions of the United States

Human activity in the last century has led to a significant increase in nitrogen (N) emissions and deposition (Galloway et al. 2004). Total N emissions in the United States have increased significantly since the 1950s (Galloway 1998, Galloway et al. 2003). As S deposition has declined in response to regulation, the rate of N deposition relative to S deposition has increased since the 1980s (Driscoll et al. 2001, 2003) followed by a general decrease in NOx emissions from electric utilities since the early 2000s. More recently, the relative proportion of NHx (NH4+ + NH3 ) to NOx (NO + NO2) emissions has also increased for many areas of the United States (Kelly et al. 2005; Lehmann et al. 2005).

Climate‐driven exceedance of total (wet + dry) nitrogen (N) + sulfur (S) deposition to forest soil over the conterminous U.S

Nitrogen (N) and sulfur (S) deposition are much mitigated over the conterminous US (CONUS) but deposition exceedance still exists on forest soil. In addition, the empirical approach is usually used but only provides a spatially constant critical load (CL). Therefore, the CL derived from steady-state mass balance equation is used to study the CL exceedance on forest soil over the CONUS. The multi-model mean (MMM) of global climate-chemistry models in 2000s indicates that total (wet + dry) N deposition alone over 10.32% of forest soil exceeds the CL, but a higher percent (30.16%) is observed by the N + S deposition, which highlights the necessity of considering S deposition. In 2050s, less CL exceeded forest soil is projected and the exceedance amount is lower as well, mainly attributed to the strong reduction of projected NOX and SO2 emissions. By firstly projecting the future CL due to the climate change, the CL exceedance could further decrease since the air temperature is projected to increase rapidly and lead to higher CL in the future. The CL exceedance by N deposition alone is likely to be dominated by NOy in 2000s but NHX in 2050s because of the enhanced NH3 emission. Moreover, both in 2000s and 2050s, using the CL generated by different aggregation methods can cause up to 33 times difference between the corresponding CL exceedance. This suggests that several regions are under the marginal threat of either N or N + S deposition and different CL can influence the results significantly.

Estimating Base Cation Weathering Rates in the USA: Challenges of Uncertain Soil Mineralogy and Specific Surface Area with Applications of the PROFILE Model

The weathering release rate of base cations (BCw) from soil minerals is fundamentally important for terrestrial ecosystem growth, function, and sensitivity to acid deposition. Understanding BCw is necessary to reduce or prevent damage to acid-sensitive natural systems, in that this information is needed to both evaluate the effectiveness of existing policies, and guide establishment of further policies in the event they are required. Yet BCw is challenging to estimate. In this study, major sources of uncertainty associated with a process-based model (PROFILE) commonly used to estimate weathering rates were quantified in the context of efforts to quantify BCw for upland forest sites across the continental USA. These include uncertainty associated with parameterization of mineral content where horizon data are not available, stoichiometry of individual minerals, and specific surface area of soil and individual soil minerals. Mineral stoichiometry was not an important influence on BCw estimates (uncertainty < 1%). Characterizing B horizon mineralogy by averaging A and C horizons was found to be a minor (< 5%) contributor to uncertainty in some areas, but where mineralogy is known to vary with depth the uncertainty can be large. Estimating mineral-specific surface areas had a strong influence on estimated BCw, with rates increasing by as much as 250%. The greatest uncertainty in BCw estimates, however, was attributed to the particle size class-based method used to estimate the total specific surface area upon which weathering reactions can take place. The resulting uncertainty in BCw spanned multiple orders of magnitude at individual sites, highlighting this as the greatest challenge to ongoing efforts to produce robust BCw estimates across large spatial scales in the USA. Recommendations for improving estimates of BCw to support robust decision making for protection against terrestrial acidification are provided.