Synthesis

Human activity in the last century has led to a substantial increase in nitrogen (N) emissions and 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 level of pollution that would be harmful to ecosystems is the critical loads approach. The critical load is dei ned as the level of a pollutant below which no detrimental ecological effect occurs over the long term according to present knowledge. The objective of this project was to synthesize current research relating atmospheric N deposition to effects on terrestrial and aquatic ecosystems in the United States and to identify empirical critical loads for atmospheric N deposition. The receptors that we evaluated included freshwater diatoms, mycorrhizal fungi and other soil microbes, lichens, herbaceous plants, shrubs, and trees. The main responses reported fell into two categories: (1) biogeochemical, and (2) individual species, population, and community responses. This report synthesizes current research relating atmospheric nitrogen (N) deposition to effects on terrestrial and aquatic ecosystems in the United States and to identify empirical critical loads for atmospheric N deposition. The report evaluates the following receptors: freshwater diatoms, mycorrhizal fungi and other soil microbes, lichens, herbaceous plants, shrubs, and trees. The main responses reported fell into two categories: (1) biogeochemical; and (2) individual species, population, and community responses. The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1 to 39 kg N ha-1 y-1. This range spans the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees

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

Eco-hydrology and physiological water relations of vegetation along coastal dune ecotones on subtropical islands

As evidence mounts that sea levels are rising, it becomes increasingly important to understand the role of ocean water in the terrestrial hydrology of coastal ecosystems. In coastal dunes, ocean water may enter soils via salt spray through the surface or by ocean water intrusion into deeper vadose layers. However, it is unclear if ocean water enters terrestrial soil of dunes and if it affects dune vegetation. The purpose of this study is to investigate the influence of ocean water on soil and vegetation of coastal dunes. Three coastal dune systems, a barrier island off the coast of southern Florida, and two islands in the Bahamian bank/platform system were investigated. Using delta 18O as a water tracer, I showed the variance of delta18O values of vegetation stem water that is closest to the ocean indicates a mixed water harvesting strategy in which plants utilize ocean, ground water and rain. In contrast, the inland vegetation relies mostly on rain.Salinity, moisture content and stable isotope date show that the vadose (surface to the water table) soil hydrology of coastal dunes is spatially and temporally dynamic. Evidence supports that ocean water enters the soil profile from above by salt spray deposition and from below by ocean water intrusion, processes that are most strongly evident in the driest months. Fluctuating periods of high soil salinity and ocean water deposition characterize dune areas closest to the ocean. Ocean water deposition across the dune drives plant function of species that are zonally distributed. Fore dune species uptake ocean water and show elevated water-use efficiency during the dry season. The few trans-dune species that are able to grow across dunes offer a special opportunity to investigate the eco-physiology of the same species at two extremes of the dune ecotone. The comparative physiology of trans-dune species Ipomoea pes-caprae (vine) and Coccoloba uvifera (shrub) in fore dune (5--12m inland) and back dune (45--55m inland) areas show that these species have different mechanisms to cope with osmotically stressful environments

An empirical method of measuring CO 2 recycling by isotopic enrichment of respired CO 2

An empirical method to measure respiratory CO 2 recycling using a fast growing agricultural cover crop as a model system was tested and compared with a theoretical method which uses a variation of the Keeling plot. Both methods gave values which were high and similar to each other. The theoretical method gave a value of respiratory based CO 2 recycling of 0.41, while the empirical method gave a value of 0.49. Therefore close to half of the respired CO 2 is refixed during daytime photosynthesis in this densely planted cover crop. Refixation of respired CO 2 during the day should lead to an isotopic enrichment of the remaining respired CO 2 leaving the canopy of the cover crop. Therefore, calculations of gross respiration and photosynthesis using isotopic mass balance equations that do not take this isotopic fractionation into account could be in error. We tested this premise by using isotopic mass balance equations to estimate average gross photosynthesis and respiration in this cover crop under two scenarios: (1) no recycling and (2) recycling of respired CO 2. Values of gross photosynthesis and respiration were unrealistically low when it was assumed that no recycling occurs. On the other hand, realistic values similar to previous publications were observed when recycling was taken into account

How inhibiting nitrification affects nitrogen cycle and reduces environmental impacts of anthropogenic nitrogen input

Anthropogenic activities, and in particular the use of synthetic nitrogen (N) fertilizer, have doubled global annual reactive N inputs in the past 50-100years, causing deleterious effects on the environment through increased N leaching and nitrous oxide (N2O) and ammonia (NH3) emissions. Leaching and gaseous losses of N are greatly controlled by the net rate of microbial nitrification. Extensive experiments have been conducted to develop ways to inhibit this process through use of nitrification inhibitors (NI) in combination with fertilizers. Yet, no study has comprehensively assessed how inhibiting nitrification affects both hydrologic and gaseous losses of N and plant nitrogen use efficiency. We synthesized the results of 62 NI field studies and evaluated how NI application altered N cycle and ecosystem services in N-enriched systems. Our results showed that inhibiting nitrification by NI application increased NH3 emission (mean: 20%, 95% confidential interval: 33-67%), but reduced dissolved inorganic N leaching (-48%, -56% to -38%), N2O emission (-44%, -48% to -39%) and NO emission (-24%, -38% to -8%). This amounted to a net reduction of 16.5% in the total N release to the environment. Inhibiting nitrification also increased plant N recovery (58%, 34-93%) and productivity of grain (9%, 6-13%), straw (15%, 12-18%), vegetable (5%, 0-10%) and pasture hay (14%, 8-20%). The cost and benefit analysis showed that the economic benefit of reducing N's environmental impacts offsets the cost of NI application. Applying NI along with N fertilizer could bring additional revenues of $163ha(-1)yr(-1) for a maize farm, equivalent to 8.95% increase in revenues. Our findings showed that NIs could create a win-win scenario that reduces the negative impact of N leaching and greenhouse gas production, while increases the agricultural output. However, NI's potential negative impacts, such as increase in NH3 emission and the risk of NI contamination, should be fully considered before large-scale application