Nitrogen is a key element controlling the species composition, diversity, dynamics, and functioning of many terrestrial, freshwater, and marine ecosystems. Many of the original plant species living in these ecosystems are adapted to, and function optimally in, soils and solutions with low levels of available nitrogen. The growth and dynamics of herbivore populations, and ultimately those of their predators, also are affected by N. Agriculture, combustion of fossil fuels, and other human activities have altered the global cycle of N substantially, generally increasing both the availability and the mobility of N over large regions of Earth. The mobility of N means that while most deliberate applications of N occur locally, their influence spreads regionally and even globally. Moreover, many of the mobile forms of N themselves have environmental consequences. Although most nitrogen inputs serve human needs such as agricultural production, their environmental conse- quences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: 1) approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; 2) increased concentrations of the potent greenhouse gas N 2O globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; 3) caused losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility; 4) contributed substantially to the acidification of soils, streams, and lakes in several regions; and 5) greatly increased the transfer of nitrogen through rivers to estuaries and coastal oceans. In addition, based on our review of available scientific evidence we are confident that human alterations of the nitrogen cycle have: 6) increased the quantity of organic carbon stored within terrestrial ecosystems; 7) accelerated losses of biological diversity, especially losses of plants adapted to efficient use of nitrogen, and losses of the animals and microorganisms that depend on them; and 8) caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries.

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Human alteration of the global nitrogen cycle: sources and consequences

This book is written as a reference on organic substances in natural waters and as a supplementary text for graduate students in water chemistry. The chapters address five topics: amount, origin, nature, geochemistry, and characterization of organic carbon. Of these topics, the main themes are the amount and nature of dissolved organic carbon in natural waters (mainly fresh water, although seawater is briefly discussed). It is hoped that the reader is familiar with organic chemistry, but it is not necessary. The first part of the book is a general overview of the amount and general nature of dissolved organic carbon. Over the past 10 years there has been an exponential increase in knowledge on organic substances in water, which is the result of money directed toward the research of organic compounds, of new methods of analysis (such as gas chromatography and mass spectrometry), and most importantly, the result of more people working in this field. Because of this exponential increase in knowledge, there is a need to pull together and summarize the data that has accumulated from many disciplines over the last decade.

Organic geochemistry of natural waters

[1] We use the Total Ozone Mapping Spectrometer (TOMS) sensor on the Nimbus 7 satellite to map the global distribution of major atmospheric dust sources with the goal of identifying common environmental characteristics The largest and most persistent sources are located in the Northern Hemisphere, mainly in a broad “dust belt” that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China There is remarkably little large-scale dust activity outside this region In particular, the Southern Hemisphere is devoid of major dust activity Dust sources, regardless of size or strength, can usually be associated with topographical lows located in arid regions with annual rainfall under 200–250 mm Although the source regions themselves are arid or hyperarid, the action of water is evident from the presence of ephemeral streams, rivers, lakes, and playas Most major sources have been intermittently flooded through the Quaternary as evidenced by deep alluvial deposits Many sources are associated with areas where human impacts are well documented, eg, the Caspian and Aral Seas, Tigris-Euphrates River Basin, southwestern North America, and the loess lands in China Nonetheless, the largest and most active sources are located in truly remote areas where there is little or no human activity Thus, on a global scale, dust mobilization appears to be dominated by natural sources Dust activity is extremely sensitive to many environmental parameters The identification of major sources will enable us to focus on critical regions and to characterize emission rates in response to environmental conditions With such knowledge we will be better able to improve global dust models and to assess the effects of climate change on emissions in the future It will also facilitate the interpretation of the paleoclimate record based on dust contained in ocean sediments and ice cores

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2000RG000095

Environmental characterization of global sources of atmospheric soil dust identified with the nimbus 7 total ozone mapping spectrometer (toms) absorbing aerosol product

The environmental conditions of Earth, including the climate, are determined by physical, chemical, biological, and human interactions that transform and transport materials and energy. This is the "Earth system": a highly complex entity characterized by multiple nonlinear responses and thresholds, with linkages between disparate components. One important part of this system is the iron cycle, in which iron-containing soil dust is transported from land through the atmosphere to the oceans, affecting ocean biogeochemistry and hence having feedback effects on climate and dust production. Here we review the key components of this cycle, identifying critical uncertainties and priorities for future research.

/pdf/global-iron-connections-between-desert-dust-ocean-43atskudjz.pdf

Global Iron Connections Between Desert Dust, Ocean Biogeochemistry, and Climate

The use of fire as a tool to manipulate the environment has been instrumental in the human conquest of Earth, the first evidence of the use of fires by early hominids dating back to 1–1.5 million years ago [1]. Even today, most human-ignited vegetation fires take place on the African continent, and its widespread, frequently burned savannas bear ample witness to this. Although natural fires can occur even in tropical forest regions [2, 3], the extent of fires has greatly expanded on all continents with the arrival of Homo sapiens. Measurements of charcoal in dated sediment cores have shown clear correlations between the rate of burning and human settlement [4]. Pollen records show a shift with human settlement from pyrophobic vegetation to pyrotolerant and pyrophilic species, testimony to the large ecological impact of human-induced fires.

https://science.sciencemag.org/content/sci/250/4988/1669.full.pdf

Biomass Burning in the Tropics: Impact on Atmospheric Chemistry and Biogeochemical Cycles

Over the past decade it has become apparent that the atmosphere is a significant pathway for the transport of many natural and pollutant materials from the continents to the ocean. The atmospheric input of many of these species can have an impact (either positive or negative) on biological processes in the sea and on marine chemical cycling. For example, there is now evidence that the atmosphere may be an important transport path for such essential nutrients as iron and nitrogen in some regions. In this report we assess current data in this area, develop global scale estimates of the atmospheric fluxes of trace elements, mineral aerosol, nitrogen species, and synthetic organic compounds to the ocean; and compare the atmospheric input rates of these substances to their input via rivers. Trace elements considered were Pb, Cd, Zn, Cu, Ni, As, Hg, Sn, Al, Fe, Si, and P. Oxidized and reduced forms of nitrogen were considered, including nitrate and ammonium ions and the gaseous species NO, NO2, HNO3, and NH3. Synthetic organic compounds considered included polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), DDTs, chlordane, dieldrin, and hexachlorobenzenes (HCBs). Making this assessment was difficult because there are very few actual measurements of deposition rates of these substances to the ocean. However, there are considerably more data on the atmospheric concentrations of these species in aerosol and gaseous form. Mean concentration data for 10° × 10° ocean areas were determined from the available concentration data or from extrapolation of these data into other regions. These concentration distributions were then combined with appropriate exchange coefficients and precipitation fields to obtain the global wet and dry deposition fluxes. Careful consideration was given to atmospheric transport processes as well as to removal mechanisms and the physical and physicochemical properties of aerosols and gases. Only annual values were calculated. On a global scale atmospheric inputs are generally equal to or greater than riverine inputs, and for most species atmospheric input to the ocean is significantly greater in the northern hemisphere than in the southern hemisphere. For dissolved trace metals in seawater, global atmospheric input dominates riverine input for Pb, Cd, and Zn, and the two transport paths are roughly equal for Cu, Ni, As, and Fe. Fluxes and basin-wide deposition of trace metals are generally a factor of 5-10 higher in the North Atlantic and North Pacific regions than in the South Atlantic and South Pacific. Global input of oxidized and reduced nitrogen species are roughly equal to each other, although the major fraction of oxidized nitrogen enters the ocean in the northern hemisphere, primarily as a result of pollution sources. Reduced nitrogen species are much more uniformly distributed, suggesting that the ocean itself may be a significant source. The global atmospheric input of such synthetic organic species as HCH,PCBs, DDT, and HCB completely dominates their input via rivers.

The atmospheric input of trace species to the world ocean

The Global Precipitation Chemistry Project collects precipitation by event to determine composition and processes controlling it in five remote areas. Compositions (excluding sea-salt) at St. Georges, Bermuda, were primarily controlled by anthropogenic processes; composition and acidities at San Carlos, Venezuela, Katherine, Australia, Poker Flat, Alaska, and Amsterdam Island were controlled by unknown mixtures of natural or anthropogenic processes. Precipitation was acidic; average volume-weighted pH values were 4.8 for Bermuda; 5.0, Alaska; 4.9, Amsterdam Island; 4.8, Australia; 4.8, Venezuela. Acidities at Bermuda and Alaska were from long-range transport of sulfate aerosol; at Venezuela, Australia, and Amsterdam Island, from mixtures of weak organic and strong mineral acids, primarily H/sub 2/SO/sub 4/. Relative proportions of weak to strong acids were largest at Venezuela and lowest at Amsterdam Island. Weak and strong acids were from mixtures of natural and anthropogenic processes. Once contributions from human activities were removed, the lower limit of natural contributions was probably greater than or equal to pH 5.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JC087iC11p08771%4010.1002/%28ISSN%292169-9291.ROAC1

The composition of precipitation in remote areas of the world

Wet-only rain samples were collected and chemically analyzed during 1980–1984 at Katherine, Australia, as part of the Global Precipitation Chemistry Project. Rainfall averaged 104 cm/yr and ranged from 74.6 cm in 1982–1983 to 135.7 cm in 1983–1984. Total cation concentration was about 29 μeq/L. The volume-weighted mean H+ concentration expressed as pH was 4.73, 64% of the free acidity was due to organic acids (formic and acetic). The pH due to strong mineral acids only was 5.08. Air mass trajectories during the wet season were predominantly from the northeast (51%), and this trajectory was most important in terms of deposition. Although there is a high frequency of lightning during the wet season, no evidence was found that it contributed significantly to the nitrate concentration in rain. Because concentrations decreased markedly with amount of precipitation, the total deposition of dissolved substances was not correlated directly with the amount of rain. Comparisons were made between the chemistry of rain at Katherine and with other remote areas and with the eastern United States.

/pdf/chemistry-of-precipitation-from-a-remote-terrestrial-site-in-4da6cl6nxq.pdf

Chemistry of precipitation from a remote, terrestrial site in Australia

A five-year climatology of back trajectories from the Mauna Loa Observatory, Hawaii

From May 1978 through February 1979, 334 samples of rain were collected on a twice-weekly basis in the summit area of Kilauea Volcano, Hawaii. Throughout the period the volcano was in a noneruptive state. The samples were analyzed for pH and conductivity as well as for the concentration of the anions F−, Cl−, NO2−, PO4−3, Br−, NO3−, and SO4=. At a distance of 0.8 km downwind of the degassing center, rainwater pH was observed to be as low as 3.0 and to have a median pH of 3.6. The maximum observed concentration of SO4= was 36 mg/l. The median pH 11 km downwind was 4.2, which approached the pH measured in precipitation on the island uninfluenced by the volcanic emissions.

John M. Miller

Papers

The atmospheric input of trace species to the world ocean

The composition of precipitation in remote areas of the world

Chemistry of precipitation from a remote, terrestrial site in Australia

A five-year climatology of back trajectories from the Mauna Loa Observatory, Hawaii

The influence on rain chemistry of the Hawaiian Volcano Kilauea