![]() ![]() Anthropogenic N is emitted to the atmosphere (via fossil fuel combustion), directly added to the soil surface (via fertilizer application), and indirectly added to the soil (via biological N fixation from crops such as soybean and alfalfa). However, this has changed dramatically, with human activities estimated to have doubled annual inputs of Nr in the biosphere (Howarth 2008 Fowler et al. The quantity of Nr available to organisms is small under natural conditions, making it a limiting nutrient in most systems (Vitousek and Howarth 1991). Simplified Model of the N Cycle in Aquatic Systems The complexity of N cycling processes makes tracking Nr from its associated sources to its effects difficult. Many of these nitrogen reactions are carried out by microbes and require varying aerobic/anaerobic conditions. The breakdown of organic molecules containing nitrogen (and other nutrients) to inorganic forms via decomposition is referred to as mineralization. Alternatively, NH 4, NO 2 or NO 3 may be converted into gaseous N forms and returned to the atmosphere through denitrification. Nr is transferred among organisms and ultimately excreted or decomposed into NH 4, which can be taken up into the food web again or converted, by nitrification, into nitrite/nitrate, which can also be taken back up into the food web. Because of its chemical characteristics, NO 3 is not as tightly retained in soils as NH 4 and is readily transported to receiving waterbodies (Schlesinger 1997). NH 4, NO 3, and organic N molecules also can enter aquatic ecosystems through atmospheric deposition and runoff from terrestrial systems, including from human sources. N 2, which constitutes approximately 78% of air, enters aquatic and terrestrial ecosystems through N fixation and becomes fixed or reactive N (Nr). N exists as inorganic molecules (e.g., nitrate, nitrite, and ammonium) and organic molecules (e.g., amino acids, urea, and nucleic acids). Figure Nut-1 depicts a simplified model of the N cycle in aquatic systems, including dinitrogen gas (N 2), nitrous oxide (N 2O), NO 3, nitrite (NO 2), and NH 4. Nitrogen: N has a complex biogeochemical cycle that is a function, in part, of the multiple oxidation states of inorganic N compounds, from nitrate (NO 3, the most oxidized form) to ammonium (NH 4, the most reduced form) (Schlesinger 1997). The effects of climate change on nutrient concentrations and loads in waterbodies will interact with these and other non-climatic drivers. Non-point sources include agricultural and residential/urban fertilizers, onsite wastewater disposal systems, organic wastes accumulated or applied diffusely to the land and atmospheric deposition. Common point sources of nutrient pollution include domestic and industrial sewage effluent urban stormwater, combined sewer overflow (CSO) events, and sanitary sewer overflow (SSO) events and concentrated animal feeding operations. Human activities, including land use change and water management infrastructure, also directly affect nutrient availability and cycling. Subsequent nutrient transport to waterbodies is largely driven by precipitation and other factors affecting runoff (Gordon et al. Climate change can affect nutrient availability and cycling through changes in temperature, humidity, and soil moisture. The availability, transport and biogeochemical cycling of nutrients into and within waterbodies are dependent on interactions between climate, hydrology, terrestrial and aquatic ecosystems and human activities (see box with "Related Links" ).
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