User:Congluo819/sandbox

Content
Overall, this Wikipedia page explained the nitrogen cycle very thoroughly, starting with different reactions/ processes involved in this cycle, elaborating the marine nitrogen cycles and ending with how human activities affect this cycle. However, the sub-titles of each section can be confusing if looking at the table of content. For example, section 2 was named as marine nitrogen cycle while section 3 was named as human influences on the nitrogen cycles with environmental impact as its subtitle. It would be more logic to name section as the human-influenced inland nitrogen cycle which is a parallel and similar grammar structure to section 2.

Tone
The tone of this page is relatively neutral but needs improvements in the flow between sentences.

Source
Citation 15 does not qualify as the most appropriate source since the link is a web page, describing the content from an NPR podcast. peer-reviewed journals can be a better choice. Since citation 16 and 17 are cited along with 15, both of which are from the very prestigious journal “ Nature”. So it is recommended to remove citation 15.

Talk Page
Several comments in the talk page are not titled and some questions are still needed to be clarified in the talk page.

Lead Section
The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition.

The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmosphere nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle. Human modification of global nitrogen cycle can negatively affect the natural environment system and also human health.

Impacts on Natural System
Increasing levels of nitrogen deposition have a number of negative effects on both terrestrial and aquatic ecosystems. Nitrogen gases and aerosols can be directly toxic to certain plant species, affecting the aboveground physiology and growth of plants near large point sources of nitrogen pollution. Changes to plant species interactions may also occur, as accumulation of nitrogen compounds increase its availability in a given ecosystem, eventually changing the plant species composition, plant diversity, and nitrogen cycling. Ammonia and ammonium - two reduced forms of nitrogen - can be detrimental over time due to an increased toxicity toward sensitive species of plants, particularly those that are accustomed to using nitrate as their source of nitrogen, causing poor development of their roots and shoots. Increased nitrogen deposition also leads to soil acidification, which increases base cation leaching in the soil and amounts of aluminum and other potentially toxic metals, along with decreasing nitrification and increasing plant-derived litter. Due to the ongoing changes caused by high nitrogen deposition, an environment's susceptibility to ecological stress and disturbance - such as pests and pathogens - may increase, thus making it less resilient to situations that otherwise would have little impact to its long-term vitality.

Additional risks posed by increased availability of inorganic nitrogen in aquatic ecosystems include water acidification; eutrophication of fresh and saltwater systems; and toxicity issues for animals, including humans. Eutrophication often leads to lower dissolved oxygen levels in the water column, including hypoxic and anoxic conditions, which can cause death of aquatic fauna. Relatively sessile benthos, or bottom-dwelling creatures, are particularly vulnerable because of their lack of mobility, though large fish kills are not uncommon. Oceanic dead zones near the mouth of the Mississippi in the Gulf of Mexico are a well-known example of algal bloom-induced hypoxia. The New York Adirondack Lakes, Catskills, Hudson Highlands, Rensselaer Plateau and parts of Long Island display the impact of nitric acid rain deposition, resulting in the killing of fish and many other aquatic species.

Ammonia (NH3) is highly toxic to fish and the level of ammonia discharged from wastewater treatment facilities must be closely monitored. To prevent fish deaths, nitrification via aeration prior to discharge is often desirable. Land application can be an attractive alternative to the aeration.

Impacts on Human Health: Nitrate Accumulation in Drinking Water
Leakage of Nr (reactive nitrogen) from human activities can cause nitrate accumulation in the natural water environment, which can create harmful impacts on human health. Excessive use of N-fertilizer in agriculture has been one of the major sources of nitrate pollution in groundwater and surface water. Due to its high solubility and low retention by soil, nitrate can easily escape from the subsoil layer to the groundwater, causing nitrate pollution. Some other non-point sources for nitrate pollution in groundwater are originated from livestock feeding, animal and human contamination and municipal and industrial waste. Since groundwater often serves as the primary domestic water supply, nitrate pollution can be extended from groundwater to surface and drinking water in the process of potable water production, especially for small community water supplies, where poorly regulated and unsanitary waters are used.

The WHO standard for drinking water is 50 mg NO3- L-1for short-term exposure, and for 3 mg NO3- L-1chronic effects. Once it enters human body, nitrate can react with organic compounds through nitrosation reactions in the stomach to form nitrosamines and nitrosamides, which are involved in some types of cancers (e.g., oral cance r and gastric cancer).

Impacts on Human Health: Air Quality
Human activities have also dramatically altered the global nitrogen cycle via production of nitrogenous gases, associated with the global atmospheric nitrogen pollution. There are multiple sources of atmospheric Nr fluxes. Agricultural sources of Nr can produce atmospheric emission of ammonia (NH3), nitrogen oxides (NOx) and nitrous oxide (N2O). Combustion processes in energy production, transportation and industry can also result in the formation of new Nr via the emission of NOx, an unintentional waste product. When those Nr are released to the lower atmosphere, they can induce the formation of smog, particular matter (PM) and aerosols, all of which are major contributors to adverse health effects on human health from air pollution. In the atmosphere, NO2 can be oxidized to nitric acid (HNO3), and it can further react with NH3 to form ammonium nitrate, which facilitates the formation of particular nitrate. Moreover, NH3 can react with other acid gases ( sulfuric and hydrochloric acids) to form ammonium-containing particles, which are the precursors for the secondary organic aerosol particles in photochemical smog.

Disclaimer: the whole section is the revision of original " environmental impacts" section. I did the include its original text and renamed the title by putting under the " Impacts on natural system". I also provide two new paragraphs related on impacts on human health. The last sentence in the lead section was added to indicate its impact on natural environment as well as human health.