Human induced emission of nitrogen trace compounds is equal or in some cases even exceeds contributions of natural sources. The three main anthropogeic source types are (i) direct agricultural emissions, (ii) emissions from livestock, (iii) indirect agricultural emissions. Industrial emission e.g. from wastewater treatment, fossil fuel and biomass combustion also contributes to anthropogenic emission. Nitrogen oxides are emitted to the atmosphere from fossil fuel burning and play important role in ozone chemistry. Emission of nitrous oxide (N2O, Figure 6.11) related to anthropogenic activity mostly due to the enhancement of natural processes, increasing rate of nitrification-denitrification as a result of fertilization.
Nitrogen is essential to synthesize e.g. nucleotids for DNA or RNA and amino acids for proteins, which are basic building blocks of plants. Nitrogen and phosphorous limitations are the most common constraints of soil fertility in agriculture, therefore fertilizers containing nitrogen compounds are widely used in agriculutral management. Agriculture is a main source of nitrogen compounds. Large amounts of ammonia and nitrogen is emitted due to fertilizer application, and from livestock waste (manure and urine).
The nitrogen related to fertilization used to increase plant productivity may lead to increased CO2 uptake but induce emission of N2O also an important GHG. Nitrogen fertilizers are useful to increase crop yield, but nitrogen pollution originated from crop production have detrimental effects on aqueous ecosystems, lead to acidification, eutrophication and GHG emission that contributes to radiative forcing. Agricultural nitrate and phosphate pollution also endangers freshwater quality. This polluting process is considered to be the main stress factor in marine and coastal ecosystems. For a sustainable agriculture farmers need to optimize fertilizer amount and management techniques in order to maximize yield and profit and minimize nitrogen emission and possibly to increase carbon sequestration as well. Nitrogen availability in a certain region depends on different factors: history of agriculture in the area, management techniques applied, atmospheric nitrogen deposition, several soil factors.
The problem of nitrogen limitation and harmful effects of the applied fertilizers show the importance of legumes in agriculture. Although most of the above ground nitrogen is harvested, some of the fixed nitrogen accumulates belowground fertilizing the soil before the next crop. Therefore using legumes in crop rotations is a popular way to improve soil fertility in environmentally conscious management systems not using fertilizers.
The effects of increased nitrogen emission include
decreased atmospheric visibility due to ammonium aerosols (fine particulate matter [PM]);
elevated ozone concentrations;
ozone and PM affects human health (e.g. respiratory diseases, cancer);
increases in radiative forcing and global climate change;
decreased agricultural productivity due to ozone deposition;
ecosystem acidification and eutrophication; and
decreasing ecosystem biodiversity.