Nitrogen overdose

Excessive nitrogen is causing "nitrogen stress" in flora and fauna

Published: Thursday 30 April 1998

researchers have known since the 1960s that nitrogenous fertilisers run-off severely affects lakes and rivers. Recently published studies show a bleaker picture. Polluting waterways of the world and displacing mineral nutrients from forest soils will result in wide-scale destruction of both forest and marine ecosystems.

Nitrogenous compounds such as ammonia and nitrous oxides ( no x ) -- from car and factory emissions -- are the main sources of nitrogen that ends up in lakes, rivers, estuaries and oceans. In water bodies, excessive nitrogen in the form of nitrogen compounds causes nutrient enrichment of the water, which leads to a sudden bloom of algae, especially red and brown algal tides that severely impair fisheries. In the case of lakes, this makes the water toxic.

"The situation is changing incredibly rapidly," says Robert Howarth, biogeochemist at Cornell University, usa . William Schlesinger, biogeochemist at Duke University, has calculated that human activity has produced 60 per cent of the fixed nitrogen deposited on land annually. This is far more than that which can be used productively in crops and other land plants. According to a study being conducted by a team led by Howarth, 20 per cent nitrogenous compounds from human activity finds its way into rivers. This leads to eutrophication in lakes, estuaries and coastal waters (eutrophication is a condition in which excessive nutrients in water cause rapid microbial growth, consuming the dissolved oxygen faster). This affects fish and plants severely.

The terrestrial ecosystem is also blanketed by nitrogen. The forest cover in Germany is decreasing due to no x emitted by fossil fuel combustion, which gets deposited as nitrates in acid rain in the forest soil. This causes the leaching of minerals like calcium, magnesium and potassium, leading to mineral deficiencies in forest trees. These minerals are essential for the normal growth of trees.

Something interesting has also been observed relating to the uptake of nitrogen directly from the air by the trees, rather than from the soil. Microorganisns in the roots of certain trees convert free nitrogen into nitrogenous compounds, which is absorbed by the trees. But a study conducted by Ernst-Detlef Schluze, plant ecologist and director of Max Plank Institute of Biotechnology in Jena, Germany, found that plants are now directly retaining excess nitrogen from the air through leaves and barks, instead of the normal route through the roots, technically termed as "above ground uptake".

Schluze and his colleagues found nitrates along with amino acids in the fluid samples of xylem (a conducting vessel which transports nutrients to various parts of trees) of trees growing in heavy nitrogen pollution areas. However, this is not true for all the tree species. Schluze estimates that 60 per cent of nitrogen found in broad-leaved trees is due to above ground uptake. This leads to rapid growth, but the trees are weak and susceptible to pest attacks.

Nitrates need to be reduced, firstly, in agricultural practices by interplanting crops that fix nitrogen and limit the need of synthetic nitrogenous fertilisers. Secondly, no x emissions from vehicles have to be controlled.

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