# Column : Increase of Nitrogen Loads Caused by Human Activities

The amount of nitrogen taken from the atmosphere and fixed through the ecosystem mechanisms is supposed to be almost equal to the amount of gaseous nitrate–nitrogen and returned to the atmosphere.

However, as a result of human activities, such as mass production of chemical fertilizers, crops and combustion of fuels, a large amount of fixed nitrogen has been accumulated and remains in ecosystems today. It is estimated that such human-produced accumulation of fixed nitrogen in ecosystems is now almost equal to the amount of naturally fixed nitrogen in terrestrial ecosystems and will further increase in the future.

According to the Millennium Ecosystem Assessment, one of the five major direct drivers of change in ecosystems and of deterioration of biodiversity is “pollution,” and nitrogen is one of the principal substances causing pollution. Accumulated nitrogen in the natural environment flows into soils, ground waters, rivers and, eventually, into the ocean while it transforms itself. In this process, it causes eutrophication in lakes, reservoirs and sea areas, creates oxygen-deficient bottom water and induces nitric-acid pollution in ground water. The Millennium Ecosystem Assessment points out that such excessive accumulation of nitrogen in ecosystems could have serious, adverse impact on biodiversity.

## (2) Nitrogen cycle

It’s not only carbon that is cycled through ecosystems. Nitrogen is an essential element for the formation of protein needed by humans and other life. Although inorganic nitrogen constitutes as much as about 78% of the Earth’s atmosphere, it cannot be of immediate use for most life; it needs to be converted into forms usable by living organisms through the process of fixation, where microorganisms, etc. combine inorganic nitrogen with hydrogen or carbon. Fixed nitrogen can be taken into plants in the form of either ammonium ions or nitrate ions, and will be transformed into amino acid or protein that will be consumed by animals. This is the only way that allows animals to take in necessary nitrogen for their nutrition. Carcasses of plants or animals are consumed by other animals or decomposed by microorganisms. Then, denitrifying microorganisms will return inorganic nitrogen to the atmosphere (Figure 2-1-3).

Figure 2-1-4: Human-produced reactive nitrogen

Source: Millennium Ecosystem Assessment

Figure 2-1-3: Nitrogen cycle

Source: EPA (U.S. Environmental Protection Agency)

## (3) Water cycle

Water is an indispensable substance for the survival of all life. To our knowledge, the Earth is the only solar planet that enjoys the presence of water, and this has much to do with the birth and existence of the Earth.

The Earth has as much as 1.386 trillion tons of water, most of which (about 97%) belongs to the oceans. Fresh water constitutes only about 3%, a large part of which takes the form of glaciers, snow or groundwater. Thus, surface-running waters such as lakes, reservoirs and rivers constitute only a small fraction of the Earth’s water, but they play a central role in the lives of most terrestrial life, including human beings.

Water comes in diverse forms, including glaciers, snow, lakes, rivers, ground water and atmospheric moisture (solid, liquid or