The Nitrogen Cycle

Nitrogen comprises approximately 79% of the Earth's atmosphere in the form of biologically unavailable dini-trogen (N2) gas. This reservoir is estimated to be in the order of 3.8 x 109kgN, approximately 90% of the global reservoir. Crustal reservoirs comprise the remaining 10% (Figure 1). By comparison the amount of N stored in the biomass (terrestrial and oceanic) and soil is small, but this, of course, is the vital component as far as living organisms are concerned.

The global nitrogen cycle (Figure 2) is driven by biological and physical processes, which depend on a variety of environmental factors such as solar energy, precipitation, temperature, soil texture, soil moisture, the presence of other nutrients, and atmospheric CO2 concentrations.

These factors control N fluxes into and out of soils and vegetation, thereby influencing the mass of N in these compartments, and therefore its availability. Figure 3 illustrates the global distribution of nitrogen in soil and vegetation. Tropical forest soils show the least amount of storage because of high decomposition rates; but vegetation in temperate and tropical forests have higher N

storage due to the higher production rates. In general terms, human activity has tended to accelerate nitrogen cycling, increasing flux rates from one store to another.

In order for nitrogen to be used for plant growth, it must be available in inorganic formal ammonia (NH3), ammonium (NH4), nitrite, (NO2), or nitrate (NO3). In the terrestrial nitrogen cycle (Figure 4), soil nitrogen cycling processes dominate, with surface application (fertilizer and manure) providing most of the nitrogen inputs. Microbes break down organic matter to produce much of the available nitrogen in soils. Mineralization/ immobilization, nitrification, nitrate leaching, denitrifica-tion, and plant uptake can then occur. Nitrate is completely soluble in water and since it is not adsorbed to clay particles, it is vulnerable to being leached out of the soil by percolating rainfall or irrigation water. Generally, the movement of nitrogen can occur in one of three directions: (1) upward - crop uptake and gaseous loss, (2) downward - as leaching to groundwater, and (3) lateral - via surface and subsurface flow to surface waters.

The nitrogen cycle is strongly influenced by anthropogenic activities. During the twentieth century land-use changes, such as intensive agriculture, over-fertilization, deforestation, biomass burning, combustion of fossil fuels, industrial activities, and energy production, have significantly disturbed 'natural' N biogeochemical cycling. In natural ecosystems plant growth rates are low and annual uptake of N is relatively small. Cultivated crops are much

Figure 3 Global distribution of nitrogen storage (kg m~2) in soil (a) and vegetation (b). Reproduced from Bin-Le Lin, Sakoda A, Shibasaki R, Gato N, and Suzuki M (2007) Modelling a global biochemical nitrogen cycle model in terrestrial ecosystems. Ecological Modelling 135(1): 89-110, with permission from Elsevier.

Figure 3 Global distribution of nitrogen storage (kg m~2) in soil (a) and vegetation (b). Reproduced from Bin-Le Lin, Sakoda A, Shibasaki R, Gato N, and Suzuki M (2007) Modelling a global biochemical nitrogen cycle model in terrestrial ecosystems. Ecological Modelling 135(1): 89-110, with permission from Elsevier.

Farm manures

Biological fixation by bacteria

Nitrite NO

Denitrification by bacteria

Assimilation by plants and microorganisms

Farm manures

Death, decay, and excretion

Biological fixation by bacteria

Assimilation by plants and microorganisms

Nitrite NO

Nitrate NO3

Ammonium NH4

Nitrification by bacteria

Ammonification by bacteria

Nitrate NO3

Ammonium NH4

Nitrification by bacteria

Excess NO3 leached to ground and surface waters

Excess NO3 leached to ground and surface waters

Figure 4 The terrestrial nitrogen cycle.

more demanding with nutrient uptake ranging from about 100kgNhayr~ for wheat and up to 450 kg N ha yr~ for sugar cane. Improved grasslands for livestock rearing typically require 250kgNhayr~. The mineralization capacity of soils is almost always insufficient to maintain optimum growth; therefore, chemical fertilizers and manures are required to supply N for intensive agriculture. This has resulted in changes to the long-term trends within the N cycle at global, regional, and local scales.

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