Living organisms (biosphere)
contain about 2.8 x 1017 g of nitrogen. The nitrogen of living organisms and of the soil is in
a constant state of flux, with some forms of nitrogen being readily transformed in this group and
some forms being inactive over a long time
(91). Transformations are insignificant in the lithosphere
and atmosphere. The amount of interchange of nitrogen among the lithosphere (not including
soil), atmosphere, and living organisms is very small.
The total amount of nitrogen in the soil to the depth of plowing is considerable relative to the
amounts required for crop production, often above 3000 kg/ha but ranging from 1600 kg/ha in sands
through 8100 kg/ha in black clay loams to 39,000 kg/ha in deep peats (Table 2.5) (92). Note that the
nitrogen in the atmosphere above a hectare of land exceeds 100 million kg at sea level. When land is
put for crop production, the nitrogen content of soils declines to a new equilibrium value (90,92). Crop
production that relies on the reserves of nitrogen cannot be effective for long, as the reserves become
exhausted. Most plants cannot tap into the large reserve of nitrogen in the atmosphere, although biological
nitrogen fixation is a means of enhancing the nitrogen content of soils. Biological nitrogen
fixation is the principal means of adding nitrogen to the soil from the atmosphere (89). More than 70%
of the atmospheric nitrogen added or returned to soils is by biological fixation, and can exceed 100 kg
of nitrogen addition per year by nitrogen-fixing legumes. |
Most of this nitrogen enters into the organic
fraction of the soils. Unless nitrogen-fixing legumes are grown, the addition of nitrogen to soils by biological
fixation, averaging about 9.2 kg/ha annually, is too small to support crop production. The
remainder is from atmospheric precipitation of ammonium, nitrate, nitrite, and organically bound
nitrogen (terrestrial dust). The amount of nitrogen precipitated is normally too small to support crop
production but might be of significance in natural landscapes
(90). Virtually no interchange of nitrogen
occurs between rocks and soils.
Organic Nitrogen in Soil
The concentrations of nitrogen range from 0.02% in subsoils to 2.5% in peats
(93). Nitrogen concentrations
in soils generally fall sharply with depth, with most of the nitrogen being in the top onemeter
layer of soils
(89). Surface layers (A-horizon, plow-depth zone) of cultivated soils have
between 0.08 and 0.4% nitrogen. Well over 90%, perhaps over 98%, of the nitrogen in the surface
layers (A-horizon, plow-depth zone) of soil is in organic matter
(93,94). Since most of the nitrogen
in soil is organic, determination of total nitrogen has been a common method of estimating organic
nitrogen. The Kjeldahl method, a wet digestion procedure
(93,95,96), provides a good estimate of
organic, soil nitrogen in surface soils, even though some forms of nitrogen (fixed ammonium,
nitrates, nitrites, some organic forms) are not determined by this analysis. In depths below the
A-horizon or plow zone, although the amounts of total nitrogen are small, inorganic nitrogen, particularly
fixed ammonium, is a high proportion of the total, perhaps 40%, and results from Kjeldahl
analysis should be treated with some caution as this fraction would not be determined
(93). The
Dumas method, a dry digestion procedure, is seldom used for determination of nitrogen in soils but
generally gives results in close agreement with Kjeldahl determinations, if certain precautions are
taken in the analysis
(93).
Soil organic matter is a complex mixture of compounds in various states of decay or stability
(97).
Soil organic matter may be classified into humic and nonhumic fractions, with no sharp demarcation
between the two fractions. The partially decayed or nonhumic portion is the major source of energy
for soil organisms. Depending on the nature of the plant materials, about half of fresh plant residues
added to soil decompose in a few weeks or months
(98,99). Humus, or humic substances, are the
degradation products or residues of microbial action on organic matter and are more stable than the
nonhumic substances. Humus is classified into three fractions, humin, humic acids, and fulvic acids,
based on their solubilities. Humin is the highest molecular weight material and is virtually insoluble
in dilute alkali or in acid. Humic acids are alkali-soluble and acid-insoluble. Fulvic acids are alkalior
acid-soluble. The humic and fulvic fractions are the major portions, perhaps 90%, of the humic soil
organic matter and are the most chemically reactive substances in humus
(100). Humus is slow to
mineralize, and unless present in large quantities may contribute little to plant nitrogen nutrition in
most soils. About 60 to 75% of the mineralized nitrogen may be obtained by a crop
(99). The turnover
rate of nitrogen in humus may be about 1 to 3% of the total nitrogen of the soil, varying with type
of soil, climate, cultivation, and other factors
(93,99). The mineralization rate is likely to be closer to
1% than to 3%. Bremner
(96) and Stanford
(101) discussed several methods to assess availability of
organic nitrogen in soils. Among these procedures were biochemical methods (estimation of microbial
growth, mineral nitrogen formed, or carbon dioxide released) and chemical methods (estimation
of soil total nitrogen, mineral nitrogen, and organic matter and application of various extraction procedures).
The chemical methods are applied more commonly than the biological methods in the estimation
of mineralization. Correlation of crop yields to estimations of mineralization generally have
not been satisfactory in the assessment of the potential for soils to supply nitrogen for crop growth.
Most studies on the fractionation of total soil organic matter have dealt with the hydrolysis of
nitrogenous components with hot acids (3 or 6 M hydrochloric acid for 12 to 24 h) (Table 2.6). The
fraction that is not hydrolyzed is called the acid-insoluble nitrogen. The acid-soluble nitrogen is
fractionated into ammonium, amino acid, amino sugar, and unidentified components. The origins
and composition of each of the named fractions are not clear. The absolute values vary with soil
type and with cultivation
(94). All of these forms of nitrogen, including the acid-stable form, appear
to be biodegradable and, hence, to contribute to plant nutrition
(94,102). Organic matter that is held
to clays is recalcitrant to biodegradation and increases in relative abundance in heavily cropped soils
(94,103,104). This fraction may have little importance in nitrogen nutrition of plants.
Cultivation reduces the total amount of organic matter in soils but has little effect on the relative
distribution of the organic fractions in soils, suggesting that the results of acid hydrolysis are of
little value as soil tests for available nitrogen or for predicting crop yields
(94). Humic substances
contain about the same forms of nitrogen that are obtained from the acid hydrolysis of soils but perhaps
in different distribution patterns
(94). Agricultural systems that depend on soil reserves do not
remain productive without the input of fertilizer nitrogen.
Inorganic Nitrogen in Soil
Soil inorganic nitrogen is commonly less than 2% of the total nitrogen of surface soils and undergoes
rapid changes in composition and quantity. Inorganic nitrogen varies widely among soils, with
climate, and with weather. In humid, temperate zones, soil inorganic nitrogen in surface soil is
expected to be low in winter, to increase in spring and summer, and to decrease with fall rains,
which move the soluble nitrogen into the depths of the soil
(105). Despite being small in magnitude,
the inorganic fraction is the source of nitrogen nutrition for plants. Unless supplied by fertilizers,
inorganic nitrogen in soil is derived from the soil organic matter, which serves as a reserve of
nitrogen for plant nutrition. Plant-available nitrogen is released from organic matter by mineralization
and is transformed back into organic matter (microbial cells) by immobilization. Absorption by
plants is the chief means of removal of inorganic nitrogen from soils, although nitrate leaching and
denitrification, ammonium volatilization and fixation, and nitrogen immobilization lead to losses of
inorganic nitrogen from soils or from the soil solution
(105).
Detectable inorganic nitrogen forms in soil are nitrate, nitrite, exchangeable and fixed ammonium,
nitrogen (N
2) gas, and nitrous oxide (N
2O gas)
(106). Nitrate and exchangeable ammonium
are important in plant nutrition. The other forms are generally not available for plant nutrition. Fixed
ammonium, entrapped in clays, is a principal nitrogenous constituent of subsoils and is probably
derived from parent rock materials; however, the fixed ammonium in surface soils may be of recent
origin from organic matter
(106). Fixed ammonium is resistant to removal from clay lattices and
has little importance in plant nutrition. The gaseous constituents diffuse from the atmosphere or
arise from denitrification and have no role in plant nutrition, other than in considerations of losses
of nitrogen from soils
(107).
Exchangeable or dissolved ammonium is available to plants, but ammonium concentrations in
soils are low, usually in a magnitude of a few mg/kg or kg/ha. In well-aerated soils, ammonium is
oxidized rapidly to nitrate by nitrification, so that nitrate is the major source of plant-available nitrogen
in soil
(108,109). Nitrite, an intermediate in nitrification, is oxidized more rapidly than ammonium
(109). Hence, little ammonium or nitrite accumulates in most soils. Ammonium and nitrite are
toxic to most plants
(110). Toxicity of ammonium or nitrite might occur if the concentration of
either rises above 50 mg N/kg in soil or in other media, especially if either is the principal source
of nitrogen for plant nutrition
(110,111). Nitrification is sensitive to soil acidity and is likely to be
inhibited in soils under pH 5; this acidity may lead to ammonium accumulation
(108).