Environmental Engineering Reference
In-Depth Information
13.3
Field experience of land application and associated
environmental impacts
13.3.1 Background on nitrogen transformations in soils
Plants require 13 mineral nutrient elements for growth. Each of these
nutrients is critical for plants to complete their life cycle. Macronutrients
(nitrogen, phosphorus, potassium, calcium, magnesium and sulphur) are
required by plants in the largest amounts, while micronutrients (iron,
copper, manganese, zinc, boron, molybdenum and chlorine) are required in
relatively smaller amounts. Nitrogen is one of the most important nutrients
for plant growth, as N is an essential component of chlorophyll molecules,
amino acids and enzymes (Olson and Kurtz, 1982). Plants can uptake N in
the inorganic forms of NH
4
+
-N and NO
3
-N. The organic nitrogen in
animal manures and plant residues must be degraded in the soil prior to
becoming available for plant growth. Proteins are converted by bacteria to
NH
4
+
-N through mineralization, also referred to as ammonification, and
other soil microorganisms oxidize NH
4
+
-N to NO
3
-N via nitrification.
Finally, to complete the nitrogen cycle, anaerobic soil bacteria reduce
NO
3
-N to N
2
and N
2
O gases through a process known as denitrification.
Nitrogen mineralization is a function of the C:N ratio. When C:N ratios
are low (less than 20:1) microorganisms will rapidly mineralize organic N
and release inorganic forms, whereas higher C:N ratios result in a slow
mineralization process (Jansson and Persson, 1982; Bengtsson et al., 2003;
Khalil et al., 2005). During the anaerobic digestion process, organic matter is
broken down to convert C to CH
4
and CO
2
, resulting in digested substrates
having low C:N ratios. Digestate samples were taken at five on-farm
digesters in Ontario, Canada, over a period of six months, and the C:N ratios
ranged between 5.6 and 7.8:1. The low C:N ratio of digestate would suggest
nitrogen mineralization can be enhanced when soil fertility is amended with
digested materials, thereby increasing the N availability for crop uptake.
The transformation of N in soils is largely influenced by soil temperature
and soil moisture. Nitrification is essentially inactive at temperatures less
than 4
C (Schmidt, 1982). A study conducted by Stark (1996) found
maximum nitrification rates to occur at temperatures between 30 and 35
8
8
C,
while Grundmann et al.
(1995)
found optimal nitrification rates at
temperatures between 20 and 25
C. Schmidt (1982) reported that optimum
temperatures for nitrification vary among soils: 20-25
8
8
C for soils in
northwestern USA and 30-40
C for soils in southwestern USA.
Nitrification in soils that are wet beyond their field capacity will be
substantially reduced, as nitrifying bacteria are sensitive to moisture stress
that reduces adequate soil aeration (Schmidt, 1982; Fierer and Schimel,
2002). Maag and Vinther (1996) investigated the effects of temperature on
8
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