Agriculture Reference
In-Depth Information
2.8.2 t
emperature
Temperature on soil surface influences N losses by volatilization, leaching, and denitrification.
When N is top dressed in a crop during growth cycle and temperature is higher (>13°C), a high rate
of N loss through NH
3
volatilization from soil-applied N fertilizer such as urea is expected (Jantalia
et al., 2012). Similarly, nitrification processes are also high under high temperatures (>13°C) and
when there is heavy rainfall for a prolonged time, more N is expected to be lost under these climatic
conditions. Maximum nitrification in soils occurs at 30-35°C (Black, 1968; Resek et al., 1971).
Mahendrappa et al. (1966) reported that attempts to agree on optimum conditions have largely been
unsuccessful, since nitrifying bacteria found in a particular soil are the result of natural selection
and adjustment to the climate. These authors found that soils from the northern states of the United
States nitrified best at a temperature of 20°C and 25°C, while soils from the southern states nitrified
best at 35°C.
Like nitrification, denitrification is also affected by temperature. Stanford et al. (1975) reported
that denitrification rate minimal at 0-5°C increased 10-fold between 5°C and 10°C. Between 15°C
and 35°C, the temperature coefficient of denitrification (Q
10
) was approximately 2. In field studies
with different tillage and cropping systems, Aulakh et al. (1983,1984) observed little influence of
temperature on denitrification rate during the crop-growing season when temperatures ranged from
10°C to 30°C. However, in early spring and late fall when temperatures were 5°C or below, virtually,
no denitrification was detectable even in wet soils with high NO
3
content. Temperature variations
may change the proportion of N oxide gases produced during denitrification. In a temperature range
of 15-30°C, the major product of denitrification in wet- or water-logged soils is N
2
. Under similar
conditions at a temperature of 4-8°C, N
2
O and NO often predominate (Bailey, 1976; Aulkh et al.,
1992).
The effect of temperature on the N losses process is influenced by influencing soil microbial commu-
nity. N losses are reported to be higher under higher temperatures as compared to lower temperatures.
Hence, soil microbial community related to both nitrification and denitrification is more sensitive to
higher temperatures. Elevated temperatures can directly alter the soil microbial community functions
by affecting the temperature-sensitive microbial enzyme activity (Von Lutzow and Kogel-Knabner,
2009; Gray et al., 2011). Elevated temperatures can also influence soil microbial abundance and com-
position by altering net primary production and therefore the pool of available substrates utilized for
microbial growth (Gray et al., 2011). Elevated temperatures also increase the evaporative flux of water
from the soil and therefore indirectly affect microorganisms through soil drying (Pendall et al., 2004;
Filella et al., 2004). The effects of elevated temperatures on the total microbial biomass are variable
(Pendall et al., 2004), and groups of microorganisms differ in their responses to elevated temperature
(Gray et al., 2011). In a recent study, Engel et al. (2011) reported that the mean daily temperature of
-2°C to 5°C did not provide protection against realizing large NH
3
emissions.
2.8.3 s
oIl
p
h
Soil pH is an important factor in determining N losses from soil-plant system. Most of the NH
3
volatilization from applied N sources occurs at alkaline soil pH (>7.0). Many studies conducted in
the United States and elsewhere have shown that heavy losses of N through ammonia gas occurred
on heavily limed or calcareous soils (Larsen and Gunary, 1962; Terman and Hunt, 1964). Pesek
et al. (1971) reviewed the literature on ammonia volatilization from various countries, including
the Netherlands. These authors concluded that results from the Netherlands obtained with 176 soil
samples from all over the world showed that the volatilization of ammonia is correlated better with
CaCO
3
content than with pH of the soil.
Generally, denitrification decreases in acid soils and increases with increasing soil pH.
Klemedtsson et al. (1977) and Muller et al. (1980) found a direct positive relationship between the
