66
S. Ambreetha et al.
2 Biogeochemistry of N in Paddy Soil
Nitrogen being associated to ecosystem stability and primary productivity, the knowledge on transformation mediated by microbes is significant in N cycling [44]. Especially the rate of N deposition on the terrestrial ecosystem is expected to project by
a factor of 2.5 in forth coming century [117]. They in turn put forth various consequences including change in microbial community structure, activities and ultimately
altering ecological functions. In paddy ecosystem, it serves as an important element
during early vegetative and panicle initiation stage and regarded as limiting nutrient
for productivity [92]. They are unique from other nutrients, since their role in formation of chlorophyll, proteins and nucleic acids is most significant. The N input for the
crop includes fertilizers, source of irrigation, precipitation and biological N 2 fixation,
while the output includes crop removal, volatilization, leaching/run off, denitrification. Hence their management is a mandate to procure high yield and knowledge on
available N from soil organic matter and added fertilizer would assist in N management technology to ensure sufficient food grain production [10]. Intensive paddy
cultivation recruits high N application which goes nearly 500 kg ha
−1 per crop [14].
Nitrogen transformation under flooded condition influences microbial structure and
function, turnover rate, availability and losses of N which differs prominently from
drained or aerated soil. N cycling in paddy soil is affected mainly by depletion of
oxygen and includes ammonification, N immobilization, nitrification, denitrification,
anaerobic ammonium oxidation (anammox) and nitrogen fixation [42]. However,
these processes were dug in detail, the significant shifts on N cycling communities
and activities stands on the fore to be explored.
2.1 N Cycling
Nitrogen is the crucial nutrient for all live forms, however most organisms bank on
bioavailable forms [57]. The network of nitrogen transforming microbes strongly
persists in paddy ecosystem and had been affected by input of intensive N fertilizers.
It is noteworthy to explore versatility of microbes involved in N transformation and
the biochemical pathway.
2.1.1 Ammonification
Organic nitrogen supplied to the soil is mineralized (ammonification / N mineralization) or broken down by anaerobic decomposition aided by heterotrophic bacteria and
fungi [11]. A key process which determines the availability of N and wetland productivity occurs in oxidized surface layer, reduced soil and also in rhizosphere, where
ammonia volatilization is followed in flooded water [50]. The rate of ammonification
is lower because of deprived O 2 status under anoxic zones. Savant and De Datta [82]
S. Ambreetha et al.
2 Biogeochemistry of N in Paddy Soil
Nitrogen being associated to ecosystem stability and primary productivity, the knowledge on transformation mediated by microbes is significant in N cycling [44]. Especially the rate of N deposition on the terrestrial ecosystem is expected to project by
a factor of 2.5 in forth coming century [117]. They in turn put forth various consequences including change in microbial community structure, activities and ultimately
altering ecological functions. In paddy ecosystem, it serves as an important element
during early vegetative and panicle initiation stage and regarded as limiting nutrient
for productivity [92]. They are unique from other nutrients, since their role in formation of chlorophyll, proteins and nucleic acids is most significant. The N input for the
crop includes fertilizers, source of irrigation, precipitation and biological N 2 fixation,
while the output includes crop removal, volatilization, leaching/run off, denitrification. Hence their management is a mandate to procure high yield and knowledge on
available N from soil organic matter and added fertilizer would assist in N management technology to ensure sufficient food grain production [10]. Intensive paddy
cultivation recruits high N application which goes nearly 500 kg ha
−1 per crop [14].
Nitrogen transformation under flooded condition influences microbial structure and
function, turnover rate, availability and losses of N which differs prominently from
drained or aerated soil. N cycling in paddy soil is affected mainly by depletion of
oxygen and includes ammonification, N immobilization, nitrification, denitrification,
anaerobic ammonium oxidation (anammox) and nitrogen fixation [42]. However,
these processes were dug in detail, the significant shifts on N cycling communities
and activities stands on the fore to be explored.
2.1 N Cycling
Nitrogen is the crucial nutrient for all live forms, however most organisms bank on
bioavailable forms [57]. The network of nitrogen transforming microbes strongly
persists in paddy ecosystem and had been affected by input of intensive N fertilizers.
It is noteworthy to explore versatility of microbes involved in N transformation and
the biochemical pathway.
2.1.1 Ammonification
Organic nitrogen supplied to the soil is mineralized (ammonification / N mineralization) or broken down by anaerobic decomposition aided by heterotrophic bacteria and
fungi [11]. A key process which determines the availability of N and wetland productivity occurs in oxidized surface layer, reduced soil and also in rhizosphere, where
ammonia volatilization is followed in flooded water [50]. The rate of ammonification
is lower because of deprived O 2 status under anoxic zones. Savant and De Datta [82]