70
S. Ambreetha et al.
2.1.6 Nitrogen Fixation
The biological conversion of atmospheric nitrogen in air to ammonia in soil is accomplished by archaea and bacteria with the help of nitrogenase encoded genes. This
reduction process in flooded ecosystem is supported by low oxidation and reduction
potentials and also by the organic matter. Biological nitrogen fixation (BNF) plays
pivotal role in flooded paddy system in maintaining the long term soil productivity
[58]. Diversity of diazotrophs in the rhizosphere is enhanced by lower NH
4+ –N
concentration. The process plays pivotal role in supplying N to the plant, where
no chemical fertilizers are used [76]. Since nitrogen fixing photosynthetic bacteria
demands anaerobic condition, reduced region supports the process than the oxidized
zone. The rate of nitrogen fixation ranges between 0.03 and 46.2 g N m
−2 y
−1 within
the redox range from −200 to −260 mV [10]. The complex and dynamic ecosystem
of paddy encompasses variety of N 2 fixers including rice associative and free living
heterotrophs, free living and symbiotic photosynthetic cyanobacteria. Metagenome
analysis of rhizosphere and phyllosphere evidenced the transcripts of nifH, nifD
and nifK. Diazotrophic community in paddy is unstable since various environmental
factors and fertilizer application influences their function [91].
2.2 Fate of N in Paddy Ecosystem
N fertilizer applied to the soil is either partly absorbed by the plant or fixed by the
soil, while the rest gets lost in different routes [5]. For producing one ton of rough
rice, the crop removes 16–17 kg of N. The amount of N added to the soil and the
quantity used by the plant is mostly unbalanced and interrupted by number of factors.
High quantity of N applied into the soil, is lost and favored by layers of the soil for
aerobic and anaerobic N metabolism. In the study of N leaching effect, [65] have
hypothesized that more loss of NO 3
− than NH 4
+ , which is due to NH 4
+ adsorbance
into the soil particles. Studies on
15 N urea for the detection of recovery and residual
rate accounted only 16–25%, while non-specified N percentage rate was higher [71].
These results suggest that continuous flooding intensifies N loss in various ways.
Besides N loss transformation process, there occurs several temporal variability as a
function of several management practices mainly irrigation and crop residues [18].
3 Microbiome of Paddy
Microbiome refers to the totality of microbial communities present in a particular
habitat. The microbial communities remain diversified within a habitat based on
various external factors including human intervention. Nevertheless, certain microbiome will be universally predominant among all the microbial assemblages of a
particular ecosystem. The microbial community which flourish in a specific habitat
S. Ambreetha et al.
2.1.6 Nitrogen Fixation
The biological conversion of atmospheric nitrogen in air to ammonia in soil is accomplished by archaea and bacteria with the help of nitrogenase encoded genes. This
reduction process in flooded ecosystem is supported by low oxidation and reduction
potentials and also by the organic matter. Biological nitrogen fixation (BNF) plays
pivotal role in flooded paddy system in maintaining the long term soil productivity
[58]. Diversity of diazotrophs in the rhizosphere is enhanced by lower NH
4+ –N
concentration. The process plays pivotal role in supplying N to the plant, where
no chemical fertilizers are used [76]. Since nitrogen fixing photosynthetic bacteria
demands anaerobic condition, reduced region supports the process than the oxidized
zone. The rate of nitrogen fixation ranges between 0.03 and 46.2 g N m
−2 y
−1 within
the redox range from −200 to −260 mV [10]. The complex and dynamic ecosystem
of paddy encompasses variety of N 2 fixers including rice associative and free living
heterotrophs, free living and symbiotic photosynthetic cyanobacteria. Metagenome
analysis of rhizosphere and phyllosphere evidenced the transcripts of nifH, nifD
and nifK. Diazotrophic community in paddy is unstable since various environmental
factors and fertilizer application influences their function [91].
2.2 Fate of N in Paddy Ecosystem
N fertilizer applied to the soil is either partly absorbed by the plant or fixed by the
soil, while the rest gets lost in different routes [5]. For producing one ton of rough
rice, the crop removes 16–17 kg of N. The amount of N added to the soil and the
quantity used by the plant is mostly unbalanced and interrupted by number of factors.
High quantity of N applied into the soil, is lost and favored by layers of the soil for
aerobic and anaerobic N metabolism. In the study of N leaching effect, [65] have
hypothesized that more loss of NO 3
− than NH 4
+ , which is due to NH 4
+ adsorbance
into the soil particles. Studies on
15 N urea for the detection of recovery and residual
rate accounted only 16–25%, while non-specified N percentage rate was higher [71].
These results suggest that continuous flooding intensifies N loss in various ways.
Besides N loss transformation process, there occurs several temporal variability as a
function of several management practices mainly irrigation and crop residues [18].
3 Microbiome of Paddy
Microbiome refers to the totality of microbial communities present in a particular
habitat. The microbial communities remain diversified within a habitat based on
various external factors including human intervention. Nevertheless, certain microbiome will be universally predominant among all the microbial assemblages of a
particular ecosystem. The microbial community which flourish in a specific habitat