Rice fields remain logged with excess water which harbour the great microbial activity. Many anaerobic bacteria also grow in these areas and release methane which is a green house gas.

Reductive Amination Ammonia combines with a keto acid (like $\alpha$-ketoglutaric acid or oxaloacetic acid) to form amino acid in presence of a reduced coenzyme (NADH, NADPH) and enzyme dehydrogenase (e.g., glutamate dehydrogenase, aspartate dehydrogenase).

$$\begin{gathered} \alpha \text {-ketoglutaric acid }+\mathrm{NH}_4^{+}+\mathrm{NAD}(\mathrm{P}) \mathrm{H} \xrightarrow[\text { dehydrogenase }]{\text { Glutamate }} \text { Glutamate }+\mathrm{H}_2 \mathrm{O}+\mathrm{NAD}(\mathrm{P}) \\ \text { Oxaloacetic acid }+\mathrm{NH}_4^{+}+\mathrm{NAD}(\mathrm{P}) \mathrm{H} \xrightarrow[\text { dehydrogenase }]{\text { Aspartate }} \text { Asparatate }+\mathrm{H}_2 \mathrm{O}+\mathrm{NAD}(\mathrm{P}) \end{gathered}$$

Manganese $\left(\mathrm{Mn}^{2+}\right)$ becomes toxic when absorbed by plants in higher amounts. The toxicity expressed in form of brown spots surrounded by chlorotic vein.

It is due to the following reasons

(i) Reduction in uptake of $\mathrm{Fe}^{3+}$ and $\mathrm{Mn}^{2+}$.

(ii) Inhibition of binding of $\mathrm{Mn}^{2+}$ to specific enzymes.

(iii) Inhibition of $\mathrm{Ca}^{2+}$ translocation in shoot apex.

Thus, excess of $\mathrm{Mn}^{2+}$ causes deficiency of iron, magnesium and calcium.

Sulphur $(\mathrm{S})$ is an important macronutrient in plants that is absorbed by the plants as $\mathrm{SO}_4^{2-}$ ion. It mainly functions as a component of vitamins (biotin, thiamine), proteins, coenzyme-A, amino acid (cystein and methionine) etc. It is also an essential component of ally sulphide (onion, garlic) and sinigrin (mustard). Deficiency of sulphur can lead to chlorosis in young leaves, extensive root growth, formation of hard and woody stem. It also causes the reduction in juice content of citrus fruit and tea yellow disease of tea. Sulphur is found in amino acids systeine, methionione, etc.

In biological nitrogen fixation, the atmospheric $\mathrm{N}_2$ gets reduced to $\mathrm{NH}_3$ by nitrogenase reductase present in some prokaryotes. $\mathrm{NH}_3$ is then oxidises to $\mathrm{NO}_2$ and $\mathrm{NO}_3$ by some other bacteria (Nitrosomonas and Nitrobacter). Various steps involved in nitrogenfixation are as follows

Pseudomonas and Thiobacillus are involved in the process of denitrification. They convert nitrate $\left(\mathrm{NO}_3^{-}\right)$and nitrite $\left(\mathrm{NO}_2^{-}\right)$into free nitrogen $\left(\mathrm{N}_2\right)$, which is released into the atmosphere.