Carbon is an essential element. Plant take it from atmosphere in the form of $\mathrm{CO}_2$. It's concentration in atmosphere is about $0.03 \%$. Plants use $\mathrm{CO}_2$ for photosynthesis (as a source of carbon) to synthesises glucose.

Azotobacter is a non-symbotic nitrogen fixing prokaryote. It flourishs in the rice fields.

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.