<h4>Nitrogen Fixation by Lightning</h4><p><strong>Lightning bolts</strong> possess immense energy, sufficient to break the strong triple bond of <strong>atmospheric nitrogen (N2)</strong>. This process allows nitrogen to combine with oxygen in the atmosphere.</p><div class="info-box"><strong>Products:</strong> Nitrogen oxides (<strong>NO</strong> and <strong>NO2</strong>)<br><strong>Mechanism:</strong> High energy from lightning breaks <strong>N2 bond</strong>, enabling reaction with <strong>O2</strong>.</div><p>These newly formed <strong>nitrogen oxides</strong> then mix with water vapour present in the atmosphere. This interaction leads to the formation of <strong>nitric acid</strong> and <strong>nitrous acid</strong>.</p><p>When these acids fall to the Earth's surface as <strong>acid rain</strong>, they deliver <strong>reactive nitrogen</strong> directly to the soil. This form of nitrogen is readily available for plant uptake and utilization.</p><h4>Biological Nitrogen Fixation</h4><p>A crucial natural process involves certain types of <strong>bacteria</strong> that can convert atmospheric nitrogen into a usable form for plants. This process is known as <strong>biological nitrogen fixation</strong>.</p><div class="key-point-box"><strong>Key Microbes:</strong> Bacteria like <strong>Azotobacter</strong> (free-living) and <strong>Rhizobia</strong> (symbiotic).<br><strong>Process:</strong> Convert inert <strong>atmospheric nitrogen (N2)</strong> into <strong>reactive nitrogen</strong> compounds (e.g., ammonia).</div><p>These bacteria often form specialized <strong>symbiotic relationships</strong> with plants. A prime example is <strong>Rhizobia</strong> bacteria associating with <strong>legumes</strong> (e.g., peas, beans, clover).</p><p>Another important symbiotic relationship is seen with <strong>aquatic ferns</strong> like <strong>Azolla</strong>, which hosts nitrogen-fixing cyanobacteria. These relationships significantly enhance <strong>nitrogen availability</strong> in the soil.</p><p>The ability of these microorganisms to fix nitrogen makes them incredibly valuable for <strong>agriculture</strong>. They naturally enrich soil fertility, reducing the need for synthetic nitrogen fertilizers.</p><div class="exam-tip-box">Understand the distinction between atmospheric (abiotic) and biological (biotic) nitrogen fixation. Both contribute significantly to the global nitrogen cycle, a recurring theme in <strong>UPSC GS Paper III (Agriculture & Environment)</strong>.</div>