Weird and wonderful members of the plant kingdom - necessary nitrogen

There would be no plants, without storms - believe it or not. Plants need nitrogen to grow and the air is about 78% nitrogen, but it's a form that plants can't use. Lightning changes nitrogen in the air into a form plants can use.

Okay, let's unpick the science a bit. The growth of all plants depends on the availability of mineral nutrients, especially nitrogen, which is required in large amounts as an essential component of proteins, nucleic acids and other cellular constituents. There is an abundant supply of nitrogen in the earth's atmosphere - nearly 79% in the form of N2 gas. However, N2 is unavailable for use because there is a triple bond between the two nitrogen atoms, making the molecule almost inert. In order for nitrogen to be used for growth it must be 'fixed' in the form of ammonium or nitrate ions. The weathering of rocks releases these ions so slowly that it has almost no effect on the availability of fixed nitrogen. This means nitrogen is often the limiting factor for growth in environments where there is suitable climate and availability of water to support life.

During a storm, lightning's intense heat forces the nitrogen to combine with oxygen in the air, this forms nitrogen oxides which are soluble in water and fall to earth as one of the components of rain - dilute nitric acid. This acid reacts with minerals in the ground to speed up the degradation of the rocks and the consequent release of ions.

Of course, lightning can do as much harm as good. When a tree is struck by lightning, the liquids inside the trunk and bark turn to gas instantly, leading to high pressure and the explosion of anything lying between the gas and the open air. Usually the lightning current runs just underneath the bark, down to ground, and the tree is scarred by a strip of blown-away bark, which it can survive. More rarely, the current runs down the heart of the trunk, and then there may be little left of the tree afterwards. This is one of the reasons it's a bad idea to shelter under a tree during a storm - the exploding timber will blow out at incredible speed, like wooden bullets. Even worse, lightning may travel radially along the path of the roots below the ground and shock you.

Lightning photograph by pewu, used under a creative commons attribution licence