Prairie fires deplete nitrogen that legumes then replenish from the air.

Evidence

But the prairie had two strings to its bow. Fires thinned its grasses, but they thickened its stand of leguminous herbs: prairie clover, bush clover, wild bean, vetch, lead-plant, trefoil, and Baptisia, each carrying its own bacteria housed in nodules on its rootlets. Each nodule pumped nitrogen out of the air into the plant, and then ultimately into the soil. Thus the prairie savings bank took in more nitrogen from its legumes than it paid out to its fires. That the prairie is rich is known to the humblest deermouse; why the prairie is rich is a question seldom asked in all the still lapse of ages. — Aldo Leopold, A Sand County Almanac, 371

Categorization

• XREF: connects to: Nutrient atoms move downhill by rain and uphill through animal bodies. — Aldo Leopold, A Sand County Almanac: With Other…, 372; Modern fertility rests on nitrogen-fixing prairie plants farmers can no longer name — Aldo Leopold, A Sand County Almanac: With Other…, 419