Carbon and Nitrogen Fixation: A Tropical Marriage: At Kupa’a Farm, a 4 acre organic farm on Maui, we make extensive use of annual and perennial nitrogen fixing plants. In addition to serving a range of direct agricultural purposes (shade for coffee, wind breaks, mulch source, nutrition) these plants also serve to store carbon and hence reduce atmospheric carbon dioxide through biologic sequestration.  Nitrogen fixing trees such as Calliandra, Gliricidia, Inga, and native Acacia (koa) provide shade and protection for orchard crops such as coffee and tropical fruit and also enrich the soil organic matter content through cyclical leaf drop and direct nitrogen fixation in the rhizosphere. When combined with a perennial ground cover such as Arachis pintoi the orchards become vibrant environments for fixing of carbon and nitrogen. Annual cover crops include N-fixers such as sunn hemp as well as grasses (sudan grass, Japanese millet, annual rye, barley and oats) and broad leafs (buckwheat and mustard). Most of these cover crops can be killed by mowing without the need for tillage incorporation leaving rich cover mulch that can be used for direct seeding or planting (for example pumpkins planted into standing dead barley-vetch covers). Although each of these processes does return some of the fixed C back to the atmosphere during biologic degradation the net increase in soil OM and the increased standing biomass serves to decrease atmospheric carbon dioxide.  

Submitted by Gerry Ross, Kupa’a Farm, Maui July 15, 2007