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Nitrogen in West Africa: The Regional Cycle

Robertson, G. Philip, Rosswall, Thomas
Ecological monographs 1986 v.56 no.1 pp. 43-72
crop rotation, crops, denitrification, fallow, fertilizers, fossil fuels, fuel combustion, grasslands, humans, hydrology, immobilization in soil, imports, legumes, models, nitrogen, nitrogen cycle, nitrogen fixation, perennials, rain forests, regrowth, rubber, savannas, sediments, soil, total nitrogen, vegetation cover, volatilization, wetlands, woodlands, Atlantic Ocean, Western Africa
We have calculated a nitrogen cycle budget for West Africa south of the northern Sahara that quantifies biologically important pools of nitrogen in the region and major fluxes associated with these pools. Major compartments of our model include noncultivated systems broken down by vegetation zone, successional status, and plant components; annual and perennial crops each broken down by crop species and plant and harvest components; wetlands; anthropic systems; and soils and sediments. Base reference year is 1978. Biological nitrogen fixation and precipitation fluxes dominated nitrogen inputs to West Africa in 1978. Approximately 12 × 10⁹ kg were fixed in noncultivated systems; about half of this was fixed in early successional (0—6 yr) rain forests, and much of the rest in grazed or fallow savanna grasslands and woodlands. Less than 0.7 × 10⁹ kg were fixed in cultivated systems; legumes accounted for ≈25% of this. Total anthropic sources (fossil fuel combustion, fertilizer production, and agricultural commodity imports) were minor (<0.3 × 10⁹ kg). Precipitation inputs to the region were ≈0.4 × 10⁹ kg. Most nitrogen leaving West Africa did so volatilized by fire (≈8.3 × 10⁹ kg), principally in non—cropped systems. Major losses also occurred via hydrologic export to the Atlantic Ocean (1.5 × 10⁹ kg) and via denitrification (1.1 × 10⁹ kg). Total N losses from West Africa exceeded 11.0 × 10⁹ kg. The nitrogen immobilized in growing woody vegetation appeared to have amounted to ≈5.6 × 10⁹ kg. Most was immobilized in forested regions by vegetation regrowth during the fallow phase of the bush fallow crop rotation cycle, the dominant cropping system in West Africa. A very small amount was immobilized by perennial crops such as cacao and rubber. Net immobilization in soil was not estimated; total soil N was assumed to be in steady state for the region as a whole. Our overall budget balances within 1%, despite independent calculations of all major fluxes. The balance portrays a nitrogen cycle dominated by pools and fluxes in noncropped systems. In contrast to N balances in developed regions, direct anthropic fluxes are a minor part of the West Africa cycle. Indirect human influences, however, mainly through effects on vegetation cover, appear to be a major determinant of both the rates at which nitrogen is cycled in West Africa and the relative importance of most pools and fluxes.