Long-Term Effects of Cattle Ranching on Soil Nitrogen and Phosphorus Balances in a Savanna Ecosystem

Abstract

We used ranch records and soil analyses to investigate the effects of cattle ranching on phosphorus (P) and nitrogen (N) balances in a humid, tallgrass savanna ecosystem in Tanzania. Over a 41-yr period between 1958 and 1999, the ranch supported an average of 10 435 cattle. These consumed an estimated 571 586 tons (t) of dry matter containing 692 t P and 6 230 t N. Of these nutrients, 162 t of P (23%) and 602 t of N (9.7%) were exported in animals leaving the ranch, while 222 t (32 %) P and 2 364 t of N (38 %) were transferred as excreta to the night corrals. The measured excesses of nutrients in the soil of the corrals were equivalent to 59% of all P and 19% of all N deposited in these areas over 41 yr. Total losses from the pastures amounted to 10.2% of P and 6.6% of N in the top 20 cm of tall grass savanna soil. These losses, especially of P, probably reduced the nutritional quality of the pastures and may have contributed to the reported decline in animal fertility. In addition, they may have promoted the spread of secondary woodland dominated by Vachellia (formerly Acacia) zanzibarica. Three general conclusions can be drawn from this study. First, humid tallgrass savannas on nutrient-poor soils are unsuitable for intensive livestock production. Second, over an extended period the loss of nutrients from cattle pastures can be ecologically significant. Ensuring the sustainability of grazing systems requires measures to counteract this loss, such as the use of shifting night corrals. Third, ranch records, while lacking the precision and detail possible in experimental studies, can provide valuable insights into long-term effects of ranching that would be difficult to obtain by other means.