An agroecosystem scale evaluation of the sustainability implications of carbon tax and carbon credit policies

In 1989, 15 fallow species were planted on a degraded Alfisol in southwestern Nigeria, but only Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and Acacia auriculiformis survived beyond 1993. After clearing different subplots of the fallowed plots in 1993 and 1995, intercropping of cassava + maize was practiced with level (minimum) and mound tillage. Nitrogen fertilizer, between 0 and 60 kg ha(-1), was also applied to subplots of natural regrowth where planted fallows did not survive. Although a maize grain yield of 2.3 t ha(-1) and a cassava root yield of 18.3 t ha(-1) were obtained with level tillage in 1995, grain yield was increased to 2.6 t ha(-1) and cassava yields to about 22 t ha(-1) by mounding and 60 kg N ha(-1) application. Effects of mounding and N application were pronounced in the second and third consecutive years of cropping. Planted tree fallows like S. siamea, A. leptocarpa and L. leucocephala were highly suitable for soil rehabilitation. For sustainable crop production in the second consecutive year of cropping after a 6-year fallow period, mound tillage was recommended while in the third year, mound tillage with 60 kg N ha(-1) was recommended.