Turnover of green manure and effects on bean yield in northern Zambia

During a three-year field experiment conducted at two locations in northern Zambia, the effects of incorporating Crotalaria zanzibarica, Crotalaria juncea, Sesbania sesban and Hyperrhenia grass spp. (Control) on bean yield was studied under the traditional mounding system. Decomposition and N mineralisation dynamics of the residues were determined in a litter bag field study and a laboratory incubation, respectively. N concentration in the green manures ranged from 0.4 % to 1.91 %, it was lowest for Hyperrhenia spp. and highest for C. zanzibarica. Bean yields were highest when C. zanzibarica was used as green manure and lowest in the control. Daily first order decomposition rates in 1995 were 0.043, 0.030 and 0.024 for C. zanzibarica, C, juncea and Hyperrhenia spp., respectively. This ranking corresponded with decreasing initial N concentration of the plant materials. Initial N immobilisation was observed with all residues. Following immobilisation a net release of N occurred after 7 days for C. zanzibarica and C. juncea, and after 21 days for Hyperrhenia spp. Whereas only 25 % of the initial N content of Hyperrhenia spp. was mineralised in five weeks, 60 and 65 %, respectively, was mineralised from C. juncea and C. zanzibarica.