Organic inputs from agroforestry trees on-farms for improving soil quality and crop productivity in Ethiopia

Indigenous agroforestry trees and shrubs on farmlands are an under-utilized resource that can be used as green manures (GM) to alleviate declining soil fertility and productivity. The aims of studies described in this thesis were to investigate: (1) the seasonal dynamics in chemical contents of foliage from selected tree species potentially available as sources of GM, (2) rates and patterns of mineralization of nutrients during decomposition of leaves, (3) nutrient limitations for microbial processes in Mollic andosols from farm and forest land-uses using microbial respiration kinetics, (4) the utility of microbial respiration kinetics for characterizing the quality of plant materials, and (5) the effect of combined GM and fertilizer inputs on crop yield and soil properties. The studies used two leguminous (Albizia gummifera G.F. Gmel and Milletia ferruginea (Hochst.) Baker) and two nonleguminous (Cordia africana Lam. and Croton macrostachyus Del.) species. Foliar contents of N, soluble polyphenols (PL) and condensed tannin (CT) were higher during the wet season while those of K and lignin were higher during the dry season. Green leaves had higher N and P contents while senesced ones had higher contents of K, cellulose, PL and CT. On average, 27% of N and 48% of P was resorbed during senescence. In a decomposition study that used leaves of Albizia and Cordia, Albizia had higher mass loss, N loss and release of CT than Cordia. However, N was immobilized for the first four weeks in both species. High rates of CT loss seemed to have facilitated decomposition in Albizia despite high initial contents of PL and CT in the leaves. Nitrogen was more limiting than P for the soil micro-biota suggesting the presence of adequate amounts of endogenous soil P. Addition of excess P resulted in respiration pattern with two peaks, suggesting different N pools being available over time. The substrate-induced respiration, specific microbial growth rate and microbially available P were higher for the farm; while the % total added C respired (49-69%) was higher for the forest land-use indicating increased C costs to utilize strongly bound nutrients. Under both laboratory and field conditions, 11%-44% and 10%42% of initial leaf C was mineralised within a month, respectively, with the species ranking as: Cordia < Albizia < Milletia < Croton. Supplemental N and P had little effect on C mineralization. Green manure (GM) from the four species caused an increase in maize yield by 10-84%, with the increase being higher for GM from Croton and Cordia which apparently had higher P and K contents in their foliage than Albizia and Milletia. Measurable improvements in pH and soil organic C due to GM inputs were also observed at the end of the short–term (2 y) field trial.