Leucaena hedgerow intercropping and cattle manure application in the Ethiopian highlands - I. Decomposition and nutrient release

A litter bag technique was used to study the decomposition and release of N , P, K, Ca, and Mg from Leucaena leucocephala and L. pallida prunings and c attle manure in a hedgerow intercropping trial conducted in the Ethiopian h ighlands. Hedgerow intercropping (also called alley cropping or alley farmi ng) is an agroforestry system in which trees are grown in dense hedges betw een alleys where short-cycle crops are grown. The hedges are pruned periodi cally during the cropping period and the prunings are added to the soil as green manure. Manure was the most resistant to decomposition, losing only 1 5% of its dry matter (DM) in 15 weeks, compared to 41-57% lost by Leucaena prunings. Large quantities of K (up to 104 kg ha(-1)) were mineralized from prunings and manure, but Ca and Mg were mostly immobilized. More N and P w ere released From prunings than from manure, which resulted in net immobili zation of these nutrients in the initial stages of decomposition and net mi neralization in later stages. Between the leucaenas more N was mineralized and less Ca and Mg were immobilized when L. leucocephala prunings were appl ied than when L. pallida prunings were applied. Fertilizer N increased DM d ecomposition and N mineralization. Mineralization of the nutrients was cons trained by lignin and polyphenol contents. It is concluded that leucaena mu lch and cattle manure may be significant sources of N and K for crop growth , but external sources of P, Ca and Mg may be required, particularly in aci d soils which have low contents of these nutrients. However, this fertility effect has to be evaluated against the competition effect of trees to pred ict crop response.