THE CONTRIBUTION OF BIOLOGICAL NITROGEN-FIXATION FOR SUSTAINABLE AGRICULTURAL SYSTEMS IN THE TROPICS

The pressing need to increase food production in the tropics to Feed t he burgeoning population of the Third World requires that crop yields ha(-1) must be increased without prejudicing the resource base for fut ure generations. Biological nitrogen fixation (BNF), especially that a ssociated with legumes, has great potential to contribute to productiv e and sustainable agricultural systems for the tropics, but more resea rch is required to investigate how biologically fixed N, and the incre ased BhTF contributions resulting from research innovations, can be in corporated into viable agricultural systems to increase crop or pastur e yields and to substitute N fertiliser inputs. A majority of the soil s of the humid and semi-humid tropics have mineral fractions composed of 1:1 lattice clays or sesquioxides of relatively low capacity to ret ain nutrients (CEC) and water (WHC). It is the soil organic matter (SO M) which has high CEC (after liming) and WHC, and soils under undistur bed climax vegetation are usually high in organic matter which is resp onsible for their fertility. The key to the long term fertility of suc h soils is to maintain their soil organic matter by the preservation o f crop residues and the selection of suitable crop rotations or fallow s. In this review we examine several types of agricultural systems uti lised in the tropics ranging from pastures, ley cropping, zero-till ro tations as well as green manuring and discuss the management options t hat can be adopted to preserve their agricultural productivity through the strategic use of legumes in these systems, and their effects on p asture and crop yields. The introduction of forage legumes into tropic al pastures can increase and sustain their productivity, with only mod est inputs of lime and P and K fertilisers. Similarly, crop and pastur e rotations (ley cropping) maintain SOM and soil fertility and crop yi elds can benefit greatly from the introduction of pasture legumes into the ley. Continuous cropping under minimum or zero tillage can mainta in soil cover, and stimulate the retention of SOM, such that nutrient losses are often minimal, and legume derived N can be efficiently tran sferred to subsequent crops. The options for the resource-poor small-h older to efficiently utilise biologically fixed N as a N supply for ce real grains are more limited and need more attention from researchers as well as less neglect from government organisations. The addition of lime and P fertiliser in modest quantities in many under-developed re gions could make large contributions to increased crop yields. If such modest fertiliser inputs were to be combined with suitable crop rotat ions including green manure or grain legume crops, larger increases in crops yields could be achieved on a sustainable basis, but in many re gions agricultural extension services are non-existent and poor farmer s have little access to even these basic chemical inputs. (C) 1997 Els evier Science Ltd.