A static model of nutrient flow on mixed farms in the highlands of western Kenya to explore the possible impact of improved management

Currently there is much interest in the potential role of agroforestry in the mitigation of nutrientdepletion in sub-Saharan Africa. Using data from farm surveys and trials static models of nutrient flowfor existing farm systems and improved agroforestry systems were constructed. These included boundary plantings of trees, hedgerow intercropping for green manure or fodder, and a well-managed zero-grazing system with moderate fertiliser inputs. The objective was to explore the possible impacton nutrient budgets of improved management options. Major nitrogen (N) losses (70% of total farmloss) occurred in the field and hedgerow compartments, principally through leaching and denitrification, which exceeded 60 kg N/ha per annum in all systems. However, there was uncertainty in predicted net mineralisation and the potential amounts of soil-N losses, and the study indicatedsubstantial potential for N mineralisation in deep subsoils. In contrast, phosphorus (P) was efficiently conserved in the farm system, and moderate additions of inorganic P fertilisers could maintain soil-P stocks. Net soil nutrient balances ranged from –39 to –118 kg N/ha per annum and from –7 to +31 kgP/ha per annum in the different simulated systems. N inputs through biological N fixation and deep N capture were significant in agroforestry systems (up to 122 kg N/ha per annum), but when trees were used for production purposes these additional inputs were offset by increases in consumable N harvested (grain, wood and milk) which ranged from 35 to 195 kg N/ha. Improved manure management reduced soil-N deficits by 70 kg N/ha per annum in a zero-grazing system with a highmanure flux. Research priorities for the humid highland farming systems include the quantification and dynamic modelling of (1) N mineralisation and N dynamics throughout soil profiles, (2) spatial and temporal patterns of N uptake by trees in agroforestry systems, and (3) nutrient budgets inlong-term systems trials

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