The Role of Agroforestry Trees in Intercepting Leached Nitrogen in the Agricultural Systems of the Central Highlands of Kenya

ABSTRACT

Application of N-rich tree biomass to crop fields in agroforestry associations can cause a build-up of mineral-N in the top soil in excess of crop demand during early stages of crop growth. It is therefore important to monitor movement of such mineral-N in the soil, so that management options to minimise its potential loss via leaching can be designed. A randomised complete block experiment to investigate mineral-N movement down a soil profile in an agroforestry system was conducted in an Ultisol in the central highlands of Kenya during the Long and Short Rains of 1998 cropping seasons. The trial consisted of 10 treatments repricated 3 times. Maize was used as the test crop, and was grown alone or in hedge row intercropping with or without fertiliser/prunings applications. The hedge row tree species were Leucaena leucocephala and Calliandra calothyrsus. The results indicated that the bulk of mineral-N in these soils was in the form of nitrate-N with ammoniumnitrate comprising less than 10%. During the first season which had plenty of rain, mineral-N progressively moved (was leached) down the soil profile as the season progressed, accumulating in the deeper soil horizons in stark contrast with the second dry season where mineral-N accumulated in the top soil layers. Treatments that received tree leafy-biomass but had no trees ± fertiliser recorded higher amounts of mineral-N in the 100-to-300 cm depth averaging 15 to 30 mg N/kg. On the other hand, soils in treatments with Leucaena leucocephala and Calliandra calothyrsus tree hedges recorded an average of 1 to 3 mg N/kg in the same depth, indicating that trees are capable of intercepting and recapturing the crop-inaccessible nutrients, below the roots of the annual crops by the action of their deep roots. Indeed, results on the total root length indicated that less than 5% of all the maize roots were located below the 90 cm soil depth while 75% leucaena and 40% calliandra roots were found below the same depth.