Impacts of vegetative contour hedges on soil inorganic-N cycling and erosional losses in Arable Steep-lands of the Central Highlands of Kenya

Abstract

Moderate to steep landscapes and severe soil, water and nutrient losses characterize over 40% of arable land in the central highlands of Kenya. To study the effectiveness of biological methods in management and enhancement of productivity of these arable steep-lands, we established contour double row hedges of sole Calliandra, Leucaena and napier and combination hedges of either Calliandra or Leucaena with napier. Hedges were established on slopes exceeding 5%, pruned regularly and the resulting biomass cut into fine pieces, which were then incorporated into the plots they served. We then evaluated these plots for inorganic-N changes with depth, soil conservation and soil loss/crop growth relationships. We observed accumulation of inorganic-N in the sub-soil in the control and napier plots but a reduction of sub-soil inorganic-N and its re-accumulation in the top-soil in the leguminous hedge plots after 20 months of trial. The first season on average, registered higher soil losses (P = 0.004) than the second season for treatments with hedges and vice versa for the control. During the first season there were significantly lower (P < 0.001) soil losses in plots with hedges relative to the control on slopes exceeding 10% but with the exception of napier, no significant differences among different types of hedges. We observed higher soil loss reduction in the combination hedge relative to individual tree hedges across the two seasons (P = 0.012). The relationship between cumulative soil loss and any of the four crop growth parameters i.e., grain weight, plant height, stover weight and total above ground biomass was negative, linear and highly significant (P < 0.0001), indicating decreased crop growth with soil loss. We conclude that there are heavy productivity losses as a result of soil erosion in arable steep-lands of the central highlands of Kenya and that well spaced, managed and combined contour hedges of leguminous trees and napier can reduce soil and nutrient losses from steep arable landscapes while simultaneously enhancing soil fertility