Spatial distribution of root length density and soil water of linear agroforestry systems in sub-humid Kenya: implications for agroforestry models

Abstract

In simultaneous agroforestry systems trees can compete with crops for water, especially in semi-arid areas. However, in the (sub)humid tropics, on P-fixing Oxisols/Ferralsols small decreases in soil water content caused a decrease in P-transport to roots and therewith a soil-drying induced P-deficiency. The aim of this study was to assess the spatial distribution of soil water content in crop fields bordering tree lines and its relation with root length density distribution of the trees throughout the soil profile. To achieve this, soil water content and tree root length densities throughout the soil profile were measured over a period of 2 years in an experiment with lines of four tree species in the middle of maize fields in sub-humid western Kenya. Soil water content was significantly reduced (2–7 vol.%) near two of the three fast-growing tree species, Eucalyptus grandis and Grevillea robusta, but not near Cedrella serrata and the slower growing Markhamia lutea. These differences were related to differences in water use. Eucalyptus and Grevillea showed high water use and Cedrella and Markhamia low water use. However, soil water content distribution was not related to root length density distribution. Root length densities hardly decreased with distance to Grevillea and clearly decreased with distance to Cedrella. Most water-uptake models, including those of agroforestry models, assume that root length density distribution throughout the profile is proportional to water extraction throughout the profile. The absence of a clear relation between root length density and water extraction near Grevillea tree lines opposed this view. It can be explained by a decrease in water-potential gradient between root and soil at increasing distance from the tree base. If the change in root length density is similar or smaller than the change in water-potential gradient between root and soil, the decrease in water-potential gradient between root and soil is of similar or larger importance for determining tree-water extraction distribution throughout the profile than root length density. Thus, modeling of spatial agroforestry systems cannot assume a direct relation between tree-water extraction and root length density, but needs to include decreasing water-potential gradient between root and soil along roots with increasing distance to the stem base, especially over the horizontal dimension.