Vertical root distribution in single-crop and intercropping agricultural systems in Central Kenya

Intercropping is an important and widespread land-management system in the tropics. At two agricultural sites in Central Kenya differing in elevation and soil type Haplic Nitisols (eutric) and Vitric Gleysols (eutric, epiclayic, endoclayic), we investigated the vertical root distributions using the trench wall profile method in single-crop systems of maize (Zea mays L.) and in intercropping systems of maize and legumes (common bean, Phaseolus vulgaris L.; pigeon pea, Cajanus cajan [L.] Millsp.) to test for possible differences in the use of water and nutrient resources. The physico-chemical soil properties of the sites were similar and imposed no restrictions to the vertical growth of the roots into soil depths of 1.4 m. The vertical distributions of the fine roots (empty set 0.5-2 mm) and very fine roots (empty set < 0.5 mm) were quantified by calculating the parameter beta which was computed from the cumulative fraction (Y) of the root densities along the depth (d) of the soil profiles (Y = 1 - beta(d)). We found no consistent differences between the single-crop and the intercropping systems in the rooting depth down to 1.4 m. However, higher beta values for fine roots of the intercropping systems were indicative of a more homogeneous vertical root distribution than in the single-crop fields. In the intercropping fields, 50% of the total number of fine roots were distributed over the uppermost 36 cm of the soil, whereas in the single-crop fields, 50% of the fine roots were concentrated in the uppermost 15-21 cm. Medium-sized roots (empty set > 2-5 mm) were detected in the intercropping fields only. The more homogeneous root distribution in the intercropping fields likely indicates a more efficient use of the limited resources nutrients and water.