Land use affects the distribution of soil inorganic nitrogen in smallholder production systems in Kenya

Abstract

 We hypothesized that the integration of trees and shrubs in agricultural landscapes can reduce NO₃ ^– leaching and increase utilization of subsoil N. A field survey was conducted on 14 farms on acid soils in the subhumid highlands of Kenya, where there is little use of fertilizers, to determine the effect of vegetation types (VT) on soil NH₄ ⁺ and NO₃ ^– to 4 m depth. The VT included maize (Zea mays) with poor growth and good growth, Markhamia lutea trees scattered in maize, natural weed fallow, banana (Musa spp.), hedgerow, and eucalyptus woodlot. The effect of VT on NH₄ ⁺ was small (<1 mg N kg^–1). NO₃ ^– within a VT was about constant with depth below 0.25 m, but subsoil NO₃ ^– varied greatly among VT. Mean NO₃ ^–-N concentrations at 0.5–4 m depth were low beneath hedgerow and woodlot (<0.2 mg kg^–1), intermediate beneath weed fallow (0.2–0.7 mg kg^–1), banana (0.5–1.0 mg kg^–1) and markhamia (0.5–1.6 mg kg^–1), and high beneath both poor (1.0–2.1 mg kg^–1) and good (1.9–3.1 mg kg^–1) maize. Subsoil NO₃ ^– (0.5–4 m) was agronomically significant after maize harvest with 37 kg N ha^–1 m^–1 depth of subsoil beneath good maize and 27 kg N ha^–1 m^–1 depth beneath poor maize. In contrast, subsoil NO₃ ^– was only 2 kg N ha^–1 m^–1 depth beneath woodlot and hedgerow. These results demonstrate that the integration of perennial vegetation and the rotation of annual and perennial crops can tighten N cycling in agricultural landscapes.