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

We hypothesized that the integration of trees and shrubs in agricultural la ndscapes can reduce NO3- leaching and increase utilization of subsoil N. A field survey was conducted on 14 farms on acid soils in the subhumid highla nds of Kenya, where there is Little use of fertilizers, to determine the ef fect of vegetation types (VT) on soil NH4+ and NO3- to 4 m depth. The VT in cluded maize (Zea mays) with poor growth and good growth. Markhamia lutea t rees 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 subsoi l 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 bene ath weed fallow (0.2-0.7 mg kg(-1)), banana (0.5-1.0 mg kg(-1)) and markham ia (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 si gnificant after maize harvest with 37 kg N ha(-1) m(-1) depth of subsoil be neath good maize and 27 kg N ha(-1) m(-1) depth beneath poor maize. In cont rast, subsoil NO3- was only 2 kg N ha(-1) m(-1) depth beneath woodlot and h edgerow. These results demonstrate that the integration of perennial vegeta tion and the rotation of annual and perennial crops can tighten N cycling i n agricultural landscapes.