Lake Naivasha is a freshwater lake in the East African Rift Valley. With continued lake level declines between 1980 and 2011, the newly exposed land areas were gradually taken for agricultural use. The resulting chronosequences allow for an analysis of the effects of land use duration on nutrient dynamics and agricultural production. Transects representing land use durations of 0-30 (cropland) and 15-30 years (pasture) were established on soils formed on alluvial deposits and lacustrine sediments. We assessed changes in topsoil nitrogen (N) stocks (t ha(-1)), ammonium mineralization potential (N-supplying capacity), and plant-available P with increasing durations of land use. An additional greenhouse experiment studied the responses of kikuyu grass (Cenchrus clandestinus) and maize (Zea mays) in potted topsoil collected from differnt land-use types and chronosequence positions. With increasing duration of land use we noted a significant decline (P < 5%) in soil N contents under both pasture and cropland uses, following a model of exponential decay. The N stocks decreased at 84kgha(-1) a(-1) and a decay rate constant of 0.019a(-1) in pasture soil within 15 years, and at 75kgha(-1) a(-1) with a decay rate-constant of 0.013 a(-1) in cropland soil within 30 years. While the ammonium-N mineralization potential also decreased with land use duration, the trends were significant only in lacustrine pasture soils. Plant-available P did not show any trends that were related to the duration of land use. Kikuyu grass and maize accumulated less dry matter and N as the duration of use increased. This biomass accumulation was significantly related to soil N. A continued mineralization of soil organic matter has possibly contributed to the observed soil N depletion over time. The continuous agricultural use of the littoral wetland zone of Lake Naivasha is likely to entail declining production potentials for both pastures and food crops.
