Nitrogen mineralization and nitri®cation following land conversion in montane Ecuador

The lower montane zone of northwestern Ecuador, like many parts of the tropics, is undergoing rapid conversion from native forest vegetation to crop and pastureland. The current landscape is a mosaic of agricultural land, forest fragments and secondgrowth vegetation in various stages of development. While there is abundant research documenting the e€ects of land-use change in the lowland tropics, such information is scarce for montane regions on young volcanic soils. We compared foxtail pasture (Setaria sphacelata, [(Schumach.) Stapf and C.E. Hubb.]) and traditional, mixed-species pasture with undisturbed oldgrowth forest, 15±20 y-old secondary forest and 5±10 y-old shrubby regrowth. At two replicates of each vegetation class, we measured soil nitrogen and carbon pools, in situ net nitrogen mineralization and nitri®cation, soil respiration and soil physical properties. Setaria pasture decreased soil NO3 -N pools and net mineralization and nitri®cation rates compared to mature forest, secondary vegetation, and mixed-species pasture. Soil NO3 -N in Setaria pastures, during wet and dry seasons, was 40 and 25% of amounts measured in other vegetation types. Net nitri®cation rates were also lower beneath Setaria during both seasons, the greatest di€erence occurring during the wet season. Net nitri®cation rates increased considerably (two-fold and greater) under wet season conditions beneath all vegetation types except Setaria pasture. Soils beneath both pasture types were wetter and had higher bulk density than mature and second-growth forests. Conversion to pasture produces widely varying e€ects on soil N dynamics depending on characteristics of the pasture species, such as Setaria's extremely dense root system. Reduced soil N availability beneath Setaria pasture will a€ect both long-term pasture productivity and subsequent forest regeneration in abandoned pastures. # 1999 Elsevier Science Ltd. All rights reserved.