Nitrogen mineralization and nitrification 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 second-growth vegetation in various stages of development. While there is abundant research documenting the effects 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 old-growth 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 nitrification, soil respiration and soil physical properties. Setaria pasture decreased soil NO−3-N pools and net mineralization and nitrification rates compared to mature forest, secondary vegetation, and mixed-species pasture. Soil NO−3-N in Setaria pastures, during wet and dry seasons, was 40 and 25% of amounts measured in other vegetation types. Net nitrification rates were also lower beneath Setaria during both seasons, the greatest difference occurring during the wet season. Net nitrification 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 effects 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 affect both long-term pasture productivity and subsequent forest regeneration in abandoned pastures.