Soil carbon and nitrogen changes following conversion of Ecuadorian lower montane forest to pasture

This dissertation describes several ecosystem-level effects of transforming native lower montane tropical rain forest for agricultural use. Forest clearing for pasture establishment is the major cause of deforestation in western Ecuador. As the region expands economically, so does the extent of livestock and dairy pastures. Setaria sphacelata has become the dominant planted pasture type in the region, due to its ease of establishment and low maintenance requirements. The attributes that make Setaria attractive as a pasture species also determine the type of effects that widespread conversion to pasture will have on key ecosystem processes.; Soil nitrogen availability is dramatically reduced in Setaria pastures. Both the extractable soil NO{dollar}sb3sp-{dollar}-N pool and net mineralization and nitrification rates decline significantly after conversion to pasture. The depressed soil N conditions in the Setaria pasture are consistent during both wet and dry season conditions. The causes for the reduction in inorganic N production are multiple and interrelated. Increased soil bulk density and moisture content may generate water-logged conditions that may inhibit the activity of aerobic nitrifying bacteria. Available soil N pools may be drawn down by plant uptake or by immobilization by soil microbes. Low soil N coupled with the physical obstacle presented by the dense root mat in the Setaria pastures form a significant barrier to natural recolonization of the pastures by forest vegetation. Nitrogen-fixing trees scattered within Setaria pastures increase soil N availability to original forest-levels. Combined with the improved microclimatic conditions in the subcanopy environment, pasture trees appear reduce the barriers of forest regeneration within the Setaria pastures.; Conversion from forest to pasture transforms the system from one where carbon inputs are received as leaf litter deposited on the soil surface to one where the dominant carbon inputs come from root turnover belowground. The effect of pasture on soil carbon stocks varies with depth. In the surface 30 cm, soil C declines by up to 20% beneath pasture vegetation, whereas in the lower depths soil C increases by 40% over original forest levels. Seventy percent of the carbon inputs in the pasture system occur in the lower 70 cm of the soil profile. For comparison, more than half of the soil carbon in the forest system is stored in the upper 30 cm of the soil profile. While the ecosystem-level losses of C following clearing of forest and establishment of pasture surpass the increment of soil C by an order of magnitude, the deep storage of C in the Setaria pastures is worth considering due to the regional importance of this land-use practice. The potential of Setaria pastures to serve as long-term C sinks is uncertain and will require investigation of how C is stored and released from volcanic soils, and how N cycling controls ecosystem production.