FOREST-TO-PASTURE CONVERSION INFLUENCES ON SOIL ORGANIC-CARBON DYNAMICS IN A TROPICAL DECIDUOUS FOREST

On a global basis, nearly 42% of tropical land area is classified as t ropical deciduous forest (TDF) (Murphy and Lugo 1986). Currently, this ecosystem has very high deforestation rates; and its conversion to ca ttle pasture may result in losses of soil organic matter, decreases in soil fertility, and increases in CO2 flux to the atmosphere. The soil organic matter turnover rate in a TDF after pasture conversion was es timated in Mexico by determining natural abundances of C-13. Changes i n these values would be induced by Vegetation changes from the C-3 (fo rest) to the C-4 (pasture) photosynthetic pathway. The rate of loss of remnant forest-soil organic matter (fSOM) was 2.9 t ha(-1) year(-1) i n 7-year-old pasture and decreased to 0.66 t ha(-1) year(-1) by year 1 1. For up to 3 years, net fSOM level increased in pastures; this incre ment can be attributed to decomposition of remnant forest roots. The s and-associated SOM fraction was the most and the silt-associated fract ion the least depleted. TDF conversion to pasture results in extremely high rates of loss of remnant fSOM that are higher than any reported for any tropical forest.