TROPICAL RAIN-FOREST CONVERSION TO PASTURE - CHANGES IN VEGETATION AND SOIL PROPERTIES

The effects of converting lowland tropical rainforest to pasture, and of subsequent succession of pasture lands to secondary forest, were ex amined in the Atlantic Zone of Costa Rica. Three replicate sites of ea ch of four land-use types representing this disturbance-recovery seque nce were sampled for changes in vegetation, pedological properties, an d potential nitrogen mineralization and nitrification. The four land-u se types included primary forest, actively grazed pasture (10-36 yr ol d), abandoned pasture (abandoned 4-10 yr) and secondary forest (abando ned 10-20 yr). Conversion and succession had obvious and significant e ffects on canopy cover, canopy height, species composition, and specie s richness; it appeared that succession of secondary forests was proce eding toward a floristic composition like that of the primary forests. Significant changes in soil properties associated with conversion of forest to pasture included: (1) a decrease in acidity and increase in some base exchange properties, (2) an increase in bulk density and a c oncomitant decrease in porosity, (3) higher concentrations of NH4+, (4 ) lower concentrations of NO3-, (5) lower rates of N-mineralization, a nd (6) in some cases, lower rates of nitrification. Chemical changes i nvolving cations associated with conversion from forest to pasture ind icated increases in soil fertility under the pasture regimes, while ch anges associated with nitrogen indicated decreases in fertility. Physi cal changes in density and porosity were deleterious with respect to i nfiltration, percolation, aeration, and, ultimately, erodability. Beyo nd the practical aspects of land management, many of these changes are very important to carbon and nitrogen cycling and to the emission and consumption of biogenic trace gases.