EFFECTS OF PASTURE INTRODUCTION ON SOIL CO2 EMISSIONS DURING THE DRY SEASON IN THE STATE OF RONDONIA, BRAZIL

Soil CO2 evolution rates, soil temperatures and moisture were measured during the dry season in two forest-to-pasture chronosequences in Ron donia, Brazil. The study included pastures ranging from 3 to 80 years- old. Mean dry-season CO2 evolution from the forest in chronosequence I , 88.8 mg CO2-C m(-2) h(-1) was lower than from the pastures which ran ged from 111 to 158 mg CO2-C m(-2)h(-1). We found that temperature was not a good predictor of CO2 emissions from pasture but that there was a significant relationship (r = 0.72, p < 0.05) between soil moisture and pasture emissions. The delta(13)C of the soil CO2 emissions also was measured on chronosequence I; delta(13)C of the CO2 emitted from t he C-3 forest was -29.43 parts per thousand. Pasture (CO2)-C-13 delta values increased from -17.91 parts per thousand in the 3 year-old past ure to -12.86 parts per thousand in the 80 year-old, reflecting the in creasing C-4 inputs with pasture age. Even in the youngest(3 year-old) pasture, 70 percent of the CO2 evolved originated from C-4 pasture-de rived carbon.