FOREST-DERIVED AND PASTURE-DERIVED CARBON CONTRIBUTIONS TO CARBON STOCKS AND MICROBIAL RESPIRATION OF TROPICAL PASTURE SOILS

The clearing of tropical forest for pasture leads to important changes in soil organic carbon (C) stocks and cycling patterns. We used the n aturally occurring distribution of C-13 in soil organic matter (SOM) t o examine the roles of forest- and pasture-derived organic matter in t he carbon balance in the soils of 3- to 81-year-old pastures created f ollowing deforestation in the western Brazilian Amazon Basin state of Rondonia. Different delta(13)C values of C3 forest-derived C (-28 part s per thousand) and C4 pasture-derived C (-13 parts per thousand) allo wed determination of the origin of total soil C and soil respiration. The delta(13)C Of total soil increased steadily across ecosystems from -27.8 parts per thousand in the forest to -15.8 parts per thousand in the 81-year-old pasture and indicated a replacement of forest-derived C with pasture-derived C. The delta(13)C of respired CO2 increased mo re rapidly from -26.5 parts per thousand in the forest to -17 parts pe r thousand in the 3- to 13-year-old pastures and indicated a faster sh ift in the origin of more labile SOM. In 3-year-old pasture, soil C de rived from pasture grasses made up 69% of respired C but only 17% of t otal soil C in the top 10 cm. Soils of pastures 5 years old and older had higher total C stocks to 30 cm than the original forest. This occu rred because pasture-derived C in soil organic matter increased more r apidly than forest-derived C was lost. The increase of pasture-derived C in soils of young pastures suggests that C inputs derived from past ure grasses play a critical role in development of soil C stocks in ad dition to fueling microbial respiration. Management practices that pro mote high grass production will Likely result in greater inputs of gra ss-derived C to pasture soils and will be important for maintaining tr opical pasture soil C stocks.