Tropical pasture carbon cycling: relationships between C source/sink strength, above-ground biomass and grazing

We measured net ecosystem CO2 exchange (NEE) in Panama over C-4 pasture plots that varied in grazing intensity. After adjusting for variation in light, there were noticeable effects of grazing-related variables on CO2 exchange that were largely dependent on the developmental stage of the plant canopy. Above-ground productivity was positively related to grazing intensity (r (2) =0.30). Two experimentally grazed fields had significantly lower standing crop biomass but no significant difference in CO2 uptake (24.2 mumol/m(2)/s) compared with two ungrazed fields (20.3 mumol/m(2)/s). Grazed fields had significantly lower ecosystem respiration rates (10.3 mumol/m(2) /s) than did ungrazed fields (17.6 mumol/m(2)/s). These results suggest that, although these pastures were possible sources of CO2 during the time intervals sampled, the size of the sources tended to be dampened by cattle grazing through reductions in ecosystem respiration. Thus, it appears that disturbance caused by cattle grazing will not always result in an increase in CO2 release from tropical pastures to the atmosphere.