| The Colombian savanna, locally called the Llanos, covers 26 million hectares and is the only available frontier for expansion of agriculture in the coming century. Current landscape includes native savanna vegetation on clay and on sandy Oxisols, gallery forest, pastures and cropland. This research reports field measurements of fluxes of methane and nitrous oxide and estimates for their annual budgets in the Llanos.; Soils under pastures were net sources of CH4, while soils under other land uses were net annual sinks. Soils of the Llanos were estimated to oxidize 6.4Gg/y of CH4. Soils of the gallery forest with 10% of the total area constitute 48% of total sinks. All soils were minor sources of N2O with upland rice cropland having the highest emission rates due to the input of N as fertilizer or green manure. Net emission by soils was estimated to be 12.4 Gg/y and soils dominate the N2O budget of the region.; Emission of CH4 by cattle is the main single source of this gas in the Llanos followed by direct emissions from biomass burning. Methane emitted by subterranean humivore termites is oxidized by soil before it reaches the atmosphere while negligible amounts were emitted from termite mounds.; Rainfall-induced soil surface sealing was found to reduce the exchange of gases between the soil and the atmosphere under laboratory conditions. Field studies found that induced soil compaction decreased methane oxidation rate from soils.; Including sources and sinks, it was estimated that the Llanos is a net source of 0.16 Tg CH4/y and 0.02 Tg N2O/y. For a 20-year time horizon, the global warming potential of the Llanos under current conditions is 9.6 Tg CO2equivalents which is less than 0.004% of estimated global planetary radiative contribution. Therefore, the Llanos can be considered as an "environmentally friendly ecosystem". The doubling in area under improved grass-legume pastures and threefold increase in cropland expected by the year 2020 will reduce this contribution to 2.4 Tg CO 2equivalents. Mainly due to the reduction in emissions from biomass burning and to CO2 sequestration as soil organic carbon by deep-rooted grasses, a phenomenon already reported in the region. |
