Almost all cultivated soils undergo some reduction in the porosity of the surface layers, and nowhere is this more evident than in tropical rainforests that have been converted to pastures. Following deforestation in an area of Costa Rica, soil bulk density has been shown to increase rapidly after conversion to pasture, leading to poor drainage and a reduced rate of gaseous diffusion1. These factors limit methane consumption and promote the anaerobic production of methane. A similar effect on methane flux has been found in upland soils in the Brazilian Amazonian basin after conversion from forest to pasture2,3. Increases in atmospheric methane are therefore not limited to emissions from flooded soils4, as forest-to-pasture conversion promotes the anaerobic mineralization of organic matter by changing the physical properties of soil.