SOIL PHYSICAL PROPERTY CHANGES DURING DUNG DECOMPOSITION IN A TROPICAL PASTURE

The contributions of soil macroinvertebrates to the creation and maintenance of soil structure in tropical rangeland ecosystems are poorly understood, in spite of the fact that compaction is frequently cited as a limiting factor in pasture production. Decomposition of cattle dung (fecal material) is associated with high levels of soil macroinvertebrate activity. We hypothesized that bulk density and surface penetrometer resistance would be reduced in soil beneath decomposing cattle dung patches and that soil water infiltration capacity and drainable porosity would be increased. Cattle dung was deposited in patches during the dry and wet seasons of 1991 in a seasonally dry pasture located 5 km north of Canas, Costa Rica, on a Typic Argiustoll. At the end of the wet season, surface 3-cm bulk density was <0.93 Mg m(-3) under dung patches, compared with 1.05 Mg m(-3) in control plots. Drainable porosity, defined as air-filled porosity at 0.006 MPa, increased from 13% in control plots to >21% under dung. Similar, but smaller, differences were recorded at dung patch edges and at a depth of 4 to 7 cm. At the end of the wet season, equilibrium infiltration rates averaged 71 mm h(-1) in the patch plots and 34 mm h(-1) in the controls. Changes in penetrometer resistance were transient and no treatment effects persisted to end of the wet season. The results of this study indicate that processes associated with dung decomposition play a role in reducing surface compaction.