Print PageMost plants have difficulty growing on shallow soils because rooting depth is insufficient for anchorage and water storage and nutrient pools are limited. Northern Yucatan is a limestone karst environment where shallow (<30 cm deep) stony soils are common. Nevertheless the weathering of bedrock promotes fractures and dissolution features allowing soil materials to move down and accumulate within the bedrock matrix forming soil pockets. These subsurface features act as pathways for root exploration and represent additional sources of water and nutrients for plants. However, their importance in ecosystem functioning is not well understood. In this study we described and characterized the topsoil and subsurface features of the vadose zone of northern Yucatan in order to analyze their importance for plant growth. We compared physical properties of shallow soils with those of the subsurface soil pockets. We also analyzed the changing physical properties of bedrock with depth and their role in soil pocket development and root distribution. Finally, we calculated the potential amount of available water through the vadose zone and the main sources of water used by trees.
Results/Conclusions
The vadose zone is composed of bedrock matrix (86%), soil pockets (8 %), topsoil (4%), and empty cavities (2%). The uppermost layer of bedrock (the ≈ 2.5-m-thick laja layer) greatly restricted root penetration; >70% of the roots did not reach the next softer layer (the ≈ 2.5-m-thick sascab layer), some roots (less than 5%) reached the lower bedrock layer (the 4-m-thick coquina layer), and only a few reached the aquifer (at 9 m). Rock properties showed that sascab and coquina layers have compressive strength and porosity values that allow root penetration, water storage, and soil pocket formation. Roots that are able to reach a soil pocket have a greater chance of success in this environment. Soil in pockets contributes a volume of 6640 m3 ha-1, whereas topsoil contributes only 3000 m3 ha-1. Potential available water was 803 m3 ha-1 for soil in pockets and 293 m3 ha-1 for topsoil. Soil in pockets could be an important source of water during the dry season since it is isolated from evaporation effects. Stable isotopes showed that some species can get water from subsurface sources (i.e. softer rock or soil pockets). Soil pockets have 3 times more clay and 10% less porosity than topsoil. Mycorrhizae were found infecting roots growing in deep soil pockets.
