Soil physicochemical properties and moisture dynamics of a large soil profile in a tropical monsoon forest

Abstract Evergreen forests in tropical monsoon Asia experience a severe dry season lasting several months. Deep soil moisture is responsible for evergreen forest transpiration during this season. This study provides preliminary information on soil physicochemical properties in a large soil profile (800 cm deep) and evaluated the influences of deep-soil moisture dynamics on the transpiration of evergreen forests in Cambodia using a water flow model. In general, changes in soil physicochemical properties with depth were small, excluding topsoil. The soil texture in the large soil profile was characterized by very little gravel and a large sand fraction. The soil carbon stock was 71.7 and 130.1 Mg C ha− 1 at depths of 0–150 and 0–800 cm, respectively. The coarse pore volume (− 100 ≤ ψ < − 0.2 cm) and saturated hydraulic conductivity (Ks) clearly decreased with depth from 100 to 600 cm and were extremely low at 600–700 cm. In the water flow simulation, a rooting depth deeper than 400 cm was necessary for the evergreen forest to maintain transpiration during the dry season. On the other hand, when the Ks of deep soil (550–750 cm depth) was assumed to be higher than that estimated by the cylinder sample, a rooting depth of 800 cm was necessary, since most of the soil water that was replenished during the rainy season was rapidly lost during the dry season. Also, it was necessary for evergreen forests to effectively use soil water within the deep root zone (400–800 cm depth). The compacted deep soil horizons played an important role in maintaining the moist conditions in the root zone of an evergreen tropical monsoon forest in Asia that experienced a severe dry season.