THE EFFECTS OF THE RAINY-SEASON AND IRRIGATION ON SOIL-WATER AND OXYGEN IN A SEASONAL FOREST IN PANAMA

The effects of irrigation and the rainy season on soil gases, water an d physical properties were studied in a lowland moist forest in Panama . Two control plots experienced a normal four-month dry season and two experimental plots were irrigated during the dry season. The forest s oils were well aerated. The average soil oxygen content at 20 cm never dropped below 15%. Irrigation in the dry season resulted in treatment effects on soil oxygen and water that persisted throughout the rainy season. The late rainy season soil oxygen content at 20 cm was 18.2% i n the control plots and 16.8% in the irrigated plots. The late rainy s eason soil water content al 0-10 cm was 0.34 m(3) m(-3) in the control plots and 0.36-0.37 m(3) m(-3) in the irrigated plots. In the control plots, in the absence of any experimental manipulation, the soil phys ical properties changed within a single rainy season. Specifically, so il oxygen dropped and soil water increased during the eight-month rain y season. The observed alterations in soil physical properties probabl y resulted from changes in soil structure and the abundance of large p ores. The rainy season or irrigation may decrease soil porosity by dis persing aggregates of soil particles, whereas soil desiccation during the dry season may enhance the formation of aggregates and large pores . As a result, the irrigated soils held more water and were less perme able to gas and water than soils experiencing a normal dry season. We conclude that the dry season may play an important role in maintaining soil structure.