Charge characteristics of soil in a lowland tropical moist forest in Panama in response to dry-season irrigation

Abstract

Although the hot, moist tropics in the Republic of Panama receive more than 2000 mm of rain per year, soils dry considerably during the 4-month dry season. We examined the effect of seasonal drought by irrigating two 2.25-ha plots of lowland tropical moist forest on Barro Colorado Island (BCI) for 5 consecutive dry seasons. Irrigation decreased soil permeability and improved soil nutrient status, which prompted this study of soil charge characteristics in the irrigated and control plots. Soil was an Alfisol, and thus it was not clear a prioriwhether variable-charge or permanent-charge components dominated. Surface soil (0–15 cm) had a pH(H₂O) of 5.5 and pH(KCl) of 4.8. Subsurface soil (30–45 cm) had a pH(H₂O) of 4.8 and a pH(KCl) of 3.5. The point of zero salt effect (PZSE), measured by titration, varied from 3.7 to 5.0 in surface soil and from 3.5 to 4.2 in subsurface soil. Variable charge of surface soil was 2.6 cmol_c/kg.pH unit after the dry season in April versus 3.2 cmol_c/kg.pH unit after the wet season in December in both control and dry-season irrigated plots, reflecting seasonal differences in pH and PZSE. The point of zero net charge (PZNC), measured by ion retention, was at pH <2.0, indicating that permanent-charge components dominated the soil surface charge. Five years of dry-season irrigation resulted in pH(H₂O) increasing by 0.6 units and pH(KCl) increasing by 0.2 units. As well, irrigation increased the amount of permanent charge and cation retention, leading to less sorption of phosphate and sulfate. The results have important ecological implications, showing mechanistically how wetter conditions affected soil surface charge leading to improved soil nutrient status.

permanent charge, soil pH, tropical forest soil, variable charge, water regime.