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 cmolc/kg.pH unit
after the dry season in April versus 3.2 cmolc/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.
Influence of Organic Based Technologies on Soil Nutrient Status in Semi-Arid Yatta Sub-County, Kenya