Spatial variations of litterfall silicon flux and plant-available silicon in highly weathered soil in a lowland mixed dipterocarp forest of Lambir Hills National Park in Borneo

Abstract

Tropical forest trees take up silicon (Si) and return it to the forest floor via leaf litterfall. Our objective was to explore to what extent litter Si flux and Si availability from the soil are spatially coupled. We examined these relationships within a 4-ha area of lowland mixed dipterocarp forest of Lambir Hills National Park in Borneo. Using leaf litter samples collected with litter traps, we found that Si concentration and flux of leaf litter ranged 2-23mgSig^－1 and 0.8-13.1gSim^－2 yr^－1, respectively, whereas water-extractable Si from 0-10cm deep soil ranged from 5.9 to 24.5mg kg^－1 (0.7 to 3.0gSim^－2) at 80 litter trap locations. There was no significant correlation among these three aspects of Si cycling via trees. Water-extractable soil Si from three 95cm deep cores showed no significant change with depth, whereas in-situ measurements with six tension lysimeters showed higher soil-water Si concentration in the upper soil layer (0-5cm depth). These results suggest that spatial variations of Si concentration and flux in leaf litter do not reflect those of soil Si availability, but are modulated by distribution of tree species that differ in Si uptake. Si returned to the soil via leaf litter did not show strong spatial signals probably because solubility of Si from dead leaves differs among species. At the stand level, our results are consistent with the perspective that litter Si input enriches plant-available Si pool in the upper soil horizons in tropical forests.