Oil-palm management alters the spatial distribution of amorphous silica and mobile silicon in topsoils

Effects of oil-palm (Elaeis guineensis Jacq.) management on silicon (Si) cycling under smallholder oil-palm plantations have hardly been investigated. As oil palms are Si accumulators, we hypothesized that management practices and topsoil erosion may cause Si losses and changes in spatial Si concentration patterns in topsoils under oil-palm cultivation. To test this hypothesis, we took topsoil samples under mature oil-palm plantations in well-drained and riparian areas of Jambi Province, Indonesia. The samples were taken from four different management zones within each oil-palm plot: palm circles, oil-palm rows, interrows, and below frond piles. We quantified mobile Si (Si-M) and Si in amorphous silica (Si-Am) by the extraction of CaCl2 and NaCO3, respectively. Both fractions are important Si pools in soils and are essential for plant-soil Si cycling. We further installed sediment traps on sloping, well-drained oil-palm plantations to estimate the annual loss of soil and SiAm caused by erosion. In well-drained areas, mean topsoil SiAm concentrations were significantly higher below frond piles (3.97 +/- 1.54 mg g(-1)) compared to palm circles (1.71 +/- 0.35 mg g(-1)), oil-palm rows (1.87 +/- 0.51 mg g(-1)), and interrows (1.88 +/- 0.39 mg g(-1)). In riparian areas, the highest mean topsoil Si-Am concentrations were also found below frond piles (2.96 +/- 0.36 mg g(-1)) and in grass-covered interrows (2.71 +/- 0.13 mg g(-1)), whereas topsoil Si-Am concentrations of palm circles were much lower (1.44 +/- 0.55 mg g(-1)). We attributed the high SiAm concentrations in topsoils under frond piles and in grass-covered interrows to phytolith release from decaying oil-palm fronds, grasses, and sedges. The significantly lower SiAm concentrations in palm circles (in both well-drained and riparian areas), oil-palm rows, and unvegetated interrows (only in well-drained areas) were explained by a lack of litter return to these management zones. Mean topsoil SiM concentrations were in the range of similar to 10-20 mu g g(-1). They tended to be higher in riparian areas, but the differences between well-drained and riparian sites were not statistically significant. Soil-loss calculations based on erosion traps confirmed that topsoil erosion was considerable in oil-palm interrows on slopes. Erosion estimates were in the range of 4-6 Mg ha(-1) yr(-1), involving SiAm losses in a range of 5-9 kg(-1) ha(-1 )yr(-1). Based on the observed spatial Si patterns, we concluded that smallholders could efficiently reduce erosion and support Si cycling within the system by (1) maintaining a grass cover in oil-palm rows and interrows, (2) incorporating oil-palm litter into plantation management, and (3) preventing soil compaction and surface-crust formation.