Phytolith content in Vietnamese paddy soils in relation to soil properties

Understanding of the fate of phytolith in soils is important because of its role as an agronomical nutrient resource and for carbon sequestration. Accumulation of phytolith derived silica occurs in paddy soils when phytolith input, here through straw incorporation, is higher than silica removal through crop offtake, dissolution and leaching. Phytolith dissolution is thought to be the main reason for diminishing of phytolith derived silica in soil, with this dissolution being highly dependent on soil chemical properties. In this study, phytolith content from 78 paddy soils sampled in the Red River (RRD) and Mekong River (MRD) Deltas, Vietnam, were quantified and related to pH, electrical conductivity (EC), oxalate-extractable Al and Fe (Alox and Feox), organic carbon (OC) and clay content. Average phytolith contents within the topsoil (0–20 cm) were 4.8 ± 1.98 and 7.74 ± 2.97 mg g−1 for the RRD and MRD, respectively. Positive correlation coefficients obtained for Alox, OC, clay content and EC, implying that these parameters might enhance phytolith resistance to dissolution. Soil pH had a negative correlation coefficient with phytolith content. Lower pH and higher Alox content explained the predominance of the phytolith in the MRD compared to the RRD. Soil pH adjustment can be proposed as an active management strategy to maintain phytolith added Si, source for crops and for carbon sequestration.