Phytolith assemblages and opal concentrations from modern soils differentiate temperate grasslands of controlled composition on experimental plots at Cedar Creek, Minnesota

Abstract

Researchers frequently assume that phytolith assemblages in modern soils reflect composition of recent vegetation because of the direct deposition of silica into the soil, once plants decay. This paper tests this assumption and determines whether temperate grasslands of different composition can be reliably detected based on their silica record in topsoil in a controlled experiment. The differences in total biogenic opal concentration (TBOC) and diversity of morphotypes were assessed in the Biodiversity II experiment (E120) at Cedar Creek Ecosystem Science Reserve, Minnesota, USA, where controlled mixtures of C3 grasses, C4 grasses, legumes, non-legume forbs, and woody shrubs (Quercus) were grown for a period of eight years. The plots have been manually maintained to contain the target species, and thus provide an opportunity to test numerous hypotheses regarding phytolith production patterns under diverse mixtures of plants. Soil samples were obtained from plots representing a variety of functional group mixtures. Pinch soil samples of 20 g from 10 random locations inside each plot were obtained. Phytoliths were extracted from each sample by chemically removing organics and carbonates and using heavy liquid flotation. A chemical dissolution method was used to obtain estimates of TBOC. Morphotypes were counted under a microscope. Morphotypes were analyzed on all plots against each other and against the morphotypes expected in the plants that grow on each plot using ANOVAs, linear regression, PCA and cluster analysis. Average above-ground biomass of expected phytolith producers was weakly but positively correlated with the TBOC values (R2 = 0.42). The morphotype analysis showed that species' composition was most accurately reflected in the phytolith assemblages on grass-dominated plots. For example, it was possible to distinguish C3, C4, or mixed grass-dominated plots from each other. Although the majority of phytoliths were from Poaceae, large shares were also from forbs and woody plants. Plots without any grasses still had some presence of grass phytoliths suggesting limited horizontal translocation and/or inheritance.