Comparing volcanic glass shards in unfertilized and fertilized Andisols derived from rhyolitic tephras, New Zealand: evidence for accelerated weathering and implications for land management

Abstract

Enhanced weathering associated with the use of phosphate fertilizers has already been identified in some of the major farming areas of New Zealand and there is need to evaluate its effects on soil properties and the implications for soil management. We assess changes in the character of volcanic glass shards in topsoils (010 cm depth) of two tephra-derived Andisols of differing ages (Udivitrands, Hapludands), and with and without long-term fertilization, to test if fertilizing accelerates the weathering of soil constituents. Using visual assessment based on scanning electron microscopy and electron microprobe analyses of glass shards from samples from paired sites, we show that the average sizes of the shards and the sharpness of the glass-shard edges (angularity) diminished with the age of the soil, and that these decreases were more marked in the soils that had been fertilized. Silica polymorphs were observed only in the older soil (Hapludand) that had been fertilized. We concluded that the addition of phosphate-containing fertilizer enhanced the dissolution of volcanic glass, consistent with phosphoric acid and F--induced dealumination and desilication, and thus soil weathering has been accelerated. The Al and Si may subsequently coprecipitate as secondary minerals, such as allophane. However, silica polymorphs may form where Al activity is low. The occurrence of neogenic silica in topsoils (despite high annual rainfall that should result in desilication of the topsoil and reprecipitation of silica deeper in the soil) is explained by the seasonality of fertilizer application and the spring-summer climate, where the soil remains moist for periods sufficiently long enough for the dissolution of volcanic glass (and possibly other aluminosilicates), the formation of Al-humus complexes, and the subsequent precipitation of silica in dry periods. Some implications of the accelerated weathering for soil management are discussed.