Comparing volcanic glass shards in unfertilised and fertilised 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 fertilisers has been identified in some of the major farming areas of New Zealand and an evaluation of its effects on soil properties and the implications for soil management are needed. We assessed changes in the character of volcanic glass shards in topsoils (0−10cm depth) of two tephra-derived Andisols of differing ages (Udivitrands, Hapludands), and with and without long-term fertilisation, to test if fertilising 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 showed 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 fertilised. Silica polymorphs were observed only in the older soil (Hapludand) that had been fertilised. We concluded that the addition of phosphate-containing fertiliser 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 fertiliser application and the spring–summer climate, where the soil remains moist for periods sufficiently long enough for the dissolution of volcanic glass (and possibly of other aluminosilicates), the formation of Al–humus complexes, and the subsequent precipitation of silica in dry periods. Implications of the accelerated weathering for soil and land management are discussed.