Fate of Applied Sulfate in Volcanic Ash‐Influenced Forest Soils

Abstract

Forests in the Inland Northwest, USA, commonly show SO4 deficiency, suggesting limited SO4 availability in the soils. Regional soils, which lie between the east slopes of the northern Cascade Range and the west slopes of the northern Rocky Mountains, are influenced to varying degrees by poorly crystalline aluminosilicates and ferrihydrite, and are often classified as Andisols (Andosols). Research has shown that SO4 retention is greatly influenced by Fe and Al oxides. However, little is known of the sorption behavior exhibited by poorly crystalline andic soils of the region. In this study we investigated the mineralogy and SO4 sorption capacity of ash‐influenced soils found in the Inland Northwest. Batch SO4 adsorption experiments showed that up to 40% of added SO4 was adsorbed. Furthermore, there were positive correlations between soil SO4 adsorption capacity and increasing ash influence as measured by (i) the andic soil parameter %Alo + 0.5%Feo (R2 = 0.89), (ii) P retention (R2 = 0.91), and (iii) NaF pH (R2 = 0.48). Soil pH, total organic C (TOC), and percentage of clay showed insignificant or inconclusive relationships with SO4 adsorption. Release of adsorbed SO4 was significantly lower in volcanic ash‐influenced soils as compared with non‐ash soils, indicating a greater affinity for SO4 These results indicate that poorly crystalline aluminosilicates and Fe oxides significantly influence the amount of SO4 present in forest soil solutions. Successful nutrient management plans must recognize the sorption behavior of these andic soils.