Forest soil mineral weathering rates: use of multiple approaches

Abstract

Knowledge of rates of release of base cations from mineral dissolution (weathering) is essential to understand ecosystem elemental cycling. Although much studied, rates remain enigmatic. We compared the results of four methods to determine cation (Ca + Mg + K) release rates at five forested soils/sites in the northcentral U.S.A. Our premise was that multiple approaches, each with their own specific strengths and weaknesses, would yield a “best” overall estimate. We used (1) a cation input-output budget on a pedon scale; (2) trends in elemental and mineral depletion in silt-size particles; (3) a laboratory batch dissolution technique, with the results adjusted for field conditions; and (4) a steady-state soil chemistry model, PROFILE. The soils included a loamy sand Typic Udipsamment, a sandy loam Spodic Udipsamment, a fine sandy loam Typic Dystrochrept, a very fine sandy loam Glossic Eutroboralf, and a clayey Glossic Eutroboralf. Weathering rates varied among both soils and methods, and neither methods nor soils could easily be grouped; the data spanned a continuum with overlapping ranges of least significant differences. Although the assumptions necessary for some methods were better suited to specific soils, we rejected only one method-soil combination as being inappropriate (input-output budget for the clay). Mean release rates for the sum of cations ranged from 470 eq ha −1 yr −1 for the clayey soil, to 460 for the fine sandy loam soil, to 430 for the very fine sandy loam soil, to 375 for the sandy loam soil, to 195 for the loamy sand soil. These rates are lower than those reported for similar soils in the literature because most reported rates are based on watershed studies. Our low rates of cation release within soil pedons, the ultimate source of nutrient ions for plant growth, has implications for estimated nutrient budgets and long-term soil sustainability.