Solutes and soil in and around an in-stream wetland on the Hubbard Brook Experimental Forest, New Hampshire, USA

Abstract

We characterized soil and the concentrations of solutes in soil solutions and in stream water in and around a wetland located within a first-order stream in a forested watershed at the Hubbard Brook Experimental Forest (New Hampshire, USA). We hypothesized that the in-stream wetland would retain solutes in stream water, especially Ca²⁺, H⁺, and strong acid anions (NO ⁻₃ , SO ²⁻₄ ). Rather, the wetland had subtle impact on stream water chemistry causing slightly greater Na⁺ concentrations and lesser concentrations of H⁺ and dissolved inorganic C (DIC). We expected anaerobic microbial denitrification and SO ²⁻₄ reduction in wetland soils to consume the strong acid anions and account for less H⁺, but Na⁺ release was responsible. The wetland soil produced dissolved organic C (DOC) but did not export it into stream water. The DOC was saturated with monomeric Al, suggesting a sink rather than source for Al. Soil extracted with 1 M NH₄Cl yielded much larger amounts of Ca²⁺ and Mg²⁺ than in the surrounding mineral forest soil, whereas NH ⁺₄ , K⁺, and Na⁺ budgets were smaller in the wetland soil than in the forest soil. Characterization of soil surface charge and organic matter fractions revealed distinct differences between wetland soil organic matter (SOM) and forest soil SOM. Our results suggest that the wetland behaves much like gelatin: the organic matrix holds water tightly, such that stream water flows mostly over the matrix rather than entering and forcing much of the old water into runoff. The subtle biogeochemical role of the wetland adds to the complex interactions that control solute mobility in small watersheds.