Comparison of the behavior of soluble organic and inorganic nutrients in forest soils

Abstract

The mechanisms by which potentially soluble inorganic nutrients are retained and eventually recycled within ecosystems have been characterized by many studies. Substantial amounts of potentially soluble organic nutrients are also released as plants grow, die, and decompose. In this study, a conceptual model was developed which shows the pools and significant processes generating and consuming dissolved organic matter and the associated nutrient elements. Experiments showed that the pool of potentially soluble organic matter in both mineral and organic horizons was much larger than the amount dissolved by any individual leaching event. Adsorption had the effect of buffering concentrations of dissolved organic matter in both mineral and organic horizons. The sorption equilibrium also slows the leaching rate of potentially soluble organic matter. By increasing the residence time of potentially soluble organic matter on solid surfaces, sorption results in a much greater time of exposure of soluble organic matter to decomposers while in the sorbed state. Literature sources indicate a bimodal distribution of decay coefficients of dissolved organic C and N with a refractory fraction dominating. A set of hypotheses comparing the factors controlling retention of inorganic vs. organic nutrients was developed. These hypotheses related to: sources of dissolved nutrients; properties of molecules controlling behavior; biological removal from solution; non-biological removal from solution and major factors allowing loss from the system. The major factor allowing loss of organic nutrients from the ecosystem is hydrologic short circuiting or absence of a mineral soil horizon rich in Fe and Al oxyhydroxides whereas, in the case of inorganic nutrients, it is hydrologic short circuiting or absence of a root network in addition to geochemical factors.