Nutrient Retention in Forest Soils: Role of Reactive Aluminum and Iron Mineral Phases.

Nutrient dynamics have been most extensively studied in agricultural soils with a near-neutral pH, a loamy texture and a predominance of crystalline phyllosilicates as surface-active components. In contrast, many forest soils are acidic and coarse-textured. Much of the chemical reactivity of such soils is accounted for by short-range order (SRO) mineral and organo-mineral products. Thus, some forest soils exhibit poorly understood, divergent geochemical behavior from agricultural soils.

The objective of this study is to improve our understanding of geochemical determinants of nutrient availability in forested Spodosols.  We identified edaphic predictors of nutrient distribution in the top and subsoil horizons of Canadian Spodosols using multivariate regression and canonical correlation approaches. Special attention was paid to the influence of reactive aluminum and iron phases on nutrient availability.

We found that organic carbon and reactive Al and Fe species were more effective predictors of nutrient distribution than the silt and clay content, suggesting that silt and clay contributed little to the surface reactivity of these soils. In contrast, SRO Al and Fe mineral phases correlated strongly with the distribution of potassium and phosphorus. Preferential association of K with SRO phases likely contributed to its retention in the profile. Phosphorus sorption to SRO phases decreased the amount of available P. Contrary to generally expected geochemical behaviour, both P lability and the effective cation exchange capacity decreased as pH increased. In this presentation, we argue that these seemingly anomalous relations result from an increase in the amount and specific affinity of SRO Al and Fe phases for oxyanions and organic anions at higher pH, which leads to P sorption in highly insoluble forms and to a decrease in cation exchange sites. This study shows that understanding the sorptive properties of naturally-occuring SRO Al and Fe species is important to our ability to predict and influence nutrient availability in managed forests, and points to the need for further investigation of the geochemical behavior of SRO minerals under field conditions.