A Comparison of Nutrient Availability Indices Along an Ombrotrophic–Minerotrophic Gradient in Minnesota Wetlands

Abstract

Despite the importance of nutrient availability in determining ecosystem structure and function, it is difficult to quantify in an absolute sense because of the complexity of nutrient cycles and methodological limitations. Others have compared nutrient availability indices for upland soils, but few comparative studies have been done in organic soils. Objectives of this study were, (i) to determine if N and P availability change in a predictable manner across an ombrotrophic–minerotrophic gradient in 16 wetlands in northern Minnesota, and (ii) to compare various laboratory and field indices of soil nutrient availability in a diverse group of organic soils. Ombrotrophic wetlands receive only atmospheric inputs of ions, while minerotrophic wetlands also receive groundwater or overland water inputs. We compared the following nutrient availability indices: 2- and 59-wk laboratory mineralization potentials, labile P and N pools determined from a kinetic mineralization model, total and extractable soil N and P pools, plant N and P concentrations, and H–OH and HCO−3 charged resins. Most indices indicated that N availability increases along the ombrotrophic–minerotrophic gradient, and correlations among indices were generally good, suggesting that they can be used somewhat interchangeably. Resins indicated a predominance of NO3–N availability during the growing season and NH4–N availability during the winter, and most indices indicated an increasing importance of nitrification in more minerotrophic wetlands. In contrast, P indices gave contrasting results across the gradient and were generally poorly correlated; however, the majority of the methods suggested that P availability is higher in minerotrophic swamp forests or beaver meadows, and that P availability is low in bogs and fens. We suggest that current methods of determining P availability may be inadequate in highly diverse organic soils. Plant nutrient concentrations did not show clear relationships with soil nutrient indices, particularly for N, which probably reflects the complicated relationship between soil nutrient availability and plant response in natural wetlands.