A comparison of nutrient availability indices along an ombrotrophic-minerotrophic gradient in Minnesota wetlands

Despite the importance of nutrient availability in determining ecosystem st ructure and function, it is difficult to quantify in an absolute sense beca use of the complexity of nutrient cycles and methodological limitations. Ot hers have compared nutrient availability indices for upland soils, but few comparative studies have been done in organic soils. Objectives of this stu dy were, (i) to determine if N and P availability change in a predictable m anner across an ombrotrophic-minerotrophic gradient in 16 wetlands in north ern Minnesota, and (ii) to compare various laboratory and field indices of soil nutrient availability in a diverse group of organic soils. Ombrotrophi c wetlands receive only atmospheric inputs of ions, while minerotrophic wet lands also receive groundwater or overland water inputs, We compared the fo llowing nutrient availability indices: 2- and 59-wk laboratory mineralizati on potentials, labile P and N pools determined from a kinetic mineralizatio n model, total and extractable soil N and P pools, plant N and P concentrat ions, and H-OH and HCO3- charged resins, Most indices indicated that N avai lability increases along the ombrotrophic-minerotrophic gradient, and corre lations among indices were generally good, suggesting that they can be used somewhat interchangeably. Resins indicated a predominance of NO3-N availab ility during the growing season and NH4-N availability during the winter, a nd most indices indicated an increasing importance of nitrification in more minerotrophic wetlands, In contrast, P indices gave contrasting results ac ross the gradient and were generally poorly correlated; however, the majori ty of the methods suggested that P availability is higher in minerotrophic swamp forests or beaver meadows, and that P availability is low in bugs 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 f or N, which probably reflects the complicated relationship between soil nut rient availability and plant response in natural wetlands.