Phosphorus Limitation in Boreal Forests: Effects of Aluminum and Iron Accumulation in the Humus Layer

Abstract

Plant growth in boreal forests is generally considered to be predominantly nitrogen (N) limited, but forested groundwater discharge areas may be exceptions. In this study, we conducted tests to determine whether highly productive forested groundwater discharge areas generally differ from adjacent groundwater recharge areas in terms of humus chemistry and the availability of phosphorus (P) and N to plants. We investigated six forested sites, divided into groundwater discharge and adjacent groundwater recharge areas, in northern Sweden. The humus layers of the forested groundwater discharge areas were clearly distinguished from the adjacent groundwater recharge areas by having higher acid-digestible calcium (Ca) and/or aluminium (Al) and iron (Fe) content and higher organic P and N content. Soil solution inorganic N (NH₄ ⁺ and NO₃ ⁻) and pH were higher in the groundwater discharge areas than in the groundwater recharge areas. The organic P content showed a positive linear relationship to the Al and Fe content in the humus layer, indicating that organic P is associated with Al and Fe compounds in the humus. A plant bioassay using humus substrate from one groundwater discharge area and the adjacent groundwater recharge area found that plants grown in groundwater discharge area humus (with a high P-fixation capacity) increased their biomass upon P fertilization, whereas no growth response was found for N additions. By contrast, plants grown in humus from the groundwater recharge area did not respond to added P unless N was added too. This study suggests that groundwater discharge can affect the nutrient availability of N and P both directly, via increased P fixation due to the redistribution of Al and Fe, and indirectly, via the inflow of groundwater high in Ca and alkalinity, maintaining a high pH in the humus layer that favors in situ N turnover processes.