Mineralogic controls are harbingers of hydrological controls on soil organic matter content in warmer boreal forests

Abstract

Boreal forests contain ∼ 30 % of the global forest soil organic carbon (SOC) stock within a region vulnerable to climate change, yet regional studies on the climate relevant controls of this large reservoir are lacking. The Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect represents a boreal climate gradient of sites ranging in temperature and precipitation similar in extent to changes expected within the region over the next century. Though forest and soil type are similar across the transect, geological parent material upon which soils have developed vary across the region. Despite a > 50 % increase in litterfall and a 3-fold increase in the dissolved organic matter inputs to the mineral soil with decreasing latitude, average surface horizon mineral SOC and N content are similar among regions. To explain this, the relationships between mineral soil characteristics and other ecosystem parameters with SOC and N content were assessed using an information theoretic approach to rank models. Rankings were used to determine the most relevant variables explaining regional variance in surface mineral SOC. Unsurprisingly, organo-metal complexes (OMCs), a weathering product, were a major component of all plausible models. However, Al complexes ranked higher than Fe, likely because of the greater solubility of reduced Fe in reducing microsites, particularly beneath the winter snowpack or within the saturated soils during the spring melt. These results highlight the potential role of seasonal hydrology on the formation of metal-stabilized C and N in boreal forests and suggest that the role of Fe OMC in C sequestration may be enhanced with warmer winters. The realization and effect of these seasonally controlled OMC mechanisms require further understanding of mineral soil characteristics and hydrology and their response to climate warming. Such efforts will improve predictions of the impact of climate change on soil C and N content in these forests.