Incorporation of fine root detritus into forest soil organic matter

Abstract Forest soils contain a large proportion of the global carbon stock in the form of relatively stable soil organic matter (SOM). One of the principal inputs of carbon to forest SOM is turnover of tree fine roots, but the process of decomposition of fine root litter and its conversion into stable SOM has received limited study. We labeled fine roots of sugar maple (Acer saccharum Marsh.) with 13C and traced the label for seven years into four contrasting soils to improve understanding of this process. After seven years we recovered an average of 8.9% of the 13C label, with about two-thirds recovered as coarse particulate organic matter (cPOM) and one-third as fine POM (fPOM) and mineral-associated organic matter (MAOM). No differences in 13C recovery were detected between 1-2 and 3-4 order fine roots. Most of the 13C in fPOM in microaggregates (53-250 µm, 58%) was occluded within macroaggregates, and the recovery increased significantly from year 2 to 7, illustrating the role of fine root detritus in the formation of microaggregates. This process was most pronounced in the A horizon of a higher pH soil (pH = 5.5) with high iron oxide content. Conversely, the lowest 13C recovery in this fraction was observed in the A horizon of an acidic, fine-textured Inceptisol. Despite the difference between leaf litter and fine root litter in terms of placement on versus within mineral soil, patterns of their contribution to MAOM appear to be similar. Our results support recent evidence that fine root litter is a modest contributor to stable SOM in acid forest soils.