Incorporation of fine root detritus into forest soil organic matter

One of the principal inputs of organic matter to forest soils is turnover of tree fine roots, but the process of decomposition of fine root litter and its conversion into stable soil organic matter (SOM) has received limited study. We labeled fine roots of sugar maple ( Acer saccharum Marsh.) with 13 C and traced the label for 7 years into four contrasting soils to improve understanding of this process. After 7 years we recovered an average of 8.9% of the 13 C label, with about two-thirds recovered as coarse particulate organic matter and one-third in microaggregates and on silt and clay particles. No differences in 13 C recovery were detected between 1–2 and 3–4 order fine roots. Most of the 13 C in microaggregates (53–250 µm, 58%) was occluded within macroaggregates, and the recovery in this fraction 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 13 C recovery in this fraction was observed in the A horizon of an acidic, fine-textured Inceptisol (Cambisol—World Reference Base). We estimate that annual input into relatively stable fractions of SOM represents about 14% of the total annual accumulation in these fractions; thus, our results support recent evidence that fine root litter is only a moderate contributor to stable SOM in acid temperate forest soils.