Rapid fine root C and N mineralization in a northern temperate forest soil

While fine roots (≤2-mm diameter) are major suppliers of carbon (C) and nitrogen (N) to northern temperate and boreal forest soils, our understanding of how long-term plant and N inputs affect fine root decomposition rates and the amount of root-derived organic matter (OM) stabilized in forest soils is incomplete. We examined the influence of long-term aboveground and/or belowground litter and inorganic N additions on mineralization and vertical transport of fine root-derived C and N during the first 2 years of decomposition of dead fine root in the field. We used an existing long-term field manipulation experiment located in a northern Michigan forest; with (i) exclusion of above and below-ground inputs, (ii) exclusion of belowground inputs alone, or (iii) inorganic N additions, for 6 years prior to the addition of dual-labeled (¹³C and ¹⁵N) Acer rubrum fine roots. After 2 years in soil, labeled fine roots rapidly decomposed in all treatments, with only 20.7 % of root ¹³C and 35.8 % of root ¹⁵N recovered in soil (0–20 cm depth). This was likely because of the combined effects of (1) root litter chemistry, (2) processing of root litter by exotic earthworms, and (3) the low stabilization potential of the coarse-textured soil at the site. Neither the long-term exclusion of litter inputs nor increased inorganic N additions influenced root mineralization rates; and there were no detectable effects of either treatment on CO₂ efflux or on dissolved organic C loss. During the 2-year study, exclusion of litter inputs did not affect root C retention in soil but lowered C:N ratios of roots recovered in that treatment. Inorganic N additions had no significant effect on root-derived C or N retention in the soil. Our results show that fine root litter turns over faster than previously thought in coarse-textured temperate forests soils that lack effective OM stabilization mechanisms.