Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with C-13 and N-15 to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of C-13 from this litter generally followed dry mass and total C loss whereas loss of N-15 (20-30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26-32%). Enrichment of C-13 and N-15 was detected in soil down to 10-15 cm depth. After 6 months of decay (November-May) 36-43% of the C-13 released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter C-13 in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m(-2) y(-1) of SOM accumulation. Recovery of litter N-15 in soil was much higher than for C (over 90%) and in May N-15 was mostly in organic horizons whereas by October it was mostly in 0-10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2-6%). Recovery of N-15 in microbial biomass was higher in May (13-15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha(-1) y(-1)) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.