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

Fahey, Timothy J., Yavitt, Joseph B., Sherman, Ruth E., Groffman, Peter M., Fisk, Melany C., Maerz, John C.
Ecosystems 2011 v.14 no.2 pp. 326-340
Acer saccharum subsp. saccharum, carbon, carbon nitrogen ratio, chronic exposure, ecosystems, forest litter, fungi, hardwood forests, leaching, microbial activity, microbial biomass, mineral soils, nitrogen, nitrogen content, organic horizons, soil organic matter, stable isotopes, stoichiometry
We used sugar maple litter double-labeled with ¹³C and ¹⁵N 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 ¹³C from this litter generally followed dry mass and total C loss whereas loss of ¹⁵N (20-30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26-32%). Enrichment of ¹³C and ¹⁵N was detected in soil down to 10-15 cm depth. After 6 months of decay (November-May) 36-43% of the ¹³C 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 ¹³C 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 ¹⁵N in soil was much higher than for C (over 90%) and in May ¹⁵N 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 ¹⁵N 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.