Trends in Carbon and Nitrogen Isotopes Associated with Particle Size Distribution in a New England Forest Soil

The quality of Soil organic matter (SOM) can vary in terms of its availability to soil microbes due to different levels of chemical resistance or protection by soil aggregates. The role SOM plays in the carbon cycle is constrained by its age. We used d13C and d15N as indicators of SOM's age and quality, and studied how depth and grain size affect age distribution of SOM in the soil at Dartmouth's Fullington Forest in Hanover. Radiocarbon dating of select samples was used to confirm the relationship between age and stable isotopic ratios. A soil profile was collected to a depth of 55 cm for ten total mineral soil layers. Each soil sample was dried and separated into 9 particle size fractions by sieving or settling. Some fractions were further separated into sinking and floating portions. In general, floating fractions are relatively high in quality indicated by high C/N ratios and low d13C and d15N values. Both d13C and d15N of sinking (mineral) fractions increased with depth. At a given depth, the d13C and d15N of the mineral fractions are relatively consistent for grain sizes >500 µm, and below this size increased with decreasing particle size. Carbon-14 dates were obtained for sinking fractions of two particle sizes (10-25 µm, 90-250 µm) at two depths (2.5-5.2 cm, 20-23 cm). For both sizes, the soil is older at depth, and larger particles are associated with younger carbon. This is consistent with our expectation that older SOM is more enriched in 13C and 15N than younger carbon. This age distribution suggests that carbon adsorbed on fine particles is protected by soil aggregates, although the stable isotopes suggest that protected SOM continues to degrade resulting in isotopic enrichment. We will discuss how soil mineralogy affects aggregate formation and consequent SOM protection.