Publication Details

Soil organic matter is principally root derived in an Ultisol under oak forest

Publication Toolbox

  • Download PDF (3.0 MB)
  • This publication is available only online.
Heckman, Katherine A.; Swanston, Christopher W.; Torn, Margaret S.; Hanson, Paul J.; Nave, Lucas E.; Porras, Rachel C.; Mishra, Umakant ; Bill, Markus

Year Published

2021

Publication

Geoderma

Abstract

To a large degree, the sources and stability of soil organic carbon remain poorly constrained. A clear understanding of links among the components of the soil C cycle is hampered by the complexity of the system as well as challenges associated with partitioning bulk soil C into meaningful fractions. A large accidental 14CO2 release at the Oak Ridge Reservation in Tennessee, USA provided a strong label pulse into adjacent, well-studied oak forests, resulting in highly elevated Δ14C values in leaf litter (~1000‰) and roots (~260–450‰). A four-year manipulative study was conducted to determine the relative contribution of litter versus roots to the bulk mineral soil C pool, as well as to free light, occluded light and heavy fractions. The heavy fraction was further split into fractions with densities of 1.7–2.4 g cm-3 and >2.4 g cm-3 to test the homogeneity of the mineralassociated fraction of C. Substantial concentrations of label were detected in all soil fractions within a year of the 14CO2 release, indicating rapid incorporation of newly fixed photosynthates in all fractions of soil organic C, regardless of differences in stability inferred by previous work. This rapid incorporation of label occurred only in treatments where roots were labeled, indicating that roots are the major source of inputs to mineral soil C stocks at these sites. Separation of the heavy fraction into subfractions of intermediate (1.7–2.4 g cm-3) and high (>2.4 g cm-3) density indicated that both subfractions incorporated label at similar rates, despite significant differences in degree of microbial processing. In general, the rate of label incorporation suggested a much faster turnover for all fractions than indicated by natural radiocarbon abundance values. This suggests that within each soil fraction there are portions of slow-cycling and fast-cycling materials, and the determination of an average turnover time or mean age is dependent on experimental approach. The rapid incorporation of label into all fractions within a year regardless of inferred stability implies a high degree of heterogeneity in all fractions regardless of how finely the soils are partitioned. Further refinement of the nature and drivers of this heterogeneity could yield important insights into the soil C cycle.

Keywords

Soil organic matter; Radiocarbon; Enriched background isotope study; Density fractionation; C stabilization

Citation

Heckman, Katherine A.; Swanston, Christopher W.; Torn, Margaret S.; Hanson, Paul J.; Nave, Lucas E.; Porras, Rachel C.; Mishra, Umakant; Bill, Markus. 2021. Soil organic matter is principally root derived in an Ultisol under oak forest. Geoderma. 403(1-2): 115385. 12 p. https://doi.org/10.1016/j.geoderma.2021.115385.

Last updated on: August 24, 2021