title: Chemical and Isotopic Thresholds in Charring: Implications for the Interpretation of Charcoal Mass and Isotopic Data.
abstract: Charcoal plays a significant role in the long-term carbon cycle, and its use as a soil amendment is promoted as a C sequestration strategy (biochar). One challenge in this research area is understanding the heterogeneity of charcoal properties. Although the maximum reaction temperature is often used as a gauge of pyrolysis conditions, pyrolysis duration also changes charcoal physicochemical qualities. Here, we introduce a formal definition of charring intensity (CI) to more accurately characterize pyrolysis, and we document variation in charcoal chemical properties with variation in CI. We find two types of responses to CI: either linear or threshold relationships. Mass yield decreases linearly with CI, while a threshold exists across which % C, % N, and δ(15)N exhibit large changes. This CI threshold co-occurs with an increase in charcoal aromaticity. C isotopes do not change from original biomass values, supporting the use of charcoal δ(13)C signatures to infer paleoecological conditions. Fractionation of N isotopes indicates that fire may be enriching soils in (15)N through pyrolytic N isotope fractionation. This influx of "black N" could have a significant impact on soil N isotopes, which we show theoretically using a simple mass-balance model.
authors.affiliation: Department of Earth Science, Rice University , 6100 Main St MS 126, Houston, Texas 77005, United States.
authors.forename: Lacey A
authors.initials: LA
authors.lastname: Pyle
authors.affiliation: Department of Geology, Baylor University , One Bear Place #97354, Waco, Texas 76798, United States.
authors.forename: William C
authors.initials: WC
authors.lastname: Hockaday
authors.affiliation: Department of Ecosystem Science and Management, Texas A&M University , 2138 TAMU, College Station, Texas 77843, United States.
authors.forename: Thomas
authors.initials: T
authors.lastname: Boutton
authors.affiliation: Department of Chemical and Biomolecular Engineering, Rice University , 6100 Main St MS-362, Houston, Texas 77005, United States.
authors.forename: Kyriacos
authors.initials: K
authors.lastname: Zygourakis
authors.affiliation: Department of Earth Science, Rice University , 6100 Main St MS 126, Houston, Texas 77005, United States.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Kinney
authors.affiliation: Department of Earth Science, Rice University , 6100 Main St MS 126, Houston, Texas 77005, United States.
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Masiello
mesh_terms: D018533:Biomass; D002247:Carbon Isotopes; D002606:Charcoal / Q000737:chemistry*; D009587:Nitrogen Isotopes; D010944:Plants / Q000737:chemistry; D012987:Soil / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D012987:Soil; C540010:biochar; D002606:Charcoal