Elsevier

Geoderma

Volume 424, 15 October 2022, 116011
Geoderma

Dissolved pyrogenic carbon leaching in soil: Effects of soil depth and pyrolysis temperature

https://doi.org/10.1016/j.geoderma.2022.116011Get rights and content

Highlights

  • 2-year laboratory decomposition and leaching study.

  • Soil depth affected soluble carbon from wood and pyrolyzed wood.

  • Greater DOC losses from PyC300 than from PyC450 in surface soil.

  • Greater DOC losses from surface than from subsurface soil for wood and PyC300.

  • Soil and PyC properties are important to assess PyC transport.

Abstract

Pyrogenic organic carbon (PyC) is found in soils as a heterogeneous mixture of thermally altered plant residues that range in their susceptibility to losses via mineralization, leaching, and erosion. Leaching of PyC as DOC (DPyC) within the soil profile is likely influenced by the chemical composition of solid PyC and the dominant soil processes and properties at a particular depth. Here we report the results of a 2-year laboratory decomposition and leaching study designed to investigate the interactive effects of pyrolysis temperature levels (no pyrolysis, 300 °C, and 450 °C) and soil depth (surface soil, 0–10 cm depth; subsurface soil, 50–70 cm depth) on losses of 13C-labeled jack pine (Pinus banksiana) wood, wood pyrolyzed at 300 °C (PyC300) and wood pyrolyzed at 450 °C (PyC450) as dissolved organic carbon (DOC). Losses of wood and PyC in the 13C- DOC pool were measured in leachates drained from soils once a month over the course of 1 year, and at one single leaching event after 2 years. We found that pyrolysis temperature levels interacted with time and soil depth to affect losses of wood and PyC as DOC throughout the 1-year incubation and leaching study, with greater DOC losses from wood in surface soil (0.73 ± 0.076% of added C) than in subsurface soil (0.40 ± 0.063% of added C) averaged across sampling time. Monthly DOC losses from PyC were not affected by soil depth. Cumulative data indicated a small contribution of wood (2.86 ± 0.07%), PyC300 (0.40 ± 0.04%), and PyC450 (0.16 ± 0.01%) to total DOC leached from soils. DOC losses from wood and PyC300 (as a proportion of added C) were greater in surface soil than in subsurface soil, whereas DOC losses from PyC450 were unaffected by differences between surface and subsurface soil. Losses of DOC were greater from PyC300 than from PyC450 in surface soil, with no significant differences in DOC losses between PyC300 and PyC450 in subsurface soil. One single leaching event at the end of the 2-year decomposition study resulted in higher DOC losses from wood in subsurface soil than in surface soil likely due to desorption, and no differences in DOC losses between PyC300 and PyC450 regardless of soil depth. Our results suggest strong interactions between the initial physicochemical composition of organic C inputs and soil properties (soil depth as a proxy) that control the mobility and transport of PyC and should be better represented in Earth system models.

Introduction

Pyrogenic organic carbon (PyC) is a heterogeneous class of thermally altered and C-rich biomass (Goldberg, 1985, Keiluweit et al., 2010, Knicker, 2011, Bird et al., 2015). Owing to the high concentration of polyaromatic ring structures in some portions of the PyC spectrum, PyC is considered less bioavailable to soil microorganisms compared to its unburned counterpart. As such, PyC is thought to represent a C sink in the global C cycle. However, in the last two decades, a rapid increase in studies of PyC’s fate in soils and aquatic systems has provided indications that PyC is more mobile and dynamic than previously thought (Hockaday et al., 2007, Jaffe et al., 2013, Abney et al., 2017, Pyle et al., 2017, Abney and Berhe, 2018). For example, roughly 2 to 6% of the total soil dissolved organic carbon (DOC) pool drained from the O, E, and B horizons following a snowmelt event in 100-year old burned and unburned sites was PyC-derived (Santos et al. 2017). Thus, PyC in the operationally-defined DOC pool (as dissolved pyrogenic carbon or DPyC) can be transported through the soil profile with the percolating water, during which it will be mineralized, accumulate in subsoil, or will be exported to rivers (Wagner et al., 2015, Velasco-Molina et al., 2016, Soucémarianadin et al., 2019). As DPyC moves downward through the soil profile, its fate in subsoil is likely driven by the same factors known to drive the accumulation of DOC in soils such as microbial recycling, physicochemical sorption processes, and physical protection within aggregates (Kaiser and Guggenberger, 2000, Ahrens et al., 2015, Hemingway et al., 2019, Roth et al., 2019). However, little is known about how DPyC mobility and transport changes with soil depth. Determining what regulates the vertical mobility (downward translocation) of DPyC along the soil profile can improve representation of PyC dynamics and transport in Earth system models, constrain global PyC budget in soils, and improve estimations of the mean residence time of PyC exported from soils to rivers. Interest in incorporating PyC to C dynamic models (Pulcher et al. 2022) and its vertical transport as DPyC within the soil profile in land surface models (Bowring et al. 2022) have started to emerge. Bowring et al. (2022) estimated 44 Tg of DPyC was exported from land to aquatic system over the period of 1910–2010, while Jones et al. (2020) estimated rivers export 18 Tg of DPyC annually, yet large uncertainties remain on the processes influencing the export of DPyC from soils to groundwater and rivers. Given that soil likely plays a critical role in PyC persistence in terrestrial ecosystems, understanding what controls PyC losses as DPyC as it vertically moves in the soil profile is important to constrain biogeochemical models and for accurate estimates of the terrestrial PyC budget.

Subsurface soils have been reported to store large amounts of PyC, with estimations reporting ∼ 200 Pg in the uppermost 2 m depth (Reisser et al., 2016, Butnor et al., 2017, Koele et al., 2017, Soucémarianadin et al., 2019), but little is known about the dissolution and leaching of PyC in subsurface soils relative to surface soils. Given that surface soil (<30 cm depth) and subsoil (>30 cm depth) differ in dominant mechanisms that operate to protect soil C against microbial decay (Rumpel and Kögel-Knabner, 2011, Lehmann and Kleber, 2015), soil depth has to be considered when assessing the dynamics of DPyC belowground. Losses of DPyC from its parent material are expected to decrease from surface to subsurface soil mainly for two reasons: the increased protection mechanisms against C mineralization and vertical transport with depth, such as sorptive interactions with reactive soil minerals or occlusion inside soil aggregates (Lützow et al., 2006, Kögel‐Knabner et al., 2008, Moni et al., 2010, Sanaullah et al., 2011); and a concomitant decline in microbial accessibility with depth, possibly associated with the decline in microbial population density with depth (Fierer et al. 2003). Thus, DPyC is likely more mobile in surface soils than in subsurface soils, given the little sorptive interactions with soil minerals at shallow depths (Lehmann and Kleber 2015). The influence of soil depth on the dynamics of PyC remains constrained in parts because most studies limit their investigation to topsoils (>30 cm of depth). It remains unclear how soil depth interacts with other PyC chemical characteristics to affect its losses as DPyC through the soil profile.

Owing to the chemical composition of the starting solid PyC material, loss of DPyC via leaching is expected to be greater from PyC formed at lower pyrolysis temperatures than from that formed at higher pyrolysis temperatures (Gibson et al. 2018). This is similar to the trend observed for losses of wood PyC as CO2 (Baldock and Smernik, 2002, Zimmerman, 2010, Fang et al., 2014), although other factors such as depositional environment might drive the overall fate of PyC at the millennial time scale (Ascough et al. 2020). In controlled experiments, an increase in pyrolysis temperature of the solid plant biomass typically leads to progressive removal of easily leachable compounds from the solid material, such as those dominated by oxygenated functional groups (i.e., polysaccharides) (Knicker 2007; Chatterjee et al., 2012, Hatton et al., 2016). Hence, the amount of leachable PyC would be expected to be influenced by pyrolysis temperature. Evidence that the composition of the solid PyC and its leached DPyC is not similar, however, suggests that the chemical composition and mineralization rates of DPyC cannot be predicted based on the chemical composition of its parent PyC alone (Bostick et al. 2018). However, aromatic cluster size and the abundance of oxygenated functional groups have been reported as the main controls of PyC solubility (Bostick et al. 2018). Indeed, oxygen-containing functional groups (e.g., carboxyl C) are known to increase the polarity, and therefore the solubility (Kleber et al. 2015). The abundance of these functional groups also increases as PyC ages (Abiven et al. 2011), which together with studies conducted in ecosystems affected by historical fire events (Hockaday et al., 2006, Hockaday et al., 2007, Marques et al., 2017, Santos et al., 2017), supports the idea that while most of PyC losses via leaching occur at early stages of decomposition (Soong et al. 2015), continuous oxidation and subsequently leaching of PyC must occur at longer (>10 years) time-scales. Measurements of PyC losses as DOC suggest PyC-derived DOC to be relatively small. For example, losses of PyC (300 °C) derived from big blue stem (Andropogon gerardii) as CO2 and DOC at the end of 1-year incubation and leaching study were 2.59% and 1.57%, respectively (Soong et al. 2015). At the end of the 2-year field incubation in a sandy Oxisol, <0.1% of the PyC (likely formed between 400 °C and 600 °C) added to soil had leached between the first 15 cm and 30 cm as DOC (Major et al. 2010). Nonetheless, significant amounts of PyC reported in lower soil depths in temperate and tropical forests could result from the accumulation and/or retention of PyC at different size fractions, including DPyC, in lower soil depths via physical or mineral protection (Jauss et al., 2015, Butnor et al., 2017, Koele et al., 2017, Soucémarianadin et al., 2019).

In a 2-year wood and PyC decomposition study, total losses of pine wood C as CO2 were greater in surface soil than in subsurface soil, while PyC losses as CO2 were unaffected by soil depth (Santos et al. 2021), raising the question on whether losses of wood and PyC as DOC in leachate collected from surface and subsurface soil would follow a similar trend. The objective of this study was to test the interactive effects of soil depth and pyrolysis temperature on losses of solid PyC as DPyC. We hypothesized that (1) DOC losses from surface soil would be greater than those from subsurface soil, regardless of the pine wood or PyC added to the soil; (2) in both surface and subsurface soils, pine wood, with most of its chemical composition dominated by oxygen-containing C (i.e., polysaccharides) and aliphatic compounds (i.e., lipids), would exhibit the greatest losses of C as DOC compared to its pyrolyzed forms (i.e., PyC); and that (3) DOC losses would be greater from PyC formed at lower pyrolysis temperatures than from PyC formed at higher pyrolysis temperatures. To test these hypotheses, we added 13C- labelled jack pine (Pinus banksiana) wood, wood pyrolyzed at 300 °C (PyC300), and wood pyrolyzed at 450 °C (PyC450) to unlabeled surface and subsurface soils and traced the 13C label in DOC leached from these soils during a 2-year decomposition study.

Section snippets

Site, soil, and dual-labeled (13C) wood and PyC

The sampling site is in the western slope of the Sierra Nevada, in central California, below the winter snowline, but with sporadic snowfall. The elevation is 1390 m (range: 1008–1688 m), and the climate is Mediterranean (hot, dry summers and cold, wet winter), with a mean annual temperature and precipitation of 11.1 °C and 910 mm, respectively. The vegetation is mixed-conifer forest, dominated by ponderosa pine (Pinus ponderosa), incense cedar (Calocedrus decurrens), California black oak (

Effects of wood or PyC amendment and soil depth on total soil DOC in leachate

DOC concentration in leachate collected from soils throughout the incubation study was affected by the interaction between time and soil depth (Fig. 1a, 1b, 1c, 1d, p < 0.001) and between time and labeled wood or PyC amendment (p = 0.0273), with differences between the two soil depths statistically significant for all sampling times, except for day 97 (p = 0.055). Overall, DOC concentration in soil leachates peaked at days 125 and 182 across soil depths. The average concentration in surface

Discussion

Our hypothesis that DOC losses from surface soil would be greater than those from subsurface soil, regardless of the pine wood or PyC added to the soil was supported by the data. At the end of 1-year incubation and leaching study, we found that surface soils had greater DOC concentration in soil leachate (Fig. 1a-d) and cumulative DOC leached from soil (Fig. 2a). The contribution of wood and PyC to the total DOC leached from soils was greater in surface soil relative to subsurface soil (Fig. 2

Conclusions

Pyrolysis temperature (wood, and PyC formed at 300 °C and 450 °C) and soil depth (surface versus subsurface soils) interacted to affect DOC lost by wood and PyC during decomposition. Losses of DOC from wood and PyC formed at 300 °C were greater in surface soil than in subsurface soil, whereas losses of DOC from PyC formed at 450 °C were unaffected by differences between surface and subsurface soil. Together with the results of the companion study (Santos et al. 2021), our results highlight the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank Benjamin Sulman for assistance in the field, Justin Van De Velde and the UC Merced Stable Isotope Laboratory, and UC-Davis Stable Isotope Facilities for support with elemental and isotopic analyses. Funding for this study was provided from a University of California, Merced Chancellor’s Fellowship awarded to F. Santos and a National Science Foundation (CAREER EAR -1352627) award to A. A. Berhe.

References (63)

  • W.C. Hockaday et al.

    Direct molecular evidence for the degradation and mobility of black carbon in soils from ultrahigh-resolution mass spectral analysis of dissolved organic matter from a fire-impacted forest soil

    Organic Geochemistry

    (2006)
  • J.S.I. Ingram et al.

    Managing carbon sequestration in soils: concepts and terminology

    Agriculture, Ecosystems & Environment

    (2001)
  • V. Jauss et al.

    Pyrogenic carbon controls across a soil catena in the Pacific Northwest

    CATENA

    (2015)
  • K. Kaiser et al.

    The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils

    Organic Geochemistry

    (2000)
  • K.H. Kim et al.

    Influence of pyrolysis temperature on physicochemical properties of biochar obtained from the fast pyrolysis of pitch pine (Pinus rigida)

    Bioresource Technology

    (2012)
  • M. Kleber et al.

    Mineral–organic associations: formation, properties, and relevance in soil environments

    Advances in agronomy

    (2015)
  • H. Knicker

    Pyrogenic organic matter in soil: its origin and occurrence, its chemistry and survival in soil environments

    Quaternary International

    (2011)
  • N. Koele et al.

    Amazon Basin forest pyrogenic carbon stocks: first estimate of deep storage

    Geoderma

    (2017)
  • C.H. Liu et al.

    Quantification and characterization of dissolved organic carbon from biochars

    Geoderma

    (2019)
  • R.L. Sinsabaugh

    Phenol oxidase, peroxidase and organic matter dynamics of soil

    Soil Biology & Biochemistry

    (2010)
  • L. Soucémarianadin et al.

    Pyrogenic carbon content and dynamics in top and subsoil of French forests

    Soil Biology & Biochemistry

    (2019)
  • M.M. Stone et al.

    Changes in extracellular enzyme activity and microbial community structure with soil depth at the Luquillo Critical Zone Observatory

    Soil Biology and Biochemistry

    (2014)
  • M. Velasco-Molina et al.

    Biochemically altered charcoal residues as an important source of soil organic matter in subsoils of fire-affected subtropical regions

    Geoderma

    (2016)
  • R.B. Abney et al.

    Pyrogenic carbon erosion: implications for stock and persistence of pyrogenic carbon in soil

    Frontier Earth Science

    (2018)
  • R.B. Abney et al.

    Post-wildfire erosion in mountainous terrain leads to rapid and major redistribution of soil organic carbon

    Frontier Earth Science

    (2017)
  • P.L. Ascough et al.

    Chemical characteristics of macroscopic pyrogenic carbon following millennial-scale environmental exposure

    Frontiers in Environmental Science

    (2020)
  • M.I. Bird et al.

    The pyrogenic carbon cycle

    Annual Review of Earth and Planetary Sciences

    (2015)
  • K.W. Bostick et al.

    Production and composition of pyrogenic dissolved organic matter from a logical series of laboratory-generated chars

    Frontier in Earth Science

    (2018)
  • S.P. Bowring et al.

    Pyrogenic carbon decomposition critical to resolving fire’s role in the Earth system

    Nature Geoscience

    (2022)
  • M.F. Cotrufo et al.

    The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: do labile plant inputs form stable soil organic matter?

    Global change biology

    (2013)
  • Y. Fang et al.

    Biochar carbon stability in four contrasting soils

    European Journal of Soil Science

    (2014)
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    Present Address: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA.

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