08 — Soil Carbon Follows Roots (`soil-carbon-root-input`)
Supplied working report; original verdicts not validated.
Review status: Needs review. This is an archival source note, not a passed proof.
REPORT.md
# 08 — Soil Carbon Follows Roots (`soil-carbon-root-input`) - Source: Incoprea Theory, Volume XXIX (`soil-carbon-root-input`) - Claim: Soil carbon stocks correlate with root biomass more strongly than with above-ground litter input across forest plots. - Prediction: Stocks should track root biomass better than litterfall. - Refutation: Stocks tracking litterfall would remove the root pathway. ## Test performed Literature-based assessment (evidence class b) of the root-vs-litter question, plus a direct cross-plot test located in the published literature. A fresh computation was attempted with the ISRaD soil radiocarbon/SOC database ([soilradiocarbon.org](https://soilradiocarbon.org/); GitHub [International-Soil-Radiocarbon-Database/ISRaD](https://github.com/International-Soil-Radiocarbon-Database/ISRaD)), but ISRaD is a radiocarbon/SOC archive and does not carry paired root-biomass and litterfall measurements for the same plots, so a new cross-plot correlation was not feasible within this run. Instead we compiled the measured quantitative claims from the key primary studies (table in `data/test08_evidence_table.csv`, figure `figures/test08_evidence_summary.png`). ## Data (sources, URLs, sample sizes) - Sokol N.W. et al. (2019) *New Phytol.* 221:233–246 — multi-year in-situ ¹³C/¹⁵N field experiment (C₄ grass in forest understory). - Jackson R.B. et al. (2017) *Annu. Rev. Ecol. Evol. Syst.* 48:419–445 — global literature synthesis. - Sokol N.W. & Bradford M.A. (2019) *Nat. Geosci.* 12:46–53 — ¹³C-labelled soil microcosms. - Clemmensen K.E. et al. (2013) *Science* 339:1615–1618 — Swedish boreal forest island chronosequence. - Cusack D.F. et al. (2018) *Biogeochemistry* 137:253–266 — 48 humid tropical forest plots, Panama, SOC to 1 m. - Lajtha K. et al. (2018) *Sci. Total Environ.* 640–641:1112–1120 — DIRT network review (10 sites, litter addition/removal and root exclusion). ## Result (key numbers; reference figure files) Evidence supporting the hypothesis: - Sokol et al. (2019): **living root inputs are 2–13× more efficient than litter inputs** in forming both slow-cycling mineral-associated SOC and fast-cycling particulate organic C (abstract). - Jackson et al. (2017): **root inputs are ~5× more likely than an equivalent mass of aboveground litter to be stabilized as SOM** (abstract). - Sokol & Bradford (2019): belowground inputs form mineral-stabilized SOC more efficiently than aboveground inputs through the microbial pathway (abstract). - Clemmensen et al. (2013): most SOC stored in these boreal forests derives from **roots and associated (mycorrhizal) fungi, not plant litter** (abstract). - Cusack et al. (2018) — the closest published test of the cross-plot claim (48 Panamanian forest plots): **soil C does not correlate with plant biomass or litter production**; a structural-equation model with base cations, clay, rainfall as exogenous factors and **root biomass as an endogenous factor predicted ~50% of the variation** in tropical soil C stocks (abstract). Counter-evidence: - Lajtha et al. (2018, DIRT network): chronic **aboveground-litter exclusion decreased SOM more than root exclusion** did, "providing evidence that root-derived C is not more critical than aboveground litter C to soil C sequestration" (abstract). Litter doubling barely changed SOM, largely due to priming. Net reading: per unit of carbon, the root pathway is repeatedly measured to be several-fold more efficient at forming/stabilizing SOC, and the one direct cross-plot correlation study we found (Cusack 2018) matches the hypothesis exactly. But DIRT's long-term manipulations show aboveground litter inputs also regulate stocks, so the hypothesis should not be read as "litter is irrelevant". ## Verdict **Support** — the strongest direct cross-plot evidence (48 tropical forest plots) finds SOC tracking root biomass while showing no correlation with litter production, and mechanistic studies quantify a 2–13× (up to ~5×) advantage for root inputs in forming SOC; however DIRT-manipulation results temper the claim, and we could not run a fresh global correlation because paired root/litter/SOC data were not obtainable. ## Limitations Evidence class (b): literature only; no new data analysis beyond compiling published quantities. Single-study support (Cusack 2018) is one region (Panama); Sokol 2019 is one site. DIRT shows the root-vs-litter balance varies with fertility and site. Efficiency ratios (2–13×, 5×) come from abstracts, not recomputed. Association is not causation, and root biomass itself responds to soil fertility (base cations), so the causal chain may be fertility → roots → SOC. ## References - Sokol N.W., Kuebbing S.E., Karlsen-Ayala E., Bradford M.A. (2019) Evidence for the primacy of living root inputs, not root or shoot litter, in forming soil organic carbon. *New Phytologist* 221:233–246. https://doi.org/10.1111/nph.15361 - Jackson R.B., Lajtha K., Crow S.E., Hugelius G., Kramer M.G., Piñeiro G. (2017) The ecology of soil carbon: pools, vulnerabilities, and biotic and abiotic controls. *Annu. Rev. Ecol. Evol. Syst.* 48:419–445. https://doi.org/10.1146/annurev-ecolsys-112414-054234 - Sokol N.W., Bradford M.A. (2019) Microbial formation of stable soil carbon is more efficient from belowground than aboveground input. *Nature Geoscience* 12:46–53. https://doi.org/10.1038/s41561-018-0258-6 - Clemmensen K.E. et al. (2013) Roots and associated fungi drive long-term carbon sequestration in boreal forest. *Science* 339:1615–1618. https://doi.org/10.1126/science.1231923 - Cusack D.F., Markesteijn L., Condit R., Lewis O.T., Turner B.L. (2018) Soil carbon stocks across tropical forests of Panama regulated by base cation effects on fine roots. *Biogeochemistry* 137:253–266. https://doi.org/10.1007/s10533-017-0416-8 - Lajtha K. et al. (2018) The detrital input and removal treatment (DIRT) network: insights into soil carbon stabilization. *Science of the Total Environment* 640–641:1112–1120. https://doi.org/10.1016/j.scitotenv.2018.05.388 - ISRaD — International Soil Radiocarbon Database. https://soilradiocarbon.org ; https://github.com/International-Soil-Radiocarbon-Database/ISRaD