Table 1 Depth variation in soil organic carbon (SOC) properties averaged across the three soil types

	Topsoil	Subsoil	Depth effect size	Partial η²
SOC concentration ([SOC], g SOC kg⁻¹ soil)	85.5 ± 5.6	34.7 ± 5.6	−59.4%^a	0.57
fPOM (% SOC)	8.3 ± 0.6	2.7 ± 0.3	−67.9%^a	0.82
fMAOM (% SOC)	91.7 ± 0.6	97.3 ± 0.3	+6.1%^a	0.82
δ¹³C (‰)	−26.85 ± 0.12	−25.88 ± 0.27	+0.98^b	0.80
Δ¹⁴C (‰)	−75.8 ± 34.2	−375.8 ± 61.1	−300^b	0.88
SOC turnover time (τ, year)	1096 ± 340	7566 ± 2033	+590%^a	0.56
Energy density (ΔE, kJ mol⁻¹ SOM)	175.1 ± 4.5	126.7 ± 4.5	−27.6%^a	0.85
Degree of reduction of SOC (γ_SOC)	3.05 ± 0.06	2.26 ± 0.08	−26.0%^a	0.83
Hydrogen Index (HI, g H_xC_y kg⁻¹ SOC)	229.1 ± 16.9	114.5 ± 10.6	−50.0%^a	0.68
Oxygen Index (OI, g O₂ kg⁻¹ SOC)	180.1 ± 9.6	222.2 ± 19.1	+23.4%^a	0.55
Mean activation energy (µE_a, kJ mol⁻¹ SOM)	159.5 ± 0.4	162.0 ± 0.9	+2.4^b	0.61
SD of activation energy (σE_a, kJ mol⁻¹ SOM)	16.10 ± 0.06	16.80 ± 0.28	+0.70^b	0.43
T₉₀-H_xC_y-pyrolysis (°C)	521.3 ± 0.7	532.8 ± 0.9	+11.5^b	0.82
T₅₀-CO₂-pyrolysis (°C)	387.2 ± 0.9	394.3 ± 2.6	+7.1^b	0.51
T₅₀-CO₂-oxidation (°C)	422.5 ± 2.1	437.8 ± 5.2	+15.3^b	0.56
Return-on-energy-investment (ROI, ΔE:µE_a)	1.10 ± 0.03	0.78 ± 0.03	−28.7%^a	0.85
Microbial biomass (C_microbial, g C kg⁻¹ soil)	1.25 ± 0.17	0.32 ± 0.03	−74.1%^a	0.73
Microbial biomass per unit SOC (g C kg⁻¹ SOC)	15.2 ± 0.9	10.9 ± 1.4	−28.1%^a	0.37
Root density (g dm⁻³)	3.21 ± 0.82	0.14 ± 0.03	−95.6%^a	0.52

fPOM fraction of particulate organic matter; fMAOM, fraction of mineral-associated organic matter; µE_a and σE_a, mean and standard deviation of the activation energy of soil organic carbon (SOC) decomposition; Return-on-energy-investment (ROI) is the ratio of ΔE to µE_a; T₉₀-H_xC_y-pyrolysis, temperature at which 90 % of H_xC_y was evolved during pyrolysis; T₅₀-CO₂-pyrolysis and T₅₀-CO₂-oxidation, temperatures at which 50 % of CO₂ was evolved during pyrolysis and oxidation, respectively. Mean ± standard errors (n = 12 soil cores, including 3 soil types with 4 replicates each).
^aRelative change in soil properties for subsoil relative to topsoil: this effect size was computed as (x̄_subsoil − x̄_topsoil)/x̄_topsoil.
^bAbsolute change in soil properties for subsoil relative to topsoil: this effect size was computed as x̄_subsoil − x̄_topsoil. Partial η² indicates the proportion of variance associated with the depth effect after accounting for soil type effect. See Supplementary Table 1 for depth variation in soil properties for each soil type, and Supplementary Table 2 for statistical results.

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