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 | +0.98b | 0.80 |
| Δ14C (‰) | −75.8 ± 34.2 | −375.8 ± 61.1 | −300b | 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 (γSOC) | 3.05 ± 0.06 | 2.26 ± 0.08 | −26.0%a | 0.83 |
| Hydrogen Index (HI, g HxCy 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 (µEa, kJ mol−1 SOM) | 159.5 ± 0.4 | 162.0 ± 0.9 | +2.4b | 0.61 |
| SD of activation energy (σEa, kJ mol−1 SOM) | 16.10 ± 0.06 | 16.80 ± 0.28 | +0.70b | 0.43 |
| T90-HxCy-pyrolysis (°C) | 521.3 ± 0.7 | 532.8 ± 0.9 | +11.5b | 0.82 |
| T50-CO2-pyrolysis (°C) | 387.2 ± 0.9 | 394.3 ± 2.6 | +7.1b | 0.51 |
| T50-CO2-oxidation (°C) | 422.5 ± 2.1 | 437.8 ± 5.2 | +15.3b | 0.56 |
| Return-on-energy-investment (ROI, ΔE:µEa) | 1.10 ± 0.03 | 0.78 ± 0.03 | −28.7%a | 0.85 |
| Microbial biomass (Cmicrobial, 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; µEa and σEa, 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 µEa; T90-HxCy-pyrolysis, temperature at which 90 % of HxCy 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 (x̄subsoil − x̄topsoil)/x̄topsoil.
bAbsolute change in soil properties for subsoil relative to topsoil: this effect size was computed as x̄subsoil − x̄topsoil. 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.