Climate and litter C/N ratio constrain soil organic carbon accumulation

Abstract

Soil organic carbon (SOC) plays critical roles in stabilizing atmospheric CO₂ concentration, but the mechanistic controls on the amount and distribution of SOC on global scales are not well understood. In turn, this has hampered the ability to model global C budgets and to find measures to mitigate climate change. Here, based on the data from a large field survey campaign with 2600 plots across China's forest ecosystems and a global collection of published data from forested land, we find that a low litter carbon-to-nitrogen ratio (C/N) and high wetness index (P/PET, precipitation-to-potential-evapotranspiration ratio) are the two factors that promote SOC accumulation, with only minor contributions of litter quantity and soil texture. The field survey data demonstrated that high plant diversity decreased litter C/N and thus indirectly promoted SOC accumulation by increasing the litter quality. We conclude that any changes in plant-community composition, plant-species richness and environmental factors that can reduce the litter C/N ratio, or climatic changes that increase wetness index, may promote SOC accumulation. The study provides a guideline for modeling the carbon cycle of various ecosystem scales and formulates the principle for land-based actions for mitigating the rising atmospheric CO₂ concentration.

Keywords: annual litterfall; litter carbon-to-nitrogen; soil organic carbon; soil texture; wetness index.

Figure 1.

Structural equation models (SEMs) showing the strong negative relationship between the litter C/N ratio and the SOC concentration (g C kg⁻¹) in the whole soil profile or in the top vs deep soil, and the contributions of the wetness index (P/PET), plant-species diversity (Shannon index) and annual litterfall (t ha⁻¹). Soil layers (a) 0–100 cm, (b) 0–20 cm and (c) 20–100 cm. The blue and red arrows indicate positive and negative relationships, respectively. The width of the arrows indicates the strength of the relationships. Numbers adjacent to arrows are standardized path coefficients and are indicative of the effect size of the relationship. *P < 0.05; **P < 0.01; ***P < 0.001. Dashed arrows indicate non-significant relationships (P > 0.05). The final models fit the data well, as suggested by the χ² and P-values. (Note P > 0.05 indicates that the final model fits the data well; see the ‘Methods’ section.)

Figure 2.

Structural equation models (SEMs) showing the connections between the litter carbon-to-nitrogen ratio (litter C/N ratio), wetness index (P/PET-precipitation/potential evapotranspiration), tree-species diversity (Shannon index), soil texture (claysilt, cumulative percentages (%) of particles <50 μm in diameter) and annual litterfall (litterfall, t ha⁻¹) with soil organic carbon concentration (SOC, g C kg⁻¹) in soil layers (a) 0–100 cm, (b) 0–20 cm and (c) 20–100 cm. The blue and red arrows indicate positive and negative relationships, respectively. The width of the arrows indicates the strength of the relationships. Numbers adjacent to arrows are standardized path coefficients and are indicative of the effect size of the relationship. *P < 0.05; **P < 0.01; ***P < 0.001. Dashed arrows indicate non-significant relationships (P > 0.05). The final models fit the data well, as suggested by the χ² and P-values (χ² = 2.48, P = 0.65, df = 4 and n = 278 in (a); χ² = 1.99, P = 0.74, df = 4 and n = 278 in (b) and χ² = 2.21, P = 0.70, df = 4 and n = 278 in (c)).

Figure 3.

Boxplot showing the relationships between SOC concentrations (g C kg⁻¹) and (a) P/PET, (b) litter C/N ratio, (c) litterfall (t ha⁻¹ yr⁻¹) and (d) claysilt (%), using data collected from the literature.

Figure 4.

Boxplot showing the relationships between SOC concentrations (g C kg⁻¹) and (a) P/PET, (b) litter C/N ratio, (c) claysilt (%) and (d) annual litterfall (t ha⁻¹ yr⁻¹), using data from both the survey of China's forest ecosystems and the published literature.