Inorganic Nutrients Increase Humification Efficiency and C-Sequestration in an Annually Cropped Soil

Area of Science:

• Agricultural Science
• Soil Science
• Environmental Science

Background:

• Global efforts prioritize atmospheric carbon dioxide (CO2) removal and carbon (C) storage in soil organic matter (SOM) for soil fertility and climate change mitigation.
• Conventional practices assume crop residue retention increases SOM, but empirical evidence is often lacking.
• Resistant SOM's microbial nature suggests a specific nutrient stoichiometry (C:N:P:S) crucial for humification efficiency.

Purpose of the Study:

• To test the hypothesis that supplementing crop residues with nutrients, matching resistant SOM stoichiometry, enhances soil carbon sequestration.
• To evaluate the impact of nutrient addition versus no addition on soil organic carbon (SOC) stocks over five years.
• To investigate the depth distribution of SOC changes and associated nutrient stoichiometry.

Main Methods:

• A five-year field study incorporating C-rich crop residues into soil.
• Application of supplementary nutrients to match predicted SOM stoichiometry in treatment plots.
• Comparison of SOC stocks and nutrient ratios (C:N, C:P, C:S) between nutrient-amended and unamended plots to a depth of 1.6 m.

Main Results:

• SOC stocks increased by 5.5 t C ha-1 with nutrient addition, contrasting with a 3.2 t C ha-1 reduction without nutrients.
• A net difference of 8.7 t C ha-1 in SOC was achieved over five years.
• Over 50% of SOC increase occurred below 0.3 m depth, with increased resistant SOC and soil N, P, and S at all depths.
• Soil C:N, C:P, and C:S ratios decreased with depth, potentially due to shifts in microbial communities (fungi to bacteria ratio).

Conclusions:

• Balancing the nutrient stoichiometry of added carbon to match resistant SOM is essential for increasing SOC sequestration, irrespective of the carbon input level.
• Nutrient addition significantly enhances carbon storage in soil organic matter, while its absence can lead to carbon loss.
• Global strategies for increasing SOC sequestration must consider the availability and stoichiometric balance of essential nutrients, not just carbon inputs.