Microbial efficiency drives depth-dependent soil carbon storage under organic fertilization

Area of Science:

• Soil Science
• Microbiology
• Biogeochemistry

Background:

• Microbial activity and turnover are key drivers of soil organic carbon (SOC) accumulation.
• The impact of fertilization on SOC storage across soil depths, mediated by microbial life-death cycles, lacks mechanistic understanding.

Purpose of the Study:

• To investigate how long-term fertilization, specifically organic manure, affects microbial carbon use efficiency (CUE) and soil organic carbon (SOC) accumulation at different soil depths.
• To elucidate the mechanisms by which microbial necromass contributes to SOC storage under various fertilization regimes.

Main Methods:

• Analysis of soil samples from a 45-year long-term fertilization experiment (Control, NPK, Manure, NPK+Manure).
• Measurement of microbial carbon use efficiency (CUE) using H218O labeling.
• Quantification of microbial necromass using amino sugar analysis.
• Assessment of soil properties including Fe oxides and Ca2+ content.

Main Results:

• Organic manure (M and NPKM) significantly increased microbial growth and CUE in surface soils by improving nutrient balance and soil structure.
• Manure application led to higher microbial necromass yield in surface soils compared to mineral fertilization (NPK).
• Microbial biomass, necromass, and CUE decreased with soil depth, but manure-enriched elements stabilized necromass, increasing its contribution to SOC in deep soils (74-76%) versus surface soils (34-43%).

Conclusions:

• Improving microbial CUE, particularly with organic manure, enhances SOC sequestration by promoting the formation of persistent, microbially-derived organic matter.
• Organic manure application transforms subsoils into dominant SOC sinks by stabilizing microbial necromass.
• Fertilization strategies should consider microbial processes and soil depth for effective SOC management.