Intensive N₂ fixation accelerates microbial turnover in cropland soils.

Area of Science:

• Soil microbiology
• Biogeochemistry
• Nitrogen cycling

Background:

• Biological nitrogen fixation (BNF) is crucial for soil fertility but is sensitive to carbon (C) and nitrogen (N) availability.
• The metabolic cost of nitrogenase activity makes the response of BNF to organic C input and its effect on microbial turnover unclear.

Purpose of the Study:

• To investigate the impact of organic carbon addition on BNF and microbial turnover in cropland soils.
• To identify the key microbial players and soil factors influencing glucose-induced BNF.

Main Methods:

• Combined 15N2 tracing with high-throughput sequencing.
• Incubated three cropland soils with glucose or glucose plus mineral N fertilizer for 12 days.
• Utilized structural equation models (SEMs) and random forest (RF) analyses.

Main Results:

• Glucose addition significantly increased BNF activity (0.76-2.51 mg N kg-1 d-1), while mineral N fertilizer completely inhibited it.
• Glucose-induced BNF supported microbial growth and organic C mineralization comparable to direct N assimilation.
• Azotobacter-affiliated diazotrophs were the primary drivers (>50% of nifH genes) of glucose-induced BNF.
• Soil C:N ratio and Azotobacter abundance were key factors influencing BNF.

Conclusions:

• Free-living diazotrophs play a critical role in soil microbial turnover of organic carbon.
• Organic carbon input, particularly in high C:N ratio soils, can stimulate BNF and support microbial activity.