In situ simultaneous measuring method for the determination of key processes of soil organic carbon cycling: Soil microbial respiration using laser spectrometry

Rationale: Soil microbial heterotrophic C-CO₂ respiration is important for C cycling. Soil CO₂ differentiation and quantification are vital for understanding soil C cycling and CO₂ emission mitigation. Presently, soil microbial respiration (SR) quantification models are based on native soil organic matter (SOM) and require consistent monitoring of δ¹³C and CO₂.

Methods: We present a new apparatus for achieving in situ soil static chamber incubation and simultaneous CO₂ and δ¹³C monitoring by cavity ring-down spectroscopy (CRDS) coupled with a soil culture and gas introduction module (SCGIM) with multi-channel. After a meticulous five-point inter-calibration, the repeatability of CO₂ and δ¹³C values by using CRDS-SCGIM were determined, and compared with those obtained using gas chromatography (GC) and isotope ratio mass spectrometry (IRMS), respectively. We examined the method regarding quantifying SR with various concentrations and enrichment of glucose and then applied it to investigate the responses of SR to the addition of different exogenous organic materials (glucose and rice residues) into paddy soils during a 21-day incubation.

Results: The CRDS-SCGIM CO₂ and δ¹³C measurements were conducted with high precision (< 1.0 µmol/mol and 1‰, respectively). The optimal sampling interval and the amount added were not exceeded 4 h and 200 mg C/100 g dry soil in a 1 L incubation bottle, respectively; the ¹³C-enrichment of 3%–7% was appropriate. The total SR rates observed were 0.6–4.2 µL/h/g and the exogenous organic materials induced -49%–28% of priming effects in native SOM mineralisation.

Conclusions: Our results show that CRDS-SCGIM is a method suitable for the quantification of soil microbial CO₂ respiration, requiring less extensive lab resources than GC/IRMS.