Design and Validation of an Assay for Isotope Ratio Mass Spectrometry Analysis of Biologically Relevant Dissolved and Heat-Extracted Organic Carbon with Neutral Potassium Phosphate Buffer

ABSTRACT

Labile soil dissolved organic carbon (DOC) and heat-extracted carbon (HEC) are sensitive indicators of changing soil organic carbon (SOC) stocks. Isotope ratio mass spectrometry (IRMS) is an important tool for studying SOC turnover and soil biological function. Several complications are involved in measuring DOC/HEC, for example salt ionic strength; solution pH; and anionic damage to elemental analyzer-IRMS. We evaluated a method for DOC/HEC analysis with 0.1 M potassium phosphate buffer (PPB). This method was strongly correlated with commonly used carbon (C) extractants for C quantification. Carbon-13 comparisons between DOC/HEC extracted with Milli-Q water and PPB were similar. The δ¹³C (‰) values of particulate OC and DOC were similar, whereas the relationship between humic OC and HEC was soil specific. An incubation experiment demonstrated that DOC/HEC δ¹³C (‰) successfully explained respired microbial carbon dioxide over 90 days. We conclude that this method represents an alternative for DOC/HEC quantification and δ¹³C (‰) analyses.

KEYWORDS:

• Carbon
• isotope ratio mass spectrometry
• potassium phosphate buffer
• soil chemistry
• testing methodology

Funding

The authors thank the National Soil Carbon Program funded by the Australian Commonwealth Department of Agriculture Farming Futures Filling the Research Gap for funding.