A UV ABSORBANCE TEST FOR MEASURING HUMIFIED ORGANIC CARBON IN SOIL

STANDARD METHODS used for measuring soil organic carbon are insensitive to short-term changes associated with soil management practices. The conventional wet oxidation and furnace induction methods measure all forms of soil organic carbon to some extent, not differentiating between rapidly recycled plant debris, organic residues, soil biomass or more resilient soil humic substances. The only accepted laboratory method for measuring resilient, sequestered carbon in soil (nuclear magnetic resonance or NMR), requires expensive laboratory equipment. Our new method uses off-the-shelf equipment developed for quantifying aquatic humic substances, measuring ultraviolet (UV) light absorbance. UV absorbance at 253.7 nm is strongly correlated with resilient aromatic compounds in aquatic humic substances quantified using NMR analysis. We have adapted water quality protocols for humified dissolved organic carbon (DOC), to measure humified organic carbon in soil. Under laboratory and field conditions, the UV method is sufficiently sensitive to detect changes in the concentration of humified DOC in soil amended with mulch only, or with mulch and compost. Results for total DOC and Specific UV Absorbance (SUVA; UV absorbance standardised per mg of DOC) for nine sugarcane soils revealed large differences in total and humified DOC that were related to soil acidity. The soil health properties associated with humic substances include pH buffering, nutrient and water holding capacity, and improved soil structure. Further testing will validate the utility of the UV method for benchmarking and monitoring the impact of land management practices on soil health.