Biodegradability of Humic Substances and Other Fractions of Decomposing Leaf Litter

Abstract

Formation of chemically resistant humic substances might be an important process controlling recycling of soil C to the atmosphere. Humic substances are believed to be resistant to microbial decomposition because they accumulate in soil, but there is little direct experimental evidence for their inherent recalcitrance. My objective was to compare the microbial mineralization rates of the humic and fulvic acid fraction to other fractions of decayed plant matter in soil. Uniformly 14C labeled Populus fremontii leaf litter that had decomposed for 180 d was fractionated into the NaOH‐insoluble residue, phenolic, humic acid, fulvic acid, hydrophilic acid, and hydrophilic neutral fractions. Humic acid comprised 22.7% of the C in the decomposed litter. These fractions were added to intact cores of soil or sand. Respired 14CO2 was collected in NaOH and the radioactivity counted. The substrate C mineralized in soil at the end of 1 yr was, in order from least to greatest percentage of added radioactivity mineralized: humic acid (12.7%), fulvic acid (29.2%), phenolic (35.4%), ground litter (38.8%), hydrophilic acid (44.6%), hydrophilic neutral (51.3%), and the NaOH‐insoluble residue (57.6%). In acid‐washed, nutrient‐amended sand, inoculated with soil microbes, the relative order of mineralization rates of the fractions was the same as in soil, but approximately 10% less C was mineralized. Results supported the hypothesis that humic substances are inherently difficult for microbes to mineralize, and this property can contribute to the sequestration of C in soil. Results also supported the hypothesis that the fulvic acid fraction is more rapidly mineralized than humic acid.