Do fulvic, humic and humin c fractions represent meaningful biological, physical and chemical carbon pools?

Authors: Sherrod, Lucretia; Vigil, Merle; Stewart, Catherine

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2019
Publication Date: 6/13/2019
Citation: Sherrod, L.A., Vigil, M.F., Stewart, C.E. 2019. Do fulvic, humic and humin c fractions represent meaningful biological, physical and chemical carbon pools? Journal of Environmental Quality. https://doi.org/10.2134/jeq2019.03.0104.
DOI: https://doi.org/10.2134/jeq2019.03.0104

Interpretive Summary: Soil organic matter chemistry was historically defined around strong alkaline extractions of soil to obtain fulvic, humic and humin soil C pools. Understanding if agronomic soil systems are managed to optimize the residence time of incoming organic matter materials to maintain or increase soil health properties such as fertility, water holding capacity, and general resilience to erosive factors, to name a few, depend on maintaining the slow cycling organic C pools that are so critical for soil aggregation and structure. Today researchers are questioning the relevance of humic fractions as being sensitive indicators of changes in management and/or climate change and its impact on C sequestration since other soil fractionation techniques may be more sensitive. We measured (15) proficiency soils ranging in texture, mineralogy, land-use, and soil organic C (SOC) content. We characterized fast cycling biological C pools of water-soluble organic C (WSOC) and soil microbial C biomass (SMBC); physical C pools of particulate organic matter C (POM-C) slow cycling pool, and passive mineral associated organic C (PMAOC) (passive cycling), and chemical C pools of permanganate oxidizable C (POX-C) as fast cycling, and fulvic acid (FA), humic acid (HA) and humin C (HC) fractions. We define FA as fast, HA as slow and HC passive for comparison. Correlations showed C pool of FA was correlated with POX-C (r=0.84) as a fast pool but was more strongly correlated with PMAOC (r=0.86) a passive pool. Slow C pools (HA) were strongly correlated to PMAOC (r = 0.98) and POXC (r=0.91). Alkaline extracted C pools of FA, HA and HC were similar in their correlations to POM-C, however the variability explained by them was low. The strongest correlations with the modern fast and slow cycling C pools were consistently found with HA pool.

Technical Abstract: Historically, humic fractions have been used to characterize soil organic matter chemistry and to identify soil C pools. Recent work questions the relevance of humic fractions as being sensitive indicators of climate change and its impact on C sequestration since other soil fractionation techniques may be more sensitive. Despite this interest, humic fractions are rarely compared with other biological, chemical and physical fractionation techniques. We measured (15) proficiency soils ranging in texture, mineralogy, land-use, and soil organic C (SOC) content. We characterized fast cycling biological C pools of water-soluble organic C (WSOC) and soil microbial C biomass (SMBC); physical C pools of particulate organic matter C (POM-C), slow cycling and passive mineral associated organic C (PMAOC) (passive cycling), and chemical C pools of permanganate oxidizable C (POX-C) as fast cycling, and fulvic acid (FA), humic acid (HA) and humin C (HC) fractions. We define FA as fast, HA as slow and HC passive for comparison. Correlations showed C pool of FA was correlated with POX-C (r=0.84) as a fast pool but was more strongly correlated with PMAOC (r=0.86) a passive pool. Slow C pools (HA) were strongly correlated to PMAOC (r = 0.98) and POXC (r=0.91). Alkaline extracted C pools of FA, HA and HC were similar in their correlations to POM-C, however the variability explained by them was low (r2 =< 0.25). This study showed that the alkaline extracted C pools were not as sensitive to more rapidly cycling C pools as determined by currently adopted methods of today.