Demonstration of chemical distinction among soil humic fractions using quantitative solid-state 13C NMR

Authors: XU, JISHENG - Chinese Academy Of Sciences; ZHAO, BINGZI - Chinese Academy Of Sciences; LI, ZENGQIANG - Chinese Academy Of Sciences; CHU, WENGYING - Old Dominion University; MAO, JINGDONG - Old Dominion University; Olk, Daniel - Dan; ZHANG, JIBAO - Chinese Academy Of Sciences; XIN, XIULI - Chinese Academy Of Sciences; WEI, WENXUE - Chinese Academy Of Sciences

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2019
Publication Date: 7/1/2019
Citation: Xu, J., Zhao, B., Li, Z., Chu, W., Mao, J., Olk, D.C., Zhang, J., Xin, X., Wei, W. 2019. Demonstration of chemical distinction among soil humic fractions using quantitative solid-state 13C NMR. Journal of Agricultural and Food Chemistry. 67(29):8107-8118. https://doi.org/10.1021/acs.jafc.9b02269.
DOI: https://doi.org/10.1021/acs.jafc.9b02269

Interpretive Summary: Carbon contributes to several essential soil processes, including nutrient retention, nourishment of microbial populations and resistance to physical degradation. Because carbon in soil can vary widely in its age, researchers often find it essential to separate younger portions of soil carbon from older portions to better study effects of recent land management. One approach for separating younger portions is to wash the soil in solutions of high pH. The suitability of this approach for studying issues under field conditions has been criticized. We found that young portions of soil carbon separated through high pH solutions changed consistently in their composition in response to the type of crop grown on the soil and to the types of fertilizers applied. These results indicate that this approach does produce portions of soil carbon that are relevant to field studies. These findings are relevant to scientists who study soil carbon or soil management.

Technical Abstract: Comprising the majority of soil organic matter, humic substances (HSs) are vital to soil fertility and carbon sequestration. Here, we quantitatively determined the chemical structures of humic fractions, i.e., fulvic acid (FA), humic acid (HA) and humin (HM), isolated from two soils (upland and paddy soils) in China under six long-term (>20 years) fertilizer treatments, i.e., OF (organic fertilizer), NPKOF (combined N, P and K mineral fertilizers with organic fertilizer), NPK (balanced NPK fertilizers), NP (NP fertilizers), NK (NK fertilizers), and Control (without fertilizer). Multiple cross-polarization/magic angle spinning nuclear magnetic resonance (multiCP/MAS NMR) combined with the dipolar dephasing technique and elemental analysis were applied to characterize all HSs, and principal component analysis was used to statistically distinguish them. These techniques revealed consistent distinctions among the FAs, HAs and HMs, and the chemical natures of humic fractions clearly reflected field conditions, i.e., soil type/cropping system/soil aeration and fertilizer treatment. The FAs showed signs of both lability and recalcitrance with abundances of COO/N–C=O groups (13–19% of total C) and high percentage (52% in average) of aromatic C being nonprotonated. The HAs featured high aromaticity (0.23–0.33), and HMs contained high proportions of aliphatic compounds and elevated C contents (51–63%). Each humic fraction showed chemical distinction between the upland and paddy soils, especially with much greater aromaticity of upland HMs than of paddy HMs (0.30 vs 0.13 in average). Fertilizer treatment exerted greater influence on the chemical natures of upland HSs than of paddy HSs, although the effect was less than that of soil type. In particular, the OF treatment decreased the percentages of aromatic C and increased those of O-alkyl C in the upland HAs and HMs. Our results show the importance of multiCP/MAS NMR techniques in evaluating the changes in chemical structures of HSs. We concluded that humic fractions do respond in chemical nature to environmental conditions, suggesting they are not merely laboratory artifacts.