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Humin is the most recalcitrant fraction of soil organic matter (SOM). However, little is known about quantitative structural information on humin and the roles of soil mircoorganisms involved in the humin formation. We applied advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to provide deep insights into humin structural changes in response to long-term balanced fertilization on a Calcaric Fluvisol in the North China plain. The relationships between humin structure and microbiological properties such as microbial biomass, microbial quotient (qmic) and metabolic quotient (qCO2) were also studied. The humins had a considerable (35-44%) proportion of aromatic C being nonprotonated and the vast majority of O-alkyl and anomeric C being protonated. Alkyl (24-27% of all C), aromatic C (17-28%) and O-alkyl (13-20%) predominated in humins. Long-term fertilization promoted the aliphatic nature of humins, causing increases in O-alkyl, anomeric and NCH functional groups and decreases in aromatic C and aromatic CO groups. All these changes were more prominent for treatments of organic fertilizer (OF) and combined mineral NPK fertilizer with OF (NPKOF) relative to the Control and NPK treatments. Fertilization also decreased the alkyl/O-alkyl ratio, aromaticity and hydrophobic characteristics of humins, suggesting a more decomposed and humified state of humin in the Control soil. Moreover, the soil microbiological properties had strong correlations with functional groups of humins. Particularly, microbial biomass C was a relatively sensitive indicator, having positive correlations with oxygen-containing functional groups, i.e., COO/NCO and protonated O-alkyl C, and negative correlations with nonprotonated aromatic C. The qmic and qCO2 were also significantly positively correlated with NCH and aromatic CO, respectively. Our results deepen our understanding of how long-term fertilization impacts the structure of humin, and highlight a linkage between microbiological properties and recalcitrant fraction of SOM besides labile fraction.
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