Glomalin amount and compositional variation, and their associations with soil properties in farmland, northeastern China

Abstract

Glomalin‐related soil protein (GRSP) is well‐known for its soil conditioning functions, but compositional traits are rarely considered. Farmland in northeastern China is the most important commercial grain basis, and soil degradation becomes the bottleneck for keeping crop productivity. The objective of this study was to uncover the possible associations between GRSP (amount and composition) and soil properties, and make suggestions for soil improvement from soil glomalin rehabilitation in northeastern China. Here, spatial variation in GRSP amount (Easily‐extractable‐GRSP, EE‐GRSP; Total‐GRSP, T‐GRSP) and its compositional traits from infrared spectroscopy, UV‐absorbance, X‐ray diffraction (XRD) and 3‐D fluorescence spectroscopy were surveyed in 360 soil samples across northeastern China, and their association with 11 soil properties were also analyzed for finding the possible influence of soil properties on GRSP composition in farmland. There about 3‐fold spatial variation in GRSP amount was observed, while functional group variations were ranged from 1.2‐fold (O–H & N–H stretching) to 2.4‐fold (C–O stretching & O–H bending of –COOH) in different locations. The XRD showed that grain size was 113–180Å and crystallinity was 0.71–1.42%, and GRSP contained seven fluorescent compounds of tyrosine‐like, tryptophan‐like, fulvic acid‐like, soluble microbial byproduct, humic acid‐like, nitrobenzoxadidole‐like, and calcofluor white‐like. Both, EE‐GRSP and T‐GRSP positively associated with soil organic carbon (SOC), soil N (SON), soil P (SOP), alkali‐hydrolyzed N (AN), available P (AP), available K (AK), and soil water, while negatively associated with soil pH and soil bulk density. Structural equation model (SEM) analysis indicates that direct effects on GRSP amounts were mainly from soil bulk density (coefficient: –0.27), soil pH (coefficients: –0.51 to –0.57), SOC (coefficients: 0.51 to 0.69) and AP (coefficients: 0.18 to 0.26), while all other soil properties had indirect effects on GRSP amounts via their close associations with these four parameters. Compared with the GRSP amounts, soil properties laid fewer effects on GRSP compositional traits. Of 16 compositional traits, five of them showed possible regulations from soil properties, which were three infrared functional groups (IR‐II: aliphatic C–H stretching; IR‐V: C–O stretching & O–H bending of –COOH; IR‐VII: O–H binding) and two fluorescent compounds (tyrosine‐like and humic acid‐like). SEM analysis indicates that soil water, pH and EC could directly affect IR‐II, IRV, tyrosine‐like and humic acid‐like, while available nutrients showed more evident influences on infra‐red functional groups than total amounts of N, P and K. Moreover, SOC, as a media of various soil nutrients, gave the strongest influence on GRSP compositional traits. As a supplement to previous studies, we found that GRSP is a mixture of different fluorescent compounds with different functional groups. Our findings highlight that soil properties could strongly change both GRSP accumulatio...

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