Abstract
Soil is one of the principal substrates of human life and can serve as a reservoir of water and nutrients. Humic substances, indicators of soil fertility, are dominant in soil organic matter. However, soil degradation has been occurring all over the world, usually by soil salinization. Sustainable soil productivity has become an urgent problem to be solved. In this study, fluorescence excitation-emission matrices integrated with parallel factor analysis (PARAFAC) and canonical correlation analysis (CCA) were applied to characterize the components of fulvic acid (FA) and humic acid (HA) substances extracted from soils from the Liaohe River Delta, China. Along the saline gradient, soil samples with four disparate depths were gathered from four aboriginal halophyte communities, i.e., the Suaeda salsa Community (SSC), Chenopodium album Community (CAC), Phragmites australis Community (PAC), and Artemisia selengensis Community (ASC). Six components (C1 to C6) were identified in the FA and HA substances. The FA dominant fractions accounted for an average of 45.81% of the samples, whereas the HA dominant fractions accounted for an average of 42.72%. Mature levels of the HA fractions were higher than those of the FA fractions, so was the condensation degree, microbial activity, and humification degree of the FA fractions. C1 was associated with the ultraviolet FA, C2 was referred to as visible FA, C3 and C4 were relative to ultraviolet HA, C5 represented microbial humic-like substances (MH), and C6 referred to visible HA. C1, C2, C5 and C6 were latent factors of the FA fractions, determined using the CCA method and could possibly be used to differentiate among the SSC, CAC, PAC and ASC samples. C3, C4, C6 and C5 were latent factors of the HA fractions, which might be able to distinguish the ASC samples from the SSC, CAC and PAC samples. Fluorescence spectroscopy combined with the PARAFAC and CCA is a practical technique that is applied to assess the humic substance content of salinized soils.
Highlights
Humic substances (HS), as typical natural supramolecular components, are predominantly stabilized by faint dispersion and originate from biodegradation [1,2,3]
The fulvic acid (FA) and humic acid (HA) fractions extracted for the Liaohe river delta soils are identified as six fluorescent components: C1 is defined as the ultraviolet FA, C2 is the visible FA, C3 and C4 are the ultraviolet HA, C5 is the microbial humic-like substances (MH), and C6 is the visible HA
The obvious variations of fluorescent components for the FA and HA fractions are represented in each soil profile, but there is no trend found in the vertical direction
Summary
Humic substances (HS), as typical natural supramolecular components, are predominantly stabilized by faint dispersion and originate from biodegradation [1,2,3]. The HS can constitute 70–80% of organic carbon [4,5,6]. They have a relatively high amount of lignin, originating from plant debris and organic materials, which is prone to be accompanied by fairly high humification degrees [7,8,9]. Despite representing a small proportion of soil isolate, they have all sorts of pivotal roles in the carbon–nitrogen cycle, availability of nutrients, and mobilization of toxic organic/inorganic substances by biochemical processes [10,11]. The FA is low in molecular weight and soluble in alkaline and acidic solutions, which can promote mineral decomposition and nutrient release [16]. The humin is insoluble in water at any pH, which exhibits recalcitrance to transformations by microorganisms and contributes to the stable carbon pool [17]
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