Which primary soil parameters control the chemical composition of DOM and POM?
Although the chemical composition of both the POM and DOM fractions in forest soils was regulated by a series of physical, chemical and biological processes, this study showed that only a small number of soil properties gave a good description of the composition, as indicated by the high constrained variances of the RDA analysis (Figs. 2 and 3).
A strong, positive correlation was found between soil pH and the relative amount of the C3 component, representing low-molecular-weight organic compounds. Higher soil pH created more favourable circumstances for microorganisms, leading to higher biological activity (Blagodatskaya and Anderson 1998; Pietri and Brookes 2008; Cao et al. 2016) which may in turn result in the enhanced degradation of high-molecular-weight compounds, such as carbohydrates, to low-molecular-weight degradates, represented by the C3 component. A recent study also found that the low-molecular-weight PARAFAC component (325/400) increased during incubation (Wang et al. 2019). The connection between higher microbial activity and the C3 component was confirmed by the fact that the C1 component was negatively correlated with pH, suggesting that the high-molecular-weight compounds in DOM could be the primary source for biological degradation. In addition to pH, low N availability was confirmed as a limiting factor for microbial degradation, because the C/N ratio of the soils was found to be in strong correlation with the C1 component, indicating a connection between N status and microbial activity. The biological activity, controlled by pH, also affected the concentration of the C4 component, which exhibited a negative correlation with pH.
Protein-like flourescences have been reported to be a useful proxy for measuring the biodegradibility of DOM (Fellman et al. 2008, 2009; Chen and Jaffé 2016), but a more complete picture emerged from the present analysis, the data of which not only revealed a possible connction between the PARAFAC components, but also how organic matter is degraded, with high-molecular-weight compounds (represented here as C1) being degraded into lower molecular-weight compounds (C4), after which the increased microbial activity results in a high level of microbial by-products such as peptides and amino acids (C6 component) (Figs. 1 and 2).
In this study the soil texture was found to be a significant controlling factor for the composition of the DOM fraction, as confirmed by the strong negative correlation between the silt and clay contents and the SUVA values of DOM and the C2 component (Fig. 2) due to the preferential sorption of large, hydrophobic organic compounds (Jardine et al. 1989; Kaiser and Zech 2000). Avneri-Katz et al. (2017) found a significant reduction in SUVA254 values, confirming the accelerated adsorption of hydrophobic compounds suggested by the present evaluation.
In the case of POM, the organic matter was found to be selectively decomposed, which could lead to an increase in the relative abundance of compounds with high resistance to microbial degradation, such as the aromatic and phenolic compounds represented by rA1630, rA1515 and rA1270, causing an enrichment in lignin-derived compounds. Although there is still debate about the preferential decomposition of organic matter in soils (Lehmann and Kleber 2015), the increasing contribution of aromatic and phenolic compounds caused by the biological degradation of plant residues is a common phenomenon. Xu et al. (2017) also found a relative increase in phenolic and aromatic carbon in residues, while Almendros et al. (2000) described the decay processes of forest biomass as the accumulation of recalcitrant, aromatic structures. In addition to pH, the cation exchange capacity of the soils proved to be a further regulator of the relative amounts of aromatic and phenolic compounds (Fig. 3). CEC is well known as a dominant factor that stimulates bacterial respiration by maintaining the pH, replacing the H+ ions produced during metabolism with basic cations (Stotzky 1966). Due to this mechanism the less degradable aromatic moieties were abundant in POM in soils with high CEC, e.g. in CEG and NYIR2.
Several studies demonstrated a strong relationship between microbial respiration and either the C/N ratio of the litter layer (Gödde et al. 1996; Michel and Matzner 2002; Spohn 2015) or the available N (Craine et al. 2007). These finding are in accordance with the present results: the strong, negative correlation between the soil C/N ratio and the relative intensity of the band at 1160 cm− 1 (representing polysachharides) clearly demonstrated the enhanced degradation of easily decomposable materials, such as sugars, in soils with higher N availability. This was confirmed by the study of Gallo et al. (2005), who reported a common microbial response to higher N: the activity of cellulase and other glycosidases tended to increase. In parallel with this, the activity of oxidative enzymes tended to decline (Saiya-Cork et al. 2002). This was clearly demostrated in the present study by the positive correlation of the C/N ratio with the relative intensity of the band at 1710 cm− 1, which represents oxidative materials; in other words, in soils with high N availability the activity of oxidative enzymes is restricted, so the amount of carboxyl groups in POM is low.
Is the chemical composition of POM linked with that of DOM?
Particulate organic matter is considered to be one of the major sources of dissolved organic carbon (Zsolnay 2003). It has long been known that several bacteria are able to decompose the native lignin in the soil (Brauns 1952; Sørensen 1962) and that the decomposition of POM in soils can result in several aromatic compounds such as lignin-derived materials, tannins and phenols (Kaiser et al. 2001; Kalbitz et al. 2006). It is thus not surprising that the relative amount of aromatic compounds (rA1515, rA1630 for aromatic rings and rA1270 for phenolic compounds) in the POM fraction of forest soils was closely correlated with the SUVA values of the DOM fraction (Fig. 4). Based on the thermochemolysis data of forest soils Klotzbücher et al. (2013) stated that at least half the aromatic carbon comes from litter. Although Matiasek and Hernes (2019) reported different solubilisation patterns for lignin phenols, indicated by the C:V ratios (ratio of cinnamyl phenols to vanillyl phenols), which were six-fold lower in DOM, the optical characteristics of these compounds were probably identical.
The degradation of lignin-carbohydrate complexes is a prerequisite for the solubilisation of lignin, a process is linked with the degradation of cellulose, resulting in a relatively labile C source partly shielded by lignin (Jeffries 1991). Furthermore, lignin oxidation is a metabolic process that requires easily degradable carbon sources (Kirk and Farrell 1987). Klotzbücher et al. (2013) also suggested that lignin degradation and solubilisation could be related to the production of soluble carbohydrates that provide energy for microbes. In the present study, these enzyme-mediated processes were confirmed by the positive correlation between rA1515 and rA1630 in POM and the C3 component of DOM, indicative of enhanced microbial activity. In addition, the relative intensity of the band at 1160 cm− 1, representing the carbohydrate content of POM, was in strong correlation with the C2 component and SUVA indexes of DOM, clearly revealing the connection between carbohydrates and the rate of solubilisation of lignin-derived compounds.
Complex coupling was revealed by RDA between the rA1710 value of POM and the C5 component (tryptophan-like materials) of DOM (Fig. 4). As discussed earlier, a lower N supply could favour oxidative enzyme processes (Saiya-Cork et al. 2002). However, the low N level could also inhibit other special microbial decomposition processes, which means that the fluorescence of proteinaceous tryptophan-like components, found to be a useful indicator of a reduction in the total microbial activity in wastewater (Cohen et al. 2014), could be less useful if nitrogen is limited. This could explain the negative correlation revealed by statistical analysis between the rA1710 value of POM and the C5 component of the DOM fraction.