Molecular signatures of soil-derived dissolved organic matter constrained by mineral weathering

Molecular-level insight into the effect of fertilization regimes on the chemodiversity of dissolved organic matter in tropical cropland

Fertilization is a critical agronomic measure for croplands in tropical regions, owing to their low fertility. However, the effects of fertilization on the quantity and chemodiversity of latosolic dissolved organic matter (DOM) in tropical regions remain largely unknown. Therefore, in this study, the variations in latosol DOM concentrations and chemodiversity induced by inorganic fertilization and the co-application of inorganic fertilization with straw return, sheep manure, biochar, and vermicompost fertilizers at a molecular level were systematically investigated using multispectral techniques and ultrahigh-resolution mass spectrometry. In line with our expectations, the results showed that combined inorganic-organic fertilization improved soil quality by increasing soil organic carbon content compared to that under inorganic fertilization. However, as the most active and bioavailable organic carbon pool, dissolved organic carbonconcentrations between the fertilization treatments were not significantly different (p = 0.07). However, the dissolved organic carbon concentrations under combined inorganic-organic fertilization treatment (NPK plus straw return, 263.45 ± 37.51 mg/kg) were lower than those under inorganic fertilization treatment (282.10 ± 18.57 mg/kg). Spectral analysis showed that the DOM in the combined inorganic-organic fertilization treatments had a higher degree of humification and lower autogenetic contributions. Furthermore, Fourier transform ion cyclotron resonance mass spectrometry analysis indicated that the combined inorganic-organic fertilization increased the chemodiversity of latosolic DOM and promoted the production of large, oxidized, and stable molecules, including lignin, aromatic, and tannin compounds, which potentially benefits soil carbon sequestration in tropical regions. This study could provide a theoretical basis for elucidating on the potentially relevant ecological functions and environmental effects of DOM under fertilization regimes.

Advancing fluorescence tracing with 3D-2D spectral conversion: A mixed culture on microbial degradation mechanisms of DOM from a large-scale watershed

Fluorescence tracing, known for its precision, rapid application, and cost-effectiveness, faces challenges due to the microbial degradation of dissolved organic matter (DOM) in aquatic environments, altering its original spectral fingerprint. This study conducted a 15-day microcosm experiment to examine the effects of biodegradation on the spectral properties of DOM from various sources: livestock excrement (EXC), urban sewage (URB), industrial wastewater (IND), and riparian topsoil (tDOM). Our findings show that while the spectral structures of DOM from different sources change during 15 days of microbial degradation, these changes do not overlap or interfere with each other. However, distinguishing between tDOM and URB in the presence of both IND and EXC is only possible at high resolution. Spectral index calculations revealed significant fluctuations and interference in FI and BIX indices among samples from different sources due to microbial degradation. In contrast, the HIX index exhibited independent fluctuations and remained a reliable spectral index for tracing. LEfSe (Linear discriminant analysis Effect Size) identified characteristic bio-indicators (CBI) for each DOM source. The CBI for tDOM and URB differed significantly; tDOM showed a marked CBI only within the first four days of microbial degradation, with a sharp decline in abundance thereafter, while URB's CBI remained abundant for 12 days. Similarly, IND's CBI maintained high relative abundance for the first 12 days. EXC's CBI was unique, showing a distinct and stable community only after six days of degradation, likely due to its high bioavailability and initial rapid microbial utilization. This study addresses the temporal variability in spectral tracing techniques caused by pollutant biodegradation. We developed a combined spectral-biological tracing technique using the "three-dimensional to two-dimensional" method along with bio-indicators, enhancing the accuracy and timeliness of spectral tracing.

Recalcitrant components accumulation in dissolved organic matter decreases microbial metabolic quotient of red soil under long-term manuring

Microbial metabolism is closely related to soil carbon dioxide emissions, which in turn is related to environmental issues such as global warming. Dissolved organic matter (DOM) affects many fundamental biogeochemical processes such as microbial metabolism involved in soil carbon cycle, not only directly by its availability, but also indirectly by its chemodiversity. However, the association between the DOM chemodiversity and bioavailability remains unclear. To address this knowledge gap, soils from two agro-ecological experimental sites subjected to various long-term fertilizations in subtropical area was collected. The chemodiversity of DOM was detected by multi-spectroscopic techniques including ultraviolet-visible spectrophotometry, Fourier transform infrared spectroscopy and excitation emission matrices fluorescence spectroscopy. Results showed that long-term manure amendments significantly decreased microbial metabolic quotient (qCO2) by up to 57%. We also observed that long-term manure amendments significantly increased recalcitrant components of DOM (indicated by the aromaticity, humification index, the ratio of aromatic carbon to aliphatic carbon, and the relative abundances of humic-like components) and decreased labile components of DOM. Negatively correlation between the qCO2 and the proportion of recalcitrant components of DOM supported that accumulation in recalcitrant components of DOM increased microbial carbon utilization efficiency. Random forest models also showed the highest contribution of the relative abundances of humic-like components and the aromaticity of DOM in affecting qCO2. Both of the redundancy analysis and structural equation modeling further indicated the decisive role of soil pH in influencing the DOM chemodiversity. Soil pH explained 56.7% of the variation in the chemodiversity of DOM. The accumulation of recalcitrant components in DOM with increasing soil pH might be attributed to the accelerated microbial consumption of bioavailability components and/or to the negative impact on the solubility of bioavailability components. Overall, this research highlights the significance of long-term manure amendments in regulating qCO2 by altering the chemodiversity of soil DOM.

Spatial distribution of sediment dissolved organic matter in oligotrophic lakes and its binding characteristics with Pb(II) and Cu(II)

Dissolved organic matter (DOM), the most active component in interstitial waters, determines the stability of heavy metals and secondary release in sediments. However, little is known about the composition and metal-binding patterns of DOM in interstitial water from oligotrophic lakes affected by different anthropogenic perturbations. Here, 18 interstitial water samples were prepared from sediments in agricultural, residential, tourist, and forest regions in an oligotrophic lake (Shengzhong Lake in Sichuan Province, China) watershed. Interstitial water quality and DOM composition, properties, and Cu(II)- and Pb(II)-binding characteristics were measured via physicochemical analysis, UV-vis spectroscopic, fluorescence excitation-emission matrix-parallel factor analysis (EEM-PARAFAC), and fluorescence titration methods. The DOM, which was produced mainly by microbial activities, had low molecular weights, humification degrees, and aromaticity. Based on EEM-PARAFAC results, the DOM was generally composed of tryptophan- (57.7%), terrestrial humic- (18.7%), microbial humic- (15.6%), and tyrosine-like (8.0%) substances. The DOM in the metal complexes was primarily composed of tryptophan-like substances, which accounted for ~42.6% of the DOM-Cu(II) complexes and ~72.0% of the DOM-Pb(II) complexes; however, microbial humic-like substances primarily contributed to the stability of DOM-Cu(II) (logKCu = 3.7-4.6) and DOM-Pb(II) (logKPb = 4.3-4.8). Water quality parameters did not significantly affect the stability of DOM-metal complexes. We demonstrated that the metal-binding patterns of DOM in interstitial water from oligotrophic lakes are highly dependent on microbial DOM composition and are affected by anthropogenic perturbations to a lesser extent.

Coupled Processes Involving Organic Matter and Fe Oxyhydroxides Control Geogenic Phosphorus Enrichment in Groundwater Systems: New Evidence from FT-ICR-MS and XANES

The enrichment of geogenic phosphorus (P) in groundwater systems threatens environmental and public health worldwide. Two significant factors affecting geogenic P enrichment include organic matter (OM) and Fe (oxyhydr)oxide (FeOOH). However, due to variable reactivities of OM and FeOOH, variable strategies of their coupled influence controlling P enrichment in groundwater systems remain elusive. This research reveals that when the depositional environment is enriched in more labile aliphatic OM, its fermentation is coupled with the reductive dissolution of both amorphous and crystalline FeOOHs. When the depositional environment is enriched in more recalcitrant aromatic OM, it largely relies on crystalline FeOOH acting concurrently as electron acceptors while serving as "conduits" to help itself stimulate degradation and methanogenesis. The main source of geogenic P enriched by these two different coupled processes is different: the former is P-containing OM, which mainly contained unsaturated aliphatic compounds and highly unsaturated-low O compounds, and the latter is P associated with crystalline FeOOH. In addition, geological setting affects the deposition rate of sediments, which can alter OM degradation/preservation, and subsequently affects geochemical conditions of geogenic P occurrence. These findings provide new evidence and perspectives for understanding the hydro(bio)geochemical processes controlling geogenic P enrichment in alluvial-lacustrine aquifer systems.

Unified Modeling Approach for Quantifying the Proton and Metal Binding Ability of Soil Dissolved Organic Matter

Soil dissolved organic matter (DOM) is composed of a mass of complex organic compounds in soil solutions and significantly affects a range of (bio)geochemical processes in soil environment. However, how the chemical complexity (i.e., heterogeneity and chemodiversity) of soil DOM molecules affects their proton and metal binding ability remains unclear, which limits our ability for predicting the environmental behavior of DOM and metals. In this study, we developed a unified modeling approach for quantifying the proton and metal binding ability of soil DOM based on Cu titration experiments, Fourier transform ion cyclotron resonance mass spectrometry data, and molecular modeling method. Although soil DOM samples from different regions have enormously heterogeneous and diverse properties, we found that the molecules of soil DOM can be divided into three representative groups according to their Cu binding capacity. Based on the molecular models for individual molecular groups and the relative contributions of each group in each soil DOM, we were able to further develop molecular models for all soil DOM to predict their molecular properties and proton and metal binding ability. Our results will help to develop mechanistic models for predicting the reactivity of soil DOM from various sources.

Combination of 3D Fluorescence/PARAFAC and UV-Vis Absorption for the Characterization of Agricultural Soils from Morocco

The present study, combining UV-Visible absorption and 3D fluorescence supported by PARAFAC chemometric analysis, focused on the characterization of soil water extractable organic matter (WEOM) in the zone of Doukkala located near the Atlantic coast of Morocco. The extracts, in water, of a set of 30 samples covering the four main types of agricultural soils in the region (commonly labeled Tirs, Faid, Hamri and R'mel) were investigated. [Formula: see text] and [Formula: see text] absorbance ratios [Formula: see text] and [Formula: see text] spectral slopes, along with their ratios[Formula: see text], as well as the fluorescence [Formula: see text] and humification [Formula: see text] indices were calculated and interpreted. In the four soil types, these parameters revealed, on the one hand, organic materials of terrigenous origin with some biological component, and showed, on the other hand, that these materials are in similar stages of humification with an important humic character. In all the soils investigated, 3D fluorescence crossed with PARAFAC chemometrics highlighted the absence of any protein component and revealed the prevalence of the fulvic acids fraction in the organic matter humic material.