Constrained sediment carbon sequestration driven by agricultural diffuse pollution: Evidence from core-derived DOM.

The burial of organic carbon (OC) in the river-dominated margin plays an important role in global carbon cycle, but its accumulation mechanism is not well understood. Here, we examined the concentration and distribution of water-extractable organic matter (WEOM) and base-extractable organic matter (BEOM) in surface sediments from the lower Yangtze River, estuary, and the East China Sea. Chemical characteristics of the WEOM and BEOM were described by multiple ultraviolet-visible and fluorescence spectral indicators. Concentrations of both WEOM and BEOM showed significant correlations with sediment grain size, suggesting that mineral surface area is a key factor for OC loadings on sediments. Three components (C1, C2, and C3) extracted from fluorescence excitation emission matrices-parallel factor analysis were assigned as terrigenous humic-like substance, mixed terrigenous/aquatic humic-like substance, and microbial protein-like substance, respectively. From the lower Yangtze River to the East China Sea, the C1%, specific UV absorbance at 254 nm (SUVA254), and humification index (HIX) of the WEOM decreased, while the C3%, fluorescence index (FI), and biological index (BIX) of the WEOM increased. This suggested the loss of terrigenous OC and addition of microbial OC in the WEOM. While for BEOM, the overall increase of C1% and HIX and the decrease of C3% and FI suggested selective removal of microbial OC and preferential preservation of terrigenous OC. Our study demonstrates complex behaviors of sediment organic matter (OM) during the land-to-sea transport that is largely controlled by the binding strength of OM–sediment association, and that the formation of BEOM is an important pathway for accumulation of terrigenous OM in the river-dominated margin.

Tree mycorrhizal type controls over soil water-extractable organic matter quantity and biodegradation in a subtropical forest of southern China

Biodegradation of dissolved organic matter and sedimentary organic matter in high arsenic groundwater system: Evidence from lipid biomarkers and compound-specific carbon isotopes

Lake sediments are considered invaluable natural archives that provide long-term records of past changes in climate and environment related to catchment processes as well as in-lake changes in biological communities. Moreover, lake sediments also register anthropogenic activities and man-made environmental problems. Lake sediments are known to accumulate different compounds during their formation and adsorption processes, and thus sediment investigations can be used as an important tool to assess the contamination of aquatic ecosystems. The organic matter in sediments is distributed between the particulate and dissolved phases, and usually the aquatic phase is named porewater (pw). The dissolved organic matter (DOM) is an important component of aquatic ecosystems and of the global carbon cycle. It is known that changes in DOM quality and quantity have effects on the whole ecosystem. Quantitative and qualitative changes in DOM are related to precipitation, runoff, and seasons. DOM consists of a mixture of macromolecular compounds with a wide range of chemical properties and diverse origins. The DOM in lakes can serve as a molecular proxy for identification of previous inputs of organic matter. Moreover, detailed knowledge about DOM is greatly needed in order to reconstruct palaeoclimate or land-use. The biogeochemical transformation of DOM helps to elucidate past and present environmental conditions. For all those reasons, detailed DOM characterization at the molecular level is of utmost importance. Since DOM is naturally a very complex mixture of molecules, the determination of its exact chemical composition is a complicated task. Only detailed chemical characterization using various analytical methods could be carried out. A part of DOM is optically active, enabling spectroscopic methods based on UV absorption to be used for the characterization. Another possible method of DOM analysis is chromatographic size fractionation using high-performance liquid chromatography (HPLC) with a size exclusion option (high-performance size exclusion chromatography – HPSEC). HPSEC has been widely used in studies of DOM together with spectroscopic methods. The reliability and sensitivity of this method have been reported and discussed previously (Chin et al., 1994; Hoque et al., 2003; Minor et al., 2002; Nissinen et al., 2001; Pelekani et al., 1999; Perminova et al., 1998, 2003; Specht & Frimmel, 2000; Zhou et al., 2000). Although HPSEC characterization of lake sediment pwDOM has demonstrated great potential for

Soil organic matter (SOM) in wetland soils, including peatlands, is crucial for maintaining ecosystem functions such as water quality, biogeochemical cycles, and regulating greenhouse gas emissions. Water-extractable organic matter (WEOM) comprises molecular compounds that dissolve in water under natural conditions. However, molecular-level studies of WEOM across wetlands in different climates and under various agricultural use intensities remain limited. We employed ultrahigh-resolution Orbitrap mass spectrometry to analyse WEOM and integrated it with data on climate types, agricultural intensities, environmental characteristics, molecular groups, microbial functional genes, and field-measured ecosystem respiration, methane and nitrous oxide fluxes. Wetland soil samples were collected from 25 regions representing four agricultural intensities: (1) no agriculture, (2) non-intensive grassland, (3) intensive grassland, and (4) arable land. Orbitrap identified 14,890 molecular formulas with masses ranging from 100 to 950 Daltons. Correlations between agricultural intensities and formula classes containing N, S, or P was visualised using Van Krevelen diagrams. We further examined the influence of climate types (tropical, temperate, continental) and agricultural intensity on WEOM molecular composition by Principal Coordinates Analysis, and linked WEOM quality changes with gas fluxes and other available environmental and microbiome characteristics. Ecosystem respiration, nitrous oxide emission, and agricultural intensity were positively correlated with the persistence of WEOM (i.e., aromaticity vs. aliphaticity) and negatively correlated with soil water content. Diversity of bacteria and archaea, as well as methane emission, were positively correlated with soil pH, but unrelated to WEOM quality. Our findings provide new insights into how WEOM chemistry changes under varying environmental and management conditions and advance our understanding of its role in global carbon and nutrient cycling.Keywords: Wetland, WEOM, GHG emissions, Orbitrap, climate, agricultural intensity

Organic Matter Budgets for Streams: A Synthesis

Knowledge of dynamic interactions between natural organic matter (NOM) and microbial communities is critical not only to delineate the routes of NOM degradation/transformation and carbon (C) fluxes, but also to understand microbial community evolution and succession in ecosystems. Yet, these processes in subsurface environments are usually studied independently, and a comprehensive view has been elusive thus far. In this study, we fed sediment-derived dissolved organic matter (DOM) to groundwater microbes and continually analyzed microbial transformation of DOM over a 50-day incubation. To document fine-scale changes in DOM chemistry, we applied high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and soft X-ray absorption spectroscopy (sXAS). We also monitored the trajectory of microbial biomass, community structure and activity over this time period. Together, these analyses provided an unprecedented comprehensive view of interactions between sediment-derived DOM and indigenous subsurface groundwater microbes. Microbial decomposition of labile C in DOM was immediately evident from biomass increase and total organic carbon (TOC) decrease. The change of microbial composition was closely related to DOM turnover: microbial community in early stages of incubation was influenced by relatively labile tannin- and protein-like compounds; while in later stages the community composition evolved to be most correlated with less labile lipid- and lignin-like compounds. These changes in microbial community structure and function, coupled with the contribution of microbial products to DOM pool affected the further transformation of DOM, culminating in stark changes to DOM composition over time. Our study demonstrates a distinct response of microbial communities to biotransformation of DOM, which improves our understanding of coupled interactions between sediment-derived DOM, microbial processes, and community structure in subsurface groundwater.

Linking groundwater dissolved organic matter to sedimentary organic matter from a fluvio-lacustrine aquifer at Jianghan Plain, China by EEM-PARAFAC and hydrochemical analyses

Dissolved organic matter (DOM) is a key component in the carbon and energy cycling of soil and aquatic ecosystems. Tracking DOM composition through soil profiles provides insight into the processes driving its transport and transformation. However, there is a lack of studies investigating whether DOM composition in deeper mineral soil is driven by topsoil inputs, or if processes during soil passage cause a rather uniform DOM quality irrespective of the source. Understanding the topsoil influence on subsoil DOM and depth-dependent transformation patterns is crucial for the transfer to and its fate within aquatic ecosystems. To address this knowledge gap, we examined the compositional features of DOM sampled in situ along depth profiles of four contrasting soil types (Peat, peaty Gleysol, Cambisol, Podzol) in a mountainous catchment (Ore Mountains, Germany). A combination of pyrolysis-gas chromatography/mass spectrometry and UV and fluorescence spectroscopy was used to characterize the molecular properties of DOM and similarities across the different soils and depths were achieved by Bray-Curtis dissimilarity analysis. Results revealed site-specific decreases in similarity with depth, driven by soil processes that progressively alter DOM composition. In Peat, composition remained rather similar between D1 and D2 or D3 (57-59%), likely due to constantly anoxic conditions that inhibit oxidative degradation and transformation of DOM. In the peaty Gleysol, moderate transformations were observed (41-59% similarity), likely driven by alternating redox conditions and sorptive interactions. The strongest compositional changes occurred in the Cambisol with similarity between D1 and D3 reaching 18%, suggesting microbial processing in conjunction with sorptive interactions with the mineral phase. In the Podzol, the formation of organo-metal complexes promoted selective preservation of aromatic structures. The site-specific processes led to decreases in both the number and abundance of identified shared compounds with depth, contrasting the assumption of DOM similarity across different soil types. Despite the changes with depth, subsoil DOM composition in Peat, peaty Gleysol, and Podzol still retained some imprint of topsoil sources. This study highlights how site-specific biotic and abiotic processing generates unique DOM composition that shape organic matter cycling in soils and its ecological implications in aquatic systems.

The excessive use of fertilisers and frequent and deep tillage are not considered good agricultural practices because they increase production costs and reduce soil fertility. Water extractable organic matter (WEOM) is the fraction of soil organic matter responsible for nutrient transport and bioavailability. The aim of this work was to investigate the effect of a 2-year rotation of faba bean–wheat, cultivated for a decade, and the agricultural practices (conventional vs no tillage, and fertilisation vs no fertilisation) on selected soil parameters and WEOM quality. Results showed that the soil organic carbon (SOC) and the total nitrogen (TN) content increased after the wheat and faba bean, respectively. Plots managed with conventional tillage showed WEOM with higher OC content with respect to no-tillage plots. A higher WEOM OC content was observed also in the fertilised plots. The WEOM of fertilised and faba bean plots was derived from decomposition of native SOC, but the microbial community decomposition was the main origin of WEOM after wheat.

Extensive processing of sediment pore water dissolved organic matter during anoxic incubation as observed by high-field mass spectrometry (FTICR-MS)

Divergent response of Chernozem organic matter towards short-term water stress in Poa pratensis L. rhizosphere and bulk soil in pot experiments: A spectroscopic study

Stable carbon (C) and nitrogen (N) isotopes (13C and 15N) in water-extractable organic matter (WEOM) derived from air-dried soils may be applicable to elucidate the microbial decomposition of soil organic matter (SOM), which is crucial in terrestrial C cycles. A total of 40 soil samples were collected from a depth of 0–6 cm from a temperate broadleaved forest in Japan with vegetation succession from grassland approximately 150 years ago. Those soil samples were air-dried before the water extraction process and organic matter analysis. The C and N concentrations of WEOM were <3.6% of those of the bulk soil and were positively correlated with those of the bulk soil at a p-value of < 0.01. A positive correlation between the two fractions (i.e., WEOM and bulk soils) was also found for natural 13C and 15N abundances (δ13C and δ15N; p < 0.01). However, the C/N ratio of WEOM was slightly correlated with that of bulk soils, exhibiting a narrow range of values of ~10. Thus, those features of the WEOM were similar to the well-known features of microbial biomass. The δ13C and δ15N enrichments in WEOM relative to bulk soil, the difference in stable isotope abundances between bulk SOM and WEOM were negatively and positively correlated, respectively, with the concentrations of organo-mineral complexes and short-range order minerals (non-crystalline oxyhydroxides of aluminum and iron, allophane, imogolite, and allophane-like constituents), which play significant roles in SOM stabilization in soils. These relationships suggest that the stable isotopic enrichments in WEOM can be a good indicator of the microbial utilization of soil C and N under different substrate availabilities, which are crucial to SOM decomposition and decomposability substantially varying from local to global scales.

Optimal tracer identification for dissolved organic matter (DOM) source tracking in watersheds using point source effluent load data

Water-extractable organic matter and enzyme activity in three forest soils of the Mediterranean area