Biogeochemistry最新文献

Deadwood represents a dynamic carbon pool in forest ecosystems where microbial decomposition causes fluxes of CO₂ to the atmosphere through respiration and organic carbon to the soil through leakage and fragmentation. This study characterises different stages of deadwood of Norway spruce (Picea abies). 35 Norway spruce trees were sampled and categorized on a 0–5 decay scale. For the 14 trees in classes 0–3, two stem discs were collected from two heights. For the 21 trees in classes 4 and 5, a single sample per tree was taken, because decay was relatively uniform throughout the stem. The relative amount of hemicellulose and cellulose declined moderately from decay class 1 to 3 and substantially from decay class 3 to class 4 but small amounts were still present in decay class 5. The relative lignin proportion increased substantially from decay class 3 to 4 and dominated in decay class 5. Relative carbon content increased from 50 to 56% during the decomposition process due to the increasing accumulation of lignin residuals being a typical signature of brown rot decay. A laboratory experiment including three species of brown rot fungi verified decomposition close to 70% of Norway spruce biomass and resulted in 55% carbon content. This was similar to the carbon content in decay class 4 and 5. A novel approach is presented to quantify the carbon flux from deadwood to the soil. First, we calculated the residual proportion of carbon in decayed wood compared to the initial carbon content of live trees. Subsequently, we extended the calculation to determine the amount of remaining carbon from non-decayed wood that was transferred to the soil during each decay class. The approach showed that Norway spruce wood decomposition under field conditions transfers at least 39–47% of the initial wood carbon to the soil carbon pool, depending on soil type. This strengthens the previously under-communicated fact that the carbon flux from deadwood to soil is higher from brown rot decomposition in boreal forests than the corresponding carbon flux in temperate and tropical forests where deadwood is more influenced by white rot fungi.

Quantifying the relative influence of factors and processes controlling riverine ecosystem function is essential to predicting future conditions under global change. Dissolved organic matter (DOM) is a fundamental component of riverine ecosystems that fuels microbial food webs, influences nutrient and light availability, and represents a significant carbon flux globally. The heterogeneous nature of DOM molecular composition and its propensity for interaction (i.e., functional diversity) can characterize riverine ecosystem function across spatiotemporal scales. To investigate fundamental drivers of DOM diversity, we collected seasonal water samples from 42 nested locations within five watersheds spanning multiple watershed sizes (~ 5 to 30,000 km²) across the United States. Patterns in DOM molecular richness, aromaticity, relative abundance of N-containing formulas, and putative biochemical transformations derived from high-resolution mass spectrometry were assessed across gradients of explanatory variables associated with watershed characteristics (e.g., watershed area, water residence time, land cover). We found that putative biochemical transformations were more strongly related to explanatory variables across watersheds than common bulk DOM parameters and that watershed area, surface water residence time and derived Damköhler numbers representing DOM reactivity timescales were strong predictors of DOM diversity. The data also indicate that catchment-specific land cover factors can significantly influence DOM diversity in diverging directions. Overall, the results highlight the importance of considering water residence time and land cover when interpreting longitudinal patterns in DOM chemistry and the continued challenge of identifying generalizable drivers that are transferable across watershed and regional scales for application in Earth system models. This work also introduces a Findable Accessible Interoperable Reusable (FAIR) dataset (> 300 samples) to the community for future syntheses.

The relative influence of seasonal patterns in hydrological flow and seasonal differences in biological and geochemical activity on stream chemistry patterns is difficult to discern because they covary; temperate systems are characterized by lower mean flow in the summer (i.e. corresponding to deeper flow paths, elevated temperature, and biological activity), and higher mean flow in the winter (i.e. corresponding to shallower flow paths, depressed temperature, and biological dormancy). Using 2018 data, when seasonal stream flow conditions reversed, and two prior conventional water years, the relationship between monthly acid-relevant analyte concentrations and streamflow were compared within and between winter and summer to provide insight into controls on characteristic seasonal chemistry patterns at two mid-Appalachian sites with distinct geology (weatherable mafic and weather resistant siliciclastic). Acid neutralizing capacity (ANC) increased (1) with lower flow, in both seasons and (2) in summer, for all flow conditions. The compounding impacts resulted in a doubling of concentration from typical winter with high flow to summer with low flow at both sites. Base cation patterns tracked ANC at the mafic site, resulting in an ~ 60% increase of from winter with high flow to summer with low flow; distinctions between summer and winter contributed more to the seasonal pattern (72%) than changes in flow. Sulfate increased at the mafic site (1) with higher flow, in both seasons and (2) in winter, for all flow conditions, resulting in an ~ 50% increase from summer with low flow to winter with high flow; distinctions between winter and summer conditions and flow contributed similarly (40–60%) to the typical seasonal chemical pattern. The biogeochemical mechanism driving differences in stream chemistry between summer and winter for the same flow conditions is likely increased rates of natural acidification from elevated soil respiration in summer, resulting in greater bedrock weathering and sulfate adsorption. Findings highlight the significance and consistency of growing vs dormant season variations in temperature and biological activity in driving intra-annual patterns of stream solutes. This data set informs parameterization of hydro-biogeochemical models of stream chemistry in a changing climate at a biologically relevant, seasonal, timescale.

Climate change may alter soil microbial communities and soil organic matter (SOM) composition. Soil carbon (C) cycling takes place over multiple time scales; therefore, long-term studies are essential to better understand the factors influencing C storage and help predict responses to climate change. To investigate this further, soils that were heated by 5 °C above ambient soil temperatures for 18 years were collected from the Barre Woods Soil Warming Study at the Harvard Forest Long-term Ecological Research site. This site consists of large 30 × 30 m plots (control or heated) where entire root systems are exposed to sustained warming conditions. Measurements included soil C and nitrogen concentrations, microbial biomass, and SOM chemistry using gas chromatography–mass spectrometry and solid-state ¹³C nuclear magnetic resonance spectroscopy. These complementary techniques provide a holistic overview of all SOM components and a comprehensive understanding of SOM composition at the molecular-level. Our results showed that soil C concentrations were not significantly altered with warming; however, various molecular-level alterations to SOM chemistry were observed. We found evidence for both enhanced SOM decomposition and increased above-ground plant inputs with long-term warming. We also noted shifts in microbial community composition while microbial biomass remained largely unchanged. These findings suggest that prolonged warming induced increased availability of preferred substrates, leading to shifts in the microbial community and SOM biogeochemistry. The observed increase in gram-positive bacteria indicated changes in substrate availability as gram-positive bacteria are often associated with the decomposition of complex organic matter, while gram-negative bacteria preferentially break down simpler organic compounds altering SOM composition over time. Our results also highlight that additional plant inputs do not effectively offset chronic warming-induced SOM decomposition in temperate forests.

Dissolved organic matter (DOM) concentrations have risen by a factor of two or more across much of Europe and North America during recent decades. These increases have affected the carbon cycle, light regime, drinking water treatability, and the energy and nutrient budgets of lakes and streams. However, while trends in DOM quantity are well characterised, information on how/whether qualitative properties of DOM have changed are scarce. Here, we describe over 40 years of monitoring data from a forested headwater stream in the Gårdsjön experimental catchment, southwest Sweden, which provides a unique record of biogeochemical change, including optical and stoichiometric DOM quality metrics, spanning the entire period of recovery from acidification. For the period 1980–2020 we find a 71% reduction in decadal mean sulphate concentrations, and a similar reduction in inorganic aluminium concentrations, alongside a 64% increase in dissolved organic carbon (DOC) concentrations. Over the same period, colour (absorbance at 420 nm) increased almost twice as much as DOC, whereas dissolved organic nitrogen (DON) increased by only one third as much. These results demonstrate a shift in stream water composition, with DOM becoming dominated by highly coloured, complex, nitrogen-poor compounds. This material is likely more resistant to biological degradation, but more susceptible to photochemical degradation. Changes in DOM stoichiometry could lead to intensified nitrogen and/or phosphorus limitation in surface waters, while increased colour/DOC ratios could intensify light-limitation of primary production beyond that expected from DOC increases alone. We observed increases in organic matter associated metals (iron 117%, organically complexed aluminium 85%) that exceeded the increase in DOC, consistent with their increased mobilisation by more aromatic organic matter. All observed changes are consistent with recovery from acidification being the primary driver of change, implying that past acidification, and ongoing recovery, have profoundly affected terrestrial and aquatic biogeochemistry, ecology and the carbon cycle.

Enhanced rock weathering (ERW), the application of crushed silicate rock to soil, can remove atmospheric carbon dioxide by converting it to (bi) carbonate ions or solid carbonate minerals. However, few studies have empirically evaluated ERW in field settings. A critical question remains as to whether additions of crushed rock might positively or negatively affect soil organic matter (SOM)—Earth’s largest terrestrial organic carbon (C) pool and a massive reservoir of organic nitrogen (N). Here, in three irrigated cropland field trials in California, USA, we investigated the effect of crushed meta-basalt rock additions on different pools of soil organic carbon and nitrogen (i.e., mineral-associated organic matter, MAOM, and particulate organic matter, POM), active microbial biomass, and microbial community composition. After 2 years of crushed rock additions, MAOM stocks were lower in the upper surface soil (0–10 cm) of plots with crushed rock compared to unamended control plots. At the 2 sites where baseline pre-treatment data were available, neither total SOC nor SON decreased over the 2 years of study in plots with crushed rock or unamended control plots. However, the accrual rate of MAOM-C and MAOM-N at 0–10 cm was lower in plots with crushed rock vs. unamended controls. Before ERW is deployed at large scales, our results suggest that field trials should assess the effects of crushed rock on SOM pools, especially over multi-year time scales and in different environmental contexts, to accurately assess changes in net C and understand the mechanisms driving interactions between ERW and SOM cycling.

The large leakage accidents of heavy metals from industrial facilities pose a serious environmental problem; however, not enough studies have been conducted to assess the long-term ecological risk associated with such accidents. This study evaluated changes in the bacterial communities within river sediment and identified the key functional microorganisms responding to the 2012 cadmium contamination incident in the Long River, Guangxi Province, China. Results revealed that after a prolonged period of pollution accidents, cadmium pollution still had a discernible effect on the bacterial communities of the river sediment. In comparison to the control site (S1), the bacterial α-diversity in sediments from the accident area (S3) and its downstream (S5) showed a significant increase following the incident. In the control site, Burkholderiaceae was dominant, while in S3 and S5, Pedosphaeraceae, Nitrosomonadaceae, Nitrospiraceae and Geobacteraceae were significantly increased. Sulfur bacteria were found to be more responsive to this cadmium contamination than other bacteria. At site S3, the abundances of Sulfuricurvum, Sulfurifustis, Thioalkalispira, Desulfobacteraceae and Desulfarculaceae were hundreds of times higher than at site S1, indicating an intensification of sulfur cycling processes. The functional prediction implied that cadmium pollution may promote methane oxidation coupled with sulfate reduction reactions and altered the processes of nitrification and denitrification. Environmental factors influencing the microbial community included the levels of metals (cadmium, arsenic, iron) in sediment, as well as other sediment characteristics like temperature and electrical conductivity. These findings contribute to our understanding of the long-term ecological consequences of environmental pollution in river ecosystems.

Graphical abstract

It is generally assumed that fertilizer addition is the prime driver of nitrogen (N) gas loss from modern cropping systems. This assumption has its basis in observations of nitrous oxide (N₂O, an important greenhouse gas) emissions, and is contrary to theory from unmanaged ecosystems, where N losses are controlled by plant physiological influence on the soil environment. However, dinitrogen (N₂) emissions are likely a major N loss pathway in both managed and unmanaged ecosystems, but these emissions are very difficult to measure. We directly measured N₂ and N₂O emissions from two temperate agricultural systems over the course of the growing season to test when total N gas losses are highest. We hypothesized that N₂ emissions mirror those of N₂O, with the largest flux immediately after fertilization, early in the growing season. Instead, we found that N₂ emissions were highest at the end of the growing season, and were most strongly correlated with soil moisture, which increased after plant senescence. Dinitrogen emissions were an order of magnitude larger than N₂O. Thus, while N₂O emissions were highest following fertilization, overall N gas loss was greatest at the end of the growing season. These data suggest that total N gas losses are high and have different temporal patterns from N₂O fluxes. Understanding the magnitude and controls over these losses are important for understanding and managing the N cycle of temperate agricultural systems.

Soil carbon (C) stability is an important consideration for management that aims to increase long-term C storage. The fraction of soil C allocated to physico-chemically protected mineral-associated organic matter (MAOM) is a common soil C stability benchmark. However, the reality of soil C persistence is more complex than MAOM content alone—particularly in ecosystems such as meadows with high rates of belowground C inputs that can stimulate MAOM decomposition. Here, we combined three metrics of soil C persistence to characterize soil C stability across a meadow restoration chronosequence averaging belowground C gains of 330 g C m⁻² y⁻¹ for ~20 y. The metrics were: (1) the fraction of soil C in MAOM and particulate organic matter (POM), (2) the susceptibility of soil C to decomposition under varying temperatures, and (3) the utilization of MAOM-C by microbes. Two metrics suggested soil C stability may increase following montane meadow restoration. As soil C concentration increased with restoration, C storage in MAOM, but not POM, increased (metric 1). The susceptibility of MAOM-C to decomposition (microbial respiration relative to MAOM-C) decreased with increasing soil C concentration across temperatures (metric 2). Stable isotope results could not definitively determine the source of carbon dioxide efflux (metric 3) but generate hypotheses for future research to address. We posit that C sequestered following montane meadow restoration could be stable, with implications for regional C storage objectives. Further, our data point toward complex mineral-associated C dynamics including the potential importance of plant inputs for MAOM formation in meadow soils.

Fecal sterols are traditionally ascribed as important biomarkers for animal excrement, and have been widely used to identify the source of organic matter and to reconstruct paleoecological changes in Antarctic terrestrial, aquatic, and marine ecosystems. However, the in situ microbial hydrogenation of cholesterol to coprostanol could have significance as a proxy to infer paleoenvironmental studies in Antarctica, particularly in anoxic sediment. Here, we report that abundant coprostanol, which was traditionally deemed as a biomarker for human sewage contamination, was found in three anoxic sediment profiles (AC2, BI, and CH1), which were strongly influenced by animal excrement at North Victoria Land, western Ross Sea, Antarctica. Our results suggest that the high concentrations of coprostanol in these three sediment profiles were not due to animal excrement, since coprostanol is not present in penguin guano and is a minor component in seal excrement. Coprostanol/cholesterol and coprostanol/(coprostanol + cholestanol) ratios suggest that coprostanol in the sediment cores of AC2 and BI was primarily derived from bacterial hydrogenation of cholesterol introduced by penguin guano. Coprostanol in CH1 sediments is related to human sewage due to intensive research activities from 1968 to 2006 in this region. However, the low abundance of coprostanol and the ratios of coprostanol/cholesterol and coprostanol/(coprostanol + cholestanol) in a relatively oxidizing sediment core (IIL1) infer that coprostanol was likely contributed by seal settlement. Together with high-throughput sequencing of the 16S rRNA gene, the conversion of cholesterol to coprostanol by anaerobic bacteria (e.g., Eubacterium coprostanoligenes) could occur in anoxic aquatic systems. Our results suggest that the presence of coprostanol in Antarctic lacustrine sediment with anoxic conditions does not necessarily indicate seal activity and human waste as the microbial hydrogenation of cholesterol to coprostanol should also be considered.