Biogeochemistry最新文献

The application of surface agricultural practices (SAPs) to agricultural soils is gaining attention as a potential valuable method for sequestering carbon and improving soil fertility. However, the impacts of SAPs on the molecular properties of dissolved organic matter (DOM) in soil leachates are poorly understood. In this study, the molecular characteristics of DOM successively leached from agricultural soils applied with control, manure fertilization, lucerne planting, and straw return were unraveled by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicated that the greater proportion of low molecular weight labile DOM (lipids-like, proteins-like and carbohydrates-like) in initial soil leachates gradually changed to higher fractions of larger recalcitrant DOM (condensed aromatics-like and tannins-like) in later soil leachates. Compared to the control, the soil leachates treated with SAPs had greater percentage of labile DOM and lower percentage of recalcitrant DOM, along with higher abundance of CHNO and CHOS compounds. Furthermore, DOM in the manure, lucerne, and straw treatments showed smaller mass weights, higher H/C ratios and fewer double bonds, rings, and aromatic structures. DOM with different physicochemical properties play different roles in the processes of nitrogen cycling and arsenic migration. The implementation of SAPs may alleviate groundwater nitrogen pollution, but it may also enhance the potential risk of arsenic mobility in groundwater. This study deepens our understanding of the molecular characterization of DOM leached from agricultural soils applied with different SAPs, which holds significant implications for evaluating the environmental impacts of soil DOM leaching.

Wetlands provide important ecosystem services, including improving surface water quality through nutrient removal. Louisiana has experienced ~ 4800 km² of coastal wetland loss between 1932 and 2016 due to high relative sea level rise and reduced sediment from the Mississippi River due to levees. The 2023 LA Coastal Master Plan aims to restore Louisiana’s degraded coastline through restoration projects, including sediment diversions or river reconnection. The Mid-Barataria Sediment Diversion Project will reconnect the river sediment-laden water with the coastal wetlands of Barataria Basin to nourish degrading marshes. However, the diversion will also deliver substantial nitrate (NO₃⁻) to the basin, potentially negatively impacting water quality. We quantified NO₃⁻ reduction rates at these high (2 mg/L) and low (0.5 mg/L) water column concentrations for marsh and submerged estuarine sediments using intact cores and a laboratory incubation. An additional treatment where 2 cm of mineral river sediment was placed over the organic marsh soil as a future, post-diversion scenario to simulate sediment deposition on the marsh once the river is reconnected. We hypothesized that NO₃⁻ reduction rates would decrease once mineral sediment is deposited on the organic marsh soil. For an aerobic water column, nitrate reduction rates for the vegetated marsh, post-diversion marsh, submerged eroded marsh, and estuarine sediment zones were 71.1 ± 2.7, 27.8 ± 4.5, 19.7 ± 1.2, and 13.0 ± 0.75 mg N m⁻² d⁻¹, respectively. Thus, the post-diversion marsh NO₃⁻ reduction rate decreased by ~ 60% compared to the current vegetated marsh. However, we predict the newly deposited sediment will increase NO₃⁻ removal by 1.17 × in the eroded marsh and estuarine sediment zones, which are always flooded and will receive river sediment. The marsh is only flooded 31–48% of the time, lessening the impact of the reduction. These findings can improve predictive water quality models used to assess nutrient loading and fate more accurately across the basin under the river reconnection scenario and inform other deltaic regions as freshwater flows are restored to coastal systems globally.

Nitrification is a key biogeochemical process, with higher rates indicative of higher soil nitrogen availability and potential nitrogen losses from soils to waterways and the atmosphere. Heterotrophic microbes and plants compete with nitrifiers for mineralized nitrogen, thereby influencing the fraction of ammonium converted by nitrifiers to nitrate. Higher soil carbon availability fuels heterotrophic microbial ammonium demand, which can weaken the positive relationship between net nitrogen mineralization and nitrification by limiting ammonium supply to nitrifiers. Whether soil carbon availability remains a central control on the coupling of these processes under altered plant nitrogen demand remains relatively unexplored even as disturbances that reduce plant biomass increase globally. Using partially disturbed forests that vary in aboveground biomass and soil carbon availability, we test the generalizability of microbially available carbon as a control on the coupling of net nitrogen mineralization and nitrification. We analyze differences between harvested and unharvested forest stands, changes over time since harvest, and the effects of retained overstory trees. Higher levels of disturbance consistently strengthened the positive relationship between net nitrogen mineralization and nitrification. Yet reduced plant biomass, rather than microbially available carbon, primarily mediated the coupling of these processes. Our findings suggest that plant-mediated nitrogen demand can be a stronger control on the decoupling of nitrogen mineralization and nitrification than heterotrophic soil microbes following partial canopy disturbances. These results have important implications for understanding coupled carbon and nitrogen cycling processes in forests globally, highlighting a need to consider how shifting disturbance regimes could influence controls on nitrification.

The signaling compound nitric oxide (NO) might play an important, yet unquantified role in mediating soil biogeochemical Carbon and Nitrogen cycles. This study quantified the effects of different soil-typical exogenous NO concentrations on the microbial community, on fertilizer N turnover, and on C and N trace gas fluxes of agricultural soil. For this, we repeatedly established soil NO concentrations of either 0, 200, 400, and ppbv˗NO in soil mesocosms for in total of 12 days, followed by high-resolution automated measurements of CO₂, NO, CH₄, and N₂O fluxes, molecular analysis of microbial community composition and ¹⁵N-isotope-tracing based assessment of fertilizer N turnover. We found no effects of different NO levels on microbial communities and CO₂, CH₄, and NO fluxes. However, at 200 ppbv concentration, exogenous NO promoted microbial assimilation of fertilizer N. In contrast, at 400 ppbv˗NO concentration, microbial biomass N was reduced, and microbial uptake of fertilizer N was inhibited, accompanied by a 33% reduction of N₂O emissions. This suggested a promoting effect of 200 ppbv˗NO on the physiology of cells involved in heterotrophic microbial N turnover, probably reinforcing the role of cell-endogenous NO. In contrast, the higher exogenous NO concentrations of 400 ppbv seemed to inhibit heterotrophic microbial inorganic N assimilation, with however no increase in N₂O emissions due to detoxification mechanisms. In conclusion, our pioneering study provides first insights into impacts of exogenous NO on soil C and N biogeochemistry in natural soil systems and reveals a NO concentration-dependent regulation of microbial N retention.

Biologically available nitrogen from human activities have altered nutrient dynamics across landscapes and aquatic ecosystems. Small spatial changes in land use and river management, may contribute to altered nutrient dynamics and influence denitrification and assimilatory uptake in river systems. Human actions can influence the stoichiometry of rivers. Construction of Libby Dam and the creation of the transboundary Koocanusa Reservoir has resulted in sequestration of approximately 60%–80% of the phosphorus entering the reservoir. Recent and ongoing expansion of surficial mining operations in one tributary upstream of Koocanusa Reservoir, the Elk River, has increased nitrate loading tenfold or more to Koocanusa Reservoir and to the Kootenai River. The combination of excessive nitrate loading and decreased phosphorus availability has skewed the N:P ratio to greater than 200:1 in both the river and reservoir. To address how this altered stoichiometry influences nitrogen spiraling in a large river, we estimated nitrate uptake over 16 years in five reaches of the Kootenai River. Reaches spanned 224 river km and types were based on natural and anthropogenically-influenced geomorphology. Although we documented a decline in nitrate moving longitudinally downstream indicating that nitrate is being used by the biota, the magnitude and timing of areal nitrate uptake varies among the reaches. Areal nitrate uptake did not differ between the early years with lower nitrate concentrations and the later years with higher nitrate concentrations suggesting that the Kootenai River is nitrogen saturated. Phosphorus addition, used as a management tool to offset P sequestration in the reservoir, increased areal nitrate uptake and extended the period of higher areal nitrate uptake. Without increases to the ecosystem functions of nitrogen transformation and removal, the ecosystem becomes saturated and the entire load is being transported downstream.

High flow and flood events are essential for sustaining river ecosystems, driving nutrient cycling, habitat diversity, and species dispersal.  However, widespread flow regulation via dams and reservoirs has disrupted natural hydrological processes, leading to river fragmentation and homogenization of flow regimes. While previous research has largely focused on the hydraulic and biological impacts of engineered flow events, less attention has been given to their influence on solute mobilization, transport, and biogeochemical transformations. This study addresses this gap by evaluating the geochemical and sediment dynamics of the first experimental spring high flow event (i.e., pulse event) on the Allegheny River (Pennsylvania, USA), conducted by the United States Army Corps of Engineers under the Sustainable Rivers Program. The pulse event, initiated on March 30, 2023, involved a sustained release of 451 cms from Kinzua Dam over 21 h. We hypothesized this experimental spring pulse would mobilize organic-rich sediment and nutrients stored behind Kinzua Dam, while also altering the geochemical signature of downstream waters through interactions with hyporheic zones, sediment scouring, and channel connectivity. To assess these impacts, we collected hourly water grab samples over a 48-h period spanning pre- and post-pulse conditions at multiple downstream locations. Samples were analyzed for dissolved metals, nutrients, total suspended solids, and nitrate isotopes. Results reveal distinct temporal shifts in water chemistry, with observed fluctuations in total suspended solids, dissolved metals, and nutrient concentrations highlighting hydrological connections between the main channel and riparian zones, reinforcing the importance of experimental pulse events in ecosystem restoration. Based on these findings, we propose a conceptual model linking controlled flood pulses to sediment and solute fluxes, which can be tested in other regulated river systems to evaluate the effectiveness of flow restoration strategies. These results provide key insights into the role of controlled high flow pulses in shaping sediment and solute dynamics, filling an important knowledge gap in understanding the biogeochemical implications of large-scale flow experiments.

Increasing concentrations of dissolved organic carbon (DOC) and changing dissolved organic matter (DOM) quality in surface waters, a phenomenon known as browning, have been observed at global scales with a range of implications for ecosystem structure and function, global carbon cycling and human health. Ecosystem recovery from chronic acidification resulting from rapid declines in acid deposition over recent decades has been the leading explanation for surface water browning. In this study, long-term dynamics of the quantity, quality, and seasonality of DOM in surface waters of an acid-resistant Adirondack lake and its forested watershed were investigated during a period of rapid regional changes in both acidic deposition and climate (1999–2018). Overall, we found that trends in DOM quality have occurred while the overall quantity and seasonality of DOC fluxes changed little during the same time frame. Lack of DOC trends was consistent with expectations for this acid-resistant ecosystem. Model reconstructions of DOM quality during this period indicated shifts towards a greater proportion of terrestrially-sourced DOM from the watershed, but with occasional ‘pulses’ of more microbially-processed DOM associated with periods of heavy rainfall and high discharge. Our findings suggest that ecologically meaningful changes in DOM quality may be occurring in acid-resistant ecosystems, aside from trends in DOC driven by ecosystem recovery from acid impairment.

We evaluated base cations, Al, Fe, Mn, REEs, DOC, anions, and P mobilization during three discharge events (E-1, E-2, E-3) at the headwater catchment W-9, Sleepers River, Vermont, USA. Peak discharge ranged from 3.696 (E-1) to 0.073 (E-3) mm h⁻¹. Eight samples from each event were speciated for total (unfiltered-acidified) and dissolved (0.45 µm filtered-acidified). During E-1, total Al, Fe, and Mn increased to maxima of 376 (1790X), 161 (194X), and 38 (45X) µmol L⁻¹, respectively. Concurrently, total La, Ce, Pr, and Nd increased to maxima of 87 (590X), 114 (671X), 15 (375X), and 53 (408X) nmol L⁻¹, respectively, greatly exceeding the dissolved fraction. Totals for Er > Yb > Tm > Lu reached comparable enrichments near or at maximum discharge. Discharge ranged from 0.41 to 0.94 mm h⁻¹ during E-2, a snowmelt event. Total Al was comparatively stable; total Fe and Mn increased 15X and 79X, remaining less than total Al. Total La, Ce, Pr, and Nd peaked with total Fe and Mn. E-3, a late summer rain, resembled E-1 but had much lower maxima for all REEs, P, Al, Fe, and Mn. Particulate, total, and dissolved REEs and P correlated with DOC and with total Al, Fe, and Mn over all discharges, with molar Al > Fe > Mn (most samples) for E-1 and E-3. Maximum total and dissolved P declined from 15.4 and 0.89 (E-1) to 0.98 and 0.27 µmol L⁻¹ (E-3), respectively. Particulate REEs correlated strongly (R² = 0.95–0.96) with Al, Fe, and Mn particulates eroded from the stream bed and continuously precipitated during and after high discharge of groundwater.