Biogeochemistry最新文献

Root exudation, the export of low-molecular weight organic carbon (C) from living plant roots to soil, influences microbial activity, nutrient availability, and ecosystem feedbacks to climate change, but the magnitude of this C flux at ecosystem and global scales is largely unknown. Here, we synthesize in situ measurements of root exudation rates and couple those to estimates of fine root biomass to estimate global and biome-level root exudate C fluxes. We estimate a global root exudate flux of 13.4 (10.1–20.2) Pg C y⁻¹, or about 9% (7–14%) of global annual gross primary productivity. We did not find differences in root mass-specific exudation rates among biomes, though total exudate fluxes are estimated to be greatest in grasslands owing to their high density of absorptive root biomass. Our synthesis highlights the global importance of root exudates in the terrestrial C cycle and identifies regions where more in situ measurements are needed to improve future estimates of root exudate C fluxes.

Beach wrack is an important supplier of nutrients and organic matter to sandy beach ecosystems and underlying subterranean estuaries (STEs), producing metabolic hotspots in these otherwise organic carbon- and nutrient-poor environments. To assess the impact of beach wrack type (e.g., marine, terrestrial, plant, animal) and environmental settings (e.g., tidal inundation, precipitation, and solar irradiation) on nutrient and dissolved organic matter (DOM) release, a series of leaching experiments was conducted. Quantities of leached nutrients and dissolved organic carbon (DOC) were determined, and DOM molecular composition was investigated using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Millimolar—to molar amounts of DOC and dissolved nitrogen were released from the beach cast per kg dry weight, with type of wrack and leaching medium (fresh- vs. saltwater) exerting the biggest influences. Exemplary for animal cast, jellyfish leached up to two 100-fold more, mostly organic, nitrogen compared to all other beach wrack types. FT-ICR-MS data of solid-phase extracted DOM indicated that beach wrack releases compounds with putative mono- and oligosaccharide-, amino acid- and vitamin-type molecular formulae, which likely serve as valuable substrate for heterotrophic microorganisms. DOM from the brown seaweed Fucus sp. was more aromatic than seawater DOM and even beach wrack of terrestrial origin, probably from structural components and secondary metabolites such as phlorotannins. We conclude that DOM and nutrient release from beach wrack strongly depends on wrack type and leaching medium, may obscure molecular provenance proxies (e.g., terrestrial indices), and adds a nutritional boost to infiltrating sea- and rainwater which likely impact microbial respiration rates in the STE.

Restorations of former cranberry farms (“bogs”) aim to re-establish native wetland vegetation, promote cold water habitat, and attenuate nitrogen (N) delivery to coastal waters. It is unclear, though, how elements of restoration design such as microtopography, groundwater interception, and plant communities affect N removal via denitrification. In a recently restored riparian cranberry bog with created microtopography, we compared denitrification potential, nitrous oxide (N₂O) yield of denitrification (ratio of N₂O:N₂O + N₂ gases), in situ N₂O fluxes, soil chemistry, and plant communities at the highest and lowest elevations within 20 plots and at four side-channel groundwater seeps. Denitrification potential was > 2 × greater at low elevations, which had plant communities distinct from high elevations, and was positively correlated with plant species richness (Spearman’s rho = 0.43). Despite detecting high N₂O yield (0.86 ± 0.16) from low elevation soils, we observed small N₂O emissions in situ, suggesting minimal incomplete denitrification even in saturated depressions. Groundwater seeps had an order of magnitude higher denitrification potentials and 100–300 × greater soil NO₃− concentrations than the typically saturated low elevation soils. Groundwater seeps also had high N₂O yield (1.05 ± 0.15) and higher, but spatially variable, in situ N₂O emissions. Our results indicate that N removal is concentrated where soils interact with NO₃–rich groundwater, but other factors such as low soil carbon (C) also limit denitrification. Designing restoration features to increase groundwater residence time, particularly in low lying, species rich areas, may promote more N attenuation in restored cranberry bogs and other herbaceous riparian wetlands.

Stemflow is a conduit for the transport of canopy-derived dissolved organic matter (DOM) to the forest floor. This study examined the character of stemflow DOM for four tree species over four phenophases (leafless, emergence, leafed, and senescence for deciduous species and leafed-winter, emergence, leafed- spring/summer, and senescence for coniferous species) occurring in temperate forests; namely, Betula lenta L. (sweet birch), Fagus grandifolia Ehrh. (American beech), Liriodendron tulipifera L. (yellow poplar), and Pinus rigida Mill. (pitch pine). American beech exhibited the lowest average specific UV absorbance at 254 nm (SUVA₂₅₄) values, while yellow poplar displayed the highest values. SUVA₂₅₄ values were largest in senescence and smallest in emergence. The spectral slope ratio was lower for pitch pine than the deciduous tree species. Humification index (HIX) values decreased across all species during the emergence phenophase. The developed and validated stemflow-specific four-component parallel factor analysis (PARAFAC) model demonstrated the combined influence of interspecific and temporal fluctuations on the composition of humic and protein-like substances within stemflow. By separating and examining stemflow DOM independent of throughfall, our study provides fresh insights into the spatiotemporal dynamics of stemflow inputs to near-trunk soils that may inform hot spots and hot moments theories.

Urban stormwater ponds (SWPs) are engineered ecosystems designed to prevent flooding and protect downstream ecosystems by retaining nutrients associated with stormwater runoff, including nitrogen (N). Despite these expectations, multiple studies have found that SWPs have low N removal efficiencies and can be sources of N to downstream ecosystems. To understand mechanisms controlling the fate of N in SWPs, we quantified dinitrogen (N₂) gas saturation to characterize net N₂ exchange as either net denitrification or net N-fixation. We assessed temporal and spatial patterns of N₂ dynamics in fifteen SWPs and six naturally occurring ponds in undisturbed watersheds (Florida, USA) by sampling in two seasons (dry and wet) and from multiple depths of the water column. Samples from SWPs were equally likely to exhibit N₂ supersaturation (net denitrification; 50%) or undersaturation (net N-fixation; 50%). In contrast, the majority (82%) of samples from natural ponds were supersaturated with N₂, indicating net denitrification. The mean SWP air–water N₂ flux was − 1.7 μg N₂-N m⁻² h⁻¹ (range − 500 to 433 μg N₂-N m⁻² h⁻¹), which was lower than clear (40 μg N₂-N m⁻² h⁻¹; range − 68 to 74 μg N₂-N m⁻² h⁻¹) and humic (202 μg N₂-N m⁻² h⁻¹; range 41 to 407 μg N₂-N m⁻² h⁻¹) natural ponds despite considerably higher variation in SWPs. These results indicate that SWPs may have low N removal efficiencies in part due to N-fixation adding new N to the system. Overall, this study shows that SWPs are less effective than natural ponds at removing reactive N from the environment, potentially impacting downstream water quality.

Increased occurrence, size, and intensity of fire result in significant but variable changes to hydrology and material retention in watersheds with concomitant effects on stream biogeochemistry. In arid regions, seasonal and episodic precipitation results in intermittency in flows connecting watersheds to recipient streams that can delay the effects of fire on stream chemistry. We investigated how the spatial extent of fire within watersheds interacts with variability in amount and timing of precipitation to influence stream chemistry of three forested, montane watersheds in a monsoonal climate and four coastal, chaparral watersheds in a Mediterranean climate. We applied state-space models to estimate effects of precipitation, fire, and their interaction on stream chemistry up to five years following fire using 15 + years of monthly observations. Precipitation alone diluted specific conductance and flushed nitrate and phosphate to Mediterranean streams. Fire had positive and negative effects on specific conductance in both climates, whereas ammonium and nitrate concentrations increased following fire in Mediterranean streams. Fire and precipitation had positive interactive effects on specific conductance in monsoonal streams and on ammonium in Mediterranean streams. In most cases, the effects of fire and its interaction with precipitation persisted or were lagged 2–5 years. These results suggest that precipitation influences the timing and intensity of the effects of fire on stream solute dynamics in aridland watersheds, but these responses vary by climate, solute, and watershed characteristics. Time series models were applied to data from long-term monitoring that included observations before and after fire, yielding estimated effects of fire on aridland stream chemistry. This statistical approach captured effects of local-scale temporal variation, including delayed responses to fire, and may be used to reduce uncertainty in predicted responses of water quality under changing fire and precipitation regimes of arid lands.

Coastal nutrient loads from point sources such as rivers are mostly well-monitored. This is not the case for diffuse nutrient inputs from coastal catchments unconnected to rivers, despite the potential for high inputs due to intensive land use. The German Baltic Sea coastline consists of numerous peatlands that have been diked and drained. However, some of the dikes have been removed in order to re-establish the hydrological connection to the Baltic Sea, restore local biodiversity, and promote natural CO₂ uptake. Since these peatlands were used for agriculture, their rewetting may release accumulated nutrients, leading to nutrient export into the Baltic Sea and intensified coastal eutrophication. Data on these potential nutrient exports are mostly lacking. Therefore, this study investigated nutrient exports from two former agricultural, coastal peatlands: Drammendorfer Wiesen, rewetted in 2019, and Karrendorfer Wiesen, rewetted in 1993. Nutrients (NO₃^–, NO₂^–, NH₄⁺, PO₄^3–), nitrous oxide (N₂O), particulate organic matter (POM, comprising POC and PON; δ¹³C-POC), chlorophyll-a, and nitrification rates were analyzed in surface water and porewater sampled weekly to monthly in 2019 and 2020 to compare the effects of different time scales after rewetting on nutrient cycling and potential exports. NH₄⁺, NO₂⁻, and PO₄³⁻ concentrations were higher in the porewater than in the overlying water at both sites, while nutrient concentrations were generally higher at the recently rewetted Drammendorfer Wiesen than at the Karrendorfer Wiesen. NO₃⁻ concentrations in porewater, however, were lower than in the overlying water, indicating NO₃⁻ retention within the peat, likely due to denitrification. Nitrification rates and N₂O concentrations were generally low, except for a high N₂O peak immediately after rewetting. These results suggest that denitrification was the dominant process of N₂O production at the study sites. Both peatlands exported nutrients to their adjacent bays of the Baltic Sea; however, N exports were 75% lower in the longer-rewetted peatland. Compared to major Baltic Sea rivers, both sites exported larger area-normalized nutrient loads. Our study highlights the need to monitor the impact of rewetting measures over time to obtain accurate estimates of nutrient exports, better assess negative effects on coastal waters, and to improve peatland management.

Methane (CH₄) emissions from wetland ecosystems are controlled by redox conditions in the soil, which are currently underrepresented in Earth system models. Plant-mediated radial oxygen loss (ROL) can increase soil O₂ availability, affect local redox conditions, and cause heterogeneous distribution of redox-sensitive chemical species at the root scale, which would affect CH₄ emissions integrated over larger scales. In this study, we used a subsurface geochemical simulator (PFLOTRAN) to quantify the effects of incorporating either spatially homogeneous ROL or more complex heterogeneous ROL on model predictions of porewater solute concentration depth profiles (dissolved organic carbon, methane, sulfate, sulfide) and column integrated CH₄ fluxes for a tidal coastal wetland. From the heterogeneous ROL simulation, we obtained 18% higher column averaged CH₄ concentration at the rooting zone but 5% lower total CH₄ flux compared to simulations of the homogeneous ROL or without ROL. This difference is because lower CH₄ concentrations occurred in the same rhizosphere volume that was directly connected with plant-mediated transport of CH₄ from the rooting zone to the atmosphere. Sensitivity analysis indicated that the impacts of heterogeneous ROL on model predictions of porewater oxygen and sulfide concentrations will be more important under conditions of higher ROL fluxes or more heterogeneous root distribution (lower root densities). Despite the small impact on predicted CH₄ emissions, the simulated ROL drastically reduced porewater concentrations of sulfide, an effective phytotoxin, indicating that incorporating ROL combined with sulfur cycling into ecosystem models could potentially improve predictions of plant productivity in coastal wetland ecosystems.