Biogeochemistry最新文献

Large mammalian grazers have regulated biogeochemical cycles across 40% of the Earth’s terrestrial surface for millennia. While grazer effects on organic matter entering the soil are becoming clear, their effect on the downstream microbial foraging through extracellular-enzymes (EE), which link processes at the cellular level to ecosystem-level flow of matter remain less clear. We assessed how large mammalian grazers influence microbial foraging in soil using two lines of evidence. First, utilising a 14-year-old long-term grazer-exclusion experiment in the Trans-Himalayan ecosystem, we find that activities of seven enzymes peak at different months during the growing-season in an asynchronous manner. Microbial deployment of enzymes responded to grazer-exclusion by shifting from nitrogen (N)-acquisition in spring to carbon (C)- and phosphorus (P)-acquisition in summer and autumn. Grazer exclusion increased peroxidase activity by 124.7% and β-glucosidase by 16.7%. About half of the variation in enzyme portfolios was explained by seasonal differences in grazer effects. Grazer-exclusion increased the variability in the enzyme portfolio, requiring the microbes to forage across a larger portion of the functional space. Structural equation models suggested that grazer-exclusion worsens the quality of biomass input to soil due to an 18% increase in acid-unhydrolysable recalcitrant C. As seen in other ecosystems, this decline in substrate quality raises microbial deployment of generic depolymerizers relative to the specialized enzymes which release assimilable end-products. As greater investment in generic depolymerizing enzymes may reduce microbial carbon use efficiency, it can affect the C cycle. Second, laboratory incubations supported that soil enzymes are indeed inducible/repressible in response to experimental manipulation of biomass quality. Microbial responses appear predictable and generalisable across spatial scales, from laboratory-incubations to whole ecosystems. Hence, fostering microbial functions can enhance the scope and efficacy of rewilding and restoration for natural climate solutions.

Litter represents the transitional stage between plant biomass and soil organic carbon, with its transformation influenced by overlying vegetation. Plant regulation of litter decomposition through priming effects in wetlands represents an important knowledge gap, as roots influence rhizosphere processes differently than in upland soils by releasing both exudates and oxygen into the otherwise water-saturated, oxygen-depleted soil matrix. To investigate the role of plants in rhizosphere priming in anoxic wetlands, two sedge species with contrasting belowground traits (rhizomatous vs. tussock-forming) were grown on uniformly carbon-13-labeled litter in waterlogged peat in a greenhouse microcosm experiment. Litter-derived respiration affected by sedge roots was quantified using a two-end-member mixing model. Rhizosphere enzyme activities were mapped using zymography, while oxygen and carbon dioxide (CO₂) dynamics were visualized using planar optode imaging. Litterbag mass loss was determined gravimetrically. Cumulative litter-derived CO₂ respiration was 87% higher under rhizomatous Eriophorum angustifolium and 106% higher under tussock-forming E. vaginatum than in the unplanted treatment, whereas litter mass loss was only 10 and 19% higher in the planted treatments, respectively. Initial positive priming effects of > + 200% declined to negative priming of < − 20% within 3 months in both planted treatments. In both species, β-glucosidase activity was elevated around mature roots, while acid phosphatase activity was elevated across the entire root system compared with background levels. Our study provides experimental evidence that sedges enhance early-stage litter decomposition in anoxic peat regardless of their belowground morphological traits, likely through root oxygen loss and stimulation of rhizosphere enzyme activity.

Invasive plant species can alter belowground biogeochemical processes, but the specific dynamics and mechanisms associated with these shifts remain unclear. We compared the effects of aqueous root extracts from the invasive Impatiens glandulifera and the native Urtica dioica on the carbon processing and functional stability of non-invaded riparian soil. In a 21-day laboratory incubation, we amended soil with two concentration levels of root extracts to simulate root lysis and exudation. We assessed microbial responses by measuring respiration rates (MicroResp™) and enzyme activities, while the stoichiometric properties of the root extracts were characterized by dissolved carbon (DC), nitrogen (DN), and total phenolics (TPC). We found that I. glandulifera extracts did not suppress microbial activity. Instead, plant origin of the root extract (U. dioica or I. glandulifera) determined the physiological efficiency of the community. The addition of native U. dioica extracts was able to support biomass accretion indicated by high substrate-induced respiration. Conversely, I. glandulifera extracts triggered a state of relatively higher basal respiration but low biomass formation resulting in lower physiological efficiency. By stoichiometric modelling we identified the dissolved carbon-to-nitrogen ratio as the primary driver of this inefficiency. Temporal dynamics were governed more by substrate concentration rather than plant origin. Low-dose treatments of both species exhibited “boom-and-bust” dynamics likely driven by rapid labile depletion and necromass recycling, whereas high-dose treatments exhibited a monotonic response. Furthermore, temporal spikes in TPC were linked to the possible enzymatic “unmasking” of reactive moieties as the labile fraction was selectively harvested. Overall, our findings indicate that changes in soil organic carbon cycling may be better explained by the stoichiometric quality of root input than by the biogeographic status of the plant itself.

Vegetated inland water habitats are important global emitters of methane (CH₄), but the variability in their CH₄ emission is poorly understood, making estimates and predictions uncertain. Methane is believed to be primarily released into the atmosphere via plant lacunal tissues in dense vegetated zones, although experiments have also shown that a large fraction of CH₄ can be consumed in the rhizosphere before it diffuses into the roots. Methane rhizospheric oxidation measurements have been limited to few species in situ, and it is still unclear how it differs between species. To determine the implications of CH₄ rhizospheric oxidation for plant-CH₄ release, I measured CH₄ rhizospheric oxidation and transport rates for 11 plant species with different gas transport efficiencies using dual isotopes in situ. Overall, the fraction of CH₄ produced that was oxidized was higher for plants that transport gases efficiently (up to 81%), and was negatively correlated to plant CH₄ release. These findings suggest that CH₄ oxidation decreases CH₄ release from plants that efficiently transport gases, although further studies are needed to assess the causality of this relationship. Such mechanistic understanding of plant-CH₄ release is necessary to define aquatic plant functional types with contrasting CH₄ emissions, and to reduce uncertainties in estimates of CH₄ freshwater emissions.

Iron (Fe) additions are widely used to suppress internal phosphorus (P) loading in eutrophic lakes by promoting the formation of vivianite (Fe(II)₃(PO₄)₂·8H₂O), a redox-stable Fe–P mineral that can sequester P in anoxic sediments over the long term. However, vivianite is unstable under sulfidic conditions, but the remobilization potential of P is unclear. In a 96-day mesocosm experiment, we examined how sulfate availability affects Fe-bound P persistence in a controlled model system of Fe-amended lake sediment. Sediment amended with Fe-P and enriched with vivianite during an anoxic pre-incubation was subsequently incubated under oxic overlying water at either low (<100 µmol L^-1) or high (~1 mmol L^-1) sulfate concentrations. Porewater profiles measured with microsensors and diffusive gradients in thin films (DGT), combined with solid-phase analyses by sequential extraction, scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD), revealed that vivianite persisted in sediment under oxic water conditions but was progressively destabilized as sulfide accumulated through microbial sulfate reduction. Sulfide drove coupled dissolution–reprecipitation reactions that transformed vivianite-bound Fe into amorphous Fe sulfides and pyrite. As a result, total sediment P-binding capacity declined. Although a portion of released P was re-adsorbed onto freshly precipitated Fe(III) phases in the oxic surface layer, this only partially compensated for the net loss of P-binding capacity, and total solid-phase P declined in both treatments. These findings highlight that P retention is linked to sulfide availability, and that monitoring porewater sulfide concentrations may serve as a practical early warning indicator of declining P retention capacity.