Biogeochemistry最新文献

In the nutrient-poor soils of the Amazon rainforest, phosphorus (P) emerges as a critical limiting factor for ecosystem productivity. Despite these limitations, the Amazon exhibits remarkable productivity that is maintained by its efficient nutrient recycling mechanisms. Central to this process is the role of organic matter, particularly its dissolved (DOM) fraction, which serves as a crucial nutrient reservoir for both plants and microorganisms. This study delves into the dynamics of nutrient-containing DOM within the soils of two contrasting rainforest ecosystems: clayey terra firme forests, known for their robust nutrient recycling and presumed P-limitation, and sandy white-sand forests, characterized by reduced nutrient recycling capacity and presumed nitrogen (N)-limitation. Utilizing ultra-high resolution mass spectrometry (HR-MS), we analyzed the molecular composition of dissolved organic nutrient species. We evidenced nutrient limitation applying innovative concepts: (1) assessing nutrient depletion in DOM via nutrient-to-carbon ratios, (2) comparing the composition of nutrient-enriched DOM pools across soil depth profiles to infer microbial nutrient processing, and (3) examining the temporal variability of nutrient-containing DOM as an indicator of nutrient uptake and production. Our results corroborate the hypothesis of P-limitation in terra firme forests, with significant processing of N-containing DOM also observed, indicating a synergistic demand for both P and N. Surprisingly, white-sand soils exhibited no signs of N-limitation but instead sulfur (S)-limitation, a novel finding for these ecosystems. This study highlights the diversity of potential nutrient limitations in the central Amazon and the importance of the bioavailable “black box” DOM for tropical nutrient cycles.

There are major gaps in our understanding of how Mediterranean ecosystems will respond to anticipated changes in precipitation. In particular, limited data exists on the response of deep soil carbon dynamics to changes in climate. In this study we wanted to examine carbon and nitrogen dynamics between topsoils and subsoils along a precipitation gradient of California grasslands. We focused on organic matter composition across three California grassland sites, from a dry and hot regime (~ 300 mm precipitation; MAT: 14.6 (boldsymbol{^circ{text{C}} })) to a wet, cool regime (~ 2160 mm precipitation/year; MAT: 11.7 (boldsymbol{^circ{text{C}} })). We determined changes in total elemental concentrations of soil carbon and nitrogen, stable isotope composition (δ¹³C, δ¹⁵N), and composition of soil organic matter (SOM) as measured through Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) to 1 m soil depth. We measured carbon persistence in soil organic matter (SOM) based on beta (({varvec{beta}})), a parameter based on the slope of carbon isotope composition across depth and proxy for turnover. Further, we examined the relationship between δ¹⁵N and C:N values to infer SOM’s degree of microbial processing. As expected, we measured the greatest carbon stock at the surface of our wettest site, but carbon stocks in subsoils converged at Angelo and Sedgwick, the wettest and driest sites, respectively. Soils at depth (> 30 cm) at the wettest site, Angelo, had the lowest C:N and highest δ¹⁵N values with the greatest proportion of simple plant-derived organic matter according to DRIFTS. These results suggest differing stabilization mechanisms of organic matter at depth across our study sites. We infer that the greatest stability was conferred by associations with reactive minerals at depth in our wettest site. In contrast, organic matter at our driest site, Sedgwick, was subject to the most microbial processing. Results from this study demonstrate that precipitation patterns have important implications for deep soil carbon storage, composition, and stability.

Wetlands are the largest natural source of methane (CH₄), but spatial variability in fluxes complicates prediction, budgeting, and mitigation efforts. Despite the many environmental factors identified as CH₄ drivers, the overall influence of wetland spatial heterogeneity on CH₄ fluxes remains unclear. We identified five dominant patch types—submersed aquatic vegetation (SAV), emergent forbs, sedges/rushes, grasses, and open water—within a freshwater wetland in Maryland, USA, and measured CH₄ fluxes using a combined chamber and eddy covariance approach from June to September 2021. Because patch types integrate co-occurring environmental factors, we hypothesized that CH₄ flux is best characterized at the patch scale. Chamber measurements from representative patches showed distinct CH₄ signals; fluxes from grasses and sedges/rushes were highest, while fluxes from SAV and forbs were lower but skewed, suggesting episodic emission pulses. Open water had the lowest fluxes. Differences between patches were consistent over time, and spatial variability was greater between patches than within them, highlighting patches as key drivers of flux variability. By combining chamber fluxes with eddy covariance data in a Bayesian framework, we provide evidence that patch-type fluxes scale over space and time. Understanding spatial heterogeneity is essential for quantifying wetland contributions to global biogeochemical cycles and predicting the impacts of environmental change on wetland ecosystem processes. Our study demonstrates the importance of vegetation patch types in structuring spatial variability and supports a patch-explicit representation to reduce uncertainty in wetland CH₄ fluxes.

Forest ecosystems contain a substantial terrestrial reservoir of soil organic carbon (SOC). Here, a “Detritus Input and Removal Treatments” experiment was conducted to explore the effects of litter and roots on soil labile, persistent, and total organic C (TOC) pools in the coniferous, broad-leaved, and coniferous-broad-leaved mixed forests (CF, BF, and CBF, respectively) in the subtropical and warm temperate transition zone in Henan province, eastern China. After 2–3 years of detritus manipulations, neither litter addition nor root exclusion affected soil temperature or moisture. In contrast, litter removal increased soil temperature but decreased soil moisture, regardless of forest types. Litter addition marginally decreased labile OC and TOC contents in the BF but not in the CF and CBF. Litter removal reduced labile OC and TOC contents in the CF and BF and persistent OC contents in the CF only. Root exclusion decreased labile OC contents in the CBF only, but reduced persistent OC and TOC contents in the CF and CBF. Structural equation models suggested that litter but not root manipulation altered SOC pools via changing soil temperature and moisture in the BF, whereas the effects of litter and root manipulation on SOC pools were not related to the changes in soil temperature and moisture in the CF and CBF. Our results suggest that the impact of litter and roots on SOC pools depends on forest types, which may indicate differential responses of SOC storage among forests under global change scenarios.

Bacteria are a key component of the critical zone, because of their role in the nutrient availability for the vegetation. There is still little knowledge on the direct role of bacteria on Ca storage/leaching in soils while it is an essential macronutrient for vegetation growth. In recent years, Ca stable isotopes have shown their potential in understanding the Ca biogeochemical cycle. Preliminary studies highlighted that in the presence of soil bacteria, the plant uptake of nutrients is increased due to the mineral bioweathering. Moreover, Ca isotope signatures of nutrient media also showed differences between growth experiments in batch in the presence and absence of bacteria. In this study, the focus is to verify if Ca adsorption and incorporation into/onto bacterial strains induce such isotopic fractionation. Batch experiments were carried out on Pseudomonas aeruginosa (a Gram-negative bacterium) and on the vegetative and sporulated forms of Bacillus subtilis (a Gram-positive bacterium). These experimentations showed that: (i) no observable isotopic fractionations were induced during calcium/bacteria contact for all experimental parameters (pH, kinetic, bacterial cell number, interaction time, dead/alive bacteria); (ii) Ca was mainly stored in the bacterial cell wall compartments. On the other hand, significant Ca isotopic differences between the spores and the sporulation medium (Δ^44/40Ca_{spores–sporulation medium} ranging from − 0.53 to − 1.15‰), suggest isotopic fractionation during the sporulation process, likely occurring during the attachment of Ca to carboxyl acid groups as calcium chelates with dipicolinic acid. The absence of Ca isotope fractionation during Ca sorption on vegetative and sporulated bacteria via passive channels indicates that the tested bacteria’s contribution to the Ca biogeochemical cycle is indirect primary enhancing bioweathering and Ca bioavailability for vegetation. If confirmed by further studies, only the sporulation mechanisms itself may directly impact the Ca biogeochemical cycle.

While organic matter (OM) interactions in the water column prevent iron (Fe) precipitation and sedimentation, Fe also acts as a precursor of aggregation and a vector of OM to sediments. This study aims to characterize Fe–OM interactions to understand the role of Fe in promoting aggregation and transport of OM. Samples of Fe and OM were collected from water, settling material, and sediment along a gradient starting from the inlet and continuing offshore within a boreal lake. Fe speciation was determined using X-ray absorption spectroscopy (XAS), and the chemical composition of OM was assessed using Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT IR) and Nuclear magnetic resonance spectroscopy (NMR). The results show a decrease in Fe and OM concentrations in the water column with increasing distance from the inlet. Winter sampling revealed a shift in Fe speciation from dominance of organically complexed Fe to an increase in Fe(oxy)hydroxide, accompanied by a loss of aromatic and carboxylate function of OM. Summer sampling revealed no significant changes along the gradient, with Fe(oxy)hydroxide and carbohydrates dominating the water phase. Interestingly, settling particles and surface sediments were dominated by Fe(oxy)hydroxides and aliphatic OM. We propose that phototransformation may be an important process that influences the interaction between Fe and OM and, as a consequence, their fate along the spatial gradient. Our study suggests a photochemically induced loss of carboxylate groups, reflected by an increased carbohydrate-to-carboxylate ratio along the gradient, particularly in winter, and generally lower levels during summer. Loss of carboxylate function promotes the formation of Fe(oxy)hydroxides, which in turn, facilitates the aggregation and sinking of OM, particularly aliphatic components. These insights contribute to a broader understanding of carbon cycling and storage in lakes. Future studies should assess the significance of photochemical processes to OM burial and it how may change given trends in Fe and OM in northern regions.

Microbial Fe(III) reduction significantly influences the fate of various elements and contaminants. Previous research has employed different Fe(III)-OM complexes and ferrihydrite to study Fe(III)-reduction-related biogeochemistry processes. However, the effects of adding specific Fe(III)-OM complexes and ferrihydrite on the Fe(III)-reducing bacterial community, Fe(III)-reducing kinetics, and Fe(III)-related functional genes remain largely unexplored. This study applied microcosm experiments and metagenomic analysis of lake sediments with and without amendments of ferrihydrite, Fe(III)-citrate, or Fe(III)-EDTA. Results showed that sediments amended with Fe(III)-citrate and Fe(III)-EDTA exhibited faster Fe(III) reduction rates and more significant changes in bacterial community structures compared to those amended with ferrihydrite. Geobacter and Clostridium were enriched in the sediments amended with Fe(III)-EDTA and Fe(III)-citrate, respectively. Despite a slower reduction rate and lack of enrichment of specific Fe(III)-reducing bacteria, ferrihydrite still led to an increase in the copy numbers of genes related to Fe(III) reduction and iron assimilation in the metagenomes, suggesting an increase in these capacities. These results suggest that introducing various Fe(III)-OM complexes and ferrihydrite into the environment would result in differences in not only Fe(III) reduction rates and Fe(III)-reducing bacterial communities but also in iron-related functional genes. Meanwhile, variations in Fe(III) reduction rates and Fe(III)-reducing bacterial communities do not necessarily correlate with changes in the abundances of functional genes relevant to Fe(III) reduction and iron assimilation in the metagenomes. These results provide a better understanding of the adaptive mechanisms of Fe(III)-reducing bacteria in different environmental systems.