Soil microbiota are key players of terrestrial ecosystem functioning, including decomposition, soil organic matter formation, and nutrient cycling, and interact strongly with plants in the rhizosphere. Several studies have demonstrated the potential of plants to alter soil microbiome assembly and functioning (i.e., through manipulation of soil organic matter pools via root exudation), which can be critical for sustaining soil ecosystem functioning. Using soil from a long-term biodiversity experiment in Germany, we investigated how soil microbial communities responded to variations in plant species richness (1–16 species), functional group richness (1–4 groups), and plant identity (grasses, legumes, small herbs, and tall herbs) using 16S rRNA gene and ITS amplicon sequencing. We examined bacterial and fungal community structure, metabolic potential, and microbial network architecture to better understand the role of the soil microbiome and its net positive relationship between biodiversity and ecosystem functioning. Plant diversity induced gradual shifts in microbial community composition, while increasing soil organic carbon and nitrogen stocks. Microbial networks exhibited increased connectivity, particularly between bacteria and fungi. Meanwhile, mutualistic and antagonistic functional guild representation increased, that is the sum total of plant-beneficial (i.e., endophytes) and plant- or fungi-detrimental (i.e., pathogens and parasites) fungal guilds, respectively. Key nodes shifted from generalist taxa at low plant diversity to more specialized communities at high plant diversity. Notably, fungi responded more strongly than bacteria, and their functional potential was driven by plant functional identity rather than species richness. At low plant diversity, generalist taxa likely exploit less complex and diverse organic carbon inputs, allowing them to dominate available niches. In contrast, higher plant diversity promotes a broader array of specialist taxa that likely benefit from the greater diversity of organic carbon compounds, and thus greater niche availability. As network complexity grows, ecosystem functions are being distributed across more taxa, leading to greater microbiome stability, and ultimately more efficient soil carbon and nutrient cycling. Our findings suggest that higher plant diversity strengthens microbial functioning and enhances microbiome resilience, that is the capacity of the microbial community to maintain soil functioning despite environmental disturbances.

Non-targeted liquid chromatography tandem high-resolution mass spectrometry (LC-MS/MS) is increasingly applied for the structure-resolved chemical analysis of dissolved organic matter (DOM). With new developments in MS instrumentation and analysis software, the approach has gained substantial momentum over the past decade. However, achieving high-quality analytical data that is reproducible and comparable across laboratories can be a bottleneck in non-targeted metabolomics and organic matter chemical analysis, especially for data reuse in repository-scale analyses. Understanding the capabilities as well as challenges of comparing LC-MS/MS data from different laboratories is necessary for inferring global trends from public data sets. To illuminate instrumentation factors that drive differences and variability, we used a standardized data analysis pipeline, including classical (CMN) and feature-based molecular networking (FBMN), to analyze data from a ring trial by 24 laboratories on identical sample sets of algal and DOM extracts that were mixed in predefined concentrations and spiked with standards. Our results showed that data sets from similar mass spectrometer types with unified instrument parameters were qualitatively comparable, resolving the same general trends and shared mass spectral features. Interlaboratory comparability was best for high-intensity features, while low-intensity features showed greater detection variability. Our analysis also highlights challenges when comparing data from instruments with different acquisition rates or operating with less standardized methods. Lastly, we provide recommendations for data integration, public data sharing, standardization, and best practices for standardized LC-MS/MS data acquisition, which will be critical for long-term time series and intercomparability of DOM chemical analyses.

White-sand forests contribute significantly to dissolved organic matter (DOM) production in the central Amazon, forming blackwater rivers that dominate organic matter export from the Amazon basin to the ocean. Despite their importance in controlling DOM export, white-sand forests are understudied, and it remains unclear whether systematic changes in the formation of blackwater DOM occur and how seasonal variations and extremes like El Niño-associated droughts impact them. We collected soil porewater from two central Amazon white-sand forests for 2 years, spanning a wet La Niña year followed by an El Niño drought year. The molecular composition of DOM was analyzed using high-resolution mass spectrometry, and correlation network analysis was employed to identify ecologically meaningful DOM subsets. Using additional chemical characterization, database annotations, correlation with 14C-age of DOM and climatic variables, and ecological null modeling, we propose five distinct DOM sources: plant litter and throughfall, soil organic matter (SOM) decomposition, root exudation, and two drought response subsets of likely microbial and plant origin. During drought conditions, aboveground plant-derived compounds decreased, while SOM products, root exudates, and drought response compounds increased. These drought responses were qualitatively similar in both years but notably amplified in the drier El Niño year. Drought amplified deterministic control over DOM composition, indicating that DOM reflected directed biological responses and that future droughts are likely to generate similar shifts. Overall, drought substantially altered belowground carbon cycling by shifting DOM sources and inducing stress responses, effects expected to recur and potentially intensify under future climate scenarios.

RATIONALE:Direct infusion mass spectrometry (DI-MS) is a rapid analytical technique widely used in omics research and other fields. However, the complexity of DI-MS spectra frequently leads to co-fragmentation of analytes with similar m/z, resulting in chimeric fragmentation spectra that complicate compound identification. A DI-based tandem mass spectrometric method (DI-MS2), which modulates the intensity of precursors and fragments by the stepwise movement of the quadrupole isolation window, has been shown to successfully deconvolute chimeric fragmentation spectra. Yet, its applicability to different instruments and optimisation has not been evaluated. METHOD:We evaluate the performance of DI-MS2 on two high-resolution instruments: a linear ion trap-Orbitrap (LIT-Orbitrap) and a quadrupole-Orbitrap (Q-Orbitrap). We examined the impact of six instrumental settings, including mass resolving power, isolation window width, step size between MS2 scans, number of microscans, collision energy and automatic gain control (AGC) target, on the analysis of isobaric mixtures with varying m/z differences. RESULTS:The LIT-Orbitrap consistently achieved high-quality chimeric spectra deconvolution with an average similarity score of 0.98 despite unexpected intensity modulation patterns. The Q-Orbitrap provided four times faster measurements but showed more variable results: It achieved a similarity score of 0.96 for isobars with a m/z difference larger than 0.02, but only 0.56 for m/z differences of 0.006. CONCLUSIONS:These findings indicate that the DI-MS2 is a robust and flexible method applicable across different MS platforms, though the Q-Orbitrap may be less suited for highly complex samples with multiple peaks per nominal mass. This highlights the potential of the DI-MS2 for structural elucidation of complex biological mixtures. Additionally, we provide initial setting optimisation guidelines to improve spectra deconvolution and measurement speed.

Abstract Soils are recognized sinks for atmospheric isoprene, but their in situ behavior remains understudied, particularly in the Amazon where emissions are globally significant. Here we show how the 2023 El Niño affected the diel and seasonal variation in soil isoprene fluxes. Under non-stress conditions, the soils acted as a persistent isoprene sink, with uptake driven primarily by ambient isoprene mixing ratios, following a diel cycle and peaking during the dry season. Soil organic matter and litter modulated the fluxes by shaping moisture, temperature, and gas-diffusion responses. During the 2023 El Niño dry season, soil moisture below ~20% constrained soil gas exchange, while associated heat extremes further reduced soil respiration and the soils’ isoprene uptake capacity. As climate change intensifies drought and heat extremes, reduced soil isoprene uptake capacity could impact atmospheric oxidation, aerosol formation, and methane lifetime. Incorporating soil isoprene uptake into atmospheric models is essential for quantifying these feedbacks.

Glacier melt increasingly delivers bioavailable yet radiocarbon-depleted dissolved organic matter (DOM) to headwaters, but controls on DOM molecular composition and radiocarbon signatures remain unclear for the Tibetan Plateau. We coupled ultrahigh-resolution mass spectrometry with Δ14C measurements for 30 samples from 16 glacier outflows, including three ablation-season time series across contrasting climatic settings. DOC concentrations were uniformly low, but DOM composition was highly heterogeneous. Multivariate analyses identified longitude and shortwave radiation as primary regional correlates, while marked differences among neighboring outlets indicated strong local controls. Seasonal DOM patterns varied by site: at the monsoon-influenced Rongbuk Glacier on the northern slope of Mount Everest, DOM shifted from lipid-rich to more lignin-, tannin-, and aromatic-enriched compositions as inorganic deposition indicators increased, consistent with enhanced allochthonous influence; however, Δ14C became more modern over the season. In contrast, the westerly-influenced, high-radiation Xiongcai Glacier showed comparatively stable DOM composition but elevated and dynamic peptide-like signals. Across the Δ14C subset (n = 23), Δ14C correlated negatively with lipid-like contributions, revealing an underappreciated "old-but-potentially labile" component. These results show that regional climate, local catchment setting, and melt-season progression jointly govern the export of mixtures of young and old, labile and compositionally complex DOM.

Soil phosphorus (P) is a limiting factor for vegetation growth in the Amazon rainforest, where plants depend on microorganisms for organic matter cycling and nutrient uptake. While forest-to-agriculture conversion fundamentally reshapes plant-microbe-soil interactions and P cycling, these dynamics are further modulated by the intensity of land management. This study examined the 30-year effects of converting a primary forest into two contrasting systems: a low-intensity agroforest and a high-intensity citrus monoculture. We investigated how microbial and low molecular weight organic compounds (LMWCs) composition interacted with soil physicochemical attributes, acid phosphatase activity, and P fractions (labile, moderately labile, non-labile, and residual). Agroforest soils retained physicochemical and enzymatic attributes similar to the primary forest, while soils of the citrus plantation showed increased P in all fractions due to mineral fertilization and reduced soil organic matter content, mainly in deeper layers. Microbial and LMWC composition patterns reflected land-use, with agroforest representing an intermediate state between primary forest and citrus monoculture. Pseudomonadota and nutrient-rich LMWC were more abundant in the agroforest, whereas Ascomycota and nutrient-poor LMWC predominated the citrus plantation. Genes related to "P acquisition" were more abundant in forest and agroforest soils, while genes related to "P-compound synthesis" were more abundant in the citrus plantation. Labile P was negatively correlated with genes related to microbial metabolism, suggesting that reduced P availability may induce a boost in microbial activity for internal P-cycling. These findings demonstrate that forest-to-agriculture conversion strongly affects microbial functions, with responses aligning with land-use intensity and LMWC resource availability. Nonetheless, microbes adapt by shifting strategies: prioritizing mineralization and solubilization or favoring biosynthesis depending on P availability.

Soils are recognized sinks for atmospheric isoprene, but their in-situ behavior remains understudied, particularly in the Amazon where emissions are globally significant. Here we show how the 2023 El Niño affected the diel and seasonal variation in soil isoprene fluxes. Under regular conditions, the soils acted as a persistent isoprene sink, with uptake driven primarily by ambient isoprene mixing ratios, following a diel cycle and peaking during the dry season. Soil organic content and litter modulated the fluxes by shaping moisture, temperature, and gas-diffusion responses. Isoprene uptake capacity and soil respiration peaked at ~27% soil water content and 26.5 °C soil temperature. El Niño heat extremes, suppressed soil respiration, and soil isoprene uptake capacity declined 4.3‑fold. As climate change intensifies heat extremes, reduced soil isoprene uptake capacity could shift atmospheric oxidation, aerosol formation, and methane lifetime. Incorporating soil isoprene uptake into atmospheric models is essential for quantifying these feedbacks.

Tropical rainforests such as the Amazon are of high importance as a global carbon sink. Due to its well-known nutrient limitation, the Amazon rainforest relies heavily on rapid microbial decomposition of biomass to release freshly available nutrients for plant growth. Despite the fundamental importance of decomposers for this ecosystem, little is known about the biodiversity of such microbiomes, their functional activity, and spatial and seasonal variability. We used 16S rDNA and ITS rDNA sequencing to analyze the microbial communities of the Amazon’s terra firme and the much drier white-sand ecosystems during the dry and wet seasons in 2022. Bacterial microbiomes differed significantly between seasons, displaying lower bacterial species richness and diversity in response to seasonal drought. In contrast, fungal richness and diversity differed strongly between sites, but were less affected by seasonal variation, suggesting their hyphae network and associations with plants as potential protectors against drought effects. Fungal and bacterial communities alike showed lower abundance of taxa involved in organic matter decomposition following seasonal drought. These changes were also reflected at the functional level, with samples collected during the dry season and at white-sand sites featuring lower abundances of decomposition and denitrification pathways. Soil hydro-chemical data also emphasizes how prolonged drought may limit soil nutrient supply via local microbiomes. Our results suggest that the reduced nutrient availability and soil connectivity during drought and within the white-sand ecosystem lower microbial activity and functional redundancy, henceforth demonstrating a strong impact of ecosystem type and drought on tropical microbiomes and their functional capacities. Our results further highlight that the observed increase in droughts in the Amazon rainforest may additionally limit nutrient supply through the microbial community, limiting carbon sequestration in the ecosystem with negative consequences for the global climate system.

The global loss of biodiversity has motivated many studies that experimentally vary plant species richness and examine the consequences for ecosystem functioning. Such experiments generally show a positive relationship between above- and below-ground biodiversity and the functioning of terrestrial ecosystems. Moreover, this relationship tends to strengthen over time, seen as enhanced functioning of diverse plant communities and reduced functioning of low-diversity plant communities. Differences in multitrophic community assembly and biotic interactions in high- versus low-diversity plant communities are hypothesized to affect plant performance by altering consumer community structure and function and driving plastic or micro-evolutionary responses of plant species in the plant communities. To resolve this complex interplay of community history, we separated these effects into plant and soil history. Plant history refers to all plant-level responses to past abiotic and biotic selection pressures experienced in their communities, while soil history relates to all abiotic and biotic soil properties developed as a legacy of plant-soil interactions under variable plant diversity. We set up a biodiversity experiment in an Ecotron, a terrestrial mesocosm facility that allows controlling environmental conditions above- and below-ground, to test whether the strengthening biodiversity-ecosystem functioning relationship is due to soil history, plant history, or a combination of both. We established a plant diversity gradient consisting of 1, 2, 3, and 6 grassland plant species and factorially nested with soil history and plant history treatments for each level of plant species richness. Representative results demonstrate the successful establishment of target treatments in the Ecotron experiment, observing the effects of plant and soil history on initial plant development and final plant growth. Additionally, we provide a case study for data analysis of individual response variables. We outline research objectives and methods to comprehensively assess the multifunctional responses to the experimental treatments necessary to ultimately address the overarching hypothesis.

Agricultural land use intensification has led to loss of soil carbon; restoring soil carbon through regenerative practices offers an opportunity to help mitigate climate change and promote soil health. The soil microbiome is central in transforming plant materials into persistent forms of soil organic carbon. However, there is a poor mechanistic understanding of how microbiomes function, assemble, interact and collectively influence soil carbon changes across land use gradients. Here we present integration of knowledge across scales from field observations and lab experiments to highlight the importance of microbial ecophysiology and their emergent traits in determining the soil carbon balance in multiple paired local contrasts of low and high land use intensity systems in the UK. Across 11 paired contrasts, we observed significantly higher microbial community-level carbon use efficiency (CUE) and increased biomass in low intensity grassland soils compared with high intensity cropland soils. We suggest that less-intensive management practices have more potential for carbon storage through increased microbial CUE. Using proteomics and extracellular enzyme analysis, we demonstrate that reduced CUE in cropland soils was linked to higher microbial investment in stress alleviation and resource acquisition traits. To examine if grassland microbiomes with higher CUE could be recruited to help accumulate soil carbon in cropland soils, in lab mesocosm we reciprocally transferred microbiomes derived from historically undisturbed grassland soil and neighbouring cropland soil into their sterile counterparts from 2 paired contrasts. We fed the microbiomes with plant litter tea and monitored community assembly over 8 months. We observed that soil conditions were more important than inoculum source in determining bacterial assemblage, inoculum source was more important than soil conditions in determining fungal assemblage, whereas both inoculum source and soil conditions mattered equally in shaping the protist assemblage. This highlights the differential response of bacteria, fungi and protists to environmental filtering and raises questions around the persistence and therefore efficacy of microbial inoculations. In terms of soil carbon accumulation, we observed that a grassland microbiome led to positive outcomes in terms of soil carbon changes in cropland soil after 8 months suggesting that the microbial emergent ecophysiology that arises from both initial inoculum as well as the soil conditions matter in determining soil carbon accumulation. Our research highlights the need for careful land management to create the right soil conditions for the promotion of beneficial microbiomes with efficient metabolism for carbon accumulation. This will aid in regenerating degraded soils for sustainable climate-smart agriculture.

Climate variability, especially monsoonal rainfall, has significantly shaped habitable areas for human populations in South Asia in the past just as it does today. Instances of climate-driven social disruptions and population movements are evident worldwide, as evidenced for example in the Classic Maya and the Indus Valley Civilization (IVC). However, climate change can manifest in very different ways in terms of vegetation and fire regimes, with important implications for regional environmental histories as well as socio-political patterns. As such, it is essential to develop a comprehensive understanding of the intricate interplay between climate, vegetation, fire, and archaeological evidence relating to changes in settlement patterns and continuities. Insights derived from such studies offer a foundation to explore and comprehend present and future human-environment interactions. Here we present multi-proxy time-series datasets derived from a 2.25-meter geological trench known as ‘Jankipura,’ located within the semi-arid Thar Desert. Jankipura, located near Didwana Lake, holds prehistoric importance, being surrounded by major archaeological sites in the Thar Desert. It is also a part of the Didwana Palaeolithic Complex, surrounded by the IVC, Jodhpura-Ganeshwar, and Ahar-Banas cultural regions. The chronology of the Jankipura trench is constructed based on four 14C AMS dates ranging from 183 to 4656 cal yr. BP, aligning with the Mature phase of the IVC – a period characterized by population migration and a severe reduction in settlement density. Our analysis encompasses measurements of sediment total organic and bulk carbon isotope (d13Cbulk) composition, alongside examinations of plant-wax molecular distributions (n-alkanes and fatty acids). Additionally, we analyzed the δ13C and δ2H values of long-chain n-alkanes (C27, C29, C31, and C33) and fatty acids (C26, C28, C30, and C32) extracted from the sediment samples. Our study also involved the assessment of macro-charcoal concentrations (>125 µm, differentiating grass from wood) to reconstruct the climate-vegetation-fire relationships during and after a major period of disruption of the IVC. The findings highlight an dry phase between 4656 and 2932 cal yr. BP, characterized by a mixed C3-C4 vegetational landscape with limited fire episodes. A significant fire episode took place during the period from 2932 to 1960 cal yr. BP, suggesting dry conditions supported by abundant C4 vegetation. Between 1960 and 183 cal yr. BP, three minor fire events occurred amid fluctuating rainfall conditions and a landscape dominated by mixed C3-C4 vegetation. The identified macro-charcoal predominantly comprised woody fragments over grass fragments. Notably, an increasing trend in isotope values, reaching its peak in macro-charcoal, is observed between 183 cal yr. BP and the present, signifying increased aridity compared to the mature phase of the IVC. Although the study is based on a single trench, our observation of a weak relationship between vegetation and fire suggests that the reconstructed fire events may have originated from anthropogenic activities. This sheds light on the significance of vegetation, especially the utilization of wood, during the Mature phase of the IVC. We recommend generating more records from this region to better comprehend the spatio-temporal interaction of the IVC population with the environment.

Groundwater health is increasingly threatened by climate change, which alters precipitation patterns, leading to groundwater recharge shifts. These shifts impact subsurface microbial communities, crucial for maintaining ecosystem functions. In this decade-long study of carbonate aquifers, we analyzed 815 bacterial 16S rRNA gene datasets, 226 dissolved organic matter (DOM) profiles, 387 metabolomic datasets, and 174 seepage microbiome sequences. Our findings reveal distinct short- and long-term temporal patterns of groundwater microbiomes driven by environmental fluctuations. Microbiomes of hydrologically connected aquifers exhibit lower temporal stability due to stochastic processes and greater susceptibility to surface disturbances, yet they demonstrate remarkable resilience. Conversely, isolated aquifer microbiomes show resistance to short-term changes, governed by deterministic processes, but exhibit reduced stability under prolonged stress. Variability in seepage-associated microorganisms, DOM, and metabolic diversity further drive microbiome dynamics. These findings highlight the dual vulnerability of groundwater systems to acute and chronic pressures, emphasizing the critical need for sustainable management strategies to mitigate the impacts of hydroclimatic extremes. ### Competing Interest Statement The authors have declared no competing interest.

Groundwater health is increasingly threatened by climate change, which alters precipitation patterns, leading to groundwater recharge shifts. These shifts impact subsurface microbial communities, crucial for maintaining ecosystem functions. In this decade-long study of carbonate aquifers, we analyzed 815 bacterial 16S rRNA gene datasets, 226 dissolved organic matter (DOM) profiles, 387 metabolomic datasets, and 174 seepage microbiomes. Our findings reveal distinct short- and long-term temporal patterns of groundwater microbiomes driven by environmental fluctuations. Microbiomes of hydrologically connected aquifers exhibit lower temporal stability due to stochastic processes and greater susceptibility to surface disturbances, yet demonstrate remarkable resilience. Conversely, more isolated aquifer microbiomes resist short-term changes, governed by deterministic processes, but exhibit reduced stability under prolonged stress. Variability in seepage-associated microorganisms, DOM, and metabolic diversity further drives microbiome dynamics. These findings highlight the dual vulnerability of groundwater systems to acute or chronic pressures and the need for sustainable management to mitigate hydroclimatic extremes.

Heavy precipitation, drought, and other hydroclimatic extremes occur more frequently than in the past climate reference period (1961-1990). Given their strong effect on groundwater recharge dynamics, these phenomena increase the vulnerability of groundwater quantity and quality. Over the course of the past decade, we have documented changes in the composition of dissolved organic matter in groundwater. We show that fractions of ingressing surface-derived organic molecules increased significantly as groundwater levels declined, whereas concentrations of dissolved organic carbon remained constant. Molecular composition changeover was accelerated following 2018's extreme summer drought. These findings demonstrate that hydroclimatic extremes promote rapid transport between surface ecosystems and groundwaters, thereby enabling xenobiotic substances to evade microbial processing, accrue in greater abundance in groundwater, and potentially compromise the safe nature of these potable water sources. Groundwater quality is far more vulnerable to the impact of recent climate anomalies than is currently recognized, and the molecular composition of dissolved organic matter can be used as a comprehensive indicator for groundwater quality deterioration.

African low-latitude regions tend to be underrepresented in global continental temperature reconstructions, limiting both our understanding and the reliability of predictions of past and future changes in temperature and precipitation in those parts of the world. The lack of continuous sedimentary archives and quantitative temperature proxies further complicates this issue, especially outside Eastern Africa. Here, we use data collected in Cameroon lakes to assess branched glycerol dialkyl glycerol tetraether (brGDGT)-based temperature proxies and examine potential confounding variables impacting temperature reconstructions. By analyzing GDGT distributions in soil watershed, water column, and surface sediment samples from a total of 11 Cameroon lakes, we could verify that the degree of methylation of brGDGTs (through the MBT ' 5Me) can serve as a reliable indicator for reconstructing mean annual air temperatures. Additionally, we could confirm that surface water conductivity represents a controlling factor for the brGDGT assemblage in surface sediments of crater lakes. Moreover, we provide the first reconstruction of Mid-to Late-Holocene GDGT-based air temperatures for a crater lake in Cameroon (Central Africa), revealing a temperature decrease of 2.5 degrees C over the last 7000 years, which agrees with recently published records for East Africa but exceeds current model predictions. These discrepancies highlight the need for additional studies to focus on this geographically underrepresented area.

The Central Amazon comprises mosaics of forest ecosystems with different water dynamics and soil characteristics. The water dynamics from each ecosystem affect the evaporation signal seasonally expressed in the water isotopes (δ18O and δD). The recognition of the evaporative signal from different forest segments is essential for the development of hydrological and eco-hydrological studies in the complex Amazon biome. In this study, we used stable isotopes to evaluate how the water dynamics of different forest ecosystems affect seasonal water evaporative signals in each environment. We monitored water isotope signals from 2018-2020 in different compartments (precipitation, soil, stream, groundwater, river, lake, and flooded areas) of two non-flooded forests (clay soils - “plato” and sandy soils – “campinarana”) and three flooded forests (pristine igapó, disturbed igapó and várzea). We found that the soil water sampled by lysimeters in the upland forest seasonally expresses the isotope signal from the rainwater (Local Meteorical Water Line-LMWL) without a strong evaporative overprint. The water isotope signal from flooded forests is more variable. The isotopic composition of pristine rivers has an overlapping signal from the rain isotope signal (LMWL). However, water from the river downstream from the very large hydropower dam (Balbina) has a strong evaporative signal. During the flooded period, the water within the flooded forests has a more evaporated signal than the signal from the source (such as the river or lakes). During the non-flooded period, the water isotope signal from the soil inside the flooded forest corresponds to the rainwater signal. To the best of our knowledge, these represent the first description of the water isotope signals from the compartments in different Central Amazonian forest ecosystems. They illustrate and identify the high variation of the evaporative signal from the complex Amazon biome, knowledge that is essential to understanding how different forest ecosystems influence water recycling in the Amazon hydrological cycle.

Dissolved organic matter (DOM) plays a crucial role in global carbon cycling, yet its molecular complexity and the factors governing its turnover and degradation in different ecosystems are poorly understood. Here, we provide an experimental assessment of structural diversity in terrestrial and marine DOM, using a multimethod approach. Terrestrial peat pore water (PPW) exhibited a similar number of COOH-groups, two times more noncarboxylic oxygen atoms (non-COOH-O, up to n = 20) as compared to surface seawater (SSW; up to n = 10), and significantly higher isomeric dispersity indices (2.5-3.0 vs 1.3-1.5), highlighting its greater structural complexity and isomeric diversity. At the level of individual molecular formulas of the widely used DOM degradation index (IDEG), we found that POSIDEG molecular formulas representing fresh DOM (i.e., they were positively correlated with radiocarbon content) share similar structural characteristics in both environments (e.g., low number of carboxyl-groups). In contrast, NEGIDEG markers for degraded DOM (i.e., negatively correlated with radiocarbon content) displayed a higher number of carboxyl-groups in the least acidic fraction for PPW but in the most acidic fraction for SSW. Our results indicate ecosystem-specific degradation pathways emphasizing how global carbon cycling is influenced by the molecular structure of DOM.

Globally, the process of atmospheric nitrogen (N2) fixation by free-living diazotrophs in soils contributes significantly to the soil N supply, yet the understanding of its driving factors, particularly the role of energy availability, is limited. In this study, we explored how two different energy sources, an artificial carbon input, simulating highly bioavailable root exudates, and a natural gradient in soil organic matter that requires decomposition, affect N2 fixation by free-living diazotrophs and soil microbial community functions through microcosm 15N2 incubation experiments. We analysed the incorporation of 15N into soil and used mass spectrometry to determine microbial lipids, which serve as indicators of microbial community functions, via an untargeted lipidomics approach. Our findings demonstrate a significant capacity for N2 fixation by free-living diazotrophs, with a potential annual storage of 111 kg N per hectare. The addition of artificial exudates yielded an extra of 51 kg N ha-1y-1. This N2 fixation was accompanied by a presumable N limitation in the microbial community, as biomass growth favoured N-free lipids with an equal synthesis of storage (triacylglycerols) and structural membrane lipids. While energy addition boosted N uptake particularly in soils with low organic matter, in soils rich in organic matter, N uptake was naturally higher (an extra 20 kg N ha-1y-1), along with increased levels of membrane-associated lipids, suggesting a larger microbial community. Our results imply that enhanced root exudation, potentially driven by more productive plant communities, could mitigate the energy constraints on free-living diazotrophic N2 fixation as part of a vital soil microbial community. These insights support the development of sustainable agricultural practices that stimulate the capacity for N2 fixation by freeliving diazotrophs, aiming to maintain ecological balance by minimising N loss from fertilisation.