Impacts of litter decay on organic leachate composition and reactivity

Litter decomposition produces labile and recalcitrant forms of dissolved organic matter (DOM) that significantly affect soil carbon (C) sequestration. Chemical analysis of this DOM can provide important knowledge for understanding soil DOM dynamics, but detailed molecular analyses on litter derived DOM are scarce. Here we use ultrahigh resolution mass spectrometry (FT-ICR MS) to characterize the molecular composition of DOM from fresh and progressively decomposed litter samples. We compared high reactive (HR) and low reactive (LR) litter sources with regard to changes in the chemistry and bioavailability of leachates throughout the early phase of litter decay. We show that litter reactivity is a driver of chemical changes in the leached DOM of litter species. Birch, alder and Vaccinium (i.e. HR) litter initially produced more DOM with a higher lability than that of spruce, pine and wood (i.e. LR) litter. Labile oxidized phenolic compounds were abundant in leachates produced during the initial HR litter decay stages, indicating litter lignin degradation. However, the similarity in chemistry between HR and LR leachates increased during the litter decay process as highly leachable structures in HR litter were depleted. In contrast, chemistry of leachates from LR litter changed little during the litter decay process. The oxygenated phenolic compounds from HR litter were driving the lability of HR leachates and the changes in relative abundance of molecules during DOM incubation. This appeared to result in the creation of stable aliphatic secondary microbial compounds. In LR leachates, lability was driven by labile aliphatic compounds, while more resistant phenolic compounds were associated with recalcitrance. These results show how DOM dynamics follow different paths depending on litter reactivity, which has important implications for soil biogeochemistry and C sequestration

Summer drought weakens land surface cooling of tundra vegetation

Siberia experienced a prolonged heatwave in the spring of 2020, resulting in extreme summer drought and major wildfires in the North-Eastern Siberian lowland tundra. In the Arctic tundra, plants play a key role in regulating the summer land surface energy budget by contributing to land surface cooling through evapotranspiration. Yet we know little about how drought conditions impact land surface cooling by tundra plant communities, potentially contributing to high air temperatures through a positive plant-mediated feedback. Here we used high-resolution land surface temperature and vegetation maps based on drone imagery to determine the impact of an extreme summer drought on land surface cooling in the lowland tundra of North-Eastern Siberia. We found that land surface cooling differed strongly among plant communities between the drought year 2020 and the reference year 2021. Further, we observed a decrease in the normalized land surface cooling (measured as water deficit index) in the drought year 2020 across all plant communities. This indicates a shift towards an energy budget dominated by sensible heat fluxes, contributing to land surface warming. Overall, our findings suggest significant variation in land surface cooling among common Arctic plant communities in the North-Eastern Siberian lowland tundra and a pronounced effect of drought on all community types. Based on our results, we suggest discriminating between functional tundra plant communities when predicting the drought impacts on energy flux related processes such as land surface cooling, permafrost thaw and wildfires

Unified understanding of intrinsic and extrinsic controls of dissolved organic carbon reactivity in aquatic ecosystems

Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land-to-ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services.publishedVersio

Unified understanding of intrinsic and extrinsic controls of dissolved organic carbon reactivity in aquatic ecosystems

Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land-to-ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services

Dissolved organic matter across terrestrial and aquatic systems : sources, chemistry and microbial processing

The movement of DOM from terrestrial to aquatic systems is a globally significant flux affecting both carbon sequestration and CO2 emissions. Here, DOM dynamics were investigated in terrestrial and aquatic systems within the context of this carbon flux. The overall aim was to investigate the source and chemistry of DOM and determine how chemistry is affected by microbial processing of DOM. In the boreal forest, understory vegetation produced more than 80 %, with the species V. myrtillus contributing more than 50 % of litter derived DOM at the forest stand level. Litter reactivity was linked to greater chemical changes in the DOM produced at different decay stages. High reactive species (V. myrtillus, birch, alder) initially produced DOM with a higher lability rich in oxidized phenolic compounds, indicating lignin degradation of the source litter. As the DOM was microbially processed the similarity between litter species increased as oxygenated phenolic compounds from high reactive litter leachates were degraded and stable aliphatic secondary microbial compounds created. Of the DOM extracted from podzols roughly a third consisted of aged (1000 yr) labile carbon as indicated by depleted 14C concentrations in Keeling plot intercepts. This fraction cannot be readily detected by bulk radiocarbon measurements without further incubation of the DOM. As such, the mobilization of aged carbon might have gone on undetected. This has important implications for e.g. predicting the future carbon balance of the boreal forest. Severe drought resulted in strongly decreased CDOM concentrations of lakes as a result of hydrological disconnection. This effect was more pronounced in large lakes. During normal conditions, temperature was a positive driver of CDOM in lakes. Drought systematically weakened the regulation of CDOM by the surrounding area. Furthermore, microbial processing of DOM in lakes with little terrestrial influence show that CDOM is systematically microbially produced while at the same time DOM is degraded. The strength of CDOM production is dependent on CDOM concentrations in these lakes. Combined these results highlight that future DOM regulation might change unexpectedly as temperatures rise, drought becomes more prevalent, understory vegetation composition changes and precipitation and runoff patterns will become more variable

The undetected loss of aged carbon from boreal mineral soils

The boreal forest is among the largest terrestrial biomes on earth, storing more carbon (C) than the atmosphere. Due to rapid climatic warming and enhanced human development, the boreal region may have begun transitioning from a net C sink to a net source. This raises serious concern that old biogenic soil C can be re-introduced into the modern C cycle in near future. Combining bio-decay experiments, mixing models and the Keeling plot method, we discovered a distinct old pre-bomb organic carbon fraction with high biodegradation rate. In total, 34 +/- 12% of water-extractable organic carbon (WEOC) in podzols, one of the dominating boreal soil types, consisted of aged (similar to 1000 year) labile C. The omission of this aged (i.e., Delta C-14 depleted) WEOC fraction in earlier studies is due to the co-occurrence with Delta C-14 enriched modern C formed following 1950s nuclear bomb testing masking its existence. High lability of aged soil WEOC and masking effects of modern Delta C-14 enriched C suggests that the risk for mobilization and re-introduction of this ancient C pool into the modern C cycle has gone undetected. Our findings have important implications for earth systems models in terms of climate-carbon feedbacks and the future C balance of the boreal forest

Different boreal terrestrial DOC sources show different δ13C signatures: implications for tracing labile doc across the land-water interface

The stable carbon isotope ratio (δ13C) is a key tool in tracing the source of carbon within and across ecosystems. In isotope mixing models dissolved organic carbon (DOC) from terrestrial sources is often assigned a fixed δ13C of roughly -28‰ to -27‰ in C3 plant-dominated areas. However, the claim of a uniform δ13C distribution for terrestrial DOC sources of different reactivity has seldom been tested. We leached DOC from 40 sources in the Fennoscandinavian boreal forest, including four decay stages of litter from five dominant species and three organic soil types. Additionally incubations were applied in combination with the Keeling plot method to resolve the δ13C of the labile DOC. The results show a clear split between litter derived (~-32‰) and wetland soil derived DOC (~-26‰). Furthermore the labile subfraction of litter derived DOC was relatively deplete in 13C compared with bulk DOC while in wetland soils the labile pool has a relatively heavier 13C signature. As a result both sources show a labile pool comparable to the initial 13C values of ~-32‰ and ~-26‰ respectively, while the resistant pools of DOC show equal 13C values around ~-27.5‰. Chemical measurements of pre- and post-incubation DOC show clear differences between labile pools of DOC from different sources, indicative of different fractionation processes contributing to the differences seen in 13C values of the respective labile pools. These results suggest that the use of 13C mixing models might lead to misjudging the true amount of terrestrial derived - and in particularly labile - DOC within boreal aquatic systems

The role of the understory in litter DOC and nutrient leaching in boreal forests

Dissolved organic carbon (DOC) derived from plant litter plays an important role in the ecosystem carbon balance and soil biogeochemistry. However, in boreal coniferous forests no integrated understanding exists of how understory vegetation contributes to litter leaching of DOC, nitrogen (N) and phosphorus (P) with different bioavailability at the forest stand level. We characterized water extractable leachates from fresh and decayed litter of dominant canopy and understory sources in a boreal coniferous forest, in order to explore the contribution of understory vegetation as a source of both total and bioavailable forms of DOC, N and P. Recently produced litter from deciduous species (including Vaccinium myrtillus) yielded the highest amounts of DOC. However, this leaching potential decreased exponentially with mass loss through litter decay. The DOC lability generally showed little interspecific variation, although wood derived DOC was more recalcitrant. Lability decreased progressively with litter aging. Water extractable nutrients increased proportionally with DOC, and roughly a quarter (N) or half (P) had directly bioavailable inorganic forms. Scaled to annual litterfall at the forest stand, understory vegetation contributed 80% of the water extractable DOC and nutrients from fresh litter, with > 60% coming from Vaccinium myrtillus alone. However, as litter decomposes, the data suggest a lower leaching potential is maintained with a larger contribution from needle, wood and moss litter. Our study shows that understory vegetation, especially V. myrtillus, is a key driver of litter DOC and nutrient leaching in boreal coniferous forests

High variability in the decay of dissolved organic carbon from different boreal litter sources; a challenge to land-water carbon flux modeling

Dynamic ecosystem modelling offers potentially groundbreaking possibilities to reconstruct and project exports of Dissolved Organic Carbon (DOC) from land to surface water. However, the balance between production, degradation and export of soil DOC remains a challenge to model in boreal forests, partly because variability in soil DOC turnover is poorly understood. Here we determined the heterogeneity in decay potentials for DOC leached from main litter sources in boreal forest. We measured 48h leaching potentials (20°C in pure water) of fresh and pre-degraded leaf and wood litter, and subsequently performed short- and long-term standardized bioassays. Leaching and decay potentials of DOC varied more than tenfold between species. Broadleaf trees and shrubs generally showed highest magnitudes and variability in both DOC leaching and subsequent decay, compared to coniferous materials. However, it appears impossible to predict differences in decay potentials without considering both the physical structure and chemical composition of source materials. We suggest that a thorough inventory of soil DOC sources with regard to decay potentials is needed to adequately model the response in DOC export to changes in climate and vegetation

Systematic microbial production of optically active dissolved organic matter in subarctic lake water

The ecology and biogeochemistry of lakes in the subarctic region are particularly sensitive to changes in the abundance and optical properties of dissolved organic matter (DOM). External input of colored DOM to these lakes is an extensively researched topic, but little is known about potential reciprocal feedbacks between the optical properties of DOM and internal microbial processes in the water. We performed 28-day dark laboratory incubation trials on water from 101 subarctic tundra lakes in northern Sweden, measuring the microbial decay of DOM and the resulting dynamics in colored (CDOM) and fluorescent (FDOM) DOM components. While losses in dissolved oxygen during the incubations corresponded to a 20% decrease in mean DOM, conversely the mean CDOM and total FDOM increased by 22% and 30%, respectively. However, the patterns in microbial transformation of the DOM were not the same in all lakes. Notably, along the gradient of increasing ambient CDOM (water brownness), the lakes showed decreased microbial production of protein-like fluorescence, lowered DOM turnover rates and decreasing bacterial growth per unit of DOM. These trends indicate that browning of subarctic lakes systematically change the way that bacteria interact with the ambient DOM pool. Our study underscores that there is no unidirectional causal link between microbial processes and DOM optical properties, but rather reciprocal dependence between the two