Soils represent the largest carbon reservoir within terrestrial ecosystems. The mechanisms controlling the amount of carbon stored and its feedback to the climate system, however, remain poorly ...resolved. Global carbon models assume that carbon cycling in upland soils is entirely driven by aerobic respiration; the impact of anaerobic microsites prevalent even within well-drained soils is missed within this conception. Here, we show that anaerobic microsites are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration, and selectively protecting otherwise bioavailable, reduced organic compounds such as lipids and waxes from decomposition. Further, shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralization rate, illustrating the sensitivity of anaerobically protected carbon to disturbance. The vulnerability of anaerobically protected carbon to future climate or land use change thus constitutes a yet unrecognized soil carbon-climate feedback that should be incorporated into terrestrial ecosystem models.
Char black carbon (BC), the solid residue of incomplete combustion, is continuously being added to soils and sediments due to natural vegetation fires, anthropogenic pollution, and new strategies for ...carbon sequestration (“biochar”). Here we present a molecular-level assessment of the physical organization and chemical complexity of biomass-derived chars and, specifically, that of aromatic carbon in char structures. Brunauer−Emmett−Teller (BET)−N2 surface area (SA), X-ray diffraction (XRD), synchrotron-based near-edge X-ray absorption fine structure (NEXAFS), and Fourier transform infrared (FT-IR) spectroscopy are used to show how two plant materials (wood and grass) undergo analogous but quantitatively different physical−chemical transitions as charring temperature increases from 100 to 700 °C. These changes suggest the existence of four distinct categories of char consisting of a unique mixture of chemical phases and physical states: (i) in transition chars, the crystalline character of the precursor materials is preserved; (ii) in amorphous chars, the heat-altered molecules and incipient aromatic polycondensates are randomly mixed; (iii) composite chars consist of poorly ordered graphene stacks embedded in amorphous phases; and (iv) turbostratic chars are dominated by disordered graphitic crystallites. Molecular variations among the different char categories likely translate into differences in their ability to persist in the environment and function as environmental sorbents.
Minerals in the rhizosphere Jilling, Andrea; Keiluweit, Marco; Contosta, Alexandra R. ...
Biogeochemistry,
07/2018, Letnik:
139, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Despite decades of research progress, ecologists are still debating which pools and fluxes provide nitrogen (N) to plants and soil microbes across different ecosystems. Depolymerization of soil ...organic N is recognized as the rate-limiting step in the production of bioavailable N, and it is generally assumed that detrital N is the main source. However, in many mineral soils, detrital polymers constitute a minor fraction of total soil organic N. The majority of organic N is associated with clay-sized particles where physicochemical interactions may limit the accessibility of N-containing compounds. Although mineral-associated organic matter (MAOM) has historically been considered a critical, but relatively passive, reservoir of soil N, a growing body of research now points to the dynamic nature of mineral-organic associations and their potential for destabilization. Here we synthesize evidence from biogeoscience and soil ecology to demonstrate how MAOM is an important, yet overlooked, mediator of bioavailable N, especially in the rhizosphere. We highlight several biochemical strategies that enable plants and microbes to disrupt mineral-organic interactions and access MAOM. In particular, root-deposited low-molecular-weight exudates may enhance the mobilization and solubilization of MAOM, increasing its bioavailability. However, the competitive balance between the possible fates of N monomers—bound to mineral surfaces versus dissolved and available for assimilation—will depend on the specific interaction between mineral properties, soil solution, mineral-bound organic matter, and microbes. Building off our emerging understanding of MAOM as a source of bioavailable N, we propose a revision of the Schimel and Bennett (Ecology 85:591–602, 2004) model (which emphasizes N depolymerization), by incorporating MAOM as a potential proximal mediator of bioavailable N.
Litter decomposition is a keystone ecosystem process impacting nutrient cycling and productivity, soil properties, and the terrestrial carbon (C) balance, but the factors regulating decomposition ...rate are still poorly understood. Traditional models assume that the rate is controlled by litter quality, relying on parameters such as lignin content as predictors. However, a strong correlation has been observed between the manganese (Mn) content of litter and decomposition rates across a variety of forest ecosystems. Here, we show that long-term litter decomposition in forest ecosystems is tightly coupled to Mn redox cycling. Over 7 years of litter decomposition, microbial transformation of litter was paralleled by variations in Mn oxidation state and concentration. A detailed chemical imaging analysis of the litter revealed that fungi recruit and redistribute unreactive Mn2+provided by fresh plant litter to produce oxidative Mn3+species at sites of active decay, with Mn eventually accumulating as insoluble Mn3+/4+oxides. Formation of reactive Mn3+species coincided with the generation of aromatic oxidation products, providing direct proof of the previously posited role of Mn3+-based oxidizers in the breakdown of litter. Our results suggest that the litter-decomposing machinery at our coniferous forest site depends on the ability of plants and microbes to supply, accumulate, and regenerate short-lived Mn3+species in the litter layer. This observation indicates that biogeochemical constraints on bioavailability, mobility, and reactivity of Mn in the plant–soil system may have a profound impact on litter decomposition rates.
► Sorption of PAEs by plant biomass-derived biochars was determined. ► Low-HTT biochars exhibited the high sorption of PAEs. ► The O and N groups jointly account for high sorption of PAEs for low-HTT ...biochars. ► Grass chars displayed higher PAEs sorption than wood chars.
Molecular variations among different biochar categories translate into differences in their ability to function as sorbents to three phthalic acid esters (PAEs) representing a gradient in hydrophobicity. The sorption capacity (KOC) for all three PAEs was the greatest for amorphous biochars (heat treatment temperature HTT=400°C), followed by biochars produced at 300°C, and was best explained by the hydrophobicity of the sorbate. Greater alkyl C content and higher polarity of grass chars versus wood chars prepared at similar temperatures explained both (a) the difference in sorbent strength between feedstocks and (b) the maximum in sorbent strength at relatively low HTTs (300–400°C). Hydrophobic partitioning into ‘soft’ alkyl carbon and specific H-bonding involving char-bound O and N groups jointly account for high affinities of PAEs for low-HTT biochars. The results highlight the influence of feedstocks and HTTs on PAEs sorption strength and mechanism.
Beyond clay Rasmussen, Craig; Heckman, Katherine; Wieder, William R. ...
Biogeochemistry,
02/2018, Letnik:
137, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial ...carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO₂ to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.
Soil organic nitrogen (N) is a critical resource for plants and microbes, but the processes that govern its cycle are not well-described. To promote a holistic understanding of soil N dynamics, we ...need an integrated model that links soil organic matter (SOM) cycling to bioavailable N in both unmanaged and managed landscapes, including agroecosystems. We present a framework that unifies recent conceptual advances in our understanding of three critical steps in bioavailable N cycling: organic N (ON) depolymerization and solubilization; bioavailable N sorption and desorption on mineral surfaces; and microbial ON turnover including assimilation, mineralization, and the recycling of microbial products. Consideration of the balance between these processes provides insight into the sources, sinks, and flux rates of bioavailable N. By accounting for interactions among the biological, physical, and chemical controls over ON and its availability to plants and microbes, our conceptual model unifies complex mechanisms of ON transformation in a concrete conceptual framework that is amenable to experimental testing and translates into ideas for new management practices. This framework will allow researchers and practitioners to use common measurements of particulate organic matter (POM) and mineral-associated organic matter (MAOM) to design strategic organic N-cycle interventions that optimize ecosystem productivity and minimize environmental N loss.
Understanding the controls on the amount and persistence of soil organic carbon (C) is essential for predicting its sensitivity to global change. The response may depend on whether C is unprotected, ...isolated within aggregates, or protected from decomposition by mineral associations. Here, we present a global synthesis of the relative influence of environmental factors on soil organic C partitioning among pools, abundance in each pool (mg C g−1 soil), and persistence (as approximated by radiocarbon abundance) in relatively unprotected particulate and protected mineral‐bound pools. We show that C within particulate and mineral‐associated pools consistently differed from one another in degree of persistence and relationship to environmental factors. Soil depth was the best predictor of C abundance and persistence, though it accounted for more variance in persistence. Persistence of all C pools decreased with increasing mean annual temperature (MAT) throughout the soil profile, whereas persistence increased with increasing wetness index (MAP/PET) in subsurface soils (30–176 cm). The relationship of C abundance (mg C g−1 soil) to climate varied among pools and with depth. Mineral‐associated C in surface soils (<30 cm) increased more strongly with increasing wetness index than the free particulate C, but both pools showed attenuated responses to the wetness index at depth. Overall, these relationships suggest a strong influence of climate on soil C properties, and a potential loss of soil C from protected pools in areas with decreasing wetness. Relative persistence and abundance of C pools varied significantly among land cover types and soil parent material lithologies. This variability in each pool's relationship to environmental factors suggests that not all soil organic C is equally vulnerable to global change. Therefore, projections of future soil organic C based on patterns and responses of bulk soil organic C may be misleading.
In the first global meta‐analysis to examine both radiocarbon and C concentrations among different soil C pools, we found that three critical carbon pools (free particulate, occluded particulate, and mineral associated) respond differently to climate. Moisture had an almost equal influence as temperature on C persistence and abundance, highlighting the need for climate change studies focused on moisture manipulations. The strong variation in pool characteristics and their relationship to environmental factors indicates that we need to go beyond bulk soil carbon measurements to understand and model the responses of soil organic carbon to global change; it is critical to evaluate distinct pools as response variables.
Despite the increasing agricultural use of biochar as a way of combining the utilization of biomass for energy production with the removal of CO2 from the atmosphere, it is not known how variations ...in pyrolysis temperature and feedstock type affect concentration and composition of polycyclic aromatic hydrocarbons (PAHs) that inevitably form and associate with biochar. To close this knowledge gap, we quantified 11 unsubstituted three- to five-ring PAHs as well as alkylated forms of phenanthrene and anthracene in grass and wood chars produced in 100 °C increments across a temperature range (100 to 700 °C). Our results show that solvent-extractable PAH concentrations in biochars produced at heat treatment temperatures (HTTs) of 400 and 500 °C greatly exceed those observed at higher and lower temperature, supporting a low HTT solid-phase formation mechanism operable at temperatures commonly used for industrial biochar production. The maximum extractable yield of ‘pyrolytic’ unsubstituted PAHs for grass (22 μg g–1 at HTT = 500 °C) greatly exceeds the value for wood (5.9 μg g–1). Moreover, PAH signatures (e.g., total monomethylphenanthrene to phenanthrene ratios, MP/P ∼2–3) at intermediate temperatures (400 °C) resemble those of fossil oils rather than that commonly attributed to pyrolytic products. Further research is needed to characterize the PAH evolution in modern pyrolysis reactors and assess the fate of biochar-bound PAHs in soils and sediments. Various commonly applied PAH ratios and indicator compounds show promise as markers for specific feedstock materials and pyrolysis conditions of biochars in environmental systems.
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