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.
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.
The deposition of aeolian, or windblown, dust is widely recognized as an important physical and chemical flux to ecosystems. Dust deposition adds exogenous mineral and organic material to terrestrial ...surfaces and can be important for the biogeochemical cycling of nutrients. There have been many studies that characterize the physical and chemical composition of dust. However, few studies have synthesized these observations in order to examine patterns geochemical fluxes. We have compiled observations of dust deposition rates, particle size distributions (PSD), mineralogy and bulk elemental and organic chemistry.
The rates of dust deposition observed across the globe vary from almost 0 to greater than 450 g m
−
2
yr
−
1
. Sites receiving dust deposition can be partitioned into broad categories based on there distance from dust source regions. When compared to global dust models our results suggest some models may underestimate dust deposition rates at the regional and local scales. The distance from the source region that dust is deposited also influences the particle size distributions, mineralogy, and chemical composition of dust; however, more consistent dust sampling and geochemical analyses are needed to better constrain these spatial patterns. On average, the concentrations of most major elements (Si, Al, Fe, Mg, Ca, K) in aeolian dust tend to be similar (±
20%) to the composition of the upper continental crust (UCC), but there is substantial variability from sample to sample. In contrast, some elements tend to be depleted (Na) or enriched (Ti) in dust, likely as a result of soil weathering processes prior to dust emissions. Trace elements, especially heavy metals, are consistently enriched in dust relative to the UCC. Ecologically important nutrients, such as N and P, are also present in dust deposition. The geochemical flux attributable to dust deposition can be substantial in ecosystems located proximal to dust source regions. We calculate estimates of elemental flux rates based on the average chemical composition of aeolian dust and varying rates of deposition. These estimated flux rates are useful as a rough gauge of the degree to which dust deposition may influence biogeochemical cycling in terrestrial ecosystems and should be utilized to better constrain deposition estimates of global dust models.
Contemporary soil organic matter (SOM) models have been successful at simulating decomposition across a range of spatial and temporal scales using first-order kinetics to represent the decomposition ...process; however, recent work suggests the simplicity of the first-order representation of decomposition is not adequate to capture the microbially-driven dynamics of SOM decomposition over short timescales. For example, the response of soils to drying-rewetting events may best be explained by microbial and/or exoenzyme controls on decomposition. To test if adding these microbial mechanisms improves the ability of SOM models to simulate the response of soils to short-term environmental changes, we developed four different SOM decomposition models with varying mechanistic complexity and compared their ability to simulate soil respiration from a pulsed drying-rewetting laboratory-based experiment. Specifically, we tested the ability of the models to capture the timing and magnitude of soil CO
2 efflux in response to rewetting or constant moisture conditions. The results of the comparison suggest that the inclusion of exoenzyme and microbial controls on decomposition can improve the ability to simulate pulsed rewetting dynamics; however, less mechanistic first-order models prevail under steady-state moisture conditions. These modeling results may have implications for understanding the long-term response of soil carbon stocks in response to local and regional climate change.
Soil organic matter (SOM) turnover increasingly is conceptualized as a tension between accessibility to microorganisms and protection from decomposition via physical and chemical association with ...minerals in emerging soil biogeochemical theory. Yet, these components are missing from the original mathematical models of belowground carbon dynamics and remain underrepresented in more recent compartmental models that separate SOM into discrete pools with differing turnover times. Thus, a gap currently exists between the emergent understanding of SOM dynamics and our ability to improve terrestrial biogeochemical projections that rely on the existing models. In this opinion paper, we portray the SOM paradigm as a triangle composed of three nodes: conceptual theory, analytical measurement, and numerical models. In successful approaches, we contend that the nodes are connected—models capture the essential features of dominant theories while measurement tools generate data adequate to parameterize and evaluate the models—and balanced— models can inspire new theories via emergent behaviors, pushing empiricists to devise new measurements. Many exciting advances recently pushed the boundaries on one or more nodes. However, newly integrated triangles have yet to coalesce. We conclude that our ability to incorporate mechanisms of microbial decomposition and physicochemical protection into predictions of SOM change is limited by current disconnections and imbalances among theory, measurement, and modeling. Opportunities to reintegrate the three components of the SOM paradigm exist by carefully considering their linkages and feedbacks at specific scales of observation.
High-latitude warming is capable of accelerating permafrost degradation and the decomposition of previously frozen carbon. The existence of an analogous high-altitude feedback, however, has yet to be ...directly evaluated. We address this knowledge gap by coupling a radiocarbon-based model to 7 years (2008-2014) of continuous eddy covariance data from a snow-scoured alpine tundra meadow in Colorado, USA, where solifluction lobes are associated with discontinuous permafrost. On average, the ecosystem was a net annual source of 232 ± 54 g C m
(mean ± 1 standard deviation) to the atmosphere, and respiration of relatively radiocarbon-depleted (i.e., older) substrate contributes to carbon emissions during the winter. Given that alpine soils with permafrost occupy 3.6 × 10
km
land area and are estimated to contain 66.3 Pg of soil organic carbon (4.5% of the global pool), this scenario has global implications for the mountain carbon balance and corresponding resource allocation to lower elevations.
Snow cover duration in a seasonally snow covered mountain range (San Juan Mountains, USA) was found to be shortened by 18 to 35 days during ablation through surface shortwave radiative forcing by ...deposition of disturbed desert dust. Frequency of dust deposition and radiative forcing doubled when the Colorado Plateau, the dust source region, experienced intense drought (8 events and 39–59 Watts per square meter in 2006) versus a year with near normal precipitation (4 events and 17–34 Watts per square meter in 2005). It is likely that the current duration of snow cover and surface radiation budget represent a dramatic change from those before the widespread soil disturbance of the western US in the late 1800s that resulted in enhanced dust emission. Moreover, the projected increases in drought intensity and frequency and associated increases in dust emission from the desert southwest US may further reduce snow cover duration.
Practical application of electrochemical water splitting demands durable, efficient, and non-noble metal catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). ...Herein, we report a new hydrogen evolution nanowire electrocatalyst, consisting of S-doped CoWP nanoparticles embedded in S- and N-doped carbon matrix (S-CoWP@(S,N)-C), which is in situ transformed from Hofmann-type (Co, W)-based metal–organic framework (MOF) nanowires. Because of S and N doping to the carbon matrix and the S doping to CoWP nanoparticles, the obtained S-CoWP@(S,N)-C catalyst reaches a current density of −10 mA cm–2 at −35 and −67 mV (vs RHE) in acidic and alkaline electrolytes, respectively. Powered by a lead halide perovskite solar cell, an unassisted two-electrode solar water-splitting device using MOF-derived S-CoWP@(S,N)-C HER electrocatalysts and S-CoW@(S,N)-C OER electrocatalysts displays a solar-to-hydrogen conversion efficiency of 10.98%. Our method is highly applicable for developing robust electrocatalysts toward efficient and low-cost solar-driven water splitting.
Oxidative weathering of sedimentary rocks plays an important role in the global carbon cycle. Rhenium (Re) has been proposed as a tracer of rock organic carbon (OCpetro) oxidation. However, the ...sources of Re and its mobilization by hydrological processes remain poorly constrained. Here, we examine dissolved Re as a function of water discharge, using samples collected from three alpine catchments that drain sedimentary rocks in Switzerland (Erlenbach and Vogelbach) and Colorado, USA (East River). The Swiss catchments reveal a higher dissolved Re flux in the catchment with higher erosion rates, but have similar Re/Na+ and Re/SO42− ratios, which indicate a dominance of Re from OCpetro. Despite differences in rock type and hydro‐climatic setting, the three catchments have a positive correlation between river water Re/Na+ and Re/SO42− and water discharge. We propose that this reflects preferential routing of Re from a near‐surface, oxidative weathering zone. The observations support the use of Re as a proxy to trace rock‐organic carbon oxidation, and suggest it may be a hydrological tracer of vadose zone processes. We apply the Re proxy and estimate CO2 release by OCpetro oxidation of 5.7 +6.6/−2.0 tC km−2 yr−1 for the Erlenbach. The overall weathering intensity was ∼40%, meaning that the corresponding export of unweathered OCpetro in river sediments is large, and the findings call for more measurements of OCpetro oxidation in mountains and rivers as they cross floodplains.
Plain Language Summary
When rocks undergo chemical weathering, breakdown of organic matter that has been stored in the rocks over time can release carbon dioxide to the atmosphere. It has remained a challenge to track and quantify this process, but the element rhenium provides a tool to do this. When organic matter in rocks undergoes oxidative weathering, rhenium is oxidized and enters river water. We can use measurements of dissolved rhenium fluxes as a proxy to estimate rock organic carbon oxidation. However, we still lack information on the sources of rhenium in river catchments and need a better understanding of the pathway that rhenium takes from rocks, through soils into streams and rivers. Here, we measured dissolved rhenium alongside other weathering products in three catchments that drain alpine landscapes made up of sedimentary rocks. The catchments behave in a remarkably similar way, where the relative amount of rhenium in river water increases with water flow. Based on our ideas of how water moves through the landscape, our results suggest that rhenium is moved from a near‐surface zone in all three catchments, where oxygen and rock organic carbon react. By applying the proxy to the catchments, we confirm that oxidative weathering rates and their carbon dioxide release increase with physical erosion rates.
Key Points
We explore the mobility of rhenium in three alpine catchments, in the context of its use as a proxy for rock organic carbon oxidation
The relative abundance of dissolved Re increases with discharge, explained by a model where Re is mobilized from near surface zone
The findings support rhenium as a proxy for oxidative weathering, and we find rock organic carbon oxidation increases with erosion rate
Soil organic matter (SOM) supports the Earth's ability to sustain terrestrial ecosystems, provide food and fiber, and retains the largest pool of actively cycling carbon. Over 75% of the soil organic ...carbon (SOC) in the top meter of soil is directly affected by human land use. Large land areas have lost SOC as a result of land use practices, yet there are compensatory opportunities to enhance productivity and SOC storage in degraded lands through improved management practices. Large areas with and without intentional management are also being subjected to rapid changes in climate, making many SOC stocks vulnerable to losses by decomposition or disturbance. In order to quantify potential SOC losses or sequestration at field, regional, and global scales, measurements for detecting changes in SOC are needed. Such measurements and soil‐management best practices should be based on well established and emerging scientific understanding of processes of C stabilization and destabilization over various timescales, soil types, and spatial scales. As newly engaged members of the International Soil Carbon Network, we have identified gaps in data, modeling, and communication that underscore the need for an open, shared network to frame and guide the study of SOM and SOC and their management for sustained production and climate regulation.
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