Soil organic matter (SOM) anchors global terrestrial productivity and food and fiber supply. SOM retains water and soil nutrients and stores more global carbon than do plants and the atmosphere ...combined. SOM is also decomposed by microbes, returning CO
2
, a greenhouse gas, to the atmosphere. Unfortunately, soil carbon stocks have been widely lost or degraded through land use changes and unsustainable forest and agricultural practices.
To understand its structure and function and to maintain and restore SOM, we need a better appreciation of soil organic carbon (SOC) saturation capacity and the retention of above- and belowground inputs in SOM. Our analysis suggests root inputs are approximately five times more likely than an equivalent mass of aboveground litter to be stabilized as SOM. Microbes, particularly fungi and bacteria, and soil faunal food webs strongly influence SOM decomposition at shallower depths, whereas mineral associations drive stabilization at depths greater than ∼30 cm. Global uncertainties in the amounts and locations of SOM include the extent of wetland, peatland, and permafrost systems and factors that constrain soil depths, such as shallow bedrock. In consideration of these uncertainties, we estimate global SOC stocks at depths of 2 and 3 m to be between 2,270 and 2,770 Pg, respectively, but could be as much as 700 Pg smaller. Sedimentary deposits deeper than 3 m likely contain >500 Pg of additional SOC. Soils hold the largest biogeochemically active terrestrial carbon pool on Earth and are critical for stabilizing atmospheric CO
2
concentrations. Nonetheless, global pressures on soils continue from changes in land management, including the need for increasing bioenergy and food production.
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.
Soils are an enticing reservoir for nature-based climate solutions, but long timescales are required to store amounts of C of relevance to mitigate warming acknowledging its impermanence. Scientific ...clarity on the controlling factors in soil C persistence should help to disambiguate debates related to permanence in the climate policy domain. However, another contributing factor that is lacking in this debate is a way to compute the climate benefits of C in terrestrial ecosystems over time in the same units as greenhouse gas emissions. We use a case study approach here to demonstrate the use of the metrics of carbon sequestration (CS) and climate benefit of sequestration (CBS) with the aim of assessing the contribution of simultaneous emissions and uptake on radiative forcing. We show how this new computational framework quantifies the climate benefit achieved in two different agricultural systems, one a managed tropical perennial grass system in Hawaiʻi, USA and the other a boreal (cold-temperate, semi-humid) agricultural soil from long term amendment trials in Sweden. Using a set of computations, we show how C inputs and persistence interact to produce different levels of radiative forcing at relevant time frames, which could greatly help to clarify issues of carbon permanence discussed in climate policy. Temporary soil C storage could help to decrease peak warming provided that ambitious emission reductions are part of the portfolio of solutions; the CS and CBS framework gives us a way to quantify it based on biogeochemical understanding of soil C persistence.
Ecosystems play a fundamental role in climate change mitigation by photosynthetically fixing carbon from the atmosphere and storing it for a period of time in organic matter. Although climate impacts ...of carbon emissions by sources can be quantified by global warming potentials, the appropriate formal metrics to assess climate benefits of carbon removals by sinks are unclear. We introduce here the climate benefit of sequestration (CBS), a metric that quantifies the radiative effect of fixing carbon dioxide from the atmosphere and retaining it for a period of time in an ecosystem before releasing it back as the result of respiratory processes and disturbances. In order to quantify CBS, we present a formal definition of carbon sequestration (CS) as the integral of an amount of carbon removed from the atmosphere stored over the time horizon it remains within an ecosystem. Both metrics incorporate the separate effects of (i) inputs (amount of atmospheric carbon removal) and (ii) transit time (time of carbon retention) on carbon sinks, which can vary largely for different ecosystems or forms of management. These metrics can be useful for comparing the climate impacts of carbon removals by different sinks over specific time horizons, to assess the climate impacts of ecosystem management, and to obtain direct quantifications of climate impacts as the net effect of carbon emissions by sources versus removals by sinks.
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.
The magnitude of carbon (C) loss to the atmosphere via microbial
decomposition is a function of the amount of C stored in soils, the quality
of the organic matter, and physical, chemical, and ...biological factors that
comprise the environment for decomposition. The decomposability of C is
commonly assessed by laboratory soil incubation studies that measure
greenhouse gases mineralized from soils under controlled conditions. Here,
we introduce the Soil Incubation Database (SIDb) version 1.0, a compilation
of time series data from incubations, structured into a new, publicly
available, open-access database of C flux (carbon dioxide, CO2, or
methane, CH4). In addition, the SIDb project also provides a platform
for the development of tools for reading and analysis of incubation data as
well as documentation for future use and development. In addition to
introducing SIDb, we provide reporting guidance for database entry and the
required variables that incubation studies need at minimum to be included in
SIDb. A key application of this synthesis effort is to better characterize
soil C processes in Earth system models, which will in turn reduce our
uncertainty in predicting the response of soil C decomposition to a changing
climate. We demonstrate a framework to fit curves to a number of incubation
studies from diverse ecosystems, depths, and organic matter content using a
built-in model development module that integrates SIDb with the existing
SoilR package to estimate soil C pools from time series data. The database
will help bridge the gap between point location measurements, which are
commonly used in incubation studies, and global remote-sensed data or data
products derived from models aimed at assessing global-scale rates of
decomposition and C turnover. The SIDb version 1.0 is archived and publicly
available at https://doi.org/10.5281/zenodo.3871263 (Sierra et al., 2020), and the database is managed
under a version-controlled system and centrally stored in GitHub (https://github.com/SoilBGC-Datashare/sidb, last access: 26 June 2020).
1. We separated the effects of plant species controls on decomposition rates from environmental controls in northern peatlands using a full factorial, reciprocal transplant experiment of eight ...dominant bryophytes in four distinct peatland types in boreal Alberta, Canada. Standard fractionation techniques as well as compound-specific pyrolysis molecular beam mass spectrometry were used to identify a biochemical mechanism underlying any interspecific differences in decomposition rates. 2. We found that over a 3-year field incubation, individual moss species and not micro-environmental conditions controlled early stages of decomposition. Across species, Sphagnum mosses exhibited a trade-off in resource partitioning into metabolic and structural carbohydrates, a pattern that served as a strong predictor of litter decomposition. 3. Decomposition rates showed a negative co-variation between species and their microtopographic position, as species that live in hummocks decomposed slowly but hummock microhabitats themselves corresponded to rapid decomposition rates. By forming litter that degrades slowly, hummock mosses appear to promote the maintenance of macropore structure in surface peat hummocks that aid in water retention. 4. Synthesis. Many northern regions are experiencing rapid climate warming that is expected to accelerate the decomposition of large soil carbon pools stored within peatlands. However, our results suggest that some common peatland moss species form tissue that resists decomposition across a range of peatland environments, suggesting that moss resource allocation could stabilize peatland carbon losses under a changing climate.
The complex structural and functional responses of agricultural soil microbial communities to the addition of carbonaceous compounds such as biochar remain poorly understood. This severely limits the ...predictive ability for both the potential enhancement of soil fertility and greenhouse gas mitigation. In this study, we utilized shotgun metagenomics in order to decipher changes in the microbial community in soil microcosms after 14 days of incubation at 23°C, which contained soils from biochar-amended and control plots cultivated with Napier grass. Our analyses revealed that biochar-amended soil microbiomes exhibited significant shifts in both community composition and predicted metabolism. Key metabolic pathways related to carbon turnover, such as the utilization of plant-derived carbohydrates as well as denitrification, were enriched under biochar amendment. These community shifts were in part associated with increased soil carbon, such as labile and aromatic carbon compounds, which was likely stimulated by the increased available nutrients associated with biochar amendment. These findings indicate that the soil microbiome response to the combination of biochar addition and to incubation conditions confers enhanced nutrient cycling and a small decrease in CO
emissions and potentially mitigates nitrous oxide emissions.
The incorporation of biochar into soil is a promising management strategy for sustainable agriculture owing to its potential to sequester carbon and improve soil fertility. Expanding the addition of biochar to large-scale agriculture hinges on its lasting beneficial effects on the microbial community. However, there exists a significant knowledge gap regarding the specific role that biochar plays in altering the key biological soil processes that influence plant growth and carbon storage in soil. Previous studies that examined the soil microbiome under biochar amendment principally characterized only how the composition alters in response to biochar amendment. In the present study, we shed light on the functional alterations of the microbial community response 2 years after biochar amendment. Our results show that biochar increased the abundance of genes involved in denitrification and carbon turnover and that biochar-amended soil microcosms had a reduction in cumulative CO
production.
Soil health is recognized as an important ecosystem property sensitive to human impact. As a concept, soil health cannot be directly measured, and so assessment and modeling efforts largely rely upon ...key biological, chemical, and physical indicators. Efforts to develop an overall soil health index are largely lacking due to significant statistical challenges and the necessity for regional calibration. Taken from the field of educational research, structural equation modeling (SEM) is an attractive approach to enhance the reliability and validity of soil health scoring. Therefore, SEM may be utilized to advance research efforts to understand management practices impacts on soil health. Our objectives were to develop a robust scoring function that (i) captures the concept of soil health and latent variables, (ii) adjusts scores by inherent soil properties and legacy of intensive land use to adequately reflect our regional conditions and contemporary land management, and (iii) meets the diverse practitioner needs. Through this process, we refined our minimum dataset of soil health indicators and reconceptualized soil health indicators into functional properties. Our results support the development of a robust single level or a multilevel SEM model–depending on the practitioner's goals–that accounts for repeated sampling or pseudoreplication. While the SEM scoring functions were highly related to the conventional scoring approach, SEM outperformed the conventional methods in terms of its wider distribution of scores–and thus enhanced discriminatory power on the lower and higher range of scores. We also confirmed that the SEM scoring function that includes adjustments for mineralogy and legacy of intensive land use successfully differentiates among contemporary management practices and land use. Therefore, we have confidence that the tool is reliable and appropriate to further examine more nuanced impacts of land use change and management practices within a given land use across time and space covering a diversity of soils. (300 words).
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•Many studies present and assess soil health indicators but very few provide a soil health score.•We explored a novel approach to soil health scoring function using structural equation modeling.•We found soil health indicators did not align with traditional biological, chemical, and physical categorizations within the stuctural equation model.•Our scoring approach outperformed the most popular and conventional method of scoring due to its greater discriminatory power.•Our function adjusts for inherent soil properties necessary to properly assess contemporary management practices across a great diversity of soils, including tropical and volcanic soils, and land use history.
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