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.
Managed grasslands have the potential to store carbon (C) and partially mitigate climate change. However, it remains difficult to predict potential C storage under a given soil or management ...practice. To study C storage dynamics due to long-term (1952–2009) phosphorus (P) fertilizer and irrigation treatments in New Zealand grasslands, we measured radiocarbon (
14
C) in archived soil along with observed changes in C stocks to constrain a compartmental soil model. Productivity increases from P application and irrigation in these trials resulted in very similar C accumulation rates between 1959 and 2009. The ∆
14
C changes over the same time period were similar in plots that were both irrigated and fertilized, and only differed in a non-irrigated fertilized plot. Model results indicated that decomposition rates of fast cycling C (0.1 to 0.2 year
−1
) increased to nearly offset increases in inputs. With increasing P fertilization, decomposition rates also increased in the slow pool (0.005 to 0.008 year
−1
). Our findings show sustained, significant (i.e. greater than 4 per mille) increases in C stocks regardless of treatment or inputs. As the majority of fresh inputs remain in the soil for less than 10 years, these long term increases reflect dynamics of the slow pool. Additionally, frequent irrigation was associated with reduced stocks and increased decomposition of fresh plant material. Rates of C gain and decay highlight trade-offs between productivity, nutrient availability, and soil C sequestration as a climate change mitigation strategy.
Carbon (C) in soils persists on a range of timescales depending on physical,
chemical, and biological processes that interact with soil organic matter
(SOM) and affect its rate of decomposition. ...Together these processes
determine the age distribution of soil C. Most attempts to measure this age
distribution have relied on operationally defined fractions using properties
like density, aggregate stability, solubility, or chemical reactivity.
Recently, thermal fractionation, which relies on the activation energy
needed to combust SOM, has shown promise for separating young from old C by
applying increasing heat to decompose SOM. Here, we investigated radiocarbon
(14C) and 13C of C released during thermal fractionation to link
activation energy to the age distribution of C in bulk soil and components
previously separated by density and chemical properties. While physically
and chemically isolated fractions had very distinct mean 14C values,
they contributed C across the full temperature range during thermal
analysis. Thus, each thermal fraction collected during combustion of bulk
soil integrates contributions from younger and older C derived from
components having different physical and chemical properties but the same
activation energy. Bulk soil and all density and chemical fractions released
progressively older and more 13C-enriched C with increasing activation
energy, indicating that each operationally defined fraction itself was not
homogeneous but contained a mix of C with different ages and degrees of
microbial processing. Overall, we found that defining the full age
distribution of C in bulk soil is best quantified by first separating
particulate C prior to thermal fractionation of mineral-associated SOM. For
the Podzol analyzed here, thermal fractions confirmed that ∼ 95 % of the mineral-associated organic matter (MOM) had a relatively
narrow 14C distribution, while 5 % was very low in 14C and
likely reflected C from the < 2 mm parent shale material in the soil
matrix. After first removing particulate C using density or size separation,
thermal fractionation can provide a rapid technique to study the age
structure of MOM and how it is influenced by different OM–mineral
interactions.
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).
Carbon (C) in soils persists on a range of timescales depending on physical, chemical, and biological processes that interact with soil organic matter (SOM) and affect its rate of decomposition. ...Together these processes determine the age distribution of soil C. Most attempts to measure this age distribution have relied on operationally defined fractions using properties like density, aggregate stability, solubility, or chemical reactivity. Recently, thermal fractionation, which relies on the activation energy needed to combust SOM, has shown promise for separating young from old C by applying increasing heat to decompose SOM. Here, we investigated radiocarbon (C-14) and C-13 of C released during thermal fractionation to link activation energy to the age distribution of C in bulk soil and components previously separated by density and chemical properties. While physically and chemically isolated fractions had very distinct mean C-14 values, they contributed C across the full temperature range during thermal analysis. Thus, each thermal fraction collected during combustion of bulk soil integrates contributions from younger and older C derived from components having different physical and chemical properties but the same activation energy. Bulk soil and all density and chemical fractions released progressively older and more C-13-enriched C with increasing activation energy, indicating that each operationally defined fraction itself was not homogeneous but contained a mix of C with different ages and degrees of microbial processing. Overall, we found that defining the full age distribution of C in bulk soil is best quantified by first separating particulate C prior to thermal fractionation of mineral-associated SOM. For the Podzol analyzed here, thermal fractions confirmed that similar to 95 % of the mineral-associated organic matter (MOM) had a relatively narrow C-14 distribution, while 5 % was very low in C-14 and likely reflected C from the < 2 mm parent shale material in the soil matrix. After first removing particulate C using density or size separation, thermal fractionation can provide a rapid technique to study the age structure of MOM and how it is influenced by different OM-mineral interactions.
In the age of big data, soil data are more available and richer than ever, but – outside of a few large soil survey resources – they remain largely unusable for informing soil management and ...understanding Earth system processes beyond the original study. Data science has promised a fully reusable research pipeline where data from past studies are used to contextualize new findings and reanalyzed for new insight. Yet synthesis projects encounter challenges at all steps of the data reuse pipeline, including unavailable data, labor-intensive transcription of datasets, incomplete metadata, and a lack of communication between collaborators. Here, using insights from a diversity of soil, data, and climate scientists, we summarize current practices in soil data synthesis across all stages of database creation: availability, input, harmonization, curation, and publication. We then suggest new soil-focused semantic tools to improve existing data pipelines, such as ontologies, vocabulary lists, and community practices. Our goal is to provide the soil data community with an overview of current practices in soil data and where we need to go to fully leverage big data to solve soil problems in the next century.
Radiocarbon is a critical constraint on our estimates of
the timescales of soil carbon cycling that can aid in identifying mechanisms
of carbon stabilization and destabilization and improve the ...forecast of soil
carbon response to management or environmental change. Despite the wealth of
soil radiocarbon data that have been reported over the past 75 years, the
ability to apply these data to global-scale questions is limited by our
capacity to synthesize and compare measurements generated using a variety of
methods. Here, we present the International Soil Radiocarbon Database
(ISRaD; http://soilradiocarbon.org, last access: 16 December 2019), an open-source archive of soil data that
include reported measurements from bulk soils, distinct soil carbon pools
isolated in the laboratory by a variety of soil fractionation methods,
samples of soil gas or water collected interstitially from within an intact
soil profile, CO2 gas isolated from laboratory soil incubations, and
fluxes collected in situ from a soil profile. The core of ISRaD is a relational
database structured around individual datasets (entries) and organized
hierarchically to report soil radiocarbon data, measured at different
physical and temporal scales as well as other soil or environmental
properties that may also be measured and may assist with interpretation and
context. Anyone may contribute their own data to the database by entering it
into the ISRaD template and subjecting it to quality assurance protocols.
ISRaD can be accessed through (1) a web-based interface, (2) an R package
(ISRaD), or (3) direct access to code and data through the GitHub
repository, which hosts both code and data. The design of ISRaD allows for
participants to become directly involved in the management, design, and
application of ISRaD data. The synthesized dataset is available in two
forms: the original data as reported by the authors of the datasets and an
enhanced dataset that includes ancillary geospatial data calculated within
the ISRaD framework. ISRaD also provides data management tools in the
ISRaD-R package that provide a starting point for data analysis; as an
open-source project, the broader soil community is invited and encouraged
to add data, tools, and ideas for improvement. As a whole, ISRaD provides
resources to aid our evaluation of soil dynamics across a range of spatial
and temporal scales. The ISRaD v1.0 dataset is
archived and freely available at https://doi.org/10.5281/zenodo.2613911 (Lawrence et al., 2019).
Earthworm population density has increased in no-till agroecosystems in the Inland Pacific Northwest (IPNW) cereal production region, but the overall impact of this increase on agricultural ...production is unknown. A field study was conducted to identify nutrient concentrations and gradients with distance from Lumbricus terrestris earthworm burrows in agroecosystems. Total carbon (C), nitrogen (N), ammonium (NH4+-N) calcium (Ca), and phosphorus (P) concentrations were greatest in drilosphere soil immediately surrounding burrows, with concentrations generally decreasing as distance from burrow walls increased. Despite large variability, weak trends toward greater nutrient concentrations were observed in active earthworm drilosphere soil compared to abandoned burrows with significant root colonization. A 13-week greenhouse study was conducted to quantify earthworm effects on the decomposition and mineralization of surface organic matter (OM) under simulated IPNW environmental conditions using 15N-labelled wheat straw. Two earthworm species common to IPNW agroecosystems were studied: the exotic endogeic species Aporrectodea trapezoides and the exotic anecic species Lumbricus terrestris, both in single species and combined treatments. Aporrectodea trapezoides stimulated microbial populations and plant-available ammonium (NH4 +-N) concentration in early weeks of the experiment. In L. terrestris and combined species treatments, straw was mostly incorporated into the soil profile and available N concentrations were significantly increased by the end of the experiment. Movement of straw-derived N into microbial and extractable N pools was most rapid in combined treatments, apparently due to the presence of A. trapezoides. However, species interactions were observed that may vary between population densities and species composition. Improved conservation management may further increase earthworm populations, while additional research on earthworm communities and distribution will improve understanding of earthworm effects on crop production.
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