Soil Carbonate Formation Research Articles

Uranium-series and strontium isotope systematics in soil carbonates from dryland Critical Zones: Implications for soil inorganic carbon storage and transformation

Soil carbonates are dominantly present in dryland Critical Zone (CZ) and their formation could lead to important long-term carbon sequestration in arid to semiarid soils if the Ca ions were derived from silicate weathering or other non-carbonate sources. In managed CZ systems such as agricultural areas converted from natural drylands, irrigation has profound effects on the dryland CZ inorganic carbon storage, especially by modifying dissolution and precipitation dynamics of soil carbonates via controls of irrigation intensity and water chemistry, soil properties, and hydrological flow paths. These processes could lead to transformation of inorganic carbon in soils and groundwater aquifers underneath drylands. One key knowledge gap in studying soil carbonates from managed dryland CZ systems is to detect the formation of soil carbonates under irrigated conditions and distinguish them effectively from soil carbonates formed under natural conditions.Here, we explore the potential of using U-series and strontium isotopes to investigate the formation timescales and conditions of soil carbonates in both natural and managed dryland CZ in Chihuahua Desert of American Southwest. We obtained new U-series and Sr isotope ratios from mature stage V natural soil carbonates in the Jornada Basin of southern New Mexico and compared to previously published U-series and Sr isotope results for younger Jornada soil carbonates as well as irrigation impacted soil carbonates from the Rio Grande floodplains in west Texas. Specifically, we applied the U-series dating method (238U-234U-230Th) to estimate the timescales of soil carbonate formation under both natural climatic conditions and impacts of modern agriculture irrigation. In addition, we showed that the initial (234U/238U) activity ratios recorded in these soil carbonates reflect the systematic changes of soil infiltration rates in both natural and managed dryland CZ. We also utilized Sr isotope ratios (87Sr/86Sr) in soil carbonates, dust, and irrigation water to trace the Ca sources in soil carbonates from natural and managed dryland CZ. Soil carbonates from both settings were characterized by slightly radiogenic 87Sr/86Sr ratios, consistent with their potential of long-term carbon storage in drylands.U-series and Sr isotope systematics characterize important differences in soil carbonates from natural and managed dryland CZ with respect to their Ca sources, timescales of carbonate formation and transformation, and availability of soil moisture. Soil carbonates formed, accumulated, and transformed in natural drylands with long time scales but under irrigated arid agricultural areas its formation is significantly modified by irrigation and other agriculture practices, with implications for long- and short-term inorganic carbon sequestration in both systems of drylands.

Microbial-explicit processes and refined perennial plant traits improve modeled ecosystem carbon dynamics

Globally, soils hold approximately half of ecosystem carbon and can serve as a source or sink depending on climate, vegetation, management, and disturbance regimes. Understanding how soil carbon dynamics are influenced by these factors is essential to evaluate proposed natural climate solutions and policy regarding net ecosystem carbon balance. Soil microbes play a key role in both carbon fluxes and stabilization. However, biogeochemical models often do not specifically address microbial-explicit processes. Here, we incorporated microbial-explicit processes into the DayCent biogeochemical model to better represent large perennial grasses and mechanisms of soil carbon formation and stabilization. We also take advantage of recent model improvements to better represent perennial grass structural complexity and life-history traits. Specifically, this study focuses on: 1) a plant sub-model that represents perennial phenology and more refined plant chemistry with downstream implications for soil organic matter (SOM) cycling though litter inputs, 2) live and dead soil microbe pools that influence routing of carbon to physically protected and unprotected pools, 3) Michaelis-Menten kinetics rather than first-order kinetics in the soil decomposition calculations, and 4) feedbacks between decomposition and live microbial pools. We evaluated the performance of the plant sub-model and two SOM cycling sub-models, Michaelis-Menten (MM) and first-order (FO), using observations of net ecosystem production, ecosystem respiration, soil respiration, microbial biomass, and soil carbon from long-term bioenergy research plots in the mid-western United States. The MM sub-model represented seasonal dynamics of soil carbon fluxes better than the FO sub-model which consistently overestimated winter soil respiration. While both SOM sub-models were similarly calibrated to total, physically protected, and physically unprotected soil carbon measurements, the models differed in future soil carbon response to disturbance and climate, most notably in the protected pools. Adding microbial-explicit mechanisms of soil processes to ecosystem models will improve model predictions of ecosystem carbon balances but more data and research are necessary to validate disturbance and climate change responses and soil pool allocation.

Soil Inorganic Carbon Formation and the Sequestration of Secondary Carbonates in Global Carbon Pools: A Review

Soil inorganic carbon (SIC), a potential carbon sink especially in arid and semi-arid environments, contributes to soil development, landscape stability, carbon (C) sequestration, and global C dynamics but due to the lack of SIC scientific reporting in most C sequestration research, its importance is unclear. A detailed overview of primary and secondary carbonate occurrence, formation, and importance is much needed to understand the role of pedogenic (PC)/secondary carbonate (a common biogeochemically derived soil mineral over time) in the SIC. The mechanisms involved in the formation of PC including carbon dioxide (CO2) from microbial respiration and precipitation, silicate mineral weathering, dissolution, and reprecipitation are highlighted. The isotopic composition of carbonates related to biological C3 or C4 carbon fixation pathways and other paleoecologic and/or climactic factors responsible for new soil carbonate formation are discussed in detail. To address the lack of knowledge associated with SIC, this review attempts to highlight the currently known aspects of the literature, and briefly describe the formation and methodologies that can aid in addressing the research gaps surrounding SIC sequestration. The authors also suggest that greater focus needs to be provided on the actual measurement of SIC to develop a more comprehensive SIC inventory to provide sound data for future research direction, and modeling efforts and to predict C terrestrial storage and change efficiently.

Grounding United States policies and programs in soil carbon science: strengths, limitations, and opportunities

The advent of “natural climate solutions” and “climate smart agriculture” has increased interest in managing agricultural lands to sequester soil carbon and mitigate climate change. This has led to enormous opportunities for soil scientists and growers alike, as new soil carbon initiatives are created by public, private, and philanthropic entities. It has also led to confusion over what is possible or practical to achieve through agricultural management, as soil carbon formation and storage is complex, and its response to management is context-dependent. This can pose challenges to decision makers tasked with creating defensible, science-informed policies and programs for building and protecting soil carbon. Here we summarize the science concerning the potential for agricultural soils to serve as a natural climate solution, in order to frame a discussion of current approaches in United States (US) policy and practice. We examine existing strategies such as soil health initiatives and direct incentive payments, as well as emerging schemes such as carbon markets and crop insurance reform. We suggest future directions for each strategy, and make recommendations for synthesizing approaches into a cohesive US policy portfolio. Guiding principles for this discussion include the notions that (i) climate change adaptation must be prioritized alongside climate change mitigation; (ii) soil carbon sequestration must be paired with greenhouse gas emission reductions; (iii) structural issues and barriers to adoption must be addressed as part of all policies and programs; (iv) practice- and place-specific programs must be administered in lieu of one-size-fits-all prescriptions; and (v) soil carbon science is not yet sufficiently advanced for the accounting and contractual frameworks proposed in cap-and-trade or regulatory approaches.

Trait-based assembly of arbuscular mycorrhizal fungal communities determines soil carbon formation and retention.

Fungi are crucial for soil organic carbon (SOC) formation, especially for the more persistent mineral-associated organic C (MAOC) pool. Yet, evidence for this often overlooks arbuscular mycorrhizal fungi (AMF) communities and how their composition and traits impact SOC accumulation. We grew sudangrass with AMF communities representing different traits conserved at the family-level: 1) competitors, from the Gigasporaceae family; 2) ruderals, from the Glomeraceae family; or 3) both families combined. We labeled sudangrass with 13 C-CO2 to assess AMF contributions to SOC, impacts on SOC priming, and fungal biomass persistence in MAOC. Single-family AMF communities decreased total SOC by 13.8%, likely due to fungal priming. Despite net SOC losses, all AMF communities contributed fungal C to soil but only the Glomeraceae community initially contributed to MAOC. After a month of decomposition, both the Glomeraceae and mixed-family communities contributed to MAOC formation. Plant phosphorus uptake, but not hyphal chemistry, was positively related to AMF soil C and MAOC accumulation. AMF contribution to MAOC is dependent on the specific traits of the AMF community and related to phosphorus uptake. These findings provide insight into how variations in AMF community composition and traits, and thus processes like environmental filtering of AMF, may impact SOC accumulation.

Phosphorus hotspots in pedogenic carbonate coatings determined by zoned microscale arrangement and organo-mineral interactions

Subsoils of calcareous soils are often characterized by pedogenic carbonate precipitates coating soil aggregate and rock surfaces. Numerous studies identified the key mechanisms of pedogenic carbonate formation in calcareous soils. Still, little is known about the ecosystem function of carbonate precipitates for nutrient supply, their microscale structure and the interaction of microscale structure with nutrient distribution such as P. We investigated the mineral composition and P speciation of lithogenic carbonate coated by pedogenic carbonate precipitates. Subsoil horizons of a Rendzic Leptosol (Tuttlingen, C horizon) and a Eutric Cambisol (Benediktenwand, BC horizon) in Central Europe were sampled. We combined wet-chemical methods with X-ray diffraction, synchrotron-based P XANES spectroscopy, and specific surface area analysis (N2-BET). To investigate the spatial arrangement and elemental distribution at the microscale, a complementary approach using light microscopy, scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDX) as well as XANES spectroscopy at the microscale (µXANES) was conducted. In both soils, the pedogenic carbonate coatings showed an intricate porous microstructure and large specific surface area. The δ13C values of the coatings were halfway between those of the primary bedrock and soil organic matter (SOM), proving their formation by re-precipitation of dissolved carbonate through organic and inorganic (H2CO3) acids. Compared to the primary rocks, the coatings were enriched in Al, Fe, Mn, and K and in organic C, N, and P; additionally, they contained silicate minerals (muscovite, illite) and quartz. Whereas the P in the rock interior was mainly present as apatite, the P in the coatings was mainly organic P bound to Ca and Al minerals. NanoSIMS and µXANES analyses at Tuttlingen revealed a zoned microstructure of the pedogenic coating: An inner zone with most P co-localized and bound to Al minerals and an outer zone enriched again in Ca and SOM. The formation of these zones indicates the infiltration and residual accumulation of Al minerals which may contribute to mitigate P losses by leaching. Intensive penetration by fine plant roots and observations of fungal hyphae within the coatings indicate the potential of pedogenically altered carbonates as microbial habitat and ecosystem nutrient source, thus contributing to the scarce ecosystem P nutrition of initial calcareous soils.

Long-term warming reduces surface soil organic carbon by reducing mineral-associated carbon rather than “free” particulate carbon

Soils in alpine ecosystems store a large amount of carbon. However, the responses of soil organic carbon (SOC) to long-term warming and the microbial mechanisms underlying SOC dynamics remain poorly understood. Here, we investigated the effects of 20-year warming (1997–2017) on SOC pools in an alpine shrubland on the Qinghai-Tibetan Plateau. We analyzed the plant and microbial communities, as well as soil chemistry, particularly the SOC of bulk soil and its two physical fractions. Warming did not alter plant biomass (either above- or below-ground). However, warming decreased overall microbial biomass carbon and nitrogen, the biomass of fungi (indicated by phospholipid fatty acids, amino sugars, and OTU richness), and potential activities of hydrolytic enzymes (especially β-1,4-glucosidase), in the surface 0–10 cm soil. Moreover, warming reduced the SOC in bulk soil (by 21%) and the heavy fraction (by 25%), but did not change the SOC in the light fraction. Importantly, there were strong positive correlations between the SOC in bulk soil and in the heavy fraction with the total microbial and fungal necromass, suggesting that the warming-induced loss of soil carbon was mainly due to the suppression of the microbial pathway of soil carbon formation. In deeper layers (10–50 cm) of the shrubland, however, warming (to a much lower degree than that in the surface soil) had only a minor effect on soil carbon pools and the microbial community. A global meta-analysis further showed that warming on average did not significantly change the SOC in different fractions (bulk SOC, active C, and stable C) in the surface soil. Taken together, these results show that long-term warming leads to significant loss of surface soil carbon (mostly in the mineral-associated heavy fraction), likely due to suppressing the microbial (particularly fungal) pathway to soil carbon formation and/or stimulating the utilization of stable soil carbon (i.e. heavy fraction) by microbes in the alpine shrubland.

Mineral-Soil-Plant-Nutrient Synergisms of Enhanced Weathering for Agriculture: Short-Term Investigations Using Fast-Weathering Wollastonite Skarn.

Enhanced weathering is a proposed carbon dioxide removal (CDR) strategy to accelerate natural carbon sequestration in soils via the amendment of silicate rocks to agricultural soils. Among the suitable silicates (such as basalt and olivine), the fast-weathering mineral wollastonite (CaSiO3) stands out. Not only does the use of wollastonite lead to rapid pedogenic carbonate formation in soils, it can be readily detected for verification of carbon sequestration, but its weathering within weeks to months influences soil chemistry and plant growth within the same crop cycle of its application. This enables a variety of short-term experimental agronomic studies to be conducted to demonstrate in an accelerated manner what could take years to be observed with more abundant but slower weathering silicates. This study presents the results of three studies that were conducted to investigate three distinct aspects of wollastonite skarn weathering in soils in the context of both agricultural and horticultural plants. The first study investigated the effect of a wide range of wollastonite skarn dosages in soil (1.5–10 wt.%) on the growth of green beans. The second study provides insights on the role of silicon (Si) release during silicate weathering on plant growth (soybeans and lettuce). The third study investigated the effect of wollastonite skarn on the growth of spring rye when added to soil alongside a nitrogen-based coated fertilizer. The results of these three studies provide further evidence that amending soil with crushed silicate rocks leads to climate-smart farming, resulting in inorganic carbon sequestration, as well as better plant growth in agricultural (soybean and spring rye) and horticultural (green bean and lettuce) crops. They also demonstrate the value of working with wollastonite skarn as a fast-weathering silicate rock to accelerate our understanding of the mineral–soil–plant–nutrient synergism of enhanced weathering.

The soil Microbial Carbon Pump as a new concept for terrestrial carbon sequestration

Soil is a huge terrestrial carbon pool, which has higher carbon storage than the sum of atmospheric and terrestrial vegetation carbon. Small fluctuations in soil carbon pool can affect regional carbon flux and global climate change. As soil organic carbon plays key roles in soil carbon storage and sequestration, studying its composition, sources and stability mechanism is a key to deeply understand the functions of terrestrial ecosystem and how it will respond to climate changes. The recently-proposed concept of soil Microbial Carbon Pump (MCP) emphasizes the importance of soil microbial anabolism and its contributions to soil carbon formation and stabilization, which can be applied for elucidating the source, formation and sequestration of soil organic carbon. This article elaborates MCP-mediated soil carbon sequestration mechanism and its influencing factors, as well as representative scientific questions we may explore with the soil MCP conceptual framework.

Middle Miocene long-term continental temperature change in and out of pace with marine climate records

Reconstructing long-term continental temperature change provides the required counterpart to age equivalent marine records and can reveal how terrestrial and marine temperatures were related during times of extreme climate change such as the Miocene Climatic Optimum (MCO) and the following Middle Miocene Climatic Transition (MMCT). Carbonate clumped isotope temperatures (T(Δ47)) from 17.5 to 14.0 Ma Central European paleosols (Molasse Basin, Switzerland) display a temperature pattern during the MCO that is similar to coeval marine temperature records. Maximum temperatures in the long-term soil T(Δ47) record (at 16.5 and 14.9 Ma) lag maximum ocean bottom water temperatures, lead global ice volume, and mark the initiation of minimum global ice volume phases. The suggested onset of the MMCT, deduced by a marked and rapid decline in Molasse Basin soil temperatures is coeval with cooling reported in high-latitudinal marine records. This is best explained by a change in the seasonal timing of soil carbonate formation that was likely driven by a modification of rainfall seasonality and thus by a major reorganization of mid-latitude atmospheric circulation across Central Europe. In particular, our data suggest a strong climate coupling between the North Atlantic and Central Europe already in the middle Miocene.

A proxy for all seasons? A synthesis of clumped isotope data from Holocene soil carbonates

Soil carbonates are important paleoclimate archives, but interpretations of their isotopic compositions (δ18O, δ13C, and Δ47) are hampered by uncertainty in the annual timing of their accumulation. Several previous studies have inferred the annual timing of soil carbonate formation by comparing modern air/soil temperatures with temperatures estimated from the clumped isotopic composition (TΔ47) of Holocene soil carbonates. Here, we compile the existing Δ47 data to first consider if recent changes in Δ47 standardization methods alters the apparent seasonal biases. Then we explore the importance of various environmental parameters on soil carbonate TΔ47 and discuss implications for associated paleoclimate reconstructions. Most soil carbonates record TΔ47 values higher than mean annual air temperature, but the residual varies from −4 to +24 °C. Δ47 values for most observed soil profiles do not vary with depth. We find that the grain size of the soil matrix, timing of precipitation, and presence of vegetation each explain a portion of the observed variance in seasonal bias and can be used to aid in interpretations of TΔ47 values from paleosols. In some soil carbonates, a warm-season bias in TΔ47 is accompanied with calculated δ18O values of soil water that appear to be biased toward the δ18O of summer rainfall. Most estimated values of δ18O of soil water are within 2‰ of the δ18O values of mean annual rainfall. Where possible, paleoclimate reconstructions should consider precipitation timing, soil texture, and vegetation cover to aid in identifying the seasonal bias of soil carbonate stable isotopic compositions.

Vegetation change in the Baringo Basin, East Africa across the onset of Northern Hemisphere glaciation 3.3–2.6 Ma

Vegetation in East Africa is generally thought to have shifted from forests to more open grasslands and savannas as global climate cooled and high-latitude ice sheets expanded during the Plio-Pleistocene. Such a shift would have greatly influenced landscape resources, and potentially hominin evolution as well. Existing records of African vegetation spanning these time-scales are generally derived from offshore marine records that record continental-scale changes, or paleosol carbonate records that record very local vegetation changes during the short time intervals of soil carbonate formation. Here we present a new record of basin-scale vegetation change from the late Pliocene (~3.3–2.6 Ma) derived from a drill core from the Chemeron Formation, located in the Baringo Basin/Tugen Hills region of the Kenya Rift Valley. Specifically, we present a new record of the relative abundance of C4 grasses and C3 vegetation based on the carbon isotopic composition of leaf wax biomarkers (δ13Cwax), which captures a signal of regional vegetation change. These data demonstrate that vegetation in the Baringo Basin varied greatly between C3 forests and C4 grasslands, and that vegetation exhibits both long-term (secular) trends and orbital-scale variations. The contribution of C3 plants was lower than estimates based on low-resolution carbon isotope data from paleosol carbonates and organic matter in the basin. C3 plants averaged ~53% of the vegetation during the late Pliocene, from ~3.3 to ~3.04 Ma, after which time δ13Cwax indicates more open vegetation and ~41% C3 plants. This transition may have been driven by changes in basin geomorphology, but also possibly occurred as part of larger-scale drying and expansion of C4 vegetation in East Africa. In addition to this secular change, we observe high amplitude variability in the δ13Cwax record including oscillations between ~80 and ~0% C3 plants. These vegetation changes are correlated with changes in precipitation inferred from δ2Hwax and lake level oscillations inferred from sedimentary facies, implying that high-amplitude, orbital-scale variations in precipitation drove significant changes in vegetation resources during the late Pliocene in the Baringo Basin. These variations have important implications for changes in terrestrial resources in light of the evolutionary innovations in the hominin fossil record related to changes in foraging strategies.

Quantifying the carbon source of pedogenic calcite veins in weathered limestone: implications for the terrestrial carbon cycle

The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate.

Seasonal Bias in Soil Carbonate Formation and Its Implications for Interpreting High‐Resolution Paleoarchives: Evidence From Southern Utah

AbstractPedogenic carbonate is commonly used as a paleoarchive, but its interpretation is limited by our understanding of its formation conditions. We investigated laminated soil carbonate rinds as a high‐resolution paleoarchive in Torrey, Utah, USA, by characterizing and modeling their formation conditions. We compared late Holocene (<5 ka) soil carbonate conventional (C and O) and “clumped” isotopes to modern soil environment and isotope measurements: soil CO2 partial pressure, soil temperature, soil moisture, δ13C‐soil CO2, δ18O precipitation, and δ18O‐soil water. Data unambiguously identified a strong summer seasonality bias, but modeling suggested soil carbonate formed several times throughout the year during infiltration events causing dissolution‐formation reactions. This apparent discrepancy resulted from preferential preservation of calcite formed from the largest annual infiltration events (summer) overprinting previously formed calcite. Soil carbonate therefore formed predominantly due to changes in soil water content. As soil CO2 was at its annual maximum during soil carbonate formation, assuming uniformly low soil CO2 formation conditions for soil carbonate in estimating paleoatmospheric CO2 is likely not viable. Additionally, we showed modern summer δ13C‐soil CO2 and soil CO2 measurements could not produce a modeled δ13C‐soil carbonate consistent with late Holocene observations. We suggest using multiple lines of evidence to identify nonanalogous modern conditions. Finally, a nearly linear radiocarbon age model from a laminated rind showed that rinds can be used as a high‐resolution paleoarchive if samples are from a single depth and the timing and conditions of soil carbonate formation can be constrained through time.

Distribution and Preservation of Root‐ and Shoot‐Derived Carbon Components in Soils Across the Chinese‐Mongolian Grasslands

AbstractThe relative contribution of roots versus shoots to soil organic carbon (SOC) is a long‐debated topic related to soil carbon formation and dynamics. However, the variation of root‐ and shoot‐derived carbon in natural soils remains poorly constrained due to limited tools to separate them. Here we overcome the difficulty by analyzing biomarkers for root (suberin) and shoot (cutin) in grassland soils spanning >7,000 km on the Qinghai‐Tibetan and Mongolian Plateaus. Coupled with a comprehensive list of climatic, edaphic, and vegetation variables, we assess their inputs, distribution, and preservation patterns. We find that while suberin was a root‐specific tracer, cutin biomarkers that occurred only in trace amount in roots also had a minor but nonnegligible contribution from belowground sources due to the massive root mass in the alpine grasslands. Both components had higher SOC‐normalized concentrations in the alpine than temperate grasslands, along with a higher preservation percentage against net primary productivity and a lower degradation parameter, suggesting a better preservation of these plant‐derived lipid components in the alpine region. Moreover, while cutin and suberin abundances were affected by plant inputs and other SOC constituents, the root mass normalized incorporation efficiency of suberin was only positively correlated to SOC in both regions with a steeper slope in the alpine grasslands and also negatively correlated to root mass in the alpine soils. These results imply that root carbon incorporation is sensitive to environmental variations in the cold region, suggesting potentially more dynamic alterations to root carbon sequestration with environmental changes.

Microbial formation of stable soil carbon is more efficient from belowground than aboveground input

The relative contributions of aboveground versus belowground plant carbon inputs to the stable soil organic carbon pool are the subject of much debate—with direct implications for how the carbon cycle is modelled and managed. The belowground rhizosphere pathway (that is, carbon exiting the living root) is theorized to form stable soil carbon more efficiently than the aboveground pathway. However, while several mechanisms have been invoked to explain this efficiency, few have been empirically tested or quantified. Here, we use soil microcosms with standardized carbon inputs to investigate three posited mechanisms that differentiate aboveground from belowground input pathways of dissolved organic carbon—through the microbial biomass—to the mineral-stabilized soil organic carbon pool: (1) the physical distance travelled, (2) the microbial abundance in the region in which a carbon compound enters (that is, rhizosphere versus bulk soil) and (3) the frequency and volume of carbon delivery (that is, infrequent ‘pulse’ versus frequent ‘drip’). We demonstrate that through the microbial formation pathway, belowground inputs form mineral-stabilized soil carbon more efficiently than aboveground inputs, partly due to the greater efficiency of formation by the rhizosphere microbial community relative to the bulk soil community. However, we show that because the bulk soil has greater capacity to form mineral-stabilized soil carbon due to its greater overall volume, the relative contributions of aboveground versus belowground carbon inputs depend strongly on the ratio of rhizosphere to bulk soil. Belowground carbon inputs form stable soil carbon more efficiently through microbial formation than carbon addition aboveground, according to soil microcosm experiments that quantitatively compare soil carbon formation efficiencies from different mechanistic pathways.

Vertical and seasonal variations of soil carbon pools in ginkgo agroforestry systems in eastern China

Agroforestry provides opportunities to decrease the levels of carbon dioxide (CO2) released into the atmosphere by increasing the carbon (C) stored in agricultural systems. In agroforestry systems, soil C pools serve as the most important and stable C sink, but there is limited information on the vertical and seasonal variations of soil C pools. In this study, the vertical and seasonal variations of soil organic C (SOC) and its labile pools were measured in five planting systems: a pure ginkgo (Gingko biloba. L) planting system, a pure wheat (Triticum aestivum L.) field, a pure metasequoia (Metasequoia glyptostroboides Hu et Cheng) seedling system, a ginkgo and wheat agroforestry system, and a ginkgo and metasequoia seedling agroforestry system. Among these systems, the ginkgo and wheat system had a significantly higher SOC content than the other systems throughout the year, particularly at depths of 0–10 cm and 10–20 cm. Additionally, the pure ginkgo and pure metasequoia systems had lower SOC contents than the other planting systems, and this decrease was attributed to the relatively limited tree litter input and lower fine root biomass. Microbial biomass C (MBC) and soil readily oxidizable C (ROC) exhibited similar vertical and seasonal variations and reached minimum values in winter. The highest MBC and ROC contents were observed in the ginkgo and wheat system at a depth of 0–10 cm, i.e., 127.3 mg kg−1 and 4.49 g kg−1, respectively. The highest water-soluble organic carbon (WSOC) content was observed in summer at a depth of 0–10 cm, i.e., 472.2 mg kg−1. A Pearson correlation analysis indicated that soil properties were significantly correlated with SOC and labile C fractions. The results suggested that an agroforestry system resulted in a greater increase in the soil C sink; in particular, the ginkgo and wheat system achieved the best results. Basic soil properties played key roles in soil carbon formation. These results provide important information about SOC and labile C fraction dynamics resulting from planting systems and depth variations and strengthen our understanding of soil C sequestration in agroforestry systems.

Loess–paleosol carbonate clumped isotope record of late Pleistocene–Holocene climate change in the Palouse region, Washington State, USA

AbstractThe Channeled Scabland–Palouse region of the Pacific Northwest (PNW) of the United States preserves geomorphic and pedosedimentary records that inform understanding of late Pleistocene–Holocene paleoclimate change in a region proximal to the last glacial period Cordilleran Ice Sheet. We present a clumped (Δ47) and conventional (δ18O, δ13C) isotopic study of Palouse loess–paleosol carbonates in combination with carbonate radiocarbon (14C) dating to provide new measures of regional late–last glacial (~31–20 cal ka BP) and Holocene soil conditions. Average clumped isotope temperatures (T(Δ47)) for last glacial Palouse loess–paleosol carbonates (9±4°C) are significantly lower than those for Holocene-aged carbonates (T(Δ47)=18±2°C) in study sections. Calculated soil water δ18OVSMOWvalues (−16±2‰) for last glacial carbonates are also offset relative to those for Holocene-aged samples (−11±1‰), whereas calculated soil CO2δ13CVPDBvalues are similar for the Holocene (−16.9±0.2‰) and late–last glacial (−16.7±1.1‰) periods. Together, these paleoclimate metrics indicate late–last glacial conditions of pedogenic carbonate formation in the C3grassland soils of the Palouse were measurably colder (9±5°C) than during the Holocene and potentially reflect a more arid last glacial paleoclimate across the Palouse, findings in agreement with previous proxy studies and climate model simulations for the region.

Substrate identity and amount overwhelm temperature effects on soil carbon formation

The size of the soil carbon sink depends on the balance between soil organic matter (SOM) formation and decomposition. Our understanding of how SOM forms and is stabilized, however, is shifting. Traditional theory maintains the formation of SOM is due to chemical complexity: difficult to decompose plant inputs persist in the soil while easily decomposable inputs are respired as CO2. However, consensus is now building around an alternative thesis, hypothesizing that the plant inputs most easily assimilated by soil decomposers are the ones stabilized as SOM because dead microbial biomass is now considered one of the primary components of stable SOM. As such, the efficiency with which the microbial community uses these plant inputs has direct implications for the amount and rate of SOM formation under both a constant and changing climate. Our study empirically tests and measures the effects of substrate quality, quantity, and temperature on SOM formation rates – a process that may have profound impact on carbon stocks. We used 13C-labeled substrates representative of plant root exudates (simple sugars, amino acids, and organic acids) to determine the proportion of substrate retained within SOM, microbial biomass, dissolved organic carbon, or evolved as 13CO2. We found that glucose, the substrate most efficiently assimilated by the microbial biomass, leads to the greatest amount of SOM formation compared to glycine and oxalic acid. In contrast to expectations, higher concentrations of substrate addition lead to proportionally less 13C label retention than lower concentrations. Temperature had a negligible impact on SOM formation, with higher temperatures actually leading to slight increases in SOM formation. While substrate quality and quantity drove the largest differences in SOM formation rates, once metabolized by the microbial biomass, eventual incorporation of carbon into the mineral associated SOM pool (thought to be the most stable of the soil C pools), was effectively equivalent across treatments. Our data suggest that changing composition and amount of labile carbon substrates supplied to soils will likely be key determinants of SOM formation rates and, hence, potentially soil carbon stock sizes.

Applying population and community ecology theory to advance understanding of belowground biogeochemistry

Approaches to quantifying and predicting soil biogeochemical cycles mostly consider microbial biomass and community composition as products of the abiotic environment. Current numerical approaches then primarily emphasise the importance of microbe-environment interactions and physiology as controls on biogeochemical cycles. Decidedly less attention has been paid to understanding control exerted by community dynamics and biotic interactions. Yet a rich literature of theoretical and empirical contributions highlights the importance of considering how variation in microbial population ecology, especially biotic interactions, is related to variation in key biogeochemical processes like soil carbon formation. We demonstrate how a population and community ecology perspective can be used to (1) understand the impact of microbial communities on biogeochemical cycles and (2) reframe current theory and models to include more detailed microbial ecology. Through a series of simulations we illustrate how density dependence and key biotic interactions, such as competition and predation, can determine the degree to which microbes regulate soil biogeochemical cycles. The ecological perspective and model simulations we present lay the foundation for developing empirical research and complementary models that explore thediversity of ecological mechanisms that operate in microbial communities to regulate biogeochemical processes.