Slow Pool Research Articles

Linear two-pool models are insufficient to infer soil organic matter decomposition temperature sensitivity from incubations

Terrestrial carbon (C)-climate feedbacks depend strongly on how soil organic matter (SOM) decomposition responds to temperature. This dependency is often represented in land models by the parameter Q10, which quantifies the relative increase of microbial soil respiration per 10 °C temperature increase. Many studies have conducted paired laboratory soil incubations and inferred “active” and “slow” pool Q10 values by fitting linear two-pool models to measured respiration time series. Using a recently published incubation study (Qin et al. in Sci Adv 5(7):eaau1218, 2019) as an example, here we first show that the very high parametric equifinality of the linear two-pool models may render such incubation-based Q10 estimates unreliable. In particular, we show that, accompanied by the uncertain initial active pool size, the slow pool Q10 can span a very wide range, including values as high as 100, although all parameter combinations are producing almost equally good model fit with respect to the observations. This result is robust whether or not interactions between the active and slow pools are considered (typically these interactions are not considered when interpreting incubation data, but are part of the predictive soil carbon models). This very large parametric equifinality in the context of interpreting incubation data is consistent with the poor temporal extrapolation capability of linear multi-pool models identified in recent studies. Next, using a microbe-explicit SOM model (RESOM), we show that the inferred two pools and their associated parameters (e.g., Q10) could be artificial constructs and are therefore unreliable concepts for integration into predictive models. We finally discuss uncertainties in applying linear two-pool (or more generally multiple-pool) models to estimate SOM decomposition parameters such as temperature sensitivities from laboratory incubations. We also propose new observations and model structures that could enable better process understanding and more robust predictive capabilities of soil carbon dynamics.

Modeling Hairy Vetch and Cereal Rye Cover Crop Decomposition and Nitrogen Release

Empirical models could help us to understand the process of plant residue decomposition and nutrient release into the soil. The objective of this study was to determine an appropriate model to describe the decomposition of hairy vetch (Vicia villosa Roth) and cereal rye (Secale cereale L.) cover crop (CC) residue and nitrogen (N) release. Data pertaining to above and belowground CC residue mass loss and N release for up to 2633 cumulative decomposition degree days (112 d) after litterbag installation were obtained from two cropping system experiments, a 1-yr study conducted in 2015 and a 2-yr study during 2017 to 2018 in the humid subtropical environment of southern IL, USA. Six exponential and two hyperbolic models were fit to percent mass and N remaining data to find the one with minimum Akaike information criterion (AIC) and residual sum of squares. Modified three-parameter single exponential and two- or three-parameter hyperbolic models best met the assumed criteria of selection for above and belowground CC residue, respectively. Fitting a double exponential model to combined data for percent mass and N remaining identified two mass and N pools, a fast and a slow pool with different rate constants. A five-parameter double exponential with an asymptote met the preset criteria and passed all tests for normally distributed population, constant variance, and independence of residuals at α = 0.05 when fit to combined data of hairy vetch shoot mass and N remaining. However, a two-parameter hyperbolic and three-parameter asymptotic hyperbolic model provided the best fit to a combined data of cereal rye shoot mass and N remaining, respectively. Both hyperbolic decay models showed a good fit for belowground mass decomposition and N release for both CCs. Cereal rye had a poorer fit than hairy vetch for mass and N remaining of both above and belowground mass. The best-selected decay models can be used to estimate the decomposition and N release rates of hairy vetch and cereal rye above and belowground residue in a similar environment.

Temperature sensitivity of soil carbon decomposition due to shifts in soil extracellular enzymes after afforestation

Afforestation of arable land has been proposed as one of the most effective strategies for climate change mitigation. However, the effects of afforestation on soil CO2 emissions have not been well investigated due to a lack of information on the capacity of carbon pools to sequester CO2 over the long term. In this study, we conducted a 414-day incubation experiment to monitor changes in CO2 emission, soil extracellular enzymes, and microbial biomass at soils collected from a site afforested with R. pseudoacacia L and an adjacent cropland system under 15 °C and 25 °C constant temperatures. The study was to reveal how soil organic carbon decomposed under different temperatures during incubation along an afforestation gradient. Our results showed that: 1) after warming, active pool respiration increased from 0.64% to 12.57%, passive pool respiration increased from 20.62% to 46.77%, and slow pool respiration decreased from 21.27% to 59.35% for cropland to afforested land; 2) active and slow carbon pool increased in young and middle-aged afforested lands while the passive carbon pool increased in the late stage; and 3) microbial biomass and enzyme activities affected the respiration rate and the fraction of respiration during warming, especially enzyme affected active and slow pools. Our findings indicate that the temperature sensitivity of soil organic carbon decomposition changed mainly due to shifts of soil extracellular enzymes after afforestation, and provides insight into how SOC decomposition in afforested ecosystems may respond to future changes in climate.

Intercropping switchgrass with hybrid poplar increased carbon sequestration on a sand soil

Switchgrass (Panicum virgatum) has been identified as an important source of cellulosic biomass to produce biofuels and improve soil carbon storage. The natural 13C abundance of soils was used to calculate the quantity and turnover of C in a hybrid poplar (Populus x generosa and P. x canadensis) plantation intercropped with switchgrass. Levels of intercrop were grass cultivars inter-planted with poplar trees (IC), monoculture switchgrass (SM) and monoculture hybrid poplar (PM). Four years of intercropping increased soil organic carbon (SOC) 18% in the surface 60 cm for switchgrass. Surface soil was enriched 1.5‰ δ13C cropped to switchgrass compared to the poplar monoculture. On average, 14% of SOC in SM was derived from switchgrass and 9% in IC. SOC accrual rates were 0.32 Mg C ha−1yr−1 for the poplar monoculture; 1.3 Mg C ha−1yr−1 for switchgrass monocultures and 1.0 Mg C ha−1yr−1 for intercrop. The CO2 evolved during laboratory incubations was used to determine the size and turnover of the functional C pools of SOC. SOC was divided into three functional pools: an active, slow and resistant pool; where the size and turnover rates were estimated by curve fitting using a three-component first-order model. The resistant C pool was 62%, slow (Cs) 34% and active (Ca) 4% of SOC. The Cs increased 31% in the 0–15 cm horizon of SM and IC compared to PM. Field mean resident times of Ca and Cs cropped to switchgrass were 50d and 15yr for surface soil, respectively, compared to 41d and 9yr for PM. Switchgrass intercropped with hybrid poplar increased SOC compared to poplar alone.

DRIFTS band areas as measured pool size proxy to reduce parameter uncertainty in soil organic matter models

Abstract. Soil organic matter (SOM) turnover models predict changes in SOM due to management and environmental factors. Their initialization remains challenging as partitioning of SOM into different hypothetical pools is intrinsically linked to model assumptions. Diffuse reflectance mid-infrared Fourier transform spectroscopy (DRIFTS) provides information on SOM quality and could yield a measurable pool-partitioning proxy for SOM. This study tested DRIFTS-derived SOM pool partitioning using the Daisy model. The DRIFTS stability index (DSI) of bulk soil samples was defined as the ratio of the area below the aliphatic absorption band (2930 cm−1) to the area below the aromatic–carboxylate absorption band (1620 cm−1). For pool partitioning, the DSI (2930 cm−1 ∕ 1620 cm−1) was set equal to the ratio of fast-cycling ∕ slow-cycling SOM. Performance was tested by simulating long-term bare fallow plots from the Bad Lauchstädt extreme farmyard manure experiment in Germany (Chernozem, 25 years), the Ultuna continuous soil organic matter field experiment in Sweden (Cambisol, 50 years), and 7 year duration bare fallow plots from the Kraichgau and Swabian Jura regions in southwest Germany (Luvisols). All experiments were at sites that were agricultural fields for centuries before fallow establishment, so classical theory would suggest that a steady state can be assumed for initializing SOM pools. Hence, steady-state and DSI initializations were compared, using two published parameter sets that differed in turnover rates and humification efficiency. Initialization using the DSI significantly reduced Daisy model error for total soil organic carbon and microbial carbon in cases where assuming a steady state had poor model performance. This was irrespective of the parameter set, but faster turnover performed better for all sites except for Bad Lauchstädt. These results suggest that soils, although under long-term agricultural use, were not necessarily at a steady state. In a next step, Bayesian-calibration-inferred best-fitting turnover rates for Daisy using the DSI were evaluated for each individual site or for all sites combined. Two approaches significantly reduced parameter uncertainty and equifinality in Bayesian calibrations: (1) adding physicochemical meaning with the DSI (for humification efficiency and slow SOM turnover) and (2) combining all sites (for all parameters). Individual-site-derived turnover rates were strongly site specific. The Bayesian calibration combining all sites suggested a potential for rapid SOM loss with 95 % credibility intervals for the slow SOM pools' half-life being 278 to 1095 years (highest probability density at 426 years). The credibility intervals of this study were consistent with several recently published Bayesian calibrations of similar two-pool SOM models, i.e., with turnover rates being faster than earlier model calibrations suggested; hence they likely underestimated potential SOM losses.

Do Fulvic, Humic, and Humin Carbon Fractions Represent Meaningful Biological, Physical, and Chemical Carbon Pools?

Humic fractions have been used to characterize soil organic matter chemistry and to identify different carbon (C) pools. The relevance of alkaline fractionation is questioned because of probable artifacts created by the extraction. Fractionation using water‐soluble organic C (WSOC), soil microbial biomass C (SMBC), permanganate‐oxidizable C (POX‐C), and particulate organic matter C (POM‐C) methodologies are sensitive to C changes. Mineral‐associated organic C (MAOC) includes WSOC and other C fractions that are not all passive C. We measured 15 proficiency soils ranging in texture, mineralogy, pH, and soil organic C (SOC) to isolate C pools with differing turnover times. We define WSOC, SMBC, POX‐C, and fulvic acid (FA) as fast pools. Slow cycling pools include POM‐C and humic acid (HA) fractions, whereas passive pools include MAOC minus WSOC (PMAOC) and humin C (HC). Data showed FA was correlated with POX‐C (r = 0.84) as a fast C pool but was slightly more correlated with PMAOC (r = 0.86), a passive pool. Slow C pools of HA and POM‐C were not strongly correlated (r = 0.49). All alkaline fractions of C showed highest correlations to PMAOC, followed by POX‐C, and all showed poor correlations with POM‐C. The conventional first extraction fraction (A) yielded 70% recovery of the total FA extracted, whereas only 45% of HA was obtain. Acid soils on average had higher total FA. The regression model using physical fractionation was the best (SOC = WSOC + POM‐C + PMAOC, R2 0.98). Only the HA fraction was a significant parameter in the regression of SOC = FA + HA + HC (p = 0.0029).Core Ideas Humic acid C showed the strongest correlation to all C pools except POM‐C. Recovery of fulvic C in conventional fraction was 70% of a total of three fractions. Recovery of humic acid C in conventional fraction was 45% of a total of three fractions. The distribution of fulvic (FA) to humic acid (HA) C changed with acid soils having higher FA. Regression of soil organic C = FA + HA + humin only showed the HA to be significant.

2126 Endoscopic Ultrasound-Guided Splenic Mass Biopsy. an Accessible Technique for Differential Diagnosis of Lymphoma

INTRODUCTION: Traditional methods for splenic mass biopsy includesurgical or percutaneous technique, however there is an increased risk of hemorrhage, hematoma or cutaneous infection associated with these techniques. In addition, these techniques do not allow us to evaluate adjacent organs. Endoscopic ultrasound (EUS) provides a novel method for splenic mass biopsy and should be considered for such procedures. Our case highlights technique for a successful and safely done EUS guided splenic mass biopsy. CASE DESCRIPTION/METHODS: Case: 68-year-old female with history of non-Hodgkin lymphoma in remission, stage IIIbsquamous cell carcinoma of anus treated with chemoradiation and on surveillance referred to GI after a surveillance CT abdomen/pelvis revealed spleen enlargement with hypodensities. This was followed by PET/CT with increased uptake on anal region, spleen, stomach and supraclavicular node. Endoscopy with EUSwas performed. Biopsies were obtained from stomach and duodenum. EUS imaging showed splenic lesion. Fine Needle Biopsy was performed with 22-gauge needle with a slow pool technique. 4 needle passes were made into the mass. Specimen was sent for pathology and flow cytometry. Patient tolerated procedure without complications. Biopsy report showed evidence of B cell lymphoma on all samples with diagnosis of relapsed B cell Non-Hodgkin lymphoma. DISCUSSION: EUS can provide a feasible and safer technique to obtain splenic mass biopsies. This technique utilizes the proximity of gastrointestinal tract and therefore a readily accessible path to obtain splenic biopsies. Endoscopic ultrasound can also provide a safe way to evaluate adjacent organs during the work up of disease extent involvement, all in one minimally invasive procedure,reducing the amount of risks and complications associated with traditional biopsy techniques.

Tundra microbial community taxa and traits predict decomposition parameters of stable, old soil organic carbon.

The susceptibility of soil organic carbon (SOC) in tundra to microbial decomposition under warmer climate scenarios potentially threatens a massive positive feedback to climate change, but the underlying mechanisms of stable SOC decomposition remain elusive. Herein, Alaskan tundra soils from three depths (a fibric O horizon with litter and course roots, an O horizon with decomposing litter and roots, and a mineral-organic mix, laying just above the permafrost) were incubated. Resulting respiration data were assimilated into a 3-pool model to derive decomposition kinetic parameters for fast, slow, and passive SOC pools. Bacterial, archaeal, and fungal taxa and microbial functional genes were profiled throughout the 3-year incubation. Correlation analyses and a Random Forest approach revealed associations between model parameters and microbial community profiles, taxa, and traits. There were more associations between the microbial community data and the SOC decomposition parameters of slow and passive SOC pools than those of the fast SOC pool. Also, microbial community profiles were better predictors of model parameters in deeper soils, which had higher mineral contents and relatively greater quantities of old SOC than in surface soils. Overall, our analyses revealed the functional potential of microbial communities to decompose tundra SOC through a suite of specialized genes and taxa. These results portray divergent strategies by which microbial communities access SOC pools across varying depths, lending mechanistic insights into the vulnerability of what is considered stable SOC in tundra regions.

Fate of atmospherically deposited NH4+ and NO3- in two temperate forests in China: temporal pattern and redistribution.

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems depend in large part on its fate. However, our understanding of the fates of different forms of deposited N as well as the redistribution over time within different ecosystems is limited. In this study, we used the 15 N-tracer method to investigate both the short-term (1week to 3months) and long-term (1-3yr) fates of deposited NH4+ or NO3- by following the recovery of the 15 N in different ecosystem compartments in a larch plantation forest and a mixed forest located in northeastern China. The results showed similar total ecosystem retention for deposited NH4+ and NO3- , but their distribution within the ecosystems (plants vs. soil) differed distinctly particularly in the short-term, with higher 15 NO3- recoveries in plants (while lower recoveries in organic layer) than found for 15 NH4+ . The different short-term fate was likely related to the higher mobility of 15 NO3- than 15 NH4+ in soils instead of plant uptake preferences for NO3- over NH4+ . In the long-term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15 NO3- than 15 NH4+ tracer persisted, suggesting that incoming NO3- may contribute more to plant biomass increment and forest carbon sequestration than incoming NH4+ . Differences between the two forests in recoveries were largely driven by a higher 15 N recovery in the organic layer (both N forms) and in trees (for 15 NO3- ) in the larch forest compared to the mixed forest. This was due to a more abundant organic layer and possibly higher tree N demand in the larch forest than in the mixed forest. Leachate 15 N loss was minor (<1% of the added 15 N) for both N forms and in both forests. Total 15 N recovery averaged 78% in the short-term and decreased to 55% in the long-term but with increasing amount of 15 N label (re)-redistributed into slow turn-over pools (e.g., trees and mineral soil). The different retention dynamics of deposited NH4+ and NO3- may have implications in environmental policy related to the anthropogenic emissions of the two N forms.

Microbial communities and their genetic repertoire mediate the decomposition of soil organic carbon pools in revegetation shrublands in a desert in northern China

AbstractRevegetation using xerophytic plants in drylands can alter soil microbial community diversity and composition; however, information about the microbial communities and their genetic repertoires that are involved in soil organic carbon (SOC) processes in drylands remain unclear. This study aimed to characterize soil microbial genes and their potential to mediate SOC decomposition in three revegetation shrublands (Artemisia ordosica, Salix psammophila and Caragana microphylla) in the Mu Us Desert of northern China using an SOC mineralization experiment and shotgun metagenome sequencing. Among the three shrublands, the S. psammophila shrubland had the highest cumulative CO2 emissions and highest rates of mineralization of the slow SOC pool. The mineralization rate of the slow SOC pool and the microbial taxonomic and functional composition displayed a significant correlation. Most C‐degradation genes were assigned to three bacterial phyla, Actinobacteria, Proteobacteria and Acidobacteria. The relative abundance of microbial functional genes involved in degrading recalcitrant C‐complexes was significantly higher in the S. psammophila shrubland, resulting in the slow accumulation of recalcitrant SOC. These results suggest that the microbial functional genes that regulate the decomposition of the SOC pool in dryland shrubland soils may be influenced by shrub species selection. These findings highlight the importance of understanding how microbial catabolic potential mediates the potential for SOC sequestration in different revegetation shrublands in drylands.Highlights We studied SOC mineralization and soil microbial functional genes in three revegetation shrublands. S. psammophila shrubland had the highest CO2 emissions and highest mineralization rates of the slow C pool. Slow SOC pool mineralization rate was related to microbial taxonomic and functional composition. Different dryland shrubs may mediate soil microbial functional genes that regulate SOC pool decomposition.

Estimating of terrestrial carbon storage and its internal carbon exchange under equilibrium state

Terrestrial carbon storage plays a crucial role in determining the global carbon cycle and regulating global climate. However, the carbon storage, as simulated by terrestrial biosphere models (TBMs), exhibits considerable inconsistency at the regional or global scale because the model structure, input dataset, and key parameter are quite different in TBMs. In this study, I proposed a detachable carbon cycle (DCC) model to simulate terrestrial carbon storage, internal carbon exchange, and the influx–efflux (IE) function of each carbon pool. The model was established based on a pool–and–flux scheme and contained 14 carbon pools, or carbon flow processes. Each process can be detached from the primary model and evaluated as an independent component. The average net primary productivity (NPP) from 1982 to 2008 was used as the influx carbon to drive the DCC model. The magnitude of the internal carbon exchanges of the DCC model was explicitly represented, and the carbon IE function of each carbon pool was fitted based on the characteristics of carbon flux. Results indicated that the terrestrial carbon storage was 2766.25 Pg, and carbon stored in vegetation and soil was 705.85 and 2022.00 Pg, respectively. Carbon stored in slow and passive pools accounted for 70.42% of the terrestrial carbon storage, and the slow pool contributes the highest amount of released CO2 among of all carbon pools during carbon decomposition. The IE functions exhibited a nonlinear curve feature and satisfactory adjust-R2. This study aimed to contribute to our understanding of the carbon cycle from non-equilibrium state to equilibrium state and can serve as a reference and framework for global carbon storage simulation research.

Controls on soil organic carbon stability and temperature sensitivity with increased aboveground litter input in deciduous forests of different forest ages

A previous 5-year long field litter manipulation study at the Smithsonian Environmental Research Center (SERC) in coastal Maryland demonstrated that forest age controls the chemical trajectory of litter decay and the extent and source of litter incorporation into soil physical fractions among young (60–74 yrs) and old (113–132 yrs) successional stands. To investigate if these ecosystem-level differences influence soil organic carbon (SOC) stability and temperature sensitivity, and to infer differences in stabilization mechanisms, a six-month laboratory incubation (15 °C and 25 °C) of soils from the experimental plots was conducted. The results showed that: 1) C mineralization of wood amended soils was lower than control soils in all forests with young and old forests exhibiting distinct, early vs. late, CO2 efflux profiles over the time course of the incubation; 2) Soils from leaf-amended old forests exhibited a proportional increase in their active SOC pool but with shorter mean residence times (MRT) and a decrease in slow pools with longer MRTs, while SOC of young forests proportionally shifted to more slow cycling SOC pools with MRTs that were unchanged from controls. Structural equation modeling combining previous field and soil property data with laboratory incubation results indicated that temperature sensitivity of the active SOC pool was related to the microbial community and lignin content, while temperature sensitivity of the slow pool was related to chemical protection from silts and clays, environmental factors like pH, and soil C/N ratio. Our results underscore how successional forests of differing age can exhibit dramatically different controls on SOC-litter dynamics, through the protection and accessibility of C, that must be taken into account when predicting forest ecosystem response to future climate change.

Cellular Responses of Human Postural Muscle to Dry Immersion.

Support withdrawal has been currently considered as one of the main factors involved in regulation of the human locomotor system. For last decades, several authors, including the authors of the present paper, have revealed afferent mechanisms of support perception and introduced the concept of the support afferentation system. The so-called “dry immersion” model which was developed in Russia allows for suspension of subjects in water providing the simulation of the mechanical support withdrawal. The present review is a summary of data allowing to appreciate the value of the “dry” immersion model for the purposes of studying cellular responses of human postural muscle to gravitational unloading. These studies corroborated our hypothesis that the removal of support afferentation inactivates the slow motor unit pool which leads to selective inactivation, and subsequent atony and atrophy, of muscle fibers expressing the slow isoform of myosin heavy chain (which constitutes the majority of soleus muscle fibers). Fibers that have lost a significant part of cytoskeletal molecules are incapable of effective actomyosin motor mobilization which leads to lower calcium sensitivity and lower range of maximal tension in permeabilized fibers. Support withdrawal also leads to lower efficiency of protective mechanisms (nitric oxide synthase) and decreased activity of AMP-activated protein kinase. Thus, “dry” immersion studies have already contributed considerably to the gravitational physiology of skeletal muscle.

Nitrogen mineralisation in sugarcane soils in Queensland, Australia: I. evaluation of soil tests for predicting nitrogen mineralisation

We explored soil properties as indices of mineralisable nitrogen (N) in sugarcane soils and whether we could increase the accuracy of predicting N mineralisation during laboratory incubations. Utilising historical data in combination with samples collected during 2016, we: (i) measured mineralised N over the course of short-term (14 days) and long-term (301 days) laboratory incubations; (ii) compared models representing mineralisation; then (iii) related model parameters to measured soil properties. We found measures representing the labile organic N pool (Hydrolysable NaOH organic N; amino sugar Illinois soil N test) best related to short-term mineralised N (R2 of 0.50–0.57, P &amp;lt; 0.001), while measures of CO2 production (3, 7, 10 and 14 days) best related to longer-term mineralised N (R2 of 0.75–0.84, P &amp;lt; 0.001). Indices were brought together to model the active and slow pools of a two-pool mineralisation model in the statistical framework of a mixed-effects model. Of the models that relied on measurement of one soil property, cumulative CO2 production (7 days) performed the best when considering all soil types; in a cross-validation test, this model gave an external R2 of 0.77 for prediction of the 301-day mineralised N. Since the mixed-effects model accounts for the various sources of uncertainty, we suggest this approach as a framework for prediction of in-field available N, with further measurement of long-term mineralised N in other soils to strengthen predictive certainty of these soil indices.

Composition of a Soil Organic Carbon Increment under Different Vegetable Cultivation Patterns: A Study Using Three SOC Pools

Previous studies suggest that vegetable cultivation increases soil organic carbon (SOC) storage. However, how stable the SOC increment is, and how greenhouse cultivation contributes to the SOC increment in terms of quantity and stability, remains unclear. Soil samples were taken from three typical vegetable cultivation pattern fields: open field (OF), seasonal greenhouse (SG), and permanent greenhouse (PG), as well as adjacent non-vegetable fields. Three conceptual SOC pools, including active (Ca), slow (Cs), and resistant (Cr) pools were fractionated to evaluate SOC sequestration and its stability in vegetable cultivation. The results indicate that vegetable cultivation is associated with greater stored SOC compared with non-vegetable cultivation (SOC increased by 57.9% on average). Using non-vegetable fields as a reference, SOC increments by vegetable cultivation were associated with a higher proportion of Ca (3.7–6.6%) than the reference fields (1.0–2.0%), indicating that the SOC increments might be easily decomposed. Among the three vegetable cultivation patterns, SG, with a higher increase in Cr, is recommended due to its relatively more stable SOC sequestration. Overall, vegetable cultivation could enhance the quantity of SOC, but the stability of the SOC increment is affected by the vegetable cultivation pattern.

Lower mineralizability of soil carbon with higher legacy soil moisture

The effect of long-term versus short-term water content on soil organic carbon (SOC) mineralizability was evaluated in a six-week incubation trial. Soils were sampled from field sites in upstate New York used for rain-fed bioenergy crop production: nitrogen (N)- fertilized reed canarygrass, switchgrass, switchgrass + N, as well as a broadleaf-grass fallow. Within each cropping system, natural moisture gradients due to topography and subsoil structure allowed us to sample across regions with high (0.5 g g−1), mid (0.4 g g−1) and low (0.3 g g−1) water content. Moisture of the laboratory incubations was adjusted mimicking the three average field moisture levels in a full factorial design. Increasing laboratory moisture in the incubations increased cumulative carbon mineralization per unit soil (C mineralization) and cumulative C mineralization per unit SOC (C mineralizability) (main effect p < 0.0001), indicating that lower average moisture as found at this site on average limited mineralization but higher average moisture did not. C mineralizability at high field moisture was 31% (25-42%) lower than at low field moisture across all cropping systems, regardless of moisture adjustment in the incubation. The mean slow C pool size of soils from high field moisture sites (997.1 ± 0.1 mg C g−1 C) was 0.2% greater than that of soils from low field moisture sites (p < 0.0001), obtained by fitting a double-exponential model. The mean residence time of the slow mineralizing C pool for soils from low field moisture sites was 5.5 ± 0.1 years, in comparison to 8.0 ± 0.1 years for soils from high field moisture sites (p < 0.0001). While permanganate-oxidizable carbon (POXC) per unit SOC (r = 0.1) was positively correlated to C mineralizability, wet aggregate stability (r = −0.2) was negatively correlated to C mineralizability. Above-ground biomass did not affect C mineralizability (p > 0.05) and root biomass marginally influenced (p = 0.05) C mineralizability after correcting for soil texture variations. Additionally, after correcting for soil texture variations and biomass inputs, C mineralizability significantly decreased with higher field moisture (p = 0.02), indicating possible stabilization mechanisms through mineral interactions of SOC under high water content. Bulk contents of pedogenic iron and aluminum determined by oxalate extraction did not clearly explain differences in mineralizability. However, exchangeable calcium and magnesium contents were significantly (p < 0.0001) greater in high moisture soils than soils with lower moisture. Additionally, cumulative C mineralizability at 6 weeks was negatively correlated to calcium (r = −0.7) and magnesium (r = −0.6) and mean residence time of the modeled slow pool correlated positively with calcium (r = 0.4). Therefore, cation bridging by retained or illuviated base ions was more important than redox changes of iron as a stabilization mechanism in this experiment.

English

English

The use of radiocarbon &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C to constrain carbon dynamics in the soil module of the land surface model ORCHIDEE (SVN r5165)

Abstract. Despite the importance of soil as a large component of the terrestrial ecosystem, the soil compartments are not well represented in land surface models (LSMs). Indeed, soils in current LSMs are generally represented based on a very simplified schema that can induce a misrepresentation of the deep dynamics of soil carbon. Here, we present a new version of the Institut Pierre Simon Laplace (IPSL) LSM called ORCHIDEE-SOM (ORganizing Carbon and Hydrology in Dynamic EcosystEms-Soil Organic Matter), incorporating the 14C dynamics into the soil. ORCHIDEE-SOM first simulates soil carbon dynamics for different layers, down to 2 m depth. Second, concentration of dissolved organic carbon and its transport are modelled. Finally, soil organic carbon decomposition is considered taking into account the priming effect. After implementing 14C in the soil module of the model, we evaluated model outputs against observations of soil organic carbon and modern 14C fraction (F14C) for different sites with different characteristics. The model managed to reproduce the soil organic carbon stocks and the F14C along the vertical profiles for the sites examined. However, an overestimation of the total carbon stock was noted, primarily on the surface layer. Due to 14C, it is possible to probe carbon age in the soil, which was found to be underestimated. Thereafter, two different tests on this new version have been established. The first was to increase carbon residence time of the passive pool and decrease the flux from the slow pool to the passive pool. The second was to establish an equation of diffusion, initially constant throughout the profile, making it vary exponentially as a function of depth. The first modifications did not improve the capacity of the model to reproduce observations, whereas the second test improved both estimation of surface soil carbon stock as well as soil carbon age. This demonstrates that we should focus more on vertical variation in soil parameters as a function of depth, in order to upgrade the representation of the global carbon cycle in LSMs, thereby helping to improve predictions of the of soil organic carbon to environmental changes.

Changes in the Rock-Eval signature of soil organic carbon upon extreme soil warming and chemical oxidation - A comparison

Soil warming can increase soil organic carbon (SOC) mineralization, triggering a positive climate‑carbon cycle feedback loop. Globally, many soil warming experiments have examined losses of bulk SOC, but few have assessed changes in quality. Accurate knowledge of the latter is required for an in-depth understanding and improved prediction of SOC feedback to climate change. In this study, we used Rock-Eval thermal analysis (RE6) to characterize shifts in SOC thermal stability and bulk chemistry after six years of geothermal warming by 0.6 °C, 1.8 °C, 3.9 °C, 9.9 °C, 16.3 °C, 40 °C, and 80 °C in an Icelandic grassland topsoil (0–10 cm). We also used the strong warming-induced depletion of SOC (up to 92% in the 80 °C soil) in comparisons of chemical oxidation-resistant and biogeochemically resistant SOC, which are generally assumed to be similar in nature. Sodium hypochlorite (NaOCl) and hydrogen peroxide (H2O2) were used for oxidation. Warming-resistant SOC was strongly depleted in hydrocarbons and enriched in oxygen, confirming that SOC oxidation state, and thus energy content, is an important driver for biogeochemical stability. This was supported by findings that thermal stability, i.e., the amount of energy (temperature) necessary to pyrolyze or oxidize SOC, strongly increased with warming intensity. Of the 31 RE6 parameters tested, the most warming-sensitive were hydrogen index (HI, ρ = −0.84), oxygen index (OIRE6, ρ = 0.83), proportion of total pyrolyzed carbon released as hydrocarbons at 200–650 °C (S2/PC, ρ = −0.86), and the temperature at which a certain proportion of CO2 evolved during pyrolysis (ρ > 0.8). Chemical oxidation of unwarmed soil caused average relative SOC losses of 61% (NaOCl) and 91% (H2O2) and shifts in RE6 properties that differed strongly from warming-induced shifts at comparable SOC losses. Chemical oxidation-resistant SOC was more enriched in oxygen, but slightly enriched in hydrocarbons, and less thermostable than comparable naturally depleted SOC at the same time. A certain overlap, especially for NaOCl-treated soils, is likely, while H2O2-oxidized soils showed very distinct RE6 properties. We concluded that i) soil warming leads to strong shifts in SOC bulk chemistry and thermal stability and ii) H2O2 should be avoided in isolation of a slow SOC kinetic pool.

Physical, chemical, and toxicological characteristics of particulate emissions from current technology gasoline direct injection vehicles

We assessed the physical, chemical and toxicological characteristics of particulate emissions from four light-duty gasoline direct injection vehicles when operated over the LA92 driving cycle. Our results showed that particle mass and number emissions increased markedly during accelerations. For three of the four vehicles tested, particulate matter (PM) mass and particle number emissions were markedly higher during cold-start and the first few accelerations following the cold-start period than during the hot running and hot-start segments of the LA92 cycle. For one vehicle (which had the highest emissions overall) the hot-start and cold-start PM emissions were similar. Black carbon emissions were also much higher during the cold-start conditions, indicating severe fuel wetting leading to slow evaporation and pool burning, and subsequent soot formation. Particle number concentrations and black carbon emissions showed large reductions during the urban and hot-start phases of the test cycle. The oxidative potential of PM was quantified with both a chemical and a biological assay, and the gene expression impacts of the PM in a macrophage model with PCR (polymerase chain reaction) and ELISA (enzyme-linked immunosorbent assay) analyses. Inter- and intra-vehicle variability in oxidative potential per milligram of PM emitted was relatively low for both oxidative assays, suggesting that real-world emissions and exposure can be estimated with distance-normalized emission factors. The PCR response from signaling markers for oxidative stress (e.g., NOX1) was greater than from inflammatory, AhR (aryl hydrocarbon receptor), or MAPK (mitogen-activated protein kinase) signaling. Protein production associated with inflammation (tumor necrosis factor alpha-TNFα) and oxidative stress (HMOX-1) were quantified and displayed relatively high inter-vehicle variability, suggesting that these pathways may be activated by different PM components. Correlation of trace metal concentrations and oxidative potential suggests a role for small, insoluble particles in inducing oxidative stress.