Slow Carbon Pools Research Articles

Biodegradability of Disposable Surgical Face Masks Littered into Soil Systems during the COVID 19 Pandemic—A First Approach Using Microcosms

The COVID-19 pandemic caused massive use and improper disposal of surgical polypropylene (PP)-based face masks. For a first evaluation of the respective environmental consequences, we performed a 6-month microcosm experiment at 25 °C to determine the microbial degradability of 10 × 10 mm cuts of single mask layers and of a complete mask mixed with topsoil (Cambisol). By analyzing the CO2 production, we identified a fast pool with a mean residence time (MRTfast) of 3 to 7 days, corresponding to approximately 4 to 5% of the total mask carbon. Solid-state nuclear magnetic resonance (NMR) spectroscopy of the degraded masks suggests a cut-off of PP units or oligomers as a main degradation mechanism. The slow carbon pool of the center mask revealed an MRTslow of 7 years and those of the remaining mask material MRTslows between 19 and 28 years, which is three to five times longer than those of soil organic matter (SOM) of the pure soil. Since the masks were not pretreated, and decomposed in the dark without UV radiation, our data support our hypothesis that in soils, microbes must exist that can decompose PP, although their nature still has to be revealed in future attempts.

Nitrogen and phosphorus additions accelerate decomposition of slow carbon pool and lower total soil organic carbon pool in alpine meadows

Nitrogen (N) and phosphorus (P) additions reduced soil organic carbon (SOC) contents and stocks in alpine meadows on the Tibetan Plateau. However, little is known about microbial mechanisms behind SOC decline. This study investigated the effects of long‐term N and P additions on microbial community composition and SOC decomposition (C mineralization (Cₘ), mean resistant times for active C pool (MRTₐ), and slow C pool (MRTₛ) in alpine meadows. Results showed that the total SOC pool was reduced by 2–9% under N and P additions, of which slow C pool decreased by 3–10%, while active C pool increased by 4–75% compared to the Control. N and P additions shortened MRTₛ by 34–40% but prolonged MRTₐ by 30–62%. The relative abundance of four bacterial families was related to Cₘ or MRTₐ, while that of most of the fungal families affected SOC decomposition (including Cₘ, MRTₐ, and MRTₛ). N and P additions increased fungal diversity, differentially affected microbial community composition and structure through modifying microbial preference, and increasing the abundance of microbes which are capable of decomposing complex carbohydrate. Soil pH, available N, and total P were main factors determining microbial abundances. Microbial changes due to N and P additions accelerated decomposition of recalcitrant SOC, thus led to declines in slow C pool and total SOC pool but increases in active C pool. Therefore, long‐term N and P additions weaken soil functioning as C pool in alpine meadows.

Active carbon pool-size is enhanced by long-term manure application

We studied the dynamics of soil organic carbon (SOC)-pool mineralisation in agricultural soil. A laboratory incubation experiment was conducted using the soil from a long-term experiment involving the following fertilisation regimes: no fertilisation (CK); mineral (NPK); organic (M), and combined organic-inorganic fertilisers (MNPK). SOC mineralisation rate decreased as follows: MNPK > M > NPK > CK. Cumulative SOC mineralisation (C<sub>m</sub>) ranged between 730.15 and 3 022.09 mg/kg in CK and MNPK, respectively; 8.81% (CK) to 20.45% (MNPK) of initial SOC was mineralised after a 360-day incubation. Soil C<sub>m</sub> values were significantly higher under NPK, M, and MNPK compared to those under the CK treatment. Dynamic variation in C<sub>m</sub> with incubation time fitted a double exponential model. Active carbon pools accounted for 2.06–6.51% of total SOC and the average mean resistant time (MRT<sub>1</sub>) was 28.76 days, whereas slow carbon pools accounted for 93.49–97.94% of SOC, with an average MRT<sub>2</sub> of 8.53 years. The active carbon pool in fertilised soils was larger than in CK; furthermore, it was larger in M- and MNPK- than under NPK-treated plots. SOC decomposed more easily in long-term fertilised plots than in non-fertilised plots.

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.

Effect of different tillage systems on aggregate structure and inner distribution of organic carbon

Tillage is a common agricultural practice affecting soil structure and biogeochemistry. Pore network geometries are crucial to oxygen concentration, gas diffusivity, water location and water movement. Soil aggregates, 4–6mm in diameter and collected from silty loam in Belgium and sandy loam in China, were scanned using a micro-computed tomography scanner. Images with a pixel size of 6.9μm were then processed with ImageJ software for pore network analysis. The treatments were no tillage (C-NT) and conventional tillage (C-CT) in China, and shallow tillage (G-ST) and conventional tillage (G-CT) in Belgium. The results showed that aggregates in conservational tillage (G-ST and C-NT) had numerous connected pores compared with conventional tillage (G-CT and C-CT). The Euler number (Ev) was significantly lower and visible total porosity and surface area (SA) were significantly higher in conservational tillage (G-ST and C-NT) than in conventional tillage (G-CT and C-CT) in both studied locations.The predominant size of pores was significantly higher in conservational tillage (G-ST and C-NT) than in conventional tillage (G-CT and C-CT) (>150μm vs 90–120μm). Pore location within the aggregates also showed differences, with porosity being evenly distributed in the aggregates under conventional tillage (G-CT and C-CT). Under conservational tillage (G-ST and C-NT), the aggregates were heterogeneous, showing higher porosity at the center of the aggregates. There was a higher soil organic carbon (SOC) content in the external layer than in the internal layer in conservational tillage in Belgium (G-ST). In no tillage in China (C-NT), the SOC in the external and internal layers, however, showed similar results.Overall, conventional tillage (G-CT and C-CT) reduced the proportion of the largest pores within soil aggregates, whereas there was no significant relationship between pore morphologies and SOC content. Further investigation is required to measure the active and slow carbon pool distribution in the different layers and under different tillage practices.

PALADYN v1.0, a comprehensive land surface–vegetation–carbon cycle model of intermediate complexity

Abstract. PALADYN is presented; it is a new comprehensive and computationally efficient land surface–vegetation–carbon cycle model designed to be used in Earth system models of intermediate complexity for long-term simulations and paleoclimate studies. The model treats in a consistent manner the interaction between atmosphere, terrestrial vegetation and soil through the fluxes of energy, water and carbon. Energy, water and carbon are conserved. PALADYN explicitly treats permafrost, both in physical processes and as an important carbon pool. It distinguishes nine surface types: five different vegetation types, bare soil, land ice, lake and ocean shelf. Including the ocean shelf allows the treatment of continuous changes in sea level and shelf area associated with glacial cycles. Over each surface type, the model solves the surface energy balance and computes the fluxes of sensible, latent and ground heat and upward shortwave and longwave radiation. The model includes a single snow layer. Vegetation and bare soil share a single soil column. The soil is vertically discretized into five layers where prognostic equations for temperature, water and carbon are consistently solved. Phase changes of water in the soil are explicitly considered. A surface hydrology module computes precipitation interception by vegetation, surface runoff and soil infiltration. The soil water equation is based on Darcy's law. Given soil water content, the wetland fraction is computed based on a topographic index. The temperature profile is also computed in the upper part of ice sheets and in the ocean shelf soil. Photosynthesis is computed using a light use efficiency model. Carbon assimilation by vegetation is coupled to the transpiration of water through stomatal conductance. PALADYN includes a dynamic vegetation module with five plant functional types competing for the grid cell share with their respective net primary productivity. PALADYN distinguishes between mineral soil carbon, peat carbon, buried carbon and shelf carbon. Each soil carbon type has its own soil carbon pools generally represented by a litter, a fast and a slow carbon pool in each soil layer. Carbon can be redistributed between the layers by vertical diffusion and advection. For the vegetated macro surface type, decomposition is a function of soil temperature and soil moisture. Carbon in permanently frozen layers is assigned a long turnover time which effectively locks carbon in permafrost. Carbon buried below ice sheets and on flooded ocean shelves is treated differently. The model also includes a dynamic peat module. PALADYN includes carbon isotopes 13C and 14C, which are tracked through all carbon pools. Isotopic discrimination is modelled only during photosynthesis. A simple methane module is implemented to represent methane emissions from anaerobic carbon decomposition in wetlands (including peatlands) and flooded ocean shelf. The model description is accompanied by a thorough model evaluation in offline mode for the present day and the historical period.

Evaluation of the CENTURY model using long-term fertilization trials under corn-wheat cropping systems in the typical croplands of China.

Soil organic matter models are widely used to study soil organic carbon (SOC) dynamics. Here, we used the CENTURY model to simulate SOC in wheat-corn cropping systems at three long-term fertilization trials. Our study indicates that CENTURY can simulate fertilization effects on SOC dynamics under different climate and soil conditions. The normalized root mean square error is less than 15% for all the treatments. Soil carbon presents various changes under different fertilization management. Treatment with straw return would enhance SOC to a relatively stable level whereas chemical fertilization affects SOC differently across the three sites. After running CENTURY over the period of 1990–2050, the SOC levels are predicted to increase from 31.8 to 52.1 Mg ha−1 across the three sites. We estimate that the carbon sequestration potential between 1990 and 2050 would be 9.4–35.7 Mg ha−1 under the current high manure application at the three sites. Analysis of SOC in each carbon pool indicates that long-term fertilization enhances the slow pool proportion but decreases the passive pool proportion. Model results suggest that change in the slow carbon pool is the major driver of the overall trends in SOC stocks under long-term fertilization.

Soil Carbon Pools, Mineralization and Fluxes Associated with Land Use Change in Vertisols of Central India

Land use affects soil carbon dynamics which influence the global carbon cycle. Understanding the response of soil organic carbon (SOC) dynamics to land-use changes require size and mean residence time (MRT) of measurable SOC pools. We measured soil carbon pool sizes and decay constants of active and slow carbon pools for different land use systems (agriculture, forest and horticulture) of Central India. Long-term incubation study was carried out for quantification of active (Ca) and slow (Cs) carbon pools and MRT. Chemical fractionation technique (acid hydrolysis) was used for quantification of resistant carbon pool (Cr). The non-hydrolysable carbon (Cr) expressed as a percentage of total C varied between 65 and 68 % in the surface soil of different land use system whereas it was 78 % in sub-surface soil. The incubation study revealed that the Ca pool in different land use system ranged between 3.3 and 10 % of the total C and MRT varied from 25 to 67 days with an average value of 46 days. Our results demonstrate that the active SOC is a sensitive indicator for changes in SOC following land use change. MRT of a slow carbon pool of forest land use was 38.5 years. The MRT of slow pool C in the upper soil layer (0–15 cm) did not differ between horticulture and agriculture land use systems and was approximately 4.5 years. In agriculture land use, MRT of a slow pool of C under was only 2.3 years in sub surface plot. However, MRT of slow carbon pool (Cs) under the Farm yard manure (organic plot) increased by fourfold (17.5 years) in comparison to inorganic treatment (recommended dose of fertilizer-RDF) plot. The results clearly depict opportunity of soil carbon buildup in agriculture with good management practices (application of organic manure). The present research work will also help in improving the SOC dynamics predictive capacity of different global bio-geochemical carbon models.

Soil and residue carbon mineralization as affected by soil aggregate size

The nature of the contact between fresh organic matter and soil depends mainly on the characteristics of the plant residues and on the physical properties of the soil. In a cultivated cropping system, changes in soil organic C cannot be entirely attributed to changes in organic matter input. Breakdown of aggregates caused by cultivation not only affects soil organic matter but also influences the rate of mineralization of added organic matter. Many models simulating organic matter decomposition in the field are calibrated with laboratory data from experiments where crop residues are ground and mixed homogeneously with soil aggregates. In the present study, soil aggregate size was used as a means of varying the contact between crop residue and the soil. The results demonstrated that cumulative soil carbon mineralization from different aggregates had a significant (r=0.60, p=0.05) and positive relationship with their oxidizable soil carbon content. Residue carbon mineralization in different aggregate size classes was inversely related to aggregate oxidizable soil carbon content (r=−0.95, p=0.01), cumulative soil carbon mineralization (r=−0.89, p=0.01) and resistant soil carbon pool (r=−0.80, p=0.01). Residue carbon mineralization in different aggregate size classes was also inversely (r=−0.61, p=0.05) related to the active carbon content (KMnO4 oxidizable carbon) of the aggregates. There was no significant difference in soil active carbon pool in different aggregate size classes. Determination of size and turnover of a slow pool showed significant difference in different aggregate size classes. The slow carbon pool in different aggregate size classes ranged from 13.7 to 25.5% with mean residence time of 1.8 to 5.4 years. Water soluble carbon and active carbon (alkaline KMnO4 oxidizable C) were significantly higher in macro-aggregates than in micro-aggregates.

Carbon dioxide emissions from Ultisol under different land uses in mid–subtropical China

Soil organic carbon (SOC) pools and turnover time are highly sensitive to site variables and land-use change. In order to ascertain mass distributions of different SOC pools influenced by land use and evaluate their relationships with carbon mineralization and different soil properties, a 62-day laboratory incubation under a range of moisture (20 to 100% of water-holding capacity) and temperature conditions (5–45 °C) was conducted to measure CO 2 evolution from four land-use types, and data from the incubation study were fitted to a three-pool first-order model that separated mineralizable soil organic carbon into active, slow and resistant carbon pools. The results indicate that: (1) The active carbon pool comprised 1.2–3.5% of SOC with a mean residence time of 49–65 days, (2) The slow carbon pool comprised 25.3–60% of SOC with a mean residence time of 2–27 yr, (3) The resistant pool accounted for 36.5–73.9% of SOC in four land-use types. The woodland had the highest resistant carbon pool and lowest active carbon pool, which indicates it to be more stable than other land-use types. Soil carbon derived respiration was well correlated with all the organic carbon pools as well as C:N ratio and soil derived respiration. SOC was positively correlated with slow and resistant carbon pool, and soil derived respiration. CO 2 emissions increased ( r 2 = 0.273–0.544, P < 0.0001) with increasing temperature (up to 40 °C), with emissions reduced at the lowest and highest soil moisture contents. The Q 10 values (the factor by which respiration rate increase for a 10 °C increase in temperature) (from 5 °C to 45 °C, at 60% of water-holding capacity) of 1.9 ± 0.2, 2.2 ± 0.3, 2.3 ± 0.3, and 3.2 ± 0.5 were observed for paddy, orchard, woodland and upland, respectively, and decreased with decreasing moisture content when soil water content was less than its optimum value, but an opposite trend was shown when soil retained water at contents higher than the optimum water content. Inclusion of both moisture and temperature in our multiple polynomial models resulted in much better predictions of CO 2 ( r 2 = 0.70–0.78, P < 0.0001) emissions than predictions obtained using temperature ( r 2 = 0.27–0.54, P < 0.01) or moisture ( r 2 = 0.29–0.45, P < 0.01) alone. Our study indicates that vegetation type and/or management practices which control soil biological and biochemical properties, were important predictors of C fluxes in this short-term experiment.

Variable use of plant- and soil-derived carbon by microorganisms in agricultural soils

In this study we used compound specific 13C and 14C isotopic signatures to determine the degree to which recent plant material and older soil organic matter (SOM) served as carbon substrates for microorganisms in soils. We determined the degree to which plant-derived carbon was used as a substrate by comparison of the 13C content of microbial phospholipid fatty acids (PLFA) from soils of two sites that had undergone a vegetation change from C3 to C4 plants in the past 20–30 years. The importance of much older SOM as a substrate was determined by comparison of the radiocarbon content of PLFA from soils of two sites that had different 14C concentrations of SOM. The 13C shift in PLFA from the two sites that had experienced different vegetation history indicated that 40–90% of the PLFA carbon had been fixed since the vegetation change took place. Thus PLFA were more enriched in 13C from the new C4 vegetation than it was observed for bulk SOM indicating recent plant material as preferentially used substrate for soil microorganisms. The largest 13C shift of PLFA was observed in the soil that had high 14C concentrations of bulk SOM. These results reinforce that organic carbon in this soil for the most part cycles rapidly. The degree to which SOM is incorporated into microbial PLFA was determined by the difference in 14C concentration of PLFA derived from two soils one with high 14C concentrations of bulk SOM and one with low. These results showed that 0–40% of SOM carbon is used as substrate for soil microorganisms. Furthermore a different substrate usage was identified for different microorganisms. Gram-negative bacteria were found to prefer recent plant material as microbial carbon source while Gram-positive bacteria use substantial amounts of SOM carbon. This was indicated by 13C as well as 14C signatures of their PLFA. Our results find evidence to support ‘priming’ in that PLFA indicative of Gram-negative bacteria associated with roots contain both plant- and SOM-derived C. Most interestingly, we find PLFA indicative of archeobacteria (methanothrophs) that may indicate the use of other carbon sources than plant material and SOM to a substantial amount suggesting that inert or slow carbon pools are not essential to explain carbon dynamics in soil.

Predicting dynamics of soil organic carbon mineralization with a double exponential model in different forest belts of China

The dynamics of soil organic carbon (SOC) was analyzed by using laboratory incubation and double exponential model that mineralizable SOC was separated into active carbon pools and slow carbon pools in forest soils derived from Changbai and Qilian Mountain areas. By analyzing and fitting the CO2 evolved rates with SOC mineralization, the results showed that active carbon pools accounted for 1.0% to 8.5% of SOC with an average of mean resistant times (MRTs) for 24 days, and slow carbon pools accounted for 91% to 99% of SOC with an average of MRTs for 179 years. The sizes and MRTs of slow carbon pools showed that SOC in Qilian Mountain sites was more difficult to decompose than that in Changbai Mountain sites. By analyzing the effects of temperature, soil clay content and elevation on Soc mineralization, results indicated that mineralization of SOC was directly related to temperature and that content of accumulated SOC and size of slow carbon pools from Changbai Mountain and Qilian Mountain sites increased linearly with increasing clay content, respectively, which showed temperature and clay content could make greater effect on mineralization of SOC.

Interpreting, measuring, and modeling soil respiration

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO2, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions.

Molecular dynamics of organic matter in a cultivated soil

The dynamics of soil organic carbon are included in global carbon (C) cycle scenarios using different, but generally arbitrary defined, kinetic pools. To improve global C models, better relationships between the chemical structure of soil organic matter (SOM) and its kinetic pools are needed. To assess the molecular residence time of SOM and the relation with plant inputs, pyrolysis–GC/MS–C–IRMS was performed on maize plants and on two samples from the same soil that had undergone a vegetation change from the C3 plant wheat to the C4 plant maize. This vegetation change has added naturally 13C-enriched material to the soil. Most pyrolysis products from the maize were derived from polysaccharides and lignins, and were not detected in soils. However, polysaccharide-derived products were also major pyrolysis products in soils, N-containing or unspecific pyrolysis products were also detected. The residence times (based on 13C natural labelling) revealed a continuum of values, that was independent of chemical structure, with only two pyrolysis products presenting a relatively long residence time (ca. 100 years). An unexpected long life-time for N-containing (∼49 years) and polysaccharide-derived (∼54 years) pyrolysis products was found. Our results suggest that mainly recycling of carbon in carbohydrates and N-containing materials in addition to physical and chemical protection is responsible for SOM stabilization in the slow carbon pool.