Effects of soil moisture fluctuation and microplastics types on soil organic matter decomposition and carbon dynamics

Storing large amounts of organic carbon, soils are a key but uncertain component of the global carbon cycle, and accordingly, of Earth System Models (ESMs). Soil organic carbon (SOC) dynamics are regulated by a complex interplay of drivers. Climate, generally represented by temperature and moisture, is regarded as one of the fundamental controls. Here, we use 54 forest sites in Switzerland, systematically selected to span near-independent gradients in temperature and moisture, to disentangle the effects of climate, soil properties, and landform on SOC dynamics. We estimated two SOC turnover times, based on bulk soil 14C measurements (τ14C) and on a 6-month laboratory soil incubation (τi). In addition, upon incubation, we measured the 14C signature of the CO2 evolved and quantified the cumulated production of dissolved organic carbon (DOC). Our results demonstrate that τi and τ14C capture the dynamics of contrasting fractions of the SOC continuum. The 14C-based τ14C primarily reflects the dynamics of an older, stabilised pool, whereas the incubation-based τi mainly captures fresh readily available SOC. Mean site temperature did not raise as a critical driver of SOC dynamics, and site moisture was only significant for τi. However, soil pH emerged as a key control of both turnover times. The production of DOC was independent of τi and not driven by climate, but primarily by the content of clay and, secondarily by the slope of the site. At the regional scale, soil physicochemical properties and landform appear to override the effect of climate on SOC dynamics.

Modelling tillage and nitrogen fertilization effects on soil organic carbon dynamics

ABSTRACTThe vertical dynamics of paddy soil organic carbon (SOC) play an important role in soil quality and carbon cycling. In this study, we used an exponential decay function to estimate the vertical dynamics of SOC content and determined its influencing factors in a typical paddy soil area of the Chengdu Plain from the 1980s to the 2010s. Our results show an overall increase in SOC stocks at 0–100 cm from 11.8 Tg in the 1980s to 13.7 Tg in the 2010s. SOC content increased at depths of 0–40 cm and declined at depths of 40–100 cm over the past three decades. The exponential decay function parameters C0 and k significantly increased by 31.4% and 18.2% respectively, which suggests the vertical pattern of SOC distribution changed. The increase in nugget effects of C0 and k and the decrease in the relative contributions of the parent material, subgroup and distance-to-river indicate that extrinsic factors played increasing roles in the vertical variation of SOC content. Our study concludes that rice planting has led to vertical variations of SOC content and decreased the effects of intrinsic factors on the vertical variation of SOC content of Chengdu Plain paddy soils over the past three decades.

We investigated the variation patterns of organic carbon in soil and soil solution of four selected Robiniapseudoacacia forests aged 10a, 25a, 31a, and 35a, as well as a contrastive tillage site in a similar topography condition in Loess Plateau, China. The purpose was to explore the dynamics of soil organic carbon (SOC) and dissolved organic carbon (DOC) in R. pseudoacacia forests. On average depths of 20, 40, and 60 cm, SOC, active organic carbon (AOC), and DOC gradually increase with increased forest age. After forest restoration, the AOC/SOC ratio and resistant organic carbon/SOC ratio increase, whereas the slow organic carbon/SOC ratio decreases. The soil solutions in the subsoil layer have low DOC:DON ratio and high UV absorption at 280 nm. At 40 and 60 cm, the depth distribution is indicated as special low values for DOC concentration in the C99 site (10a site), as well as for soil water content, SOC, and AOC in the 25a forest site. Our results provide evidence that during forest restoration, SOC does not consistently increase linearly. The change points of different SOC proportions and DOC concentrations at various depths are not same, i.e., asynchronous changes exist.

<p>In the last years, soil organic carbon (SOC) dynamics have been explored for agronomic and environmental issues in different agro systems. Many soils of the world, especially in arid and semi-arid environments, contain large stocks of soil inorganic carbon (SIC) as carbonates. Yet, the SOC dynamics has been poorly investigated in these soils, due to the complexity of measurements and of the processes involved. Indeed, few previous studies have shown links between SIC and SOC dynamics. Theses interactions are initiated by biological activities, i.e. CO<sub>2</sub> production, are explained through equilibrium equations between soil carbonates and bicarbonates. However, few data were available on the specific impact of SIC on SOC mineralization especially at increasing soil depth.</p><p>Alley agroforestry systems increased SOC content in the tree rows without any change in the SIC content.  The heterogeneity in organic inputs and SOC contents induced by alley agroforestry allows the investigation of the interactions between SIC and SOC on CO<sub>2</sub> emissions.</p><p>To assess contributions of SIC to CO<sub>2</sub> emissions with depth, we incubated carbonaceous soil samples coming from an 18-year-old agroforestry system (both tree row and alley) and an adjacent agricultural plot. Soil samples were taken at four different depths: 0-10, 10-30, 70-100 and 160-180 cm. Total CO<sub>2</sub> emissions, the isotopic composition (δ<sup>13</sup>C, ‰) of the CO<sub>2</sub> and microbial biomass were measured. The SIC concentrations were from 48 to 63 g C kg<sup>-1</sup> soil and the SOC concentrations from 4 to 17 g C kg<sup>-1</sup> soil. The total amounts of CO<sub>2</sub> emissions from soil were correlated to C contents and decreased with depth (from 183-569 µgC g<sup>-1</sup> soil in top soil vs 21-25 µgC g<sup>-1</sup> soil in subsoil).</p><p>The contribution of SIC-derived CO<sub>2</sub> was not homogenous along the soil profile. It represented about 20% in the topsoil and 60% in the subsoil of the total soil CO<sub>2</sub> emissions. As the SOC content and the microbial biomass, the SOC-derived CO<sub>2</sub> emissions were larger in the topsoil especially in the tree row compared to the alley and the agricultural plot. The SIC-derived CO<sub>2</sub> emissions were also larger in topsoil and in tree rows at 0-10 cm than in alleys or agricultural plots (71 µg C g<sup>-1</sup> soil vs 45-48 µg C g<sup>-1</sup> soil) or in the subsoil (13-15 µg C g<sup>-1</sup> soil), whereas the amount of SIC was similar in top and subsoil and in tree rows, alleys or agricultural soils. This indicate that CO<sub>2</sub> emissions from SIC were linked to the SOC content and its mineralization.  In addition, our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO<sub>2</sub> is not taken into account. It also advocates more in-depth studies on carbonate dissolution-precipitation processes and their impact on CO<sub>2 </sub>emissions.</p><p>Reference:</p><p>Cardinael, R., Chevallier, T., Guenet, B., Girardin, C., Cozzi, T., Pouteau, V., and Chenu, C. 2019 Organic carbon decomposition rates with depth and contribution of inorganic carbon to CO2 emissions under a Mediterranean agroforestry system, Eur J Soil Sci, https://doi.org/10.1111/ejss.12908.</p>

The Mediterranean region, warming 20% faster than the global average, is experiencing significant climate change impacts, including rising temperatures, altered precipitation patterns, and increased frequency of extreme weather events. To investigate the implications of these changes on soil organic carbon (SOC) dynamics, we utilized an ensemble of four agricultural system models—EPIC, DSSAT, CropSyst, and APSIM. These models were calibrated and validated using approximately 9,000 observations of crop yields and residues, along with 110 specific SOC measurements, collected from five long-term experiments across a north-to-south pedoclimatic transect in Italy, covering the period from the 1970s to 2022. This comprehensive dataset enabled us to accurately simulate SOC dynamics under varying conditions. We examined three representative concentration pathways (RCPs): RCP2.6 (very low future emissions), RCP7.0 (high future emissions), and RCP8.5 (very high future emissions) for the period 2023-2100, utilizing three bias-corrected EURO-CORDEX climate models with local corrections. Projections revealed significant variations in SOC stocks based on location and agricultural practices.  In Southern Italy, SOC stocks remained stable over time, showing only little variation according to the climate scenario. Conversely, in Central Italy, 30-cm SOC stocks increased until 2070 and then diverged according to the RCPs: a decrease under RCP8.5 (-7.1 t/ha), stabilization under RCP7.0 (+1.0 t/ha), and a sharp increase under RCP2.6 (+11.5 t/ha). In Northeast Italy, SOC stocks decreased under all scenarios, with slightly lower decreases under RCP2.6. Regarding different management systems, conservation agriculture proved to be the most effective in terms of SOC increase or stabilization, while maize and wheat monocultures were the most negatively affected by RCP7.0 and RCP8.5. This study underscores the critical need for location-specific management strategies to address the challenges posed by climate change in the Mediterranean region. Our findings highlight the importance of tailored agricultural practices to mitigate SOC losses and promote soil health under changing climatic conditions.This research was conducted within the Agritech National Research Center and received funding from the European Union Next-GenerationEU (PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR) – MISSIONE 4 COMPONENTE 2, INVESTIMENTO 1.4 – D.D. 1032 17/06/2022, CN00000022).

Quantifying soil organic carbon (SOC) dynamics is critical for monitoring agricultural sustainability and mitigating global climate change. A study was done to investigate the SOC changes between 2003 and 2016 by DeNitrification-DeComposition (DNDC) model in an intensively cultivated region of eastern China, using a detailed high-resolution soil database, which contained 4875 sites (0–20 cm depth) with the sampling density of one sample per km2 in 2003. Results showed the DNDC model had good performance for simulating the SOC dynamics in the study area. Spatial analysis showed that 71% of the study area gained C, whilst 29% maintained their C balance with no C loss in all areas. Results suggested that the current management practice facilitated soil C sequestration in this region although soil type also played an important role. SOC density (SOCD) in lime concretion black soil was significantly higher than those in other soils. The averaged global warming potential ranged from 3865.8 to 7110.8 kg CO2 equivalent ha−1 yr−1 from 2003 to 2016 with CH4 contributing the largest proportion, which might be associated with the high input of fertilizer. Spatially high-resolution estimates of SOC using DNDC can support dual agricultural and carbon management in this region.

Soil carbon efflux and sequestration as a function of relative availability of inorganic N pools in dry tropical agroecosystem

The surface of the continental Earth is constantly reworked at different scales of time and space by weathering and erosion. Erosion, although perceived as destructive process, can produce spectacular landscapes giving rise to an outstanding geodiversity. Particularly in southern Mediterranean regions, soils have become highly dynamic owing to climate change and intense land use. Soil redistribution rates (erosion/accumulation) determine soil evolutional trajectories, weathering and organic carbon dynamics in such landscapes. The interaction among these factors, however, remains poorly understood. The area around Corleone (north-western Sicily, Italy), a potential new UNESCO geopark, consists of a worldwide peculiar geological condition, i.e. massive glauconitic calcarenites. We investigated soil erosion rates by using 239+240Pu as tracer, the weathering state of soils, soil organic carbon dynamics and its chemistry in this area. Although the soils showed clear signs of strong degradation and erosion with up to 39 t ha-1 yr-1, org. C stocks remained on a surprisingly high level with 7 to 25 kg C m-2. Erosion removed the strongly weathered part of the soil and left behind a younger and fresher soil matrix with a low org. C content, but an organic carbon fraction that was enriched in aliphatic chains and lignin-like compounds and having, therefore, a lower maturity. The investigated soils developed on a parent material that promoted the stabilisation and sequestration of organic matter so that even highly eroded and shallow soils still contained a considerable amount of org. C, which is rather unique for Mediterranean areas. Due to the presence of glauconite, smectite and oxyhydroxides in the parent material, the soils were able to retain a high amount of soil organic carbon. The determination of erosion rates on such soils was, however, challenging, and the major difficulties are discussed. Furthermore, when using 239+240Pu as a tracer for soil erosion, only point information is usually obtained and an extrapolation to a larger area is difficult or very time and resource consuming. A new, promising procedure will be presented, based on 239+240Pu, on how to overcome this problem in future and how to extrapolate to a whole catchment area.

Agricultural practices (e.g. tillage, crop rotation and fertiliser application) have a strong influence on the balance between carbon (C) input and output by altering physicochemical and microbial properties that control decomposition processes in the soil. Recent studies suggest that the mechanisms by which agricultural practice impacts soil organic carbon (SOC) dynamics in the topsoil may not be the same as those in the subsoil. Here, we assessed SOC stock, soil organic fractions and nitrogen availability to 1.0 m in soils under a cotton (Gossypium hirsutum L.)-based cropping system, and assessed the impact of agricultural management (three historical cropping systems with or without maize (Zea mays L.) rotation) on SOC storage. We found that the maize rotation and changes in the particulate organic fraction influenced SOC stock in the topsoil, although the overall change in SOC stock was small. The large increase in subsoil SOC stock (by 31%) was dominated by changes in the mineral-associated organic fraction, which were influenced by historical cropping systems and recent maize rotation directly and indirectly via changes in soil nitrogen availability. The strong direct effect of maize rotation on SOC stock, particularly in the subsoil, suggests that the direct transfer of C into the subsoil SOC pool may dominate C dynamics in this cropping system. Therefore, agricultural management that affects the movement of C within the soil profile (e.g. changes in soil physical properties) could have a significant consequence for subsoil C storage.

Soil carbon sequestration potential in a Vertisol in central India- results from a 43-year long-term experiment and APSIM modeling

Irrigation alters biogeochemical processes to increase both inorganic and organic carbon in arid-calcic cropland soils

Abstract. Montane tropical soils are a large carbon (C) reservoir, acting as both a source and a sink of CO2. Enhanced CO2 emissions originate, in large part, from the decomposition and losses of soil organic matter (SOM) following anthropogenic disturbances. Therefore, quantitative knowledge of the stabilization and decomposition of SOM is necessary in order to understand, assess and predict the impact of land management in the tropics. In particular, labile SOM is an early and sensitive indicator of how SOM responds to changes in land use and management practices, which could have major implications for long-term carbon storage and rising atmospheric CO2 concentrations. The aim of this study was to investigate the impacts of grazing and fire history on soil C dynamics in the Peruvian montane grasslands, an understudied ecosystem, which covers approximately a quarter of the land area in Peru. A density fractionation method was used to quantify the labile and stable organic matter pools, along with soil CO2 flux and decomposition measurements. Grazing and burning together significantly increased soil CO2 fluxes and decomposition rates and reduced temperature as a driver. Although there was no significant effect of land use on total soil C stocks, the combination of burning and grazing decreased the proportion of C in the free light fraction (LF), especially at the lower depths (10–20 and 20–30 cm). In the control soils, 20 % of the material recovered was in the free LF, which contained 30 % of the soil C content. In comparison, the burnt–grazed soil had the smallest recovery of the free LF (10 %) and a significantly lower C content (14 %). The burnt soils had a much higher proportion of C in the occluded LF (12 %) compared to the not-burnt soils (7 %) and there was no significant difference among the treatments in the heavy fraction (F) ( ∼ 70 %). The synergistic effect of burning and grazing caused changes to the soil C dynamics. CO2 fluxes were increased and the dominant temperature driver was obscured by some other process, such as changes in plant C and N allocation. In addition, the free LF was reduced when these two anthropogenic activities took place on the same site – most likely a result of reduced detritus being incorporated into the soil. A positive finding from this study is that the total soil C stocks were not significantly affected and the long-term (+10 years) C storage in the occluded LF and heavy F were not negatively impacted. Possibly this is because of low-intensity fire, fire-resilient grasses and because the grazing pressure is below the threshold necessary to cause severe degradation.

A multitude of models for soil organic carbon (SOC) dynamics are available to predict management impacts on SOC dynamics in its interaction with environmental factors. Most models include one SOC pool that has no or a very slow, negligible turnover. For this pool of long‐term stabilized (LTS) organic matter, we assumed a dynamic physical protection, whereby organic matter is protected in micropores and exposed to microbial access due to soil structure dynamics. Dynamics of the LTS pool is of special interest because of its very long turnover time. Therefore, it is of special importance whether the size of this pool is unlimited or if it reaches a saturation state. We investigated this topic by applying the CCB (CANDY Carbon Balance) model on a dataset from a long‐term experiment that started in 1983 in Bad Lauchstädt, Germany on a Haplic Chernozem and includes bare fallow treatments with the application of farmyard manure at different rates from 5 to 20 kg m −2 year −1 . Observations of SOC development over time supported our hypothesis about the existence of a saturation effect and were used to calibrate the CCB model with the hypothetical assumption of a limited LTS pool size. The model results suggest an upper limit of the LTS pool of 1.07 M% (percent of dry soil mass) SOC for the Bad Lauchstädt site. Furthermore, model calibration of LTS turnover revealed that organic matter in this pool is released from its protected state at a very low rate of 0.003 year −1 . Highlights Long‐term bare fallow field experiment with FYM input rates up to 20 kg m −2 Predicted long‐term SOC dynamics reflect observations Proof of non‐linear relationship between SOC storage and carbon input rate Evidence for limited protection capacity for SOC (saturation effect)

Climate change may significantly impact crop yields and soil. In this study the DNDC model, together with climatic outputs from Hadley Centre’s general circulation model (HadCM3), was used to investigate the influence of projected climate change and management practices on soil organic carbon (SOC) dynamics and crop yield of the Chinese Loess Plateau. The results identify management practices with the greatest potential to mitigate climate change and to increase SOC in this area. Field experiments on winter-wheat (Triticum aestivum L.) and summer maize (Zea mays L.) rotation included a control and four types of fertilization treatments: T1 (control), T2 (inorganic fertilizer), T3 (NPK inorganic fertilization combined with wheat or maize residue return), T4 (NPK inorganic fertilization combined with low amount of manure) and T5 (NPK inorganic fertilization combined with high amount of manure). DNDC model was calibrated using the field data from 1991 to 2000 and validated from 2001 to 2010. Furthermore, a baseline climate and three future climate scenarios (A1B, A2 and B1) were considered. DNDC model effectively simulated the SOC and crop yields. The findings showed that in 1991–2010, T1 maintained its initial SOC level but reduced crop yields, while T2 promoted crop production with less effect on soil carbon storage. However, T3, T4 and T5 enhanced both crop yield and soil carbon, and the best results were observed under T5. The investigated climate scenarios substantially affect SOC content and crop yields. In terms of SOC content, B1 had great effects on T1, T4 and T5, while A1B on T2 and T3. Considering crop yields, in all treatments, the trends are B1 > A1B > A2 for winter-wheat and A2 > A1B > B1 for summer maize, respectively. The impacts of climate changes on SOC dynamics and crop yields were different depending on the management applied. Thus, the adoption of certain management practices in the Chinese Loess Plateau agroecosystems could be critical in maximizing SOC sequestration and reducing CO2 in the atmosphere. Reasonably low temperature and high precipitation can enhance winter-wheat yields, while maize yields need medium temperature and precipitation. We recommended the combined application of inorganic and organic fertilizers to achieve a balance between food security and soil carbon sequestration objectives.