The residence time of carbon (C) and nitrogen (N) in soil is a fundamental parameter reflecting the rates of soil organic matter (SOM) transformation and the contribution of soils to greenhouse gases fluxes. Based on the global database of the stable isotope composition of C (δ¹³C) and N (δ¹⁵N) depending on soil depth (171 profiles), we assessed С and N turnover and related them to climate, biome types and soil properties. The ¹³C and ¹⁵N discrimination between the litter horizon and mineral soil was evaluated to explain the key litter transformation processes. The ¹³C and ¹⁵N discrimination by microbial utilization of litter and SOM, as well as the continuous increase of δ¹³C and δ¹⁵N with depth, enabled to assess C and N turnover within SOM. N turnover was two times faster than that of C, which reflects i) repeated N recycling by microorganisms accelerating the N turnover, ii) C loss as CO₂ and input of new C atoms to cycling, which reduces the C turnover, and iii) generally slower turnover of N free persistent organic compounds (e.g. lignin, suberin, cellulose) compared to the N containing compounds (e.g. amino acids, ribonucleic acids). An increase in temperature and precipitation accelerated C and N turnover because: i) higher microbial activity and SOM decomposition rate, ii) larger soil moisture and fast diffusion of dissolved organics towards exoenzymes, iii) downward transport of ¹³C-enriched organic matter (e.g. sugars, amino acids), and iii) leaching of ¹⁵N-depleted nitrates from the topsoil and losses from the whole soil profile. Temperature accelerates SOM turnover stronger than precipitation. The temperature increase by 10 °C accelerates the C and N turnover for 40%. SOM turnover is boosted by decreasing C/N ratio because: i) SOM with a high C/N ratio originated from litter is converted to microbially-derived SOM in mineral soil characterized by a low C/N ratio; ii) litter with a low C/N ratio is decomposed faster than litter with a high C/N; iii) microbial carbon-use efficiency increases with N availability. The biome type affects SOM decomposition by i) climate: slower turnover under wetter and colder conditions, and ii) by litter quality: faster utilization of leaves than needles. Thus, the fastest C turnover is common under evergreen forests and the lowest under mixed and coniferous ones, whereas temperature and C/N ratio are the main factors controlling SOM turnover. Concluding, the assessment of SOM turnover by δ¹³C and δ¹⁵N approach showed two times faster N turnover compared to C, and specifics of SOM turnover depending on the biomes as well as climate conditions.

Soldatova E, Krasilnikov S, Kuzyakov Y 2024. Soil organic matter turnover: global implications from δ¹³C and δ¹⁵N signatures. Science of the Total Environment 912, 169423. https://doi.org/10.1016/j.scitotenv.2023.169423

How to cite: Kuzyakov, Y. and Soldatova, E.: Soil organic matter turnover: global implications from δ13C and δ15N signatures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11857, https://doi.org/10.5194/egusphere-egu24-11857, 2024.

Soil nitrogen (N) is a key component of plant nutrition but our ability to predict organic N mineralization potential remains incomplete.  Several methods are commonly used to characterize and measure mineralizable N; however, they are generally lacking because of required lab resources and poor predictive power. Pyrolysis is an emerging technology used to characterize soil organic matter and the thermal stability of soils. However, the idea of using Pyrolysis technology to characterize soil N and measure soil N release is novel. We adopted a novel online pyrolysis coupled with FTIR (Fourier-transform infrared spectroscopy) technology to investigate soil N. The soil samples used were collected from a long-term field trial involving different crop rotations and fertilization to include a wide array of samples. Samples were pyrolyzed from 25 to 850 °C with a heating rate of 10 K min^-1. The temperature at which 50% of the material underwent pyrolysis, referred to as T50, was determined to quantify the thermal stability. The focus was to look at mass loss characteristics, identify volatile matter released, T50, and the correlation of TG-FTIR data with a 12-week lab mineralization study. We found a negative correlation (R²= -0.67) between the T50 and mineralized N at week 12. In conclusion, this study elucidates the intricate interplay between temperature kinetics and nitrogen mineralization. The negative correlation between T50 and mineralizable N underscores the potential of the material to release N over time. This research offers a valuable foundation for optimizing Pyrolysis application in the context of soil nitrogen. 

How to cite: Kaur, S. and Gillespie, A.: Predicting Soil Nitrogen Mineralization Potential using Pyrolysis-coupled FTIR, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20796, https://doi.org/10.5194/egusphere-egu24-20796, 2024.

Abundant use of chemical fertilizer causes reactive nitrogen load to the environment. Leaching of reactive nitrogen occurs mainly in nitrate (NO₃−N) form, which moves into groundwater and other water bodies causing human health hazard and harming ecosystems, especially in tropical islands. Biochar application, which is known as a measure of carbon sequestration, can mitigate the leaching of NO₃−N. We have accumulated knowledge regarding the effective application rate of biochar. However, the information on the effect of biochar application depth remains unclear, although it would affect the leaching of NO₃−N and crop growth. The objective of this study was to evaluate the effect of biochar application depth on the leaching of NO₃−N and crop growth. Using biochar made from bagasse, which is a major organic waste in tropical islands, we conducted a pipe experiment with upland rice (NERICA). We set four treatments: control (no biochar application); surface application (0−5 cm); plow layer application (0−30 cm); subsurface application (25−30 cm). Regarding leaching of NO₃−N, the result under surface application showed a 12% decrease, while that under plow layer application showed an 11% increase against that under the control. Whereas the leaching of NO₃−N was the same under subsurface application as that under the control. Total nitrogen uptake by crop was the highest under surface application, whereas those under plow layer and subsurface applications were smaller than those under the control. By comparing the leaching of NO₃−N with the total nitrogen in the root, we obtained a clear relationship that the higher the total nitrogen in the root was, the lesser the leaching became. The result of the matric potential head in each pipe revealed that soil water condition was stressless for crops under the surface application. On the other hand, dry stress occurred more frequently under plow layer and subsurface applications. These results indicated that, depending on biochar application depth, soil water stress conditions differed and affected root growth positively/negatively. Consequently, crop growth and the leaching of NO₃−N were changed. The surface application can be considered as an effective application, which mitigates the leaching and promotes crop growth simultaneously. We believe the finding of this study encourages the establishment of sustainable agriculture.

How to cite: Hamada, K. and Nakamura, S.: Effect of biochar application depths on leaching of nitrate and crop growth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13568, https://doi.org/10.5194/egusphere-egu24-13568, 2024.

Sulfur (S) in soils mainly occurs in organic forms, and its cycling should be primarily controlled by the same factors as those for carbon (C) and nitrogen (N), two other main constituents of soil organic matter. Here, we aim to test this assumption based on a global meta-analysis of soil organic C, N and S contents in grassland soils. We reviewed existing literature with a focus on grassland soils as one of the major global ecosystems, including both native grasslands and managed grasslands with additional fertilization. Element concentrations and supplementary parameters (mean annual temperature and precipitation, texture, pH, soil group, management) were retrieved from the studies, while C:S and N:S element ratios were either directly obtained from the studies or calculated. Additionally, we analyze isotope ratios of the respective elements (δ¹³C, δ¹⁵N and δ³⁴S) in soil samples collected from native and managed grassland sites along climatic transects across Europe and North America.

In literature data concentrations of OC and N, but not S, were significantly higher in pastures compared to native grassland. As a consequence, C:S and N:S ratios were significantly lower in cultivated grassland than in native sites. Further, climatic conditions and soil group significantly affected C:S and N:S ratios, with significantly lower ratios in arid climate and in soil groups typically occurring there (e.g. Kastanozems) compared to more humid conditions and respective soil groups (e.g. Luvisols). However, the variation of C:S and N:S ratios was considerably higher than for C:N ratios. This was also evident in the isotope data obtained from the soil samples along the continental transects. Here, compared to δ¹³C and δ¹⁵N, δ³⁴S values showed strong variation that was only partially explained by climate and land use, and was additionally affected by the specific parent material at each site. We therefore suggest that the availability and turnover of S in organic matter of grassland soils is not strictly analogous to C and N cycling. 

How to cite: Bauke, S. L., Iser, J., Schimmel, H., Burger, D., and Amelung, W.: Divergent patterns of Carbon, Nitrogen and Sulfur storage in grassland soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12106, https://doi.org/10.5194/egusphere-egu24-12106, 2024.

Wetlands account for roughly 20-30% of global methane (CH₄) emissions, in part due to (permanently) anoxic conditions that promote methanogenesis. Low-lying permafrost peatlands are expected to be an increasing source of CH₄ due to permafrost thaw in the future, due to waterlogging, anoxia and organic carbon (OC) mobilization. These processes can force an increase of the extent of wetlands and higher total CH₄ emissions. Net release of CH₄ depends on the availability of more energetically favorable electron acceptors in soil, such as ferric iron (Fe(III)). Fe(III) may be present as Fe(III) oxyhydroxides or Fe(III)-OC phases in peatlands. Since Fe(III)-reducing microbes and methanogens compete for the same substrates (e.g., small organic molecules such as acetate), CH₄ production is often suppressed as long as there is bioavailable Fe(III) present. However, the dissolution of Fe(III)-OC phases during Fe(III) reduction would release the previously bound OC and make it more bioavailable to fermenting microorganisms. This would produce more substrates for methanogens and could therefore increase CH₄ production. It is currently unknown to what extent microbial reduction of Fe(III)-OC phases and the coupled OC release effects CH₄ emissions across permafrost thaw.

In this study, we therefore aim to elucidate the extent of reduction of Fe(III)-OC phases upon permafrost thaw and the corresponding effect on CH₄ emissions across three distinct thaw stages: (i) recently collapsed palsa hills, (ii) partly thawed bog and (iii) fully thawed fen habitats. We simulated permafrost thaw by a series of incubation experiments with soils from each thaw stage from a permafrost peatland (Stordalen Mire, Abisko, Sweden). Soils were incubated under anoxic, flooded conditions for 30 days, after which synthesized ⁵⁷Fe-labelled Fe(III)-OC coprecipitates were added as representative Fe-OC phases. Over the course of the incubations (42 days), we followed Fe speciation and ⁵⁷Fe fractions in the dissolved and solid phases using geochemical and synchrotron-based spectroscopy techniques in addition to quantification of greenhouse gas production. Results show that added coprecipitates were completely reduced (within 1 day) in palsa and bog soils, leading to increases in dissolved Fe²⁺, OC concentrations and CO₂ emissions. CH₄ production was not detected in palsa soils over the course of the incubation and CH₄ suppression in bog soils due to Fe(III) reduction was only short-term. In contrast, added coprecipitates in fen soils were only reduced by 10% after 42 days, likely due to low dissolved OC concentrations. However, this led to a significantly higher inhibition of methanogenesis than in the palsa and bog soils. We also studied the microbial community by 16S rRNA amplicon (gene) sequencing and quantified mcrA gene copy numbers to assess the potential activity of methanogens. Overall, these results help to understand the influence of Fe-OC coprecipitates on methane emissions in thawing permafrost peatlands.

How to cite: Voggenreiter, E., Straub, D., Thomas Arrigo, L., Kappler, A., and Joshi, P.: Microbial iron(III) reduction of iron-organic carbon phases across a permafrost thaw gradient and its impact on methane emissions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3409, https://doi.org/10.5194/egusphere-egu24-3409, 2024.

Extreme summer droughts can drastically lower water tables and lead to oxygenation of normally anoxic soils in boreal ecosystems. In organic rich riparian soils, this creates a dynamic redox environment, driving changes in soil organic matter stability and the export of redox sensitive elements (e.g., C, N, S, Fe, etc.) to surface waters.  We hypothesized that the destabilization of redox cycles and the activation of oxidative soil enzymes during drought periods can lead to prolonged periods of altered soil biogeochemical processes that drive element export from terrestrial to surface water systems upon rewetting. Here we simulated a soil core drying-rewetting event, to ask how riparian soil solution biogeochemistry changes during two months post drought. To three drought treatments (dry, semi-dry, wet), we also added a root exudate treatment (exudates or no exudates) to simulate the effects of riparian vegetation on microbial organic matter decomposition. We found that following drought, dissolved organic carbon (DOC) concentrations initially decreased, due to the increased acidity caused by the oxidation of reduced S to SO₄^2-. As other preferred electron acceptors (O₂, NO₃^2-, Fe³⁺) were gradually reduced, reduction of SO₄^2- lead to increases in DOC concentrations, which after 2 weeks surpassed concentrations in the control (wet) treatment, and continued increasing until the end of the experiment. Once SO₄^2- was depleted and CO₂ became the preferred electron acceptor, methane (CH₄) in solution also increased to concentrations higher than those in control treatments. Peroxidase activity was increased post drought and remained elevated throughout the experiment, suggesting that microbial organic matter breakdown was enhanced, and could explain why DOC concentrations in drying treatments eventually surpassed those in wet controls. While the root exudate treatments produced mixed results, an increase of labile C supply appeared to increase extracellular enzymatic activity and serve as an alternative electron acceptor, thereby suppressing methanogenesis. Our results show that drought drastically changes the biogeochemistry of boreal riparian soils and that upon rewetting this can eventually lead to increased lateral exports of both organic and inorganic C. Changes in C biogeochemistry are seemingly caused by shifts in redox chemistry and by changes in microbial decomposition of soil organic matter induced by the oxygenation of riparian soils. Since this has implications for surface water chemistry, further study is needed on the length of drought effects to establish the duration of this influence of stream and riparian biogeochemistry.

How to cite: Škerlep, M., Reidy, M., Laudon, H., and Sponseller, R.: Drought induces changes to redox chemistry and C exports from boreal riparian soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18712, https://doi.org/10.5194/egusphere-egu24-18712, 2024.

The coastal terrestrial-aquatic interface (TAI) is a highly dynamic system characterized by strong physical, chemical, and biological gradients. In particular, shifting soil redox conditions, due in part to dynamic water conditions, is a strong driver of carbon availability and transformations across TAIs. However, one of the important unknowns across TAIs is how soils with different characteristics and inundation regimes respond quantitatively to water saturation and resulting shifts between oxic and anoxic subsurface conditions. We used field measurements, laboratory incubations, and model simulations to investigate oxygen consumption and redox transformations following short-lived oxygenation events under different environmental conditions. Soils were collected along a coastal gradient (upland to wetland) from the Western Lake Erie region (freshwater TAI) and the Chesapeake Bay (estuarine TAI) and incubated in microcosms for two weeks. When inundated in MilliQ water, the upland A horizon soils went anoxic in 24 hours, whereas the wetland and transitional soils went anoxic in 0.5 - 10 hours. In contrast, the upland B horizon soils did not go anoxic during the 2-week incubation. Model simulations suggested stronger abiotic controls of oxygen consumption in the wetlands vs. biotic controls in the upland soils. These simulations also suggested nutrient limitation in the subsurface soils. Subsequent incubations with glucose and acetate additions showed increased rates of oxygen consumption in the B horizon soils, suggesting that these soils were indeed carbon limited. These experiments provide insight on shifting redox conditions during flooding events, especially relevant in coastal systems that experience rapidly shifting hydrological conditions and are becoming increasingly vulnerable to sea level rise and episodic disturbances (e.g., storm surges, king tides).

How to cite: Patel, K., Rod, K., Zheng, J., Regier, P., Machado-Silva, F., Kaufman, M., Kemner, K., Megonigal, J. P., Ward, N., Weintraub, M., and Bailey, V. and the COMPASS-FME Team: Time to Anoxia: Oxygen Consumption in Soils Varies Across a Coastal Gradient, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6628, https://doi.org/10.5194/egusphere-egu24-6628, 2024.

Global fresh groundwater resources are threatened due to increasing withdrawal and salinization. Managed Aquifer Recharge (MAR) is an effective approach to augment overexploited aquifers and can also be applied to improve the groundwater quality by infiltration of desalinated water. But MAR can also bear risks when the soil passage or aquifer contains trace metals (e.g. As, V, Co, Cd) with their mobilization and subsequent human uptake. Understanding of metal mobilization (especially As) due to mineral dissolution at MAR sites with desalinated water was subject of investigation in many studies. Ionic strength and divalent ions concentration of the infiltrating water are known to influence the transport behaviour of trace metals by controlling dissolution and stabilization. As a new approach for water desalination, the aim of the cooperative project “innovatION” is the development of a monovalent-selective membrane capacitive deionization method to remove monovalent ions from brackish water. Other than fully desalinated water, the product water is still enriched in divalent ions. Here, we present first insights on how trace metal transport during MAR depends on the chemistry of the infiltrating water. Trace elements, as for example As, were found in sands of the East Frisian Island Langeoog, Northern Germany, and could be mobilized during potential MAR. We conducted column experiments with infiltration of a fresh groundwater (fGW), monovalent partial desalinated water (mPDW) and pure water (PW) into grey dune sand from Langeoog.

Our results show As mobilization due to shifting redox conditions and iron mineral dissolution up to a maximum of 16 µg/l in the outflow. Notably, the infiltration of mPDW, with a higher ionic strength than fGW and PW, lead to a temporary retention of As with a concentration decline to 2 µg/l and subsequently a slow increase. Whereas As is further mobilized with a very slow decrease with infiltration of fGW and PW. Arsenic concentrations were positively connected to dissolved organic carbon concentrations of the outflow, an indication that organic complexation of As takes place after dissolution of Fe-minerals. Clearly, the infiltration of mPDW can mitigate potentially harmful colloidal trace element transport. These results help to understand the mechanisms of sorption, desorption and transport of trace metals in environments with changing pore water chemistry during MAR. Our research also aids to better assess the functionality of this novel desalination technique that has high potential to improve groundwater quality.

How to cite: Braeunig, L., Longman, J., Gäng, F., and Massmann, G.: Trace metal transport during managed aquifer recharge with monovalent-partial desalinated water: The role of divalent ions and organic matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15009, https://doi.org/10.5194/egusphere-egu24-15009, 2024.