SSS5.3

This work proposes a new view of soil organic matter (SOM) formation: microorganisms use most of the organics entering the soil as energy rather than as a source of carbon (C), while SOM accumulates as a residual by-product because the microbial energy investment in its decomposition exceeds the energy gain. Considering the annual sequestration of C from litter into SOM of 0.4-5% of the total SOM pool, the energy input is equivalent to 1-10% of the total energy of SOM. Thus, more than 90% of the energy added to the soil by plants is lost in microbial transformation, with SOM representing the residual fraction. The conversion of plant litter accumulates approximately ~ 2% of the energy per unit of persisting plant organic matter. This is the proportion of biochemically stable litter-derived compounds and microbial necromass that get accumulated, while oxidized compounds are completely decomposed or recycled. As a result, SOM has more energy per unit C than plant residues, but the availability of that energy is low. This is because SOM composition is more diverse with a non-regular structure compared to plant residues and thus requires a wider range of enzymes to break it down.

The microbial transformation of plant residues into SOM is a never-ending continuum governed by processes such as mineralization, recycling, microbial necromass, and residue accumulation, all of which determine the energy content, fluxes, and nominal oxidation state of C (NOSC) values of the residual litter and the resulting SOM. NOSC and energy content of SOM are narrower in range than litter, with an average NOSC of -0.3, and a higher energy per unit C. Meanwhile, the NOSC values of available compounds (mainly low molecular weight) released from decomposed polymers play a role in the partition of C between catabolism and anabolism in microorganisms. They also affect the energy investment of microorganisms in nutrient mining from SOM.

The conversion of rhizodeposits and plant litter, considered to be the main sources of C in soil, therefore needs to be re-examined from an energy perspective, including energy quality and availability. This would also require the assessment of energy loss and conservation, as almost all microbial processing is directed towards energy acquisition rather than actual C demand. The small amount of plant-derived C and energy that persist in the form of SOM is only an intermediate phase to ensure energy fluxes in the soil system. Thus, the transformation of rhizodeposits and plant litter represents a process of utilization of the energy stored in them, while SOM is the residual material that persists because its microbial utilization is energetically inefficient.

How to cite: Gunina, A. and Kuzyakov, Y.: From energy to (soil organic) matter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5496, https://doi.org/10.5194/egusphere-egu22-5496, 2022.

Maize monoculture with low external inputs, as frequently practiced in sub-Saharan Africa, usually leads to the long-term loss of soil fertility. This threatens the already poor yields in the region. Practices that add organic and mineral resources to the soil therefore promise to counteract soil fertility loss by providing the potential feed-stock for microbes to build new soil organic matter. We studied the effect of organic and mineral resource addition from five organic amendment types of different quality (relative N, lignin and polyphenol contents) and quantity and from chemical nitrogen fertilizer, on soil organic carbon (SOC) and soil nitrogen in the 15 cm topsoil of four long-term trials in contrasted sites in Kenya. They had different climate and soil texture and lasted between 16 and 19 years. Treatments were identical among sites, the organic resources were Tithonia diversifolia (high quality and fast turnover) and Calliandra calothyrsus (high quality and slow turnover), stover of Zea mays (low quality and fast turnover), sawdust from Grevillea robusta trees (low quality and slow turnover) and locally available farmyard manure (undefined quality and slow turnover). The organic resources were added in the quantities of 1.2 and 4 t C ha^-1 yr^-1 and the experiments included a split-plot treatment of ±N addition (120 kg ha^-1 in each of the two growing seasons per year).

Despite site-specific differences, the general trend across sites indicated that SOC is usually lost with all treatments. Typical losses ranged from 1.9% to 0.6% loss of initial SOC yr^-1 for the control and the farmyard manure (at 4t C ha^-1 year^-1) respectively. Adding Calliandra or Tithonia at 4t C ha^-1 yr^-1 also enable to slow the loss (about 1.1% of initial SOC yr^-1 lost). Nevertheless, the addition of 4t C ha^-1 yr^-1 farmyard manure and Calliandra calothyrsus, together with mineral N addition, achieved a gain in SOC over time only in the site which had lowest initial SOC contents (about 6 g C kg^-1), a sand of 31% content and a climate that was suitable for maize growth. In contrast, another site with low initial SOC content, high sand content, but a less suitable climate, with frequent failures of the maize crop, lost SOC in all treatments. In the site with initially 25 g C kg^-1, the farmyard manure treatment at 4t C ha^-1 yr^-1 with N addition was the only treatment that could maintain SOC, while in the site with initially highest SOC (about 30 g C kg^-1), all treatments lost SOC. The mineral N addition, with the exception of two treatments in the lowest fertility site, had no significant effect on the response of SOC to the different organic resource treatments. Our results indicate that farmyard manure may be the most suitable resource to reduce losses of SOC, but increases may only be possible in sites with initially low SOC contents, e.g. where, because of sufficiently long cultivation activities, a new steady state with low SOC contents has already been attained.

How to cite: Six, J., Laub, M., Van de Broek, M., Couedel, A., Mathu, S., Necpalova, M., Waswa, W., Mugendi, D., Mucheru-Muna, M., Corbeels, M., and Vanlauwe, B.: Long-term effects of different organic resource rates, quality and nitrogen fertilizer on SOC development and conversion efficiency across Kenya. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9057, https://doi.org/10.5194/egusphere-egu22-9057, 2022.

Climate change mitigation strategies require long time removal and storage of carbon; thus, enhancing soil carbon stock is an appealing way to increase carbon sink potential and control emissions owing to associated ecosystem benefits. Understanding soil organic carbon (SOC) stock in the semiarid landscape is vital for natural based climate solutions and mechanisms. The carbon stock in soil represents 25% of the potential of natural climate solutions and wetlands have around 72% mitigation potential for soil carbon. Wetlands have a very complex natural system and provide a potential sink of atmospheric carbon. Particularly the role of wetlands in arid and semiarid lands has become vital as they not only provide a water source and livelihood options to the local community but also play an important role in maintaining ecosystem services. However, only limited studies have been conducted to assess the roles and potentials of wetlands in carbon sequestration in a semiarid region. The geospatial technologies provide a cost-effective and more accurate estimation of SOC stock in these ecosystems. The SOC distribution in wetland ecosystems and their carbon sequestration potential studies are crucial to understanding the global carbon budget. The present study area Keoladeo National Park is an ecologically important forested wetland situated in semiarid India with a heterogeneous landscape. Current research work illustrated a hybrid interpolation method for estimating the distribution of soil carbon in different vegetation type/land cover (VT/LC) using point survey data (prepared after laboratory test) with remote sensing. The map prepared has given satisfactory results with more than 80 present accuracy. SOC distribution data were collected from 130 plots from both the surface (0-15 cm) and subsurface soil (15-30 cm) covering all the 15 VT/LC classes. SOC was found to be significantly related to VT/LC type and water availability. The spatial distribution of SOC shows a wide range with an average value of around 1.5%; the seasonal distribution shows an increased amount of carbon in pre monsoon season and a high amount of carbon in the surface soil. The concentration of SOC (around 2.5%) has been observed to be more in wetland and grassland soils in both the seasons that cover about 13% and 27% area of the park, respectively. SOC stock management in this region is vital in observing the local community needs, which is mainly dependent on the park for livestock food. Further geospatial analysis of soil carbon stock potential will add value to the study. Synergising climate change mitigation strategies and community requirements are needed to enhance vulnerable communities' benefits. 

Keywords: Soil carbon, semiarid region, remote sensing, climate change mitigation.

How to cite: Deval, K. and Joshi, P. K.: Distribution of soil carbon stock in a forested wetland in the semiarid region of India: implications for climate change mitigation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-527, https://doi.org/10.5194/egusphere-egu22-527, 2022.

Climate change is expected to shift the home range of sugar maple (Acer saccharum Marsh.) northward, thereby encroaching onto the southern range of present-day balsam fir (Abies balsamea (L.) Mill.) forests. Such a shift from coniferous to deciduous forest stands will affect several edaphic properties and potentially modify soil organic carbon (SOC) storage and stability. For example, the more labile deciduous litter should decompose faster than coniferous litter, potentially resulting in lower SOC storage in forest floors. On the other hand, labile deciduous litter may result in a greater microbial turnover of SOC, leading to more stable SOC in mineral-associated organic matter (C-MAOM) in the subsoil. To test these hypotheses, we surveyed 30 mature forest stands in three regions along the sugar maple–balsam fir ecotone in southern Quebec, Canada. We dug three soil pits in each stand and measured SOC stocks in the organic forest floor as well as across five depth increments (0-5, 5-10, 10-20, 20-30 and 30-40 cm) in the mineral soil. We incubated mineral soil samples from each depth for 51 weeks and monitored CO₂ emissions rates, from which we quantified the bioreactive SOC pool. We derived two indices of microbial turnover of SOC at different soil depths based on δ¹³C signatures. Finally, we used a wet sieving procedure to assess the proportion of C-MAOM at each soil depth. Results revealed that SOC stocks were 27% greater in balsam fir than in sugar maple forests. Most of this difference was attributable to the thicker forest floors under balsam fir, in accordance with slower litter decomposition rates. CO₂ emission rates in the first 10 weeks of incubation were higher in soil samples collected under sugar maple; thereafter, CO₂ emission rates were higher in soil samples collected under balsam fir.  As a result, the bioreactive SOC pool over the course of 51 weeks did not differ significantly between stand types. We found significant region × stand type interactions on both indices of microbial turnover as well as on the proportion of C-MAOM in the mineral soil. More specifically, only in one region was microbial turnover higher under sugar maple than under balsam fir. Likewise, the effect of stand type on the proportion of C-MAOM was significant in only one region, and this effect was contrary to expectations (i.e. balsam fir > sugar maple). We ascribe this unexpected result to the presence of earthworms, which we only found in sugar maple stands in this region. Although we did not find generalizable effects of stand type on SOC turnover and stability, we did find significant generalizable patterns of decreasing SOC bioreactivity, increasing microbial turnover and increasing C-MAOM with increasing soil depth. Taken collectively, our results suggest that a northward shift of sugar maple will cause a net release of CO₂ to the atmosphere and potentially create a positive feedback on global warming.

How to cite: Boilard, G., Bradley, R., and Houle, D.: Evidence that a northward range shift of sugar maple (Acer saccharum Marsh.) causes a net release of CO2 from soil, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1196, https://doi.org/10.5194/egusphere-egu22-1196, 2022.

Effects of the construction of the lower Yarlung Tsangpo River tunnel project on the stability of organic carbon in forest soils

Abstract: Tunnels are widely used in road construction in areas such as the highlands and mountains, however, their effect on soil organic carbon stability has been less studied. Soil organic carbon stability is a sensitive index to evaluate the response of soil ecosystem to environmental changes. In order to detect the soil organic carbon (SOC) the anti-interference ability of the engineering construction of the tunnel, the stability of soil organic carbon was analyzed by using labile soil organic carbon（LOC）, soil aggregates and enzyme activities. Based on the construction of the lower Yarlung Tsangpo River tunnel, fixed monitoring plots were set up in the Engineering disturbance areas (ED) and undisturbed areas (CK) as a control to investigate the influence of tunnel construction on SOC stability . Results showed that the SOC and LOC in the ED were 291.40 mg/kg and 110.28 mg/kg, respectively, which were slightly higher than those in the CK area 255.31 mg/kg and 91.19 mg/kg, but the difference was not significant (p=0.6). The proportion of >0.25 mm aggregates in all soil fractions was more than 80%. With the decrease of aggregate size, the content of organic carbon in aggregate showed a decreasing trend, but there was no significant difference between ED and CK areas. This study showed that tunnel construction has no significant effect on soil organic carbon stability, which may be associated to the abundant precipitation in the study area. Because vegetation mainly absorbed soil water in top layer and the input and output of soil organic matter were not affected. The results of the study provide a reference basis for the evaluation of the impact of tunnel construction on the environment and for the management of the forest ecosystem in the lower Yarlung Tsangpo River.

Keywords: Tunneling; Forest soils; Organic carbon stability; labile organic carbon; Soil enzyme; soil aggregates

How to cite: Zhao, R. and Tang, X.: Effects of the construction of the lower Yarlung Tsangpo River tunnel project on the stability of organic carbon in forest soils, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1615, https://doi.org/10.5194/egusphere-egu22-1615, 2022.

Soil organic carbon (SOC) saturation is generally defined as soil’s unique limit to stabilize C, which is dependent on soil physio-chemical characteristics. It is claimed that if once the mineral fraction is saturated, no additional carbon input leads to mineral-associated organic carbon (MAOC) accrual. However, the capacity of a specific soil to store MAOC may depend on many factors, and experimental evidence of actual saturation is scarce. Earlier defined saturation points appear to be statistically biased since especially agricultural soils with high SOC contents are rare so that the strong linear relationship between MAOC and total SOC becomes less clear at higher SOC contents.

To assess, whether the amount of MAOC saturates at a certain point, we used the opportunity of a comprehensive soil archive. From a total of 3104 topsoil samples collected during the German Agricultural Soil Inventory, we took a systematic random sample (n=190) with a wide range in SOC (0.54 - 11.7 %) and clay content (3-77%). We thereby ensured, that an equal number of samples were selected in each SOC content class, which were defined in steps of 1% SOC to ensure an unbiased evaluation of a potential saturation point.  Those soils are being fractionated into Particulate Organic Carbon (POC) and MAOC. Firstly, soils are subjected to ultrasonic dispersion followed by particle size separation via wet sieving (50 µm). SOC and total nitrogen will be determined in each sample. The first results of this study will be presented, which will shed more light on an important aspect of SOC dynamics.

How to cite: Neha, N. and Poeplau, C.: Fractionating German agricultural soils to assess if the mineral associated carbon fraction saturates , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2775, https://doi.org/10.5194/egusphere-egu22-2775, 2022.

Tropical soils are increasingly subjected to both site conversion and intensification of agricultural practices, leading to cultivation-induced losses of soil organic matter (SOM) and associated nutrients. Hence, robust techniques for the qualitative characterisation of SOM in heavily weathered tropical soils are required. While thermogravimetric methods are widely used for the characterisation of temperate soils, thermal degradation features of pedogenic oxides typical for many tropical soils can confound the analyses, particularly in thermolabile SOM fractions. We used thermogravimetry coupled to differential scanning calorimetry and mass spectrometry (DSC-TGA-MS) to discern mineral and organic thermal degradation patterns in a kaolinitic soil from Cameroon receiving different mineral and organic amendments. We quantified endothermic mineral degradation features overlapping with OM combustion and thus corrected the exothermic OM degradation signal for pedogenic oxide dehydroxylation. The addition of thermostable biochar interfered with the identification and quantification of clay mineral dehydroxylation features. Between three and four thermal OM fractions of different energy density were identified, among which a distinct cellulosic fraction marked the continuous C4 vegetation on the site. The addition of compost led to a reduction of the thermolabile fraction, while the absence of organic input resulted in a reduction of the thermostable fraction. We conclude (I) that the addition of nutrient-rich fresh OM (compost) may lead to faster OM turnover as indicated by a reduction of the thermolabile OM fraction, and (II) while DSC-TGA-MS is generally suitable for OM characterisation in tropical soils, the presence of pyrogenic C represents a challenge if clay dehydroxylation is to be determined simultaneously.

How to cite: Schnee, L., Kaufhold, S., Ngakou, A., and Filser, J.: Thermogravimetric-calorimetric characterisation of organic matter in oxide-rich tropical soils, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2916, https://doi.org/10.5194/egusphere-egu22-2916, 2022.

The aim of this work is to investigate the mechanisms of soil organic carbon (SOC) sequestration as a function of time and depth. A chronosequence, consisting of two orders (T2 and T1) of the Adige river terraces (Veneto region, North of Italy) and 3 sites (Q2, Q3, and Q4), has been investigated. The highest and oldest terrace (T1) is located in Montalto di Gaium, 125 m above the current Adige riverbed level. This terrace was probably formed during the last interglacial (ca. 125,000 years BP) and was characterized by Paleudalf soils. Conversely, T2 represents the youngest order of terraces (probably formed during the early Holocene) and is situated 15 m above the current riverbed level. The Q2 site was located in T1 whereas Q3 and Q4 in T2; all sites have a common vegetation. From each site, soil samples have been collected (1 profile and 2 cores per site) by soil horizon, and each horizon sub-sampled by depth (each 5 cm). Five-cm thick sub-samples have been characterized for pH, electrical conductivity, total organic C (C_org), total N (N_TOT), texture, and micro and macro nutrients. Particulate organic matter (POM) and mineral-associated organic matter (MAOM) have been isolated using a physical fractionation method and characterized by elemental (CHNS) and thermal analysis (TGA-DSC).

The average C_org content in the topsoil (20 cm) is quite constant in the three sites (27.4 mg/g), whereas the average N_TOT concentration ranges between 2.7 and 3.1 mg/g. In all sites, the C_org concentration along the profile is positively correlated with N_TOT (p<0.001); moreover, a positive and significative correlation between C_org and clay (p<0.001) was observed exclusively in Q2, while in all sites Ca, instead of Al or Fe, seems to play a major role in C_org sequestration. SOC stock in topsoil is 47% higher in Q2 (T1) (72±3 MgC/ha) than in Q4 (T2) (49±5 MgC/ha), but such a difference decreases at 35 cm (96±2 and 76±7 MgC/ha, respectively). Furthermore, in the site showing the deepest soil profile (Q3), the SOC accumulated between 35 and 80 cm (42 MgC/ha) represents the 33% of the total. The average content of the MAOM pool is constant along the T2 (Q3 and Q4) profiles (52%), while increases with depth in T1 (up to 62% in deeper layers).

Thermal indices (e.g., WL_{400-550/200-300}, TG-T₅₀, DSC-T₅₀) suggest that the stability of bulk SOM generally increases with depth in the three sites. Moreover, a general increase in the thermal stability of both MAOM and POM is observed with depth in all sites, with Q2 (i.e., the site in the oldest terrace) showing a larger increase of MAOM thermal stability in deeper soil compared to Q3 and Q4 (located on the youngest terrace).

While most of the studies on SOC sequestration and stabilization focuses on topsoils, our preliminary data show that a significant stock of more recalcitrant organic C accumulates in deeper soils. Future data will help to better understand the effect of time on SOC distribution among different pools and as a function of depth.

How to cite: Galluzzi, G., Plaza, C., Priori, S., Giannetta, B., and Zaccone, C.: Soil organic carbon sequestration and dynamics along a chronosequence on fluvial terraces, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3027, https://doi.org/10.5194/egusphere-egu22-3027, 2022.

In order to understand the coupling of land ecosystem carbon (C), nitrogen (N), and phosphorous (P) cycles, it is necessary to understand microbial element use efficiencies (C, N and P) of soil organic matter (SOM) decomposition. While important controls of those efficiencies by microbial community adaptations have been shown at the scale of a soil pore, an abstract simplified representation of community adaptations is needed at the ecosystem scale. The conceptual soil enzyme allocation model (SEAM) explicitly models community adaptation strategies of resource allocation to extracellular enzymes and enzyme limitations on SOM decomposition. It thus provides a scaling from representing several microbial functional groups to a single holistic microbial community. The model has been further abstracted using quasi-steady-state assumption for extracellular enzyme pools to the SESAM model. While initially, P optimality considerations have been treated analogue to N, we found with simulating a sequence of sites with a P availability gradient that model extensions were required for P. Here we discuss effects of explicitly considering two assumptions on SOM dynamics: (1) oxidative enzymes can acquire P from SOM without necessary stoichiometric decomposition of C and N, and (2) for the case where P is limiting, in addition to P cost, also the C and N cost of enzyme production are important for optimality. We found that neglecting these two assumptions did not significantly change system behavior and predictions in the case where P was not limiting soil microbes. However, it changed model predictions of ecosystem-scale SOM dynamics for the case where P started to become limiting.

This modeling study links knowledge of constraints at soil microbial scale to SOM dynamics at ecosystem scale. It highlights the important role of adaptability of soil microbial communities to resource supply and stoichiometry for the development of SOM stocks and nutrient availability.

How to cite: Wutzler, T., Yu, L., Zaehle, S., Schrumpf, M., Ahrens, B., and Reichstein, M.: Upscaling microbial stoichiometric adaptability in SOM turnover using the SESAM model: specifics of phosphorous dynamics., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3399, https://doi.org/10.5194/egusphere-egu22-3399, 2022.

Ecological stoichiometry theory plays a crucial role in linking ecosystem process. The interactions between organisms and/or between organisms and abiotic environment are strongly affected by the biological demand for elements and their supply by environment. Therefore, the complex feedback between the elemental stoichiometry of organism and their resources maintains the functioning of ecosystem. However, whether and how the multiple responses of soil micro-food webs to stoichiometric imbalance affect the soil multifunctionality have never been assessed so far. In this study, we tested the soil multifunctionality along the forest-steppe ecotone and assessed several potential adaptation mechanisms of soil micro-food web responding to elemental limitations including soil microbial stoichiometry, extracellular enzyme activities and the composition of soil communities as well as ecological network. The soil multifunctionality gradually decreased from forest towards steppe. The microorganisms invested more C-acquiring enzymes over nutrient-acquiring enzymes with decreasing soil C:N:P ratios, while the increasing C limitation in steppe soil enhanced microbial threshold element ratio and carbon-use efficiency. The changes in extracellular enzyme activity and community structure of soil micro-food web had a stronger impact on soil multifunctionality. The multiple adaptive pathways of soil micro-food web to the stoichiometric imbalance of resources, jointly affected the multifunctionality of soil. Our study provides deeper insights into how stoichiometric constraints may induce shifts in soil micro-food web and then influence the ecosystem functioning. Our findings have important implications for integrating shifts in individual physiological metabolism as well as changes in community composition of soil biota and for better understanding the relations of soil biodiversity and soil multifunctionality in terrestrial ecosystems.

How to cite: Li, B., Li, Y., Han, X., Li, Y., and Li, Q.: Soil micro-food web adaptations to stoichiometric imbalance regulate soil multifunctionality, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3506, https://doi.org/10.5194/egusphere-egu22-3506, 2022.

Mechanistic simulation models are an essential tool for predicting soil functions such as nutrient cycling, water filtering and storage, productivity and carbon storage as well as the complex interactions between these functions. Most soil functions are driven or affected by soil organisms. Yet, biological processes are often neglected in soil function models or implicitly described by rate parameters. This can be explained by the high complexity of the soil ecosystem with its dynamic and heterogeneous environment, and by the range of temporal and spatial scales these processes are taking place at.

Here, we meet these challenges and present our approach for integrating biological microscale processes into soil modelling. We introduce our systemic soil model BODIUM which simulates relevant microbial processes at the profile scale to analyse the impact of different agricultural management and climatic conditions on soil functions. For this, BODIUM includes different microbial pools as well as C:N stoichiometric considerations but does not explicitly account for microbial community structure or composition dynamics. In our approach, soil structure dynamics at the pore scale is a critical factor for controlling the availability of carbon and nutrients as well as the distribution of water and oxygen, which in turn jointly drive microbial growth and activity. To explore the potential advantage of BODIUM, we compare our model approach with traditional modelling approaches without explicit microbial activity under different simulation scenarios. We further analyse the impact of changing microclimatic conditions of water, oxygen and nutrient availability as dedicated by a dynamic soil structure on microbial activity and the corresponding soil functions.

Finally, we discuss ongoing developments to additionally consider, e.g., microbe-fauna-interactions, microbial feedback on soil structure dynamics, and phosphor dynamics.

How to cite: König, S., Reitz, T., Weller, U., and Vogel, H.-J.: Accounting for microbial dynamics to simulate soil functions under agricultural management, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4920, https://doi.org/10.5194/egusphere-egu22-4920, 2022.

Soil mineral characteristics have been shown to play a dominant role in stabilizing soil organic matter over medium to long term timescales. However, while great strides have been made (Kleber et al, 2021) toward understanding organic matter stabilization processes, there remain uncertainties about the chemistry, time scales, and age of carbon that is stored on soil minerals. We applied modern thermal analysis methods to investigate soil mineral effects on the thermal stability, chemical composition, and age distribution of soil organic matter. We selected subsoil mineral fractions that contained a single dominant stabilizing pathway (e.g. 2:1 clays, iron oxides, short-range order minerals, crystalline minerals) to isolate effects of individual minerals. We paired thermal fractionation with pyrolysis-GC/MS to describe the relationships of SOM age and chemical composition. Early results show that while certain minerals display heterogeneous thermal stabilities, single mineralogies contain generally narrow age ranges. In addition, organic matter chemistry associated with diverse minerals varies widely and indicates that certain minerals provide higher stability to complex, energy-rich molecules. Associated with this work, we also present novel continuous SOM radiocarbon distributions from thermal fractionation.

How to cite: Stoner, S., Sierra, C., Doetterl, S., Schrumpf, M., Hoyt, A., and Trumbore, S.: How diverse minerals affect soil organic matter age distribution and chemical composition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5461, https://doi.org/10.5194/egusphere-egu22-5461, 2022.

Soil organic matter (SOM) is widely recognized as of critical importance for both soil quality and climatic mitigation. The quality and quantity of SOM are key to assess the characteristics of soils, and thus must be accurately monitored in order to protect the integrity of soils. In the last few years, a thermal analysis technique called Rock-Eval® that provides insights on bulk SOM chemistry and thermal stability has been recognized as a powerful method for SOM characterization. It can moreover be applied on large sets of samples.

The RMQS is the French monitoring network of soil quality. The first sampling campaign took place from 2000 to 2015 and resulted – among others – in about 2200 composite surface (0-30 cm) samples taken all over France. It represents an unprecedented collection of precise and complete data on French soils.

We observed significant effects of land cover on both SOM thermal stability and bulk chemistry. The mean values of hydrogen index (HI, which is a proxy for SOM H/C ratio) for arable lands (190.5 ± 43.4 mg HC per g of SOC, n=786) was lower than for grassland soils (228.4 ± 46.3 mg HC per g of SOC, n=486) and forest soils (240.4 ± 66.4 mg HC per g of SOC, n=528). Regarding the oxygen index (OIre6, which is a proxy of SOM O/C ratio), we observed significantly different values (P<0.001) for arable land soils (188.8 ± 30.4 mg O₂ per g of SOC), grassland soils (172.4 ± 26.8 mg O₂ per g of SOC) and forest soils (164.2 ± 29.6 mg O₂ per g of SOC). We also observed that thermal stability of SOM was significantly higher in cropland soils compared to grassland and forest soils. Our data suggest that topsoil SOM is on average more oxidized and biogeochemically stable in croplands. Further analyses will investigate the influence of pedo-climatic conditions on SOM characteristics.

The high number and even repartition of data on the French territory allow for the constitution of a national interpretative referential for these indicators. The Rock-Eval® parameters will also be used to calculate the centennially stable SOC fraction using the PARTYsocv2.0 model and map it at the scale of France.

How to cite: Delahaie, A., Barré, P., Baudin, F., Arrouays, D., Bispo, A., Boulonne, L., Chenu, C., Jolivet, C., Martin, M., Saby, N., Savignac, F., and Cécillon, L.: Rock-Eval®-RMQS: Monitoring the Characteristics of SOM on the French Territory with Rock-Eval® 6 Thermal Analysis to Assess its Stability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5871, https://doi.org/10.5194/egusphere-egu22-5871, 2022.

Humin fraction of soil organic matter is assigned to play an important role in carbon sequestration and sorption of xenobiotics. This study concerns luminescence properties (fluorescence and delayed luminescence) of humin fraction isolated from mollic horizons of eight Chernozems and Phaeozems, used as arable soils in various regions of Poland. Isolation procedure was described by Weber et al. (2022). Investigated soils differed in the content of TOC, ranging from 13.3 to 41.7 g kg⁻¹, as well as texture from loam (Magnice, Pyrzyce) through silt loam (Trzebnik, Ciepłowody, Hrubieszów) and sandy clay loam (Psary) till clay (Ziemnice, Kętrzyn). They also differed in their pH values (from 5.64 to 7.71), and CEC (from 21.6 to  53.2 cmol(+)kg^-1). Ash content of humin varied between 22.89%  - 54.50%, which is typical for humin originated from mineral soils (Stevenson 1994).

References:

Stevenson FJ. 1994. Humus chemistry: Genesis, composition, and reactions. New York: John Wiley and Sons, p 512.

Weber J., Jamroz E., Kocowicz A., Dębicka M., Bekier J., Ćwieląg-Piasecka I., Ukalska-Jaruga A., Mielnik L., Bejger R., Jerzykiewicz M. (2022). Optimized isolation method of humin fraction  from mineral soil material. Environmental Geochemistry and Health, 1-10 https://doi.org/10.1007/s10653-021-01037-3 

Acknowledgements

This work was supported by the National Science Center (NCN) Poland (project No 2018/31/B/ST10/00677 “Chemical and spectroscopic properties of soil humin fraction in relation to their mutual interaction with pesticides")

How to cite: Weber, J., Mielnik, L., Bejger, R., Ukalska-Jaruga, A., Jamroz, E., Jerzykiewicz, M., Ćwieląg-Piasecka, I., Kocowicz, A., Dębicka, M., and Bekier, J.: Luminescence properties of the humin fraction isolated from Chernozems and Phaeozems from various regions of Poland , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6144, https://doi.org/10.5194/egusphere-egu22-6144, 2022.

Soil organic matter (SOM) plays vital importance for biological, chemical, and physical soil improvement and productivity. Organic matter composition also depends on different types of vegetation covers. Thus the study was aimed to estimate and characterize the soil organic matter (SOM) under different land uses (cropland, grassland, and forestland) and depths (0-10 cm, 10-20 cm, and 20-30 cm) in Prague Suchdol. Ninety samples of disturbed soil were collected within different land uses and within different depths. The soil organic matter (SOM) composition was assessed by diffuse reflectance infrared fourier transform spectroscopy (DRIFT). Humic and fulvic acid were extracted from soils and their composition was also assessed by DRIFT. Additionally, dissolved organic carbon (DOC), humus quality as ratio between absorbances of soil extract with sodium pyrophosphate at the wavelengths 400 and 600 nm, soil organic carbon (by the rapid dichromate oxidation technique), were determined on the samples as well as physical characteristics as bulk density. The data were analysed statistically by statistical package for the social sciences (SPSS) version 20. The results indicated that pH_KCl is significantly different among land uses. Cropland had the highest values of pH_KCl with a range from 7.76 to 6.86, followed by grassland with a range from 5.72 to 5.93 and forestland with 3.34 to 3.65, respectively. However, the humus quality was significantly different for all depths where forestland had the lowest humus quality compared to grassland and cropland, respectively. The soil organic carbon deviates statistically in depth 0-10 cm and 20-30 cm, while the depth in between from 10 to 20 cm showed no substantial difference among the land uses. Nonetheless, the result revealed that the largest differences of the spectra in the composition of organic matter were observed in the upper parts of the soil profile. The forest soil spectra had more intense aliphatic bands, carboxylic, and CH bands than spectra of grassland and cropland soils. The difference of HAs spectra was at 3 010 to 2 800 cm^-1 where the most intensive aliphatic bands were in forest soil HAs, followed by grassland and cropland soil HAs. The grassland topsoil FAs spectrum differs most from the other land uses. It has lower peaks around 1 660–1 600 cm^-1 and 1 200 cm^-1 than cropland and forest. The concentration of low molecular mass organic acid (LMMOA) was the highest in the forest soil and the most abundant acid was citrate.

How to cite: Thai, S., Pavlů, L., and Tejnecký, V.: Comparison of soil organic matter composition under differentland uses by DRIFT spectroscopy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6222, https://doi.org/10.5194/egusphere-egu22-6222, 2022.

Soil Organic Matter (SOM) is composed of a complex and heterogeneous mixture of organic compounds. It is of great importance to understand its molecular structure, the conformations and water accessibility, as well as the interfaces and reactivity of SOM with its surrounding. SOM extracts permitted for decades a systematic way of studying SOM via the use of standardized samples.  We used such standardized samples of the International Humic Substances Society (IHSS) to computationally explore the properties of SOM.

We used the Vienna Soil Organic Matter Modeler 2 (VSOMM2; Escalona et al. (2021); https://somm.boku.ac.at/) to produce representative, condensed-phase, atomistic models of IHSS samples. This online tool ensures greater chemical diversity of the models and reproduces the carbon distribution or organic composition estimated by NMR. Generated atomistic models were subjected to molecular dynamics simulations. We characterized these systems in order to observe differences in their structure and dynamics.

Our results indicate the importance of carboxyl and aromatic groups in the molecular interactions, specifically for their interactions with cations and indirectly for their aggregation properties. We also investigated the sorption properties of these systems by calculating the free energy of absorption of inserting a water molecule to the system, which values were affected by the water content, compaction and phases of the organic matter.

These investigations help improve our understanding of properties and behavior of soil organic matter at a molecular level that is not attainable to experiments. We hope that such studies will have a great impact on basic research involving SOM.

How to cite: Petrov, D., Escalona, Y., Galicia, E., Tunega, D., Gerzabek, M., and Oostenbrink, C.: Exploring macroscopic properties of soil organic matter using modeling and molecular simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7504, https://doi.org/10.5194/egusphere-egu22-7504, 2022.

The concept of distinct soil organic matter (SOM) fractions – with differing formation pathways, stabilization mechanisms and responses to change – is a promising avenue to improve our understanding of soil carbon (C) dynamics. While there is widespread consensus on the general usefulness of conceptual fractions with specific functional implications, there is still a lack of information on the patterns with which they contribute to bulk soil organic carbon (SOC) stock at larger scales and across climatic and soil physicochemical gradients. In this study, we aimed to assess first the quantitative importance of three key SOM fractions across a diverse range of 12 soil groups with global significance. Secondly, we wanted to gain insights on the environmental drivers that shape the contribution of these fractions to SOC stocks.

Here we sampled a set of 35 grassland topsoils (0 – 10 cm) along a 3000 km north-south transect in Chile ranging from subpolar to Mediterranean climate, and covering 12 WRB major soil groups. Following a modified version of the protocol in Zimmermann et. al (2007), we partitioned the soils into three functional SOM fractions defined by particle size and density (free silt and clay, free particulate organic matter, stable microaggregates), enabling us to quantify SOC stocks and the relative contribution to SOC in these three fractions. In order to identify links between fractions and potential drivers of C stabilization, we further characterized extensively relevant physico-chemical properties of the soils, compiled climatic data of the sites and characterized OM maturity (DRIFT spectroscopy and Rock-Eval pyrolysis) as well as pedogenic, secondary Fe-, Al- and Mn-oxide concentrations through sequential extraction.

We found that the contributions of mineral-associated SOM fractions to bulk SOC varied strongly across the soil gradient, while the contribution of free particulate organic matter was comparatively stable and low. SOM associated with free silt and clay sized particles are the most important C reservoir in soils with less than 4 % SOC, whereas in soils with higher SOC content, the majority of the SOC is contained in stable microaggregates. The SOC stock in various fractions was sensitive to changes in temperature, pedogenic oxides, and OM input vs. decomposition. Comparison of OM maturity showed that free particulate OM and free silt and clay associated OM can be clearly distinguished, while OM in microaggregates is likely a mixture of both. However, drivers of OM composition in microaggregates could not be identified.

This study demonstrates that in SOC-rich soils, microaggregates represent a major fraction of bulk SOC, and that SOC stocks in key SOM fractions can be linked to distinct climatic and soil physicochemical factors.

How to cite: Wasner, D., Abramoff, R., Zagal, E., Griepentrog, M., and Dötterl, S.: Drivers of the distribution of soil organic matter fractions along a geo-climatic gradient, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9088, https://doi.org/10.5194/egusphere-egu22-9088, 2022.

Although most organic matter (OM) in soil is mineralized by microorganisms, the nonmicrobial processes, e.g., Fenton reactions and photo-degradation, strongly contribute to OM decomposition and CO₂ emission and are the chemical background of many biotic transformations. Fenton oxidation is a catalytic reaction chain of hydrogen peroxide (H₂O₂) with ferrous iron (Fe(II)) and Fe (oxyhydr)oxides that generates highly reactive hydroxyl radicals (HO^•) oxidizing OM to CO₂. Reactive Fe (oxyhydr)oxides store at least one quarter (~600 Gt) of organic C in soil, which may be subjected to Fenton reactions in which nano-sized Fe (oxyhydr)oxides act as nanocatalysts. The Fenton mechanisms depend on the sources of reactive oxygen species (ROS): O₂^•−, H₂O₂ and HO^•. Because microorganisms continuously produce ROS, biotic Fenton chemistry is ubiquitous in all soils, especially with strong UV radiation, fluctuating O₂ concentrations and redox, microbial hotspots such as rhizosphere and detritusphere, and high contents of Fe (oxyhydr)oxides. Charcoal and biochar catalyze ROS formation in soil as an electron shuttle or by electron transfer from the valence to the conduction band under UV irradiation. Despite the extremely short lifetime (from nanoseconds to a few days), ROS are continuously produced and sustain the ubiquity of chelators and Fe(III) reduction. For the first time we calculated the fundamental Eh-pH diagrams for ROS species and showed their relevance for Fenton reactions under soil conditions. HO^• as one of the most powerful oxidants (E^o = 2.8 V) provides the most energy release from Fenton reactions in soil. In some ecosystems (hot deserts; red soils in the tropics and wet subtropics) Fenton reactions contribute to OM oxidation to 30% and even exceed 50% of total CO₂ emissions. Fenton reactions are omnipresent and play a dual role for soil C cycling: stimulate OM mineralization (including the most stable pools) and facilitate long-term C stabilization due to the increased recalcitrance of remaining OM and organo-mineral complex formation. Summarizing, Fenton reactions and their effects on OM decomposition and formation are an emerging research field that explains the chemical background of many oxidative enzymatic processes, may crucially change our views on C, energy and nutrient cycling in soils.

How to cite: Kuzyakov, Y. and Yu, G.-H.: Reactive oxygen species in soil: Abiotic mechanisms of biotic processes and consequences for organic matter and nutrient cycling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11390, https://doi.org/10.5194/egusphere-egu22-11390, 2022.

Terrestrial ecosystems are continuously exposed to dry periods and rainfall events. These cycles of drying-rewetting cause strong variations in biochemical processes that alter the balance of soil carbon (C), affecting both its inputs and losses. The rewetting of dry soils results in large pulses of C dioxide to the atmosphere that can constitute a major fraction of the annual emissions in some ecosystems and, at the same time, promotes the sequestration of C into growing microorganisms. After rewetting, microbial growth and respiration can follow decoupled patterns depending on the intensity of the rewetting and the physiological status of the microbes—in turn, this decoupling can lead to contrasting fates of C between emission and stabilization into soil organic matter. Moreover, these patterns can be classified as either ‘resilient’ or ‘sensitive’, depending on the way C is used over time. Despite the significance of these dynamics for the C budget, the mechanisms controlling them are still not well understood.

To shed some light on this challenging problem, we simulated the soil-microbial response to drying-rewetting by using the process-based model EcoSMMARTS. The results indicated that the history of soil moisture affected the responses to rewetting by promoting microbial groups with specific survival strategies. The soils regularly exposed to ‘severe’ conditions (e.g., shallow horizons in semi-arid or Mediterranean ecosystems) exhibited resilient responses, whereas sensitive responses were obtained in soils from ‘milder’ environments (e.g., humid climates and deep horizons). The resilient responses were obtained when soil microbial communities could cope well with water-stress and could started synthesizing new biomass right after rewetting, which also triggered large respiration peaks induced via osmoregulation. In contrast, sensitive responses were found in communities that could not withstand the effects of drying-rewetting, which led to a delay in microbial growth and sustained C mineralization by cell residues. The disruption of soil aggregates during drying-rewetting was also identified as the major contributor of the C sources fuelling the rewetting responses. By allowing us to attribute rewetting responses to individual processes (physiological, physical, or ecological), these model results improve our understanding of the mechanisms that govern the emission and stabilization of C in soils during drying-rewetting.

How to cite: Brangarí, A. C., Manzoni, S., and Rousk, J.: Mechanisms controlling the emission and stabilization of carbon during soil drying-rewetting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11799, https://doi.org/10.5194/egusphere-egu22-11799, 2022.

The storage of organic carbon in soils is regulated by different physicochemical mechanisms. The physical fractionation of soil organic matter (OM) into particulate and mineral-associated pools has advanced our knowledge based on these operationally-defined different storage forms of organic carbon in soils. Attempts to integrate a thermodynamic perspective to decipher mechanisms of soil organic carbon storage require the integration of our understanding of different physicochemical mechanisms. Here we analyzed the energy contents related to different pools of OM using differential scanning calorimetry. Various pools of OM were isolated by combined density and size fractionation differentiating free particulate OM (fPOM), occluded particulate OM larger than 20 µm (oPOM_>20 µm), occluded particulate OM smaller than 20 µm (oPOM_<20 µm) and mineral-associated OM smaller than 20 µm (MOM_<20 µm). We compared cropland and grassland sites from long-term monitoring sites across Bavaria (Germany). Our aim was to relate the novel energy analyses with characterizations of the carbon storage and OM composition. In the cropland sites we found an energy gradient with increasing energy content (J/mg C) across fPOM, oPOM_>20 µm, oPOM_<20 µm, MOM_<20 µm. The increasing energy gradient was independent of different carbon contents and correlated with decreasing C:N ratios. These results indicate an important role of energy accumulation through association of OM with minerals along the gradual decomposition of different OM pools. A relationship of the energy content with the alkyl/O-alkyl ratio serving as a proxy of decomposition based on ¹³C NMR spectroscopy was also related with the energy content but to a lower extent. In the grassland sites, the energy content gradient of different OM pools was in a similar range and the C/N ratios of the POM fractions were also lower compared to the cropland sites. By comparing energy stored in OM pools from cropland and grassland sites, we will discuss potential implications of energy analyses for our understanding of soil organic carbon storage.

How to cite: Andre, N., Schugmann, M., Kühnel, A., Wiesmeier, M., and Schweizer, S.: Energy content of soil organic matter pools under cropland and grassland sites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12247, https://doi.org/10.5194/egusphere-egu22-12247, 2022.

Soil organic carbon (SOC) is known to play a crucial role in soil quality. The general approach to enhance SOC is to minimise soil disturbance and ensure fresh C-inputs to the soil. However, current sustainable land management practices do not always result in an increase in SOC and are not precise enough to prescribe C-inputs to achieve a target soil C stock and management of soil quality. Recently, CNPS stoichiometry has been shown to limit the stabilised SOC pool. The aim of this study was to test CNPS stoichiometry to increase organic matter (OM) mineralization and examine the effect on soil properties following straw incorporated with supplementary nutrients in a soil incubation experiment. The objectives were to (i) quantify the dynamic change in SOC and particulate organic matter (POM) in response to straw incorporation with and without supplementary nutrients based on CNPS stoichiometry and (ii) determine if the limits of detection for visible near-infrared spectroscopy (vis-NIR) can capture short-term change in SOC and POM. Five soils (40g) varying in clay content were incubated for 12 weeks at 25℃ and 70 % field capacity. Soils received straw at a rate of 8 t/ha with and without supplementary nutrients (N, P and S) based on stoichiometric inputs. Vis-NIR measurements were collected for the soil samples post incubation with soil structure intact and and removed (sieved to <2 mm). Laboratory analysis of soil properties is underway. Preliminary exploratory analysis of the spectra was performed by Principal Component Analysis (PCA). Preliminary results of the PCA show that the first two principal components captured the soil variability (PC1 56.09%, PC2 36.0%) however no obvious treatment effect was observed. Further modelling work will investigate if the straw treatments with and without nutrient supplementation produced a measurable change in SOC and POM and if the dynamic change in soil carbon can be detected in the spectra using regression analysis.

How to cite: Panthi, S., Amin, N., and O'Rourke, S.: Assessment of short-term effect of CNPS stoichiometry on SOC and soil properties using Vis-NIR spectra, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12819, https://doi.org/10.5194/egusphere-egu22-12819, 2022.

Wildfires can promote changes in soil organic carbon pools (SOCp) mainly as consequence of the input of ashes and charred materials from the scorched vegetation; and/or the removal of litter layer and organic matter from the upper soil centimetres affected by high temperatures. Moreover, post-fire management practices can also cause changes in the different forms of organic carbon in the soil (from the most labile to the most recalcitrant).

In the REMAS project, a methodology to study the different SOCp is proposed to assess the effects of the application of different management post-fire practices over the burned areas: (1) cut and remove burned trunks, (2) shrub clearing letting the masticated debris on the soil carried out 6-8 years after the fires and, (3) no intervention treatment. The SOCp analysed include hot-water extractable C, particulate organic C, associated to the mineral fraction and total organic C. The study areas include diverse forest ecosystems from France (Pinus pinaster Ait.), Portugal (Quercus suber L.) and Spain (Pinus halepensis Mill.and Pinus sylvestris L.). Results show variable effects of the management practices on the different organic C pools, mainly over the most labile ones.

Acknowledgements: The REMAS project SOE3/P4/E0954 is co-financed by the Interreg Sudoe Program through the European Regional Development Fund (ERDF).

How to cite: Gimeno-García, E., Carbó, E., Ruiz-Peinado, R., López Senespleda, E., Jalabert, S., Chéry, P., Pétillon, T., Castro Rego, F., Marques Duarte, I., and Lerma-Arce, V.: Soil carbon pools in forested areas affected by fires after the application of restoration measures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13453, https://doi.org/10.5194/egusphere-egu22-13453, 2022.