Identifying global controls on soil carbon (C) storage, as well as where soil C is most vulnerable to loss, are essential to realizing the potential of soils to mitigate climate change via C sequestration. However, we currently lack a comprehensive understanding of the global drivers of soil C storage, especially with regards to particulate (POC) and mineral-associated organic carbon (MAOC). To better understand global controls on these two C fractions, we synthesized climate, and net primary production (NPP), and soils data from 73 published studies and databases. This large dataset is representative of multiple land cover types, including broadleaved and coniferous forests, grasslands, shrublands, wetlands, tundra, and wetlands. We then applied structural equation modeling (SEM) to assess hierarchical, interactive controls on global POC and MAOC pools (i.e., g POC or MAOC per kg soil) in topsoils. Our SEM tested relationships between NPP and climate (i.e., mean annual temperature (MAT) and effective moisture, assessed as mean annual precipitation minus potential evapotranspiration), as well as the extent to which climate and NPP, along with soil texture and pH, govern POC and MAOC storage. We found that NPP is positively related to MAT and effective moisture. Additionally, POC storage is negatively related to both MAT and pH, while MAOC storage is positively related to NPP and effective moisture, but negatively related to soil % sand. Given that temperature and pH impose constraints on microbial decomposition, these results indicate that POC storage is primarily controlled by C output limitations. In contrast, strong relationships with variables related to plant productivity constraints and to mineral surfaces available for sorption indicate that MAOC storage is primarily controlled by climate-driven C input limitations and C stabilization mechanisms. Together, we demonstrate that divergent controls govern C storage in POC and MAOC, and that these controls are consistent across multiple ecosystem types.

How to cite: Hansen, P., King, A., Lavallee, J., Schipanski, M., and Cotrufo, M. F.: Divergent controls on particulate and mineral-associated organic carbon formation and persistence, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10362, https://doi.org/10.5194/egusphere-egu23-10362, 2023.

A wide range of image-based techniques revealed mounting evidence of a heterogeneous arrangement of mineral-associated organic matter (OM) in soils at the microscale and nanoscale. Spectromicroscopic approaches using such as NanoSIMS, STXM-NEXAFS, AFM, STEM-EELS, and others have provided insights about a patchy and piled-up arrangement of OM. This arrangement is determined by different local OM properties and mineral composition as well as OM-OM interactions. The emerging conceptual framework of the microscale arrangement of OM affects our understanding of soil functions: By compartmentalizing and decoupling local carbon sequestration in the mineral soil matrix, by localizing the mechanical stabilization of soil structure, by altering surface properties and re-distributing ion exchange sites, and by shaping distinct biotic microenvironments. After an overview on the spectromicroscopic evidence, this contribution will illustrate the emerging conceptual framework of localized soil functions, and highlight opportunities for research approaches based on the patchy and piled-up arrangement of OM at the microscale and nanoscale.

How to cite: Schweizer, S.: Taking a closer look: How spectromicroscopic imaging of organo-mineral associations leads to a novel perspective on interrelated soil functions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11969, https://doi.org/10.5194/egusphere-egu23-11969, 2023.

Soil organic matter (SOM) is a key player in soil functioning and services in forest lands, which had been subject to accelerated land degradation particularly in karst terrain in Southwest China. So far, there had been poor knowledge of pool and molecular composition of SOM associated with soil aggregates across lithologic origins of karst soil. In this study, undisturbed topsoil (0-10 cm) samples were collected in forest lands on sandstone (SS), dolomite (DS) and limestone (LS) sedimentary rocks in a karst terrain from Guizhou, Southwest China. Changes in SOM pool distribution and molecular composition of water-stable aggregates were explored using size and density fractionation and GC/MS detection of extracted biomarkers. The OC content ranged from 41.05 g kg^-1 on SS to 50.94 g kg^-1 while the mean weight diameter of sand-free soil water-stable aggregates ranged from 420.9 μm on SS to 544.4 μm on DS, across the lithologic sequence. With biomarker molecular assay, the higher SOC storage was relevant to the higher abundance of plant-derived organics (lignin, cutin, suberin, wax and phytosterols) in macro- and micro- aggregates. Whereas, the higher OC in silt & clay fraction of topsoil on DS and LS could be explained by the higher abundance of microbial lipids plus cutin and suberin. Also, the higher ratio of (Ad/Al)v to (Ad/Al)s of silt-clay fraction pointed to a stronger degradation of lignin thereby. Thus, the forest soil of dolomite and limestone origin preserved a relatively high level of SOC storage in topsoil, mainly with accumulation of POC physically protected in macro- and micro- aggregates. Moreover, the high SOC of topsoil on dolomite could also be attributed to enrichment of SOC in the clay silt fraction, mainly with mineral association of microbially degraded OCs.

How to cite: Lin, Q., Chen, S., Feng, X., and Pan, G.: Pool distribution and molecular composition of organic matter among water-stable aggregate size fractions of karst topsoil across a lithologic sequence from Guizhou, Southwest China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3957, https://doi.org/10.5194/egusphere-egu23-3957, 2023.

This study aims to investigate the mechanisms of soil organic carbon (SOC) sequestration with depth as a function of time and climate. Two chronosequences located along a climate gradient were investigated. The first chronosequence (ADI) consisted of fluvial terraces, whereas the second (LED) of fluvio-glacial terraces. Four sites (Q2, Q3, Q4 and Q5) located in 3 terraces (T1, T2, and T3), with age ranging from about 125,000 to 2,000 yr, were investigated for ADI, while 3 sites (Q1, Q2, and Q3) in 3 terraces (T1, T2, and T3, respectively), with age range from about 16,000 to 10,000 yr, were selected for LED. All sites were grasslands. Soil samples were collected (1 profile and 2 cores per site) by horizon, and each horizon sub-sampled by depth (each 5 cm). The sub-samples were characterized for pH, EC, total organic C, total N, texture, mineralogy, total and extractable elements, and for soil respiration. Particulate organic matter (POM) and mineral-associated organic matter (MAOM) were isolated and characterized by elemental and thermal analyses.

In ADI, the oldest site (ADIQ2) stocks 2 times more C in the topsoil (15 cm) than the youngest site (ADIQ5) (60 and 27 MgC/ha, respectively). Furthermore, in ADIQ3, 38% of the total SOC accumulated between 30 and 80 cm (48 MgC/ha). In LED, the youngest site (LEDQ3) shows the highest SOC stock to both 15 and 30 cm (86 and 138 MgC/ha, respectively). In LEDQ1, 46% of the total SOC accumulated between 30 and 90 cm (94 MgC/ha). Among sites having same age but different climate, LEDQ3 (the wettest and coldest site) stocks ~2 times more carbon than ADIQ3 (the driest and warmest site) to the first 30 cm of depth.

In LED, the ratio between the organic C in MAOM/POM in the topsoil ranges between 0.6 and 1.8, while in ADI between 1.1 and 3.9. Thermal indices (e.g., WL_{400-550/200-300}, TG-T₅₀) show that the stability of bulk SOM and pools generally increased with depth in ADI sites, whereas remained constant in LED. ADI soils had similar cumulative respiration (RH_CUM), whereas LEDQ3 exhibited the highest RH_CUM along the first 30 cm. Indeed, LEDQ3 had a 3× higher RH_CUM than ADIQ3 in topsoil.

Our data show that significant amounts of organic C were accumulated in deeper soils (>30cm). Moreover, soil organic matter (SOM) stability, and especially that of MAOM, in ADI increased with depth. The relative contribution of POM to C storage was more important in LED than in ADI, especially in the topsoil. Overall, our data suggest that climate has a greater influence on the size of SOC stocks than age, which in turn exerts a major influence on the stability of SOM.

How to cite: Galluzzi, G., Plaza, C., Priori, S., Giannetta, B., and Zaccone, C.: Dynamics and stability of soil organic matter: climate vs. time, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12839, https://doi.org/10.5194/egusphere-egu23-12839, 2023.

Urbanization is a major land use change factor affecting soils. There is little understanding of how expansions of tropical West African cities have affected soil organic matter (SOM) composition and dynamics. In such cities, urban agriculture is common, playing an essential role in food security and urban sustainability. However, tropical soils tend to have low nutrient contents and cation exchange capacity. Thus, management strategies that enhance soil fertility and carbon (C) sequestration are needed. Developing such strategies requires a thorough understanding of how SOM dynamics alter in response to urban growth. Here, our objective was to assess how urbanization has affected the relatively stable mineral-associated-SOM (MAOM) and the labile particulate-SOM (POM) fractions in rainfed urban arable maize fields of Kumasi, a typical expanding city in Ghana (West Africa).

Using a grid-based satellite approach, and keeping other factors constant (including climate, topography, parent material and soil type), we took topsoil samples (0–10 cm) along an urban-intensity (UI) gradient, distinguishing: (i) low UI, located >400 m away from any primary road and having been under urbanization for <30 years, (ii) mid-low UI, located ≤400 m from a primary road and having been under urbanization for <30 years, (iii) mid-high UI, located >400 m from primary road and having been under urbanization for ≥30 years, (iv) high UI, located within ≤400 m from a primary road and having been under urbanization for ≥30 years. SOM fractions were isolated from the soils using a size separation approach, whereby the sand-sized fraction (0.063 - 2 mm) was regarded as POM and the clay- and silt-sized fraction (<0.063 mm) as MAOM. Prepared samples were ultimately analyzed for SOC using a Leco temperature ramp C analyzer, where a temperature threshold of 600 ºC was used to separate organic from inorganic C.

We found that mean SOC contents of the POM fraction increased markedly from 7.7 g kg^-1 in the low UI topsoils to 13 g kg^-1 in their high UI counterparts, suggesting an increase in POM with increasing urbanization. This trend was not observed for the MAOM that showed SOC contents of 4.5, 4.1, 4.9, and 4.1 g kg^-1 for the low, mid-low, mid-high, and high UI topsoils, respectively. Moreover, the share of SOC contents of POM in the bulk SOC increased from 51% in the low UI topsoils to 64% in the high UI topsoils, whereas that of MOAM decreased by 6% from 31% to 25%, respectively. These findings suggest that while there is evidence of strong anthropogenic contributions of SOM to urban arable soils, urbanization does not seem to promote SOC storage in the relatively stable MAOM fraction. Consequently, rainfed urban arable soils in Kumasi will need management interventions for keeping appropriate long-term SOM levels to maintain soil functions.       

How to cite: Asabere, S. B., Don, A., Peplau, T., and Sauer, D.: Soil organic matter stability decreases with increasing urbanization in highly weathered rainfed tropical arable soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7318, https://doi.org/10.5194/egusphere-egu23-7318, 2023.

The capability of coastal wetland soils to store large amounts of organic carbon (OC) has been increasingly recognised. Stabilisation mechanisms (e.g. aggregation or mineral association) and stability of organic matter (OM) (recalcitrant vs. labile) are important features for the long-term storage of soil organic carbon (SOC). In estuarine marshes, SOC storage is dominated by a complex and dynamic interaction of abiotic conditions such as tidal inundation or changes in salinity. However, little is known on OC stabilisation and stability in these transitional ecosystems and how they are affected by system-specific characteristics. Therefore, our aim was to assess the effect of flooding and salinity on (i) OC stabilisation by aggregation and mineral association and (ii) the stability of the OC pool in estuarine marsh soils.

We analysed topsoil (0 – 10 cm) and subsoil (10 – 30 cm) samples from 9 marsh zones along the salinity gradient (salt, brackish and freshwater) and flooding gradient (pioneer zone, low and high marsh) of the Elbe Estuary for their SOC contents, OC stabilisation mechanisms (density fractionation), OC stability (incubation with one- and two-compartment model fits) and dissolved organic carbon (DOC) concentrations.

Total SOC contents were highest in the freshwater marsh and decreased towards topsoils with higher salinity. Flooding frequency had no uniform effect on SOC contents: While there was a positive tendency with decreasing flooding frequency, subsoils of the freshwater marsh showed the opposite trend. Total SOC contents were positively correlated with mineral-associated OC (C_MAOM) and pedogenically unprotected particulate OM (C_fPOM). The highest proportion of C_MAOM was found in topsoils of freshwater marshes and it decreased towards higher salinities in topsoils of high marshes and pioneer zones. The OM protection by aggregation (C_oPOM) increased in topsoils of high marshes. The proportion of C_fPOM was less directly affected by salinity and flooding than by the CN ratio of the aboveground biomass (CN_litter). Furthermore, C_fPOM correlated positively with the potential mineralisable C (C_pot) and labile C (C_labile) and negatively with the recalcitrant C pool (C_recalcitrant) that were derived from the one- and two-compartment models. Labile C, C_pot and C_recalcitrant were also strongly influenced by CN_litter. Moreover, C_recalcitrant was linked to the proportion of C_MAOM. Concentrations of DOC increased with total SOC and C_pot but decreased with C_oPOM.

We conclude that SOC stabilisation in the Elbe Estuary is mainly related to mineral association of OM. With increasing terrestrial influence, physical protection in aggregates becomes more important. Besides these pedogenic stabilisation mechanisms, recalcitrance is strongly determined by vegetation characteristics.

How to cite: Neiske, F., Becker, J. N., Seedtke, M., Schwarze, D., and Eschenbach, A.: Organic carbon stabilisation mechanisms in estuarine marsh soils: Effect of salinity and flooding frequency, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8543, https://doi.org/10.5194/egusphere-egu23-8543, 2023.

Forest soils are recognized to be important organic carbon storage, but the role of surface and subsurface soil horizons on such function and its drivers are still field of debate. In this context, we examined the dynamics of soil organic carbon (SOC) for a chestnut forestry system in a temperate area of northern part of Apennine mountain range in Italy. Specifically, we questioned: what are the main i) SOC forms both in mineral surface and subsurface soil horizons? ii) factors affecting SOC stabilization?. Soil samples were collected by horizon and SOC was separated into organic C of the particulate organic matter (POM_C), sand–size aggregates (sand_C) and the mineral–associated organic C (MAOM_C). The easily oxidizable C (EOC), water–soluble organic C (WSOC), the microbial biomass–C and its respiration, and the total and easily extractable glomalin–related soil protein (T–GRSP and E–GRSP, respectively) were also estimated. Then, the E–GRSP–to–T–GRSP and E-GRSP–to–SOC ratios, the metabolic (qCO₂) and microbial (qMIC) quotients were calculated. The POM_C, sand_C and MAOM_C showed the highest concentrations in A horizon (26.5, 14.6 and 13.9 g kg^–1, respectively) highlighting the importance of the litter floor on the organic matter pools quantity in the topsoil. Further, the A horizon was enriched of the most labile organic C forms (i.e., EOC and WSOC) indicating the key role of A horizon for the development and growth of chestnut forest ecosystems. In fact, the labile organic C forms provide several ecosystem services, such as plant growth and yield. Unlike A horizon, the subsurface horizons preserved SOC mostly in the most stable form (63.8 %, on average). Because of the role of fungal biomass and its exudates to increase SOC capture and stabilization, the great potential of the subsurface horizons to store MAOM_C can be attributed both to the higher release of exogenous GRSP (higher E–GRSP–to–T–GRSP ratio) by mycorrhizal fungi and fungal mycelium expansion (higher E-GRSP–to–SOC ratio) within such horizons (0.504  and 0.061, respectively) compared to the A horizon (0.244 and 0.034, respectively). Therefore, the subsurface soil horizons seemed to have more favourable conditions for microorganisms compared to surface one as shown by the lower qCO₂ and the higher qMIC values found in the former than in the latter. Overall, the present investigation highlighted the importance of subsurface soil horizons of chestnut forests on C stabilization processes compared to the A horizon likely due to the better edaphic conditions for the microbial communities. Thus, our results pointed out the key role that the subsurface soil horizons of chestnut forests could have for mitigating the current climate change.

How to cite: De Feudis, M., Falsone, G., Trenti, W., Vianello, G., and Vittori Antisari, L.: The subsoil horizons are the preferential location for organic carbon stabilization in chestnut forests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2423, https://doi.org/10.5194/egusphere-egu23-2423, 2023.

The conversion of native vegetation into agricultural lands is often associated with a decrease in soil C. The soils from the Brazilian savannah (named Cerrado), with 200 million hectares, are rich in Fe and Al (hydr)oxides, which could result in more organo-mineral associations and lead to particularly high C storage. The changes in the C stocks from the conversion of native forest into degraded pasture (DP), and the adoption of proper management to recover DP and increase C stocks in such Ferralsols are not well understood. To provide insights into the drivers of C storage, this study compared the C stocks across depth in the top 1m and the distribution of C in the soil fractions 24 years after the adoption of different management systems in degraded pastures in the Brazilian Cerrado.

A DP area located in São Carlos, São Paulo, Brazil was converted into different management systems: (i) RMS: rainfed pasture with moderate animal stocking rate, (ii) RHS: rainfed pasture with higher animal stocking rate, and (iii) IHS:  irrigated pasture with higher stocking rate. As a control, the adjacent native vegetation (FO) was also evaluated. The adoption of management started in 1996 with RMS and in 2002 for RHS and IHS. Except for the DP, all areas were limed and N-fertilized. RMS with 200 kg N ha, RHS 400 kg N ha, and IHS with 600 kg N ha. Soil sampling was carried out in 2020 and the C stocks were evaluated up to 1 m deep. To state vegetation change from C3 (native forest) to C4 (introduced pasture) the isotopic natural abundance of ¹³C was analyzed. To evaluate the contribution of mineral-associated and particulate organic matter forms to C storage, we performed a physical fractionation by size and density with SPT 1.8 g cm^-3, respectively.

Our results showed that the conversion of FO into DP decreased soil C stocks.  Otherwise, the adoption of management in DP with RMS and RHS increased C stocks achieving levels similar to FO. RMS showed the highest C stocks with the lower dosage of N-fertilizer and animal stocking rate. IHS area did not increase their C stocks compared to DP, which may be related to limited root growth after irrigation decreasing the C input. Around 50% of the C stocks in RHS and RMS systems are pasture-derived (C4 plants) according to the ¹³C abundance. This shows that half of C stocks from rainfed pastures is of preserved organic matter from previous FO. While in the IHS and DP systems, the organic matter composition is mainly pasture-derived. Our preliminary data showed that the RMS topsoil contained more free particulate organic matter than the FO, suggesting that the C stocks were enhanced mainly by pasture-derived biomass input. The contribution of mineral-associated organic matter still will be evaluated.

Our study shows that the recovery of degraded pasture soils by management leads to increased OC stocks derived from fertilized pasture but also higher maintenance of OC from FO.

How to cite: Bento, L. R., Schweizer, S. A., Oliveira, P. P. A., Pezzopane, J. R. M., Bernardi, A. C. D. C., Kögel-Knabner, I., and Martin-Neto, L.: Managed pastures enhance soil carbon stocks from degraded pasture in Ferralsol of Brazilian Cerrado, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15729, https://doi.org/10.5194/egusphere-egu23-15729, 2023.

ABSTRACT: Global nitrogen (N) deposition has impacted the structure and functioning of soil microbial communities, translating into important changes to the cycling of soil organic matter (SOM). Recent frameworks have proposed that portioning the particulate and mineral-associated organic matter (POM and MAOM) fractions can help us better understand SOM cycling. However, how N deposition affect the fractionation of SOM into MAOM and POM forms, and how soil microbes process these across soil profile all remain unclear. Here we examined the microbial phospholipid fatty acids and determined N and soil organic carbon (SOC) content in POM and MAOM at depths of 0–10, 30–40 and 70–100 cm after 10-year N addition at rates of 0, 2, 10 and 50 g m^-2 yr^-1 in a temperate steppe. We found that N addition remarkably shifted microbial communities by increasing the relative abundances of bacteria and gram-positive (GP) bacteria, and decreasing gram-negative bacterial across the three soil layers. These effects of N addition tended to increase with the N addition rate but diminished with soil depth probably as pH decreased with the N addition rate but increased with soil depth. Both N addition and soil depth may cause similar microbial community shifts, through which fungi and GP bacteria become dominant, but may through different mechanisms. More than 60% of total SOC and N are stored as MAOM in this grassland. The share of SOC and total N in the MAOM was slightly decreased by N addition in 0-10 cm but significantly increased in deeper soils. The ratios of POM-C/MAOM-C and POM-N/MAOM-N significantly decreased with soil depth regardless of N addition treatments. Moreover, N addition increased the two ratios in 0-10 cm soil, but decreased them in deeper soil layers. N addition increased the stocks of SOC (MAOM: +11 %; POM: +23 %) and total N (MAOM: +10 %; POM: +27 %) in 0–10 cm soil, but increased only in MAOM in 30–40 cm (SOC: +24 %; total N: +24 %) and 70–100 cm (SOC: +15 %; total N: +13 %) soils. Soil physicochemical features exerted stronger controls than microbial properties in the distribution of SOC and total N in the two fractions regardless of soil depth because of eight soil features explaining more of the total variation than eight microbial properties. Our findings imply that increase in N deposition may make more SOC stabilized as MAOM fraction in grassland soils.

Keywords: Nitrogen deposition, Soil microbiome, Mineral-associated organic matter, Subsoil

How to cite: Niu, G., Huang, J., Lu, X., and Rousk, J.: Decadal nitrogen addition enhanced soil C and N storage in mineral-associated organic matter by altering soil abiotic and microbial properties in a temperate grassland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3272, https://doi.org/10.5194/egusphere-egu23-3272, 2023.

Formation of mineral-associated organic matter (MAOM) supports accumulation and stabilization of carbon in soil, and thus, is a key factor in the global carbon cycle. Little is known about the interplay of mineral type, land use, and management intensity on the extent of MAOM formation. We addressed this research question by exposing mineral containers with pristine minerals (goethite, as a representative of oxide-type mineral phases, and illite, representing layered aluminosilicate minerals) for five years to ambient soil conditions at 5 cm depth in 150 grassland and 150 forest plots in three regions across Germany. After recovery, the content of organic carbon (OC) of the minerals was determined by dry combustion. Results show that irrespective of land use and management intensity, more OC accumulated on goethite than illite (on average 0.23 and 0.06 mg m^-2 mineral surface, respectively), demonstrating that mineral type was the most crucial factor for MAOM formation. Carbon accumulation was consistently greater in coniferous forests than in deciduous forests and grasslands. Structural equation models revealed that in grasslands, fertilization had contradictory effects on carbon accumulation, with the positive effect being mediated by enhanced plant productivity and the negative effect by reduced plant species richness. Overall, our results suggest that OC stabilization in soil is primarily driven by mineral type, in particular iron and other metal oxides. The mineral-driven MAOM formation is further modified by land use and management intensity.

How to cite: Ulrich, S., Bramble, D. S., Schöning, I., Mikutta, R., Kaiser, K., and Schrumpf, M.: Mineral type, land use, and management intensity drive the formation of mineral-associated organic matter in temperate soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12085, https://doi.org/10.5194/egusphere-egu23-12085, 2023.

Under conservation agriculture (CA), soil aggregates physically protect soil organic C, creating microhabitats with heterogeneities in nutrient availability. These may become rich in microbial taxa with structured interconnections, and thus maintain the equilibrium between C sources and sinks. A long-term experiment on tillage and N fertilization located in the Mediterranean was used to investigate the microbiota within small macroaggregates (sM), and occluded microaggregates (mM). At surface layer N fertilization was the main driver of diversity of prokaryotes and fungi in soil aggregates, whereas at subsurface layer tillage intensity was the primary driver. Moreover, although along the soil profile a conserved core microbial community was found across managements in soil aggregates, some taxa were unique to certain managements. At surface layer, N fertilization significantly modified the prokaryotic community structure in sM and mM under conventional tillage, whereas in the subsurface layer, tillage modified the community structure of prokaryotes in both soil aggregates, and of fungi in mM. The fungal community structure in sM was strongly modified by the interaction between tillage and N fertilization at both soil layers and in mM only at surface layer. Overall sM had a higher diversity of prokaryotes and a lower diversity of fungi than mM. Small macroaggregates and mM had distinctive microbial community structures. Prokaryotic taxa, such as Actinobacteria, Chloroflexi and Thermomicrobia, and fungi, such as Agaricomycetes, Dydimellaceae, and Mortierellaceae, characterized sM, whereas others prokaryotes (Betaproteobacteria, Sphingobacteriia, Blastocatellia) and fungi (Sordariales, Lasiosphaeriaceae and Glomeraceae) characterized mM. Within- and cross-domain network were more complex in mM than sM at surface layer, and the opposite occurred at subsurface. Some prokaryotic and fungal taxa (Chloroflexi and Sordariomycetes), found abundant in hubs within soil aggregate networks, were consistently positively related to C cycling and soil structuring. We can therefore conclude that soil aggregation should be included in a more complete ‘multifunctional’ perspective of soil ecology, and that a full understanding of soil processes requires analyses emphasizing feedbacks between soil structure and soil microbiota, rather than a unidirectional approach simply addressing single members in bulk soil. As CA systems and soil structure were strongly connected to soil microbiome and function, the application of CA practices should be supported for the restoration of disturbed soils, the prevention of soil erosion and the enhancement of SOC storage. Overall, the higher diversity and differentiated soil microbial structures observed in minimum and fertilized tillage systems may offer biological buffering capacity and maintain agriculturally relevant soil functions. This study allows to improve the knowledge on taxa resistant and sensitive to modifications induced by tillage and N fertilization, according to soil aggregation size. We also demonstrate that linking taxonomy to function is a priority for explaining the ecological interactions that promote SOC accumulation in soil aggregates.

How to cite: Ercoli, L., Piazza, G., Helgason, T., and Pellegrino, E.: Microbiome structure and interconnection in soil aggregates across conservation and conventional agricultural practices allow to identify taxa related to soil functioning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16928, https://doi.org/10.5194/egusphere-egu23-16928, 2023.

Plastic film mulching (PFM) is critical for agricultural planting and production in semi-arid and arid areas. Particulate organic matter (POM) is assumed to be a sensitive indicator of evaluating the effects of different agricultural practices on soil fertility and soil organic carbon (SOC) pool. Soil aggregates are the main storage sites for POM. However, there is limited information regarding how PFM and fertilization influences the dynamic changes of newly added stalk-derived POM in Brown earth. Consequently, a depth-study of the fate of carbon (C) and nitrogen (N) derived from maize stalk residues as the POC and PON fractions in soil aggregates will help in predicting the active organic matter component sequestration in the soil. The dynamics and contribution of the newly added maize stalk C and N as POC and PON in different soil aggregates (using dry sieving method divided to > 2, 1-2, 0.25-1and < 0.25 mm) was analyzed by an in-situ ¹³C¹⁵N-tracing technique under 27-year long term PFM and different fertilization treatments. Over the 360 d cultivation, the POC and PON contents were significantly (P < 0.05) higher in the nitrogen (N) and organic manure (M) treatments than other fertilizer addition treatments. Compared with no PFM, PFM accelerated the decomposition of maize stalk C in the N fertilizer treatment, exhibiting an increase of 64% in stalk-derived POC in the initial cultivation time. In addition, stalk-derived POC tended to accumulate in 1-2 mm aggregates in the summer and fall as a result of long-term PFM coupled with fertilization. However, the stalk-derived PON was decreased with the cultivation time in different four aggregates. Stalk-derived POM was tended to accumulate in the macroaggregate size fraction (> 0.25 mm) over 360 days of cultivation in the field conditions. Accordingly, PFM application and fertilization practices had important effects on accumulation of newly added stalk-derived POM in soil aggregates.

How to cite: Jin, X., Bol, R., An, T., Zheng, L., Li, S., Pei, J., and Wang, J.: Long-term fertilization and PFM changed the accumulation of stalk-derived POM in soil aggregates under field conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2051, https://doi.org/10.5194/egusphere-egu23-2051, 2023.

The implementation of agroecological practices can lead to an additional soil organic carbon (SOC) storage. The carbon sink effect will be more effective, even in the short and medium term, if the additional storage is realized in the form of persistent organic carbon (OC) and not in labile OC. The objective of this study was to evaluate the biogeochemical stability of additionally C stored by agroecological practices. Biogeochemical stability was assessed using particles size and density fractionation and Rock-Eval (RE) thermal analyses with PARTYsoc machine learning model. Samples were collected from the QualiAgro experiment, where organic wastes products (OWPs) including biowaste compost (BIOW), residual municipal solid waste compost (MSW) and farmyard manure (FYM) were applied, and from the La Cage experiment, where conservation (CA) and organic (ORG) agriculture had been established for 20 years. The plots that received the OWPs showed that 60-66% of the additional C was stored in mineral-associated organic matter (MAOM-C) and 29 - 39% in particulate organic matter (POM-C), whereas in CA and ORG, 77 - 84% of the additional C was stored in MAOM-C versus 15 - 23% in POM-C. While leading to additional C stocks of similar sizes, MSW and FYM exhibited higher proportions of the additionally stored C as POM-C (39 and 29% respectively) compared to CA (15%). This suggests a recalcitrance of POM under OWPs management compared to CA. The PARTY_SOC model using RE thermal analysis parameters allows to predict the active (30 - 40 years) and stable (>100 years) carbon pools as defined in the AMG model. The results revealed that most, if not all, of the additional C belonged to the active C pool. These findings suggest that although additional SOC is mainly associated with MAOM-C, it is probably not stored in a form with a mean residence time exceeding ~30 years. The agroecological practices implemented in both long-term field experiments have resulted in substantial short-term additional C storage, but this storage will only be maintained at a high level if these storing practices are continued.

How to cite: Kpemoua, T. I., Barré, P., Houot, S., Baudin, F., Plessis, C., and Chenu, C.: Is the additional organic carbon stored thanks to alternative cropping systems and organic waste products application predominantly stable at a decadal timescale?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8025, https://doi.org/10.5194/egusphere-egu23-8025, 2023.

Subsoils have gained increasing attention due its slower soil organic carbon (SOC) turnover than in topsoil. Thus, subsoil with low content of mineral associated organic matter but a large number of exchange sites on mineral surfaces represents the potential to accumulate and sequester carbon (C). Generally, it has been assumed that the carbon turnover mechanism in topsoil and subsoil is influenced by similar environmental factors, with the difference of a lower C content in subsoil. In contrast, diverse abiotic variables prevalent in subsoils, like low temperature, high moisture, nutrient availability, etc., have been shown to imply different processes influencing C turnover in subsoils. Therefore, differences in processes and factors affecting SOC turnover in topsoil and subsoil are incompletely identified and understood.

Our objective is to investigate whether C decomposition and stabilisation mechanisms in topsoil and subsoil differ given the same added substrate content, as well as how it responds to increasing substrate C content. To assess these questions, a long-term (total duration 20-year) field incubation experiment was conducted at three different locations with varied soil textures in which soil was mixed and labelled with isotopically (13C) enriched beech litter substrate with different C contents of 8, 16, 32, and 64 g substrate kg^-1 in topsoil (10 cm) and 2, 4, 8, and 16 g substrate kg^-1 in subsoil (60 cm), filled in mesocosms, and buried. Soil samples were collected after one, two, and four years. Soil was fractionated into particulate organic matter (>20µm) and mineral-associated organic matter (<20µm) to find out how carbon is stabilised in these fractions, and stable C isotopes were measured. Our results indicate that the decomposition of the identical litter substrate strongly depends on the soil depth. The results of four years of buried field-microcosms will be presented, which will shed more light on differences in mechanisms responsible for SOC dynamics and the fate of litter substrate into different SOC pools of topsoil and subsoil.

How to cite: Begill, N., Don, A., and Poeplau, C.: Investigating soil organic matter dynamics in topsoil and subsoil by burying isotopically labelled litter substrate for four years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3538, https://doi.org/10.5194/egusphere-egu23-3538, 2023.

Intercropping maize along with forages fertilized with N can potentially increase soil carbon sequestration, contributing to climate change mitigation. However, there is a lack of knowledge if the input of new C sources in this production system impacts the cycling of the original soil C and SOM fractions, especially in tropical soils. To investigate this, soil samples were taken up to 80 cm depth from a 7-year experiment where ruzigrass (Urochloa ruziziensis), palisadegrass (Urochloa brizantha) and Guinea grass (Megathyrsus maximus) were intercropped with maize fertilized with (270 kg N ha^-1) or without N. In these samples, SOM was fractionated by size into particulate (POM) and mineral-associated (MAOM) organic matter and submitted to ¹³C natural abundance measurements. Intercropping with Guinea grass reduced the δ¹³C values in comparison to ruzigrass and palisadegrass, especially under N fertilization. Forage grasses reduced the δ¹³C values up to 40cm, indicating the contribution of the grasses for the cycling of the original carbon of the soil. Nitrogen supply increased the contribution of C from the grasses to the POM fraction if compared to the no N application. Further, ¹³C  in POM at 0-10 and 10-20 cm differed from deeper layers, probably due the above- and belowground C inputs on the uppermost soil layers. Under N supply, Guinea grass lowered the δ¹³C value, which did not occur in the palisade and ruzigrass treatments. In contrast to POM, the δ¹³C values of MAOM decreased in all depths, with the highest change at the uppermost soil layer. Our findings showed that intercropping influenced the cycling of total C and SOM fractions , with differences in the soil profile. However, only Guinea grass changed δ¹³C values under N supply.

How to cite: Vieira da Mota Neto, L., Valadares Galdos, M., Eliodoro Costa, V., and Antonio Rosolem, C.: Assessing soil carbon cycling as a function of intercropped maize-forage systems and nitrogen rates using 13C natural abundance, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10208, https://doi.org/10.5194/egusphere-egu23-10208, 2023.

Soil organic matter (SOM) originates from various sources such as foliar litter, roots and microbial (e.g. fungal) components. The relative sources contribution represents one of the key unknowns in SOM dynamics. Our study aimed to explore whether stable isotope ratios of non-exchangeable hydrogen (H_n) bound to organic matter can be used to differentiate SOM sources, since natural ²H_n abundance can strongly differ between root and foliar tissues. We also investigated if long-term irrigation with ²H-depleted water in a pine forest can be used to track H_n incorporation into organic matter inputs and eventually in the soil pools.

In a 17-year-long irrigation experiment in a dry pine forest, we assessed variations in natural abundance of ²H_n, ¹³C, and ¹⁵N in SOM sources (foliar litter, fine roots, fungal mycelia), decomposing litter, soil (organic layers and uppermost 5 cm-mineral soil) and particle-size fractions. We then applied a Bayesian mixing model (including δ²H_n,δ¹³C, and δ¹⁵N) to estimate the relative sources contribution to SOM.

Natural ²H_n abundance was significantly higher in roots vs. foliar litter (up to +39‰), and in fungal mycelia vs. roots (up to +41‰). Results from Bayesian mixing model suggest that foliar litter contributed to approximately 68 ± 10% of SOM in organic layers and in coarse particulate organic matter (POM). Foliar litter and roots contributed similarly to upper 2 cm of mineral soil (46 ± 11%), while 2-5 cm of mineral soil were largely derived from roots (61 ± 13%). Fungal mycelia contributed to 18 ± 8% of mineral-associated organic matter (MOM), while only to 1-2% of coarse and fine POM. Bayesian mixing models provided only a general indication of the sources contribution to SOM, also considering that isotopic signatures shifted during decomposition. Measurements of isotope signatures in microbial necromass might allow a more accurate assessment of the different SOM sources contribution.

The δ²H_n depletion of soil water under irrigation was paralleled by a comparable decrease in δ²H_n of roots (~12‰). In comparison, the natural ²H_n abundance in fresh needles and foliar litter decreased less strongly (~ 7‰ and 4‰, respectively), likely due to photosynthetic adjustments that may have counterbalanced the irrigation water ²H-depletion. Similar to soil water ²H-depletion, δ²H_n values in coarse POM were 11‰ lower in irrigated vs. dry plots, suggesting that nearly all organic H_n turned over or exchanged with soil water in less than two decades. In contrast, δ²H_n values in fine POM and MOM decreased only by 3‰ under irrigation, which indicate that these fractions comprise slower cycling H_n pools.

Our study showed that the natural ²H_n abundance represents a promising tool to differentiate among SOM sources. While ¹³C and ¹⁵N did not clearly separate between roots and foliar litter, H_n isotopic signatures allowed a good discrimination between SOM sources. In addition, long-term irrigation can provide a potential in situ ²H-labelling of SOM, which may help to examine organic H_n turnover rates across SOM pools.

How to cite: Guidi, C., Lehmann, M., Meusburger, K., Saurer, M., Vitali, V., Peter, M., Brunner, I., and Hagedorn, F.: Tracing sources and turnover of soil organic matter in a long-term irrigated dry forest - a non-exchangeable hydrogen isotope approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5557, https://doi.org/10.5194/egusphere-egu23-5557, 2023.

How to cite: Otero-Fariña, A., Brown, H., Xiao, K.-Q., Antelo, J., Fiol, S., Chapman, P., Holden, J., Banwart, S., and Peacock, C.: Carbon preservation in soils: The role of carbon chemistry in soil aggregate formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1525, https://doi.org/10.5194/egusphere-egu23-1525, 2023.

Understanding the molecular make-up of recalcitrant organic matter (rSOM) is important to postulate the capability of soil organic matter (SOM) to sequester carbon and mitigate climate change. Humic acid (HA) extracted from the river bed sediment (RS) from the West Holland river was analyzed, aiming to characterize and quantify the fused ring aromatic structures (FRA) portion. FARs can be formed through condensation and polymerization reactions and act as an important skeletal structure of the rSOM which has a mean residence time >1000 years. We conducted a series of nuclear magnetic resonance (NMR) experiments, 13C Direct Polarization Magic Angle Spinning (DP-MAS) NMR spectroscopy, and Dipolar dephased (dd) DPMAS NMR, chemical shift anisotropy (CSA) cross-polarization (CP) total sideband suppression (TOSS) NMR experiment and a dd-CSA filtered CPTOSS to accurately quantify the proportion of FRAs in the sediment HA sample. We compared the proportions of the functional groups of the RS with the surface (0-20 cm, TS) and deep (>90 cm, CS) soil HAs of the nearby Holland Marsh, Muck Crops Research Station to understand the linkages and the transformations of SOM happened while transportation (wind erosion and horizontal seepage) to the muck river sediment. We found that 90% of the aromatic C in the RS is non-protonated, and 32% of the aliphatic region was non-protonated. The DPMAS spectral comparison between RS, TS and CS clearly showed that RS contains characteristic peaks of both TS and CS. Moreover, the proportion of non-protonated aliphatics in RS (32%) is high compared to TS (18%) and CS (29%). Our results indicate that in muck river sediment soil HA, non-protonated aliphatics (CRAM-like structures) contribute to the rSOM more than FRAs, while in TS and CS, FRAs' contribution is higher than the non-protonated aliphatics. Collectively our results show the link between terrestrial organic matter transportation to the river sediment and the transformation that occur in the rSOM fraction in the river sediment SOM. This new knowledge allows us to understand the structural changes that happen in the sequestered carbon in different soil environments.

How to cite: Gamage, J., Longstaffe, J., Gillespie, A., Lo, A., Al-Abdul-Wahid, S., and Voroney, P.: Evidence for the diagenetic formation of fused aromatic ring structures in an organic soil, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17105, https://doi.org/10.5194/egusphere-egu23-17105, 2023.

The amount of carbon stored in boreal forests soil as Soil organic matter (SOM) is significant. Carbohydrate polymers such as cellulose and hemicellulose constitute 40-50% of the SOM mass in the surface mor layer, even in SOM that has been decomposed for decades to centuries. This is in contrast to conceptual decomposition models assuming aromatic and aliphatic polymers to constitute the fraction of recalcitrant SOM. One prevailing view for support is that lignin manifest itself as a factor in the stabilization of carbohydrate polymers as SOM. However, detailed elucidation of how the complex array of molecular moieties making up SOM decompose over time is lacking. Here we investigated the effect of lignin content and composition during the progressive degradation of polymeric carbohydrates, lignin, and lipids in the lab during a year-long soil decomposition study using Aspen (Populus tremula) wood as a model substrate. To specifically address lignin decomposition we used a range of Aspen clones that varied naturally in their lignin content (high lignin 30% - low lignin 25%) with boreal coniferous forest soil obtained from the surface moor layer (O-horizon). The decomposition of the different molecular moieties of the model substrate was evaluated by Two-dimensional (2D) liquid state 1H–13C nuclear magnetic resonance (NMR) spectroscopy. In addition, the CO₂ production during decomposition was monitored continuously and assays for exo-enzymatic activity was carried out at selected time points.

The NMR spectroscopy revealed that for different periods of decomposition, saprotrophic microorganisms preferred different monomers of polymeric lignin, carbohydrates, and lipids. The relative degradation of resinol, spirodienone, and cinnamyl alcohol were higher among lignin interlinkages and the relative degradation of p-hydroxybenzoate and syringyl were higher among lignin subunits. For carbohydrates, the relative degradation of mannose and glucose were higher than that of e.g. xylose. The relative degradation of unsaturated fatty acids was higher among lipids. The lignin: carbohydrates ratio decreased linearly over the period of decomposition. This showed that the initial degradation of lignin compounds was greater compared to the decomposition of carbohydrate compounds. The significant difference in the relative degradation of mannose among model substrate with different lignin content showed that lignin had no effect on cellulose degradation but may have had an effect on the preferential degradation of hemicelluloses. The high-resolution decomposition patterns we observe are crucial for obtaining a detailed mechanistic understanding of plant polymer decomposition by soil microorganisms during the initial stages of SOM genesis.

Keywords: Soil organic matter (SOM), Lignin, Carbohydrates, 2D NMR, Decomposition

How to cite: Thirunavukkarasu, A., Öquist, M., Schleucher, J., Sparrman, T., Hedenstrom, M., Nilsson, M., and Bertilsson, S.: The role of lignin in the saprotrophic degradation of plant biomass in boreal forest soil., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13926, https://doi.org/10.5194/egusphere-egu23-13926, 2023.