Mineral-associated Organic Matter Research Articles

Soil organic matter and aggregate stability dynamics under major no-till crop rotations on the Canadian prairies

There is a need to develop and adopt novel, more resilient, or “climate-smart” cropping strategies on the Canadian prairies, which are crucial for sustainable agroecosystem management. To examine how crop rotation influences soil organic matter and aggregate stability (AS) dynamics, six crop rotations: (i) conventional (control), (ii) pulse/oilseed intensified, (iii) diversified, (iv) market-driven, (v) high-risk and high-reward, and (vi) soil health-enhanced were established in 2018 at three field sites: Lethbridge, Alberta and Swift Current and Scott, Saskatchewan under no-till management. After 4-year, soil organic carbon (SOC), total nitrogen (TN), particulate organic matter carbon (POM-C) and nitrogen (POM-N), or mineral-associated organic matter C (MAOM-C) and N (MAOM-N) concentrations were not significantly affected by crop rotation in the 0–7.5 cm soil depth. However, crop rotation significantly altered AS at two of three sites, with both soil health-enhanced and high-risk and high-reward rotations having the highest AS at Lethbridge and Swift Current. Across all three sites, strong positive correlations were found among SOC, TN, MAOM, and AS. Moreover, MAOM-C and MAOM-N showed stronger relationships with AS than POM, perhaps suggesting a positive feedback loop on the stability of SOC and aggregation. Overall, the inclusion of pulses in rotations showed the potential to sustain soil quality, likely by offsetting low residue quantity with better residue quality and diversity, thereby supporting SOC accrual and AS similar to or greater than conventional cereal- or oilseed-dominated rotations.

Subtropical forest macro-decomposers rapidly transfer litter carbon and nitrogen into soil mineral-associated organic matter

BackgroundForest soils in tropical and subtropical areas store a significant amount of carbon. Recent frameworks to assess soil organic matter (SOM) dynamics under evolving global conditions suggest that dividing bulk SOM into particulate and mineral-associated organic matter (POM vs. MAOM) is a promising method for identifying how SOM contributes to reducing global warming. Soil macrofauna, earthworms, and millipedes have been found to play an important role in facilitating SOM processes. However, how these two co-existing macrofaunae impact the litter decomposition process and directly impact the formation of POM and MAOM remains unclear. MethodsHere, we set up a microcosm experiment, which consisted of 20 microcosms with four treatments: earthworm and litter addition (E), millipedes and litter addition (M), earthworm, millipedes, and litter addition (E+M), and control (only litter addition) in five replicates. The soil and litter were sterilized prior to beginning the incubation experiment to remove any existing microbes. After incubating the samples for 42 days, the litter properties (mass, C, and N contents), soil physicochemical properties, as well as the C and N contents, and POM and MAOM 13C abundance in the 0–5 and 5–10 ​cm soil layers were measured. Finally, the relative influences of soil physicochemical and microbial properties on the distribution of C and N in the soil fractions were analyzed. ResultsThe litter mass, C, and N associated with all four treatments significantly decreased after incubation, especially under treatment E+M (litter mass: −58.8%, litter C: −57.0%, litter N: −75.1%, respectively), while earthworm biomass significantly decreased under treatment E. Earthworm or millipede addition alone showed no significant effects on the organic carbon (OC) and total nitrogen (TN) content in the POM fraction, but joint addition of both significantly increased OC and TN regardless of soil depth. Importantly, all three macrofauna treatments increased the OC and TN content and decreased the 13C abundance in the MAOM fraction. More than 65% of the total variations in the distribution of OC and TN throughout the two fractions can be explained by a combination of soil physicochemical and microbial properties. Changes in the OC distribution in the 0–5 ​cm soil layer are likely due to a decrease in soil pH and an increase in arbuscular mycorrhizal fungi (AMF), while those in the 5–10 ​cm layer are probably caused by increases in soil exchangeable Ca and Mg, in addition to fungi and gram-negative (GN) bacteria. The observed TN distribution changes in the 0–5 ​cm soil likely resulted from a decrease in soil pH and increases in AMF, GN, and gram-negative (GP) bacteria, while TN distribution changes in the 5–10 ​cm soil could be explained by increases in exchangeable Mg and GN bacteria. ConclusionsThe results indicate that the coexistence of earthworms and millipedes can accelerate the litter decomposition process and store more C in the MAOM fractions. This novel finding helps to unlock the processes by which complex SOM systems serve as C sinks in tropical forests and addresses the importance of soil macrofauna in maintaining C-neutral atmospheric conditions under global climate change.

Land use determines the composition and stability of organic carbon in earthworm casts under tropical conditions

Environmental conditions play an important role in controlling the fate of organic carbon (OC) in earthworm casts. However, the mechanisms that lead to the destabilization of OC in casts deposited in different land use systems are poorly understood. In this study, we investigated the impact of land use on the fate and composition of particulate organic matter (POM) and mineral-associated organic matter (MAOM) in earthworm casts under tropical conditions. We conducted a 400-day field exposure experiment in a woodland and a meadow in northern Vietnam with earthworm casts and soil aggregates without trace of earthworm activity (reference soil) under natural rainfall conditions. We analyzed the element content, stable carbon isotope composition and mid-infrared signatures of the two types of materials after 9 and 400 days of field exposure. The results showed that the casts were initially enriched in OC as compared to the reference soils, with the MAOM fraction accounting for over 90% of the earthworm-induced OC accumulation. The POM fraction occluded in casts consisted of fresh plant material and disappeared during the 400 days of field exposure. Enrichment and composition of OC differed between woodland and meadow casts. POM and MAOM showed contrasting persistence in the two land use systems. While woodland casts showed the highest potential to stabilize OC, the actual amount of cast OC stabilized for more than 400 days was more important under meadow. The processes affecting cast OC dynamics were contrasting in both systems. Under woodland, cast OC destabilization was most probably related to microbial degradation, while under meadow, OC accumulation was most probably related to root activity. Our study highlights that the impact of earthworms on the origin, composition, and fate of cast OC in tropical environments is strongly influenced by land use.

Dynamic changes in soil organic carbon induced by long-term compost application under a wheat-maize double cropping system in North China

Soil organic carbon (SOC) plays a vital role in improving soil quality and alleviating global warming. Understanding the dynamic changes in SOC is crucial for its accumulation induced by compost application in agroecosystem. In this study, soil samples were collected from three treatments: high-rate bio-compost (BioMh), low-rate bio-compost (BioMl), and control (CK, no fertilization) during 2002–2020 in a wheat-maize double cropping system in North China. The soils were separated into three functional fractions, i.e., coarse particle organic matter (cPOM, >250 μm), microaggregates (μAgg, 53–250 μm) and mineral-associated organic matter (MAOM, < 53 μm), and the associated SOC contents were determined. During 1993–2002, SOC contents in bulk soil significantly increased with the duration in the BioMh and BioMl plots. However, there was no significant correlation between SOC content and duration during 2002–2020. These results suggested that compost application positively improved SOC sequestration, while the duration of SOC sequestration (i.e., the longevity of increased SOC with time) under compost inputs maintained only 9 years. Moreover, there was a significant increase in mean annual SOC contents in bulk soil with compost application rate during 2002–2020, indicating that carbon saturation did not occur. Additionally, the SOC contents in the cPOM fraction increased with time (p < 0.01), but the corresponding μAgg and MAOM associated SOC was insignificant (p > 0.05). The MAOM fraction exhibited no additional carbon accumulation with expanding compost application, confirming a hierarchical carbon saturation in these fractions. We concluded that soils under wheat-maize double cropping system in North China have greater potential to sequester C through additional compost inputs, despite showing hierarchical saturation behavior in the non-protected coarse particulate fraction.

Cover crop benefit bacteria and increase aggregate-associate soil C and N storage

The aim of this study was to analyze the effect of four crop sequences with different aboveground biomass and crop residue biochemical composition on biological soil variables, fractions of soil organic matter, and soil aggregation. The study was carried out in a long-term experiment under no-till, in the Argentinean Pampas (31°51′ S; 60°32′ W), on an Aquic Argiudoll. The treatments were: i) fertilized soybean monocropping (Syf), ii) winter cover crop (CC)/ fertilized soybean (CC/Syf), iii) N-fertilized winter CC/fertilized soybean (CCN/Syf), and iv) N-fertilized winter CC/fertilized soybean in a crop rotation: CCN/Syf - wheat [Triticum aestivum L.] for grain production/soybean - maize [Zea mays L.] (CCN/Syf rot). Wheat was used as winter CC. Crop residues were sampled in four moments, and were separated into different fractions. Furthermore, crop residue quality was analyzed according to Van Soest (Cellulose, Hemicellulose, Lignin, Non- structural carbohydrates), and %C and %N was also determined. In soil samples obtained at 0–5 cm depth, we determined: soil organic C, total N, particulate organic matter C and N, mineral-associated organic matter C and N, C and N stocks in different aggregate-size classes, microbial biomass C and N (MBC and MBN), fungal and bacterial biomass-C (FBC and BBC), and N released after a 7-d anaerobic incubation. From June to November, Syf exhibited slower decomposition of soybean stems and an increase in the non-identifiable residue biomass. Intensified crop sequences (i.e., with CC) increased MBC (∼38%) and BBC (∼115%), but had no effect on FBC. The BBC was positively correlated with Hemicellulose (r = 0.86, P < 0.001), but negatively associated with lignin (r = −0.84, P < 0.001) and the lignin:N ratio (r = − 0.89, P < 0.001) in the crop residue. The C and N stocks in aggregates >2000 μm were 263% and 227% greater in treatments with CC than Syf, respectively, and correlated with improvements in the MBC and MBN (r = 0.87, P < 0.001 and r = 0.88, P < 0.001, respectively), particularly the BBC. Intensification of crop sequences through the incorporation of CC provided crop residues with a higher availability of easily decomposable C compounds. This enhancement stimulated the soil biota responsible for decomposition, particularly bacteria. Our study highlights the importance of increasing cropping intensity and covering the soil with live vegetation, resulting in a healthier soil.

Accumulation of century-old biochar contributes to carbon storage and stabilization in the subsoil

Soil organic carbon (C) is a key component of the global C budget. As soil organic C turnover rates decrease with depth, agricultural practices favoring deep organic C storage will gain importance as long-term climate change mitigation strategies. In addition, amendment of pyrogenic organic matter (biochar) is considered a promising practice for sequestering C in croplands. However, so far the >30 cm depth soil organic C pool, and subsoil biochar dynamics in particular, has been understudied.To address this, we focused on leftovers from pre-industrial charcoal kilns as a proxy to study the accrual of century-old biochar in the subsoil (30–100 cm) in comparison to adjacent control soil. Using thermal and elemental analyses as well as size and density fractionations (i.e., separating particulate and mineral-associated organic matter), we determined the distribution of pyrogenic organic C within the soil profile and investigated its stabilization in depth. We measured the dissolved organic carbon (DOC) concentrations as well as absorbance and fluorescence properties of dissolved organic matter (DOM) at different depths to characterize the effects of century-old biochar accumulation on the current leaching of DOM.Our results showed that the presence of century-old biochar resulted in an increase of 53.8 ± 25.1 t C ha−1 in 0–100 cm, and 12 % of the pyrogenic organic C was stored in 30–60 cm. No difference in C stocks was observed in 60–100 cm between kiln sites and reference soils. Most of the pyrogenic organic C has been translocated as particulate organic matter, either free or occluded, to subsoils. This led to a change in the dominant fraction of organic matter in the E horizon; in reference soils 64.7 % of the total C was associated with mineral phases as opposed to only 42.3 % in soils enriched with biochar. The thermal analysis of the mineral-associated organic matter revealed that pyrogenic organic C was associated with mineral phases. With depth, DOC concentrations decreased and the relative contribution of microbial byproducts to fluorescent DOM increased. The soils enriched with century-old biochar displayed lower DOC concentrations and more aromatic DOM in the Ap horizon, which suggests biochar dissolution is still an on-going process. Our results suggest that the vertical transfers of century-old biochar mainly occurred as particle leaching in macroporosity and bioturbation but continuous dissolution has also contributed in these fine-textured cropland soils. In conclusion, the association of pyrogenic organic C with mineral phases as well as its migration in subsoil horizons promoted its physical disconnection from abiotic and biotic degrading agents, which likely contribute to the long-term stability of biochar.

Formation of mineral-associated organic matter in temperate soils is primarily controlled by mineral type and modified by land use and management intensity.

Formation of mineral-associated organic matter (MAOM) supports the accumulation and stabilization of carbon (C) in soil, and thus, is a key factor in the global C cycle. Little is known about the interplay of mineral type, land use and management intensity in MAOM formation, especially on subdecadal time scales. We exposed mineral containers with goethite or illite, the most abundant iron oxide and phyllosilicate clay in temperate soils, for 5 years in topsoils of 150 forest and 150 grassland sites in three regions across Germany. Results show that irrespective of land use and management intensity, more C accumulated on goethite than illite (on average 0.23 ± 0.10 and 0.06 ± 0.03 mg m-2 mineral surface respectively). Carbon accumulation across regions was consistently higher in coniferous forests than in deciduous forests and grasslands. Structural equation models further showed that thinning and harvesting reduced MAOM formation in forests. Formation of MAOM in grasslands was not affected by grazing. Fertilization had opposite effects on MAOM formation, with the positive effect being mediated by enhanced plant productivity and the negative effect by reduced plant species richness. This highlights the caveat of applying fertilizers as a strategy to increase soil C stocks in temperate grasslands. Overall, we demonstrate that the rate and amount of MAOM formation in soil is primarily driven by mineral type, and can be modulated by land use and management intensity even on subdecadal time scales. Our results suggest that temperate soils dominated by oxides have a higher capacity to accumulate and store C than those dominated by phyllosilicate clays, even under circumneutral pH conditions. Therefore, adopting land use and management practices that increase C inputs into oxide-rich soils that are under their capacity to store C may offer great potential to enhance near-term soil C sequestration.

Divergence in soil particulate and mineral-associated organic carbon reshapes carbon stabilization along an elevational gradient

Variations in climates and vegetation types along elevations in mountain ecosystems strongly reshape soil organic matter (SOM) stabilization. However, soil carbon (C) stabilization pathways from litter to SOM and potential mechanisms along elevations in the subalpine area remain unclear. Here, we investigated the C stabilization pathway from litter to particulate and mineral-associated organic matter (POM and MAOM) and associated drivers along elevational gradients in the Hengduan Mountains, China, based on δ13C signatures. Our results showed C stabilization pathways from litter to POM to MAOM, irrespective of elevation. Unexpectedly, the δ13C values of SOM and its fractions (POM and MAOM) were not significantly related to litter δ13C across elevations although litter was the main source of SOM, suggesting that other potential drivers could weaken the coordination of δ13C among litter and SOM fractions. We observed that the△13CLitter→POM (△13C between litter and POM) was more dependent on the direct effect of soil texture (i.e., sand, silt and clay). However, the △13CPOM→MAOM (△13C between POM and MAOM) was more related to microbial attributes (i.e., microbial community and enzyme activities).These results emphasizing the different stabilization procedures between POM and MAOM. Particularly, soil temperature dominantly determined △13CLitter→POM, but soil pH was the primary factor regulating the △13CPOM→MAOM. Overall, our results suggested that POM and MAOM formation occurred with different regulation processes, the POM formation was primarily via more physical pathways and MAOM formation mainly via more microbial pathways, thus providing novel insights into the better understanding of SOM formation and stabilization mechanisms in mountain ecosystems.

Destructive selective logging in tropical forests causes soil carbon loss through forest degradation and soil redox change

Soil organic C (SOC) stock in tropical forests can be reduced significantly after logging, not only due to lowered above ground biomass (AGB) and consequentially reduced organic matter (OM) input, but also consumption on sequestrated soil C protected by OM-mineral association. In this study, we studied the loss of mineral-associated OM after logging in the main rooting zone (e.g., 0–30 cm), and examined a hypothetical twofold underlying mechanism of SOC reduction: first, decrease in AGB limiting OM input; second, decrease in soil redox potential (Eh) leading to the reductive dissolution of short-range-ordered iron (SRO Fe) minerals that destabilize some mineral-associated OM. Thirty-five plots at different degradation levels were investigated for both above- and belowground properties. AGB and tree community composition were employed as indicators of forest degradation gradient; multiple physicochemical properties in top-30 cm soils were measured every 10 cm depth. We analysed the relationship between AGB and soil C with the mediator SRO Fe, and investigated the role of soil Eh in linking the changes in AGB and SRO Fe. Our results revealed simultaneous decreases in AGB, SRO Fe and soil C in top-30 cm soils, and SRO Fe mediated 43 % of the hypothetical causal effect of AGB on soil C in deeper (20–30 cm) soil. Soil Eh decreased with decreasing AGB in a logistic pattern, dropping to a reductive regime (Eh7 ≤ 0.05 V) when more than half of the AGB was removed from the pristine level. Mean SRO Fe stock in the reductive soils was significantly lower than that in the oxic soils (Eh7 > 0.05 V), and the loss of SRO Fe correspond to the decomposition of OM in 10–30 cm soils. We suggest that the reductive shift in soil is caused by the weakened evapotranspiration as destructive selective logging inflicts canopy openness and succession failure. This study presents the possibility that forest degradation may reduce the size of protected soil C pool resulting from the loss of soil SRO Fe minerals.

Macrofauna amplify plant litter decomposition and stabilization in arctic soils in a warming climate

The soil organic carbon (SOC) pool of the Arctic region is currently protected by low temperatures, but is likely to decrease due to greater organic matter (OM) decomposition under a warmer climate. Negative feedback for climate warming can, however, be reversed by SOC accrual as climate warming leads to shifts in arctic vegetation (from grass to shrub) and soil faunal (introduction of macrofauna) communities affecting plant-soil C allocation. To decipher these contrasting effects, we performed a laboratory experiment with soils from dry tundra to test the interacting effects of plant litter quality (high-quality grass litter vs. the intermediate- and low-quality litter of shrubs) and soil fauna functional grouping (micro-, meso- and macrofauna [millipede]) on the processes of litter decomposition and OM stabilization.Our findings showed that macrofauna largely promoted decomposition of shrub litter, while soil micro- and mesofauna were mainly responsible for the decomposition of grass litter. Our study thus confirmed that, when introduced and established in a warmer Arctic, macrofauna may become an important agent in shrub litter decomposition. Our data also showed that with shrub litter, higher C content was stabilized as particulate OM (POM) in aggregates, whereas in grass litter and low-quality shrub litter, higher C content was stabilized as mineral-associated OM (MAOM). Both these effects were larger in the presence of macrofauna and with a higher abundance of fungi. This suggests that consequent shrub OM stabilization in occluded POM and MAOM fractions will be carried out jointly by macrofauna and fungi, which will probably lead to more efficient OM stabilization in Arctic soils than in the case of grass litter OM stabilization by micro- and mesofauna and bacteria. In conclusion, our study suggests that vegetation changes and the introduction of macrofauna in a warming climate will most probably lead to higher OM stabilization in Arctic soils.

Distribution of Soil Organic Carbon Density Fractions in Aggregates as Influenced by Salts and Microbial Community

Understanding soil organic carbon (SOC) at the aggregate level is crucial for soil health in secondary-salinized greenhouse development. Nevertheless, the specific patterns and contributions of ion content and microbial communities on SOC density fractions at the aggregate level remain unclear in secondary-salinized soil. We investigated variations in salts [electrical conductivity (EC) and ions] and microbial communities across various aggregate classes in both a 16-year-use greenhouse and open-field soils. We also examined SOC density fractions, including the light fraction (LF), the heavy fraction of particulate organic matter (POM), and mineral-associated organic matter (MOM) across different aggregates. The findings revealed that a lower Ca2+/K+ along with elevated EC levels (average 2.49 mS cm−1) reduced the macroaggregate percentage in greenhouse compared to open-field conditions, with a lower EC of 0.58 mS cm−1. Bacterial diversity and community composition exhibited no variation across different aggregate sizes at both sites. Conversely, fungal diversity and relative abundance (primarily dominated by Ascomycota of 78.50%) substantially increased in microaggregates (&lt;0.25 mm) compared to macroaggregates (&gt;0.25 mm). Macroaggregates exhibited a higher proportion of LF and MOM (3.3–18.2%, 24.9–34.5%, and 2.9–4.0% for LF, MOM, and POM, respectively) than microaggregates. Correlation and redundancy analyses revealed that fungal diversity, particularly the relative abundance of Ascomycota in aggregates &lt; 0.25 mm, significantly and positively influenced (p &lt; 0.05) the proportion of MOM carbon in terms of the overall SOC (J-type). This study provides valuable insights into the distribution patterns of SOC within the secondary salt-affected soils.

Contributions of fine mineral particles and active Al/Fe to stabilization of plant material in neutral-to-alkaline soils of Indo-Gangetic Plain

Factors controlling organic carbon stabilization are elusive in neutral-to-alkaline soils, thereby hindering the assessment of carbon sequestration potential across vast agricultural regions like the Indo-Gangetic Plain (IGP). This study investigated controls over mineralization and stabilization of added organic matter in tropical neutral-to-alkaline soils with low organic carbon (SOC). Using topsoil and subsoil samples from 12 sites of upper-to-lower IGP, we conducted a one-year incubation with and without adding 13C-labeled maize material. We tracked CO2 release and residual C remaining in soil organic matter fractions (free, occluded particulate (oPOM), and mineral-associated organic matter (MAOM)) and analyzed organic matter molecular compositions in incubated soils using pyrolysis-GC/MS. Our results revealed that 48 ± 7 % of added maize C was mineralized, mostly within the first 70 days. Higher active Al/Fe, notably Al, retarded primary maize mineralization by facilitating aggregation. High SOC content and SOC saturation degree resulted in more maize mineralization. The disappearance of maize-unique compounds (e.g., neophytadiene) revealed substantial degradation of added maize. Regarding SOC composition, maize addition increased the relative abundance of fatty acids and decreased that of N-containing compounds. Most residual maize-derived C was found in stabilized fractions, MAOM (77 ± 15 % of residual maize C) and oPOM (8 ± 4 %). Clay fraction contributed to most maize-derived C stabilization as MAOM (path coefficient (β) = 0.81**). Moreover, the significant correlation (P < 0.001) between maize-derived oPOM C and active Al/Fe or clay + silt suggested that active Al/Fe contributed to the stabilization of maize-derived C as oPOM (β = 0.62***) probably by bonding clay and silt particles to form stable aggregates since active Al/Fe content was low (<14 cmol kg−1). Our study highlighted the importance of active Al/Fe in stabilizing SOC, by promoting aggregation and retarding degradation of residue-derived C in neutral-to-alkaline soils.

Unexpected high alkyl carbon contents in organic matter-rich sandy agricultural soils of Northwest Central Europe

Carbon sequestration in Plaggic Anthrosols is most often investigated by bulk soil carbon inventories, without considering the form in which the carbon is stored (e.g., particulate or mineral-associated organic matter (OM), its capacity, or its chemical composition). Here, we focus on the unusual high organic carbon (OC) accumulation in sandy Plaggic Anthrosols and adjacent reference soils under agricultural use. In these soils, the mineral fraction ≤20 µm which is commonly assumed to be the major factor for OC stabilization, are very low in mass proportion. Soil organic matter (SOM) physical fractionation was done to evaluate the quantity and quality of OC in the topsoil (Ap horizon). For the fraction ≤20 µm (medium and fine silt-, and clay-sized particles), we measured the concentration of OC and calculated its OC storage capacity and contribution. The OC of the fraction ≤20 µm was radiocarbon-dated and analyzed for its chemical composition by solid-state 13C NMR spectroscopy. The highly sandy (∼90 % sand and coarse silt) soils showed an accumulation of OC much higher than the conventionally calculated saturation level controlled by the proportion of the fraction ≤20 µm. Unexpectedly, Plaggic Anthrosols and the respective reference soils showed similar fractional OC concentrations, radiocarbon ages, and OM composition. The isolated fraction ≤20 µm contained, on average, 81 % of the total soil OC in only 9 % of the corresponding soil mass. All soil fractions ≤20 µm are characterized by a high mean OC concentration in the topsoil (reference soils: 226 ± 66.5 mg OC g−1, Plaggic Anthrosols: 202 ± 59.0 mg OC g−1) with a C/N ratio of 15 on average for both soils. The OM composition of the fraction ≤20 µm was specifically rich in alkyl-C, with unusually low proportions of O-alkyl-C and low contents of aryl-C. The radiocarbon concentration (F14C) indicated that topsoil OM of the ≤20 µm fraction is stored for long time periods with high mean conventional radiocarbon ages (14C) not only for Plaggic Anthrosols (F14C: 0.92 ± 0.04; 14C: 639 yBP) but also for the reference soils (F14C: 0.93 ± 0.04; 14C: 575 yBP) and received low inputs of OC derived from recent photosynthesis. Our data indicate the existence of specific SOM accumulation processes in the investigated sandy agricultural soils, resulting particularly large SOM stocks which cannot be explained by mechanistic association of OM with mineral surfaces. It is not clear, if this inherited OM is stable under present-day soil and management conditions.

Distribution of soil organic carbon between particulate and mineral-associated fractions as affected by biochar and its co-application with other amendments

The application of biochar to soils, either alone or combined with other amendments, represents a management practice aimed at storing carbon (C) while enhancing soil fertility. However, the long-term effects of biochar application on soil organic C protection against microbial decomposition are uncertain. This study investigated, in a 9-year-long-term field experiment, the protection of organic C by minerals and iron (Fe) in soils amended with biochar alone or combined with either municipal solid waste compost or sewage sludge. Particulate (not protected by minerals) and mineral-associated organic matter fractions were separated, quantified, and finally the Fe-mediated protection mechanisms were examined by Fe K-edge extended X-ray absorption fine structure spectroscopy. With respect to the unamended control soil, soils amended with biochar, especially when combined with municipal solid waste compost and sewage sludge, had 3 times greater content of particulate organic C. Biochar combined with municipal solid waste compost and sewage sludge also increased mineral-associated organic C content (1.5×), although the magnitude of the effect was smaller than for the particulate organic C fraction. The contents of Fe(III)-organic matter complexes in particulate and mineral-associated organic matter fractions of the amended soils were similar to those of the unamended soils. Hematite represented the main Fe oxide phase in the particulate organic matter fraction of all the soils as well as in the mineral-associated organic matter fraction of the unamended soils, whereas ferrihydrite was more abundant in the mineral-associated organic matter fraction of the amended soils. As a whole, the obtained results in general, and the positive effect on the mineral-associated organic matter possibly mediated by ferrihydrite occurrence in particular, highlight the potential of biochar, alone or in combination with other amendments, as a strategy to store and preserve C in soils.

Evidence of Potential Organo-Mineral Interactions during the First Stage of Mars Terraforming

Future space missions to Mars will depend on the development of bioregenerative life support systems. Mars regolith contains most of the nutrients needed for plant growth, but not organic matter (OM). Although Mars simulants have been deeply characterized and tested as growing media, no data are available about their possible modification occurring during terraforming, including the interaction of exogeneous OM with iron (Fe) oxides, particularly abundant in Mars regolith. The aim of this study was to investigate the mineral transformation and the OM evolution occurring in the early stages of the terraforming process. Potato was grown for 99 days on Mojave Mars Simulant MMS-1, alone (R100) and mixed with a compost 70:30 v:v (R70C30), and on a fluvial sand, alone (S100) and mixed with compost (S70C30), for comparison. Bulk (BK) and potato tubero/rhizo-sphere (RH) soils were fractionated to obtain particulate OM (POM) and mineral-associated OM (MAOM). Bulk samples and corresponding fractions were characterized for total nitrogen and organic carbon (C) and analyzed by Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Organic C increased by 10 and 25 times in S70C30 and R70C30, respectively, compared to S100 and R100. Most of the organic C accumulated in the POM fraction of both growing substrates, while its content in the MAOM was 3 times higher in R70C30 than in S70C30. No significant differences between BK and RH were found. Finally, ferrihydrite mediated exogenous OM stabilization in regolith-based substrates, while Fe(III)-OM complexes were detected exclusively in sand-based growing media. Understanding mechanisms and testing potential sustainable practices for creating Mars regolith similar to terrestrial soil will be fundamental to sustain food crop production on Mars.

Mercury Accumulation and Sequestration in a Deglaciated Forest Chronosequence: Insights from Particulate and Mineral-Associated Forms of Organic Matter.

Understanding mercury (Hg) complexation with soil organic matter is important in assessing atmospheric Hg accumulation and sequestration processes in forest ecosystems. Separating soil organic matter into particulate organic matter (POM) and mineral-associated organic matter (MAOM) can help in the understanding of Hg dynamics and cycling due to their very different chemical constituents and associated formation and functioning mechanisms. The concentration of Hg, carbon, and nitrogen contents and isotopic signatures of POM and MAOM in a deglaciated forest chronosequence were determined to construct the processes of Hg accumulation and sequestration. The results show that Hg in POM and MAOM are mainly derived from atmospheric Hg0 deposition. Hg concentration in MAOM is up to 76% higher than that in POM of broadleaf forests and up to 60% higher than that in POM of coniferous forests. Hg accumulation and sequestration in organic soil vary with the vegetation succession. Variations of δ202Hg and Δ199Hg are controlled by source mixing in the broadleaf forest and by Hg sequestration processes in the coniferous forest. Accumulation of atmospheric Hg and subsequent microbial reduction enrich heavier Hg isotopes in MAOM compared to POM due to the specific chemical constituents and nutritional role of MAOM.

No detectable upper limit when predicting soil mineral-associated organic carbon stabilization capacity in temperate grassland of Central Argentina mountains

Soil organic carbon (SOC) is the major active organic carbon (OC) reservoir within terrestrial ecosystems. SOC is stored in different soil organic matter fractions, among which mineral-associated organic matter is considered the more stable. It has been proposed that the OC stored in this fraction (i.e., MAOC-mineral associated organic carbon) depends on the proportion of fine soil particles. This idea has been challenged by studies that found soils with significantly higher MAOC values than predicted by several models. We assessed the soil MAOC contents in central Argentina mountain grasslands. These soils store high OC contents, and thus may constitute a valuable case to evaluate the soils' saturation behavior. We compared the actual soil MAOC content of these mountain grasslands with the predictions from different MAOC saturation models, and we explored if there were indications of saturation behavior by: a) plotting the MAOC against the total SOC contents, and b) by plotting the OC contents in the fine fraction against the fine soil particles content. We found a positive relationship between MAOC and the fine particle content in the soils studied. However, we also found that even the model with the highest upper limit of MAOC stabilization underestimated the MAOC contents of these grasslands. In addition, we found the absence of an inflection point when plotting the MAOC against total SOC contents. Finally, we observed a weak but significant negative relationship between the concentration of OC in the fine fraction and the proportion of the fine fraction of the soil, with high OC loading values in the coarser soils. These last patterns could be indicators of non-saturation behavior. Our results provide additional evidence that many of the models currently used for predicting the OC saturation deficit of soils may be underestimating the potential MAOC stabilization capacity of certain soils. Additionally, our findings contribute to the evidence challenging the ideas of OC saturation on a fine soil particles basis. We highlight some lines of research that should be developed to confirm the existence of a MAOC saturation behavior, as well as to continue exploring the drivers of OC accrual in different systems.

Root but not shoot litter fostered the formation of mineral-associated organic matter in eroded arable soils

Erosion leads to a decline in carbon (C) stocks in arable soils and negatively impacts soil functions worldwide. For soil restoration, it is critical to identify the factors that link crop residue quality to effective C sequestration in the soil, primarily through the formation of mineral-associated organic matter (MAOM) and through incorporation into aggregates (oPOM). The widely accepted concept links effective C stabilization with input of high-quality substrates, but studies of C-deficient soils do not support this assumption. Therefore, we aimed to determine the potential of eroded arable soils to stabilize C from barley shoot and root residues, which represent high- and low-quality inputs, respectively. In a year-long laboratory experiment, we added the residues to two soil pairs (eroded and non-eroded) with different soil textures, observed the formation of oPOM and MAOM and identified microbial groups important for substrate transformation. We found that eroded soils retained added residues very efficiently (35–65% bound residue C), making them a high-priority target for C sequestration. Root residues caused more efficient MAOM formation than shoot residues, primarily by direct binding of depolymerized root-C to mineral surfaces without subsequent microbial transformation. This root C stabilization in MAOM was more pronounced in eroded (highly C-undersaturated) soils than in non-eroded soils and in fine-textured soils, which provided more space for microbial colonization and C sorption, than in coarse-textured soils. Shoot residues were decomposed and metabolized by a microbiome rich in efficient bacterial decomposers (Actinobacteria, Xanthomonadales). This led to inevitably higher C losses related to their growth and biomass turnover, and probably also to an intense priming effect on pre-existing MAOM that lowered the efficiency of MAOM formation. Our results argue for crops with robust root systems, or for the inclusion of deep-rooted plants in crop rotations, which could help rapidly restore the C stocks in arable soils.

Legume cover crops enhance soil organic carbon via microbial necromass in orchard alleyways

Cover crops may enhance soil organic carbon (SOC) sequestration, but the links among SOC fractions, microbial necromass carbon (C), and SOC sequestration potential in legume and non-legume cover-cropped orchards are unclear. We leveraged data from seven orchards with varying climatic and edaphic properties in China to assess the effects of legume and non-legume cover crops on SOC and microbial necromass C in bulk soil, particulate organic matter (POM), and mineral-associated organic matter (MAOM). Legume cover crops led to a significantly greater increase in SOC than non-legumes (23 % vs. 2 %), compared with no cover crops. The MAOM-C with legumes and non-legumes were similar but were both significantly greater than the control plots without cover crops. Legume cover crops positively influenced bacterial and fungal necromass C in all SOC fractions, while non-legume cover crops only affected fungal necromass C in bulk soil and MAOM. The effect of cover crops on the microbial necromass C to SOC ratio was negligible. Fungal necromass contributed most of the microbial necromass C to SOC accumulation, particularly in MAOM. Linear regression showed SOC, POM-C, and MAOM-C accumulated with increasing microbial necromass C. Microbial necromass C was the key factor explaining the variation in POM-C and MAOM-C compared to other climatic and edaphic factors. Partial least squares path modeling showed a causal relationship among cover crops, climate, soil substrates, soil properties, microbial necromass, and SOC and SOC fractions. Our study suggests that changes in microbial necromass C and composition may explain most of the increase in SOC in cover-cropped orchards and that MAOM-C is the principal sink of microbial necromass. This information is valuable for improving our understanding of the potential impact of microbial necromass C on enhancing SOC sequestration potential in orchard soils under cover crop management.

Response to: "The robust concept of mineral-associated organic matter saturation: A letter to Begill et al. (2023)".

In this response to a letter to the editor, we provide evidence that the findings regarding a non-detectable limit of mineral-associated organic carbon as published in Begill et al. (2023) are robust. This is mainly done by showing that no methodological bias was present and that the main correlation was not driven by a few exceptional soils.