Soil Organic Matter Stabilization Research Articles

Soil organic matter components and characteristics of forest soil in spruce and sycamore plantations in the temperate region

The stability of soil organic matter (SOM) that governs soil organic carbon (SOC) storage depends on its characteristics and components, but little is known about how tree species in forest ecosystems affect SOM components and characteristics. In this study, we used FTIR spectroscopy to investigate plantations of two ecologically and economically significant tree species—namely, spruce (Picea spp.) and sycamore (Acer pseudoplatanus)—in order to determine how the different litter inputs and root-microbe interactions of these two plantations affect the functional groups, components, and characteristics of their SOM. Soil samples were taken from the topsoil (0–10 cm) and subsoil (10–20 cm). In the 0–10 cm soil depth, the SOM's hydrophilic, hydrophobic, and aromatic components differ between the spruce and sycamore plantations. The hydrophobic components constitute the primary constituents of the SOM of the two forest plantations, in contrast to the expected predominance of the hydrophilic component of the SOM. Also, the high hydrophobicity (hydrophilic/hydrophobic) in the subsoil of the spruce plantations was attributed to a decrease in hydrophilic components and a subsequent increase in hydrophobic components of the SOM. The sycamore plantations exhibited a higher SOM aromaticity and a greater degree of decomposition than the spruce plantations. The aforementioned distinctions emphasise the contrasting mechanisms involved in transforming and turnover of the two-tree species' soil organic matter (SOM).

Molecular Composition of Humic Acids of Different Aged Fallow Lands and Soils of Different Types of Use in Northwest of Russia

Post-agrogenic transformation of fallow soils leads to changes in soil carbon content, the molecular composition of humic substances, and rates of organic matter stabilization, which can affect climate change on the planet. In this regard, we analyzed the molecular composition of humic acids isolated from natural and fallow soils in the southern Taiga zone of northwest Russia. Different-aged soils on fallow lands represent a model of soil transformation in time, and data on the transformation of soil humic acid molecular composition make a significant contribution to the understanding of soil organic matter stabilization aspect issues. In this case, the molecular structure of humic acids isolated from natural and fallow soils in northwest Russia was analyzed. To study the molecular composition of HAs, the elemental composition was analyzed, and 13C (CP/MAS) NMR spectroscopy of HAs isolated from different aged abandoned soils and soils of different types of use was carried out. The obtained data showed that with the increasing age of soils in the fallow state, there is an increase in the carbon content of humic acids as well as a decrease in nitrogen content. As a result of the increasing age of soils in the fallow state, there are dynamics in the content of aromatic structural fragments in humic acids: 34% for 40 years old, 28% for 80 years old, and 31% for 120 years old. This is due to changes in the precursors of humification and the further transformation of plant residues in the soil. Re-involved fallow land soils lead to an increase in the content of aromatic structural fragments in the composition of HA in relation to HA extracted from mature soils. The lowest content of aromatic structural fragments was observed in the humic acids of 130-year-old agricultural soil, which is associated with the long-term application of organic fertilizers.

The Use of Products from Leonardite to Improve Soil Quality in Condition of Climate Change

Abstract The principle of sustainable fertilisation in modern agriculture, which ensures that the soil is maintained in a fertile state requires the search for alternatives to manure fertilisation for products based on humic substances to increase the organic matter content of soils.The advantage of leonardite over conventional natural and organic fertilisers is mainly due to its high content of organic matter and humic substances (humic and fulvic acids). Leonardite, due to the presence of humic acids in it, can be suitable for soil amendment and yield of barley. In the conducted studies, the yield increase after adding Rosahumus fertilizer to the soil at a rate of 6 kg.ha−1 + NPK (Nitrogen, Phosphorus, Potasssium) was found to be 11% compared to the control (NPK).There were no changes in the Corg. content of the soil as a result of the application of Rosahumus fertilizer with NPK. The fertilizer only slightly modified the fractional composition of humus. In the future, we should expect an increase in the use of leonardite-derived humic substances in fertilizers and soil conditioners to stabilize soil organic matter.

Conversion of Natural Soil to Paddy Promotes Soil Organic Matter Degradation in Small-Particle Fractions: δ13C and Lipid Biomarker Evidence

The stabilization mechanism of soil organic matter (SOM) has received considerable attention. It is widely accepted that mineral sorption/protection is important for SOM stabilization. However, it remains unclear which organic carbon component is beneficial for mineral protection. We collected soil samples from a paddy field (TP) to compare with natural soil (NS). To illustrate the behavior of different SOM pools and their protection by particles, we separated the soils into different particle-size fractions and then removed the active minerals using an acid mixture (1 M HCl/10% HF). The different carbon pools were analyzed using stable carbon isotopes and lipid biomarkers. Our study showed that acid treatment evidently increased the extractability of free lipids, usually over 60%, which confirmed the predominant role of minerals in SOM protection. For NS, the δ13C values increased with decreasing soil particle sizes and soil depths, indicating that 13C-enriched SOM was selectively preserved. However, this trend disappeared after cultivation, which was mainly attributed to the combined effects of the input of 13C-depleted fresh SOM and decomposition of the preserved 13C-enriched SOM. Meanwhile, based on the degradation parameters of the overall lipid biomarkers, SOM showed higher degradation states in clay and silt fractions than in the sand fraction before cultivation. It is possible that the small particle-size fractions could selectively absorb highly degraded SOM. The clay-associated SOM showed a low degradation state, but its carbon content was low after cultivation. We propose that the previously protected SOM was degraded after cultivation and was replaced by relatively fresh SOM, which should be carefully monitored during SOM management.

Soil organic matter dynamics and stability: Climate vs. time

Climate and time are among the most important factors driving soil organic carbon (SOC) stability and accrual in mineral soils; however, their relative importance on SOC dynamics is still unclear. Therefore, understanding how these factors covary over a range of soil developmental stages is crucial to improve our knowledge of climate change impact on SOC accumulation and persistence. Two chronosequences located along a climate gradient were investigated to determine the main interactions among time (age) and climate (precipitation and temperature) on SOC stability and stock with depth. Considering a common depth (0–15 or 0–30 cm), in the drier chronosequence, the older soil showed the highest SOC stock, while the younger exhibited the lowest carbon accumulation. Considering the whole profile, the SOC stock increased with age. In the wetter chronosequence, the younger soil showed the highest SOC stock considering a common depth, whereas, when the entire profile is taken into account, the older one accumulated 2–3 times more SOC than the others. In both chronosequences, significant stocks of SOC (~42 %) were accumulated >30 cm. Soil organic matter stability, assessed by thermal analysis and heterotrophic respiration, increases with depth and age only in the drier chronosequence. Soils from the wetter chronosequence were instead characterized by a greater quantity of labile and/or not-stabilized SOC; here, the amorphous Fe/Al-rich secondary mineral weathering products showed an essential predictor function of SOC storage, although they do not seem to be involved in SOC stabilization mechanisms. Otherwise, the interaction of SOC with fine particles, short-range order minerals, and organo-metal complexes represent the significant stabilization mechanisms in soils from drier climate. The results highlighted how the age factor plays an unassuming role in geochemical processes influencing SOC dynamics; however, climate determines different trajectories of soil development and SOC dynamics for a given soil age. Thus, soil age shows a key role in SOC stabilization especially in drier climatic conditions, while wetter conditions determine an accumulation of a higher yet more labile amount of SOC.

Interaction between ectomycorrhizal and ericoid mycorrhizal plants decelerates stable soil organic matter decomposition

Boreal forests are N-limited ecosystems storing globally significant amounts of carbon (C) belowground as soil organic matter (SOM). The significant role of mycorrhizal fungi, both ericoid (ERM) and ectomycorrhizal (ECM) in decomposition and building up SOM has been proposed, but it is still largely unknown how these two types of mycorrhiza interact in terms of SOM changes. Here we try to elucidate the combined effect of these mycorrhizal types on the SOM decomposition and accumulation in a pot study with ECM-pine (Pinus sylvestris L.) and ERM-heather (Calluna vulgaris L.). Experimental design included plants growing alone or in a mixture, and a non-planted control. In contrast to the mixture, heather growing alone exerted higher biomass, higher uptake of N from stable SOM pool, slightly higher microbial biomass C and fungal biomass. Moreover, heather soil had the highest dissolved organic carbon and nitrogen. On the contrary, mixed cultures showed lower growth of heather and stable SOM decomposition. Our results suggest that interaction between ERM and ECM plants may lead to stable SOM accumulation in boreal forest soils.

No-tillage farming for two decades increases plant- and microbial-biomolecules in the topsoil rather than soil profile in temperate agroecosystem

The impact of conservation tillage on the composition, stability, and origin (plant- or microbial-derived) of soil organic matter (SOM) within soil profiles remains unclear. In this study, we characterized the impact of different tillage practices on the molecular composition and status of SOM. These tillage practices included conventional tillage (CT) and conservation tillage such as rotary tillage (RT) and no-tillage (NT). Our investigation spanned a two-decade period within a temperate agroecosystem. Soil samples were collected down to a 30 cm profile, and four biomarkers, namely free lipids, lignin phenols, neutral sugars, and amino sugars, in conjunction with 13C NMR techniques were used to quantify SOM characteristics. The results revealed that conservation tillage (especially NT) led to improvements in both the quantity and composition of SOM when compared to CT in the topsoil (0–5 cm), but not in deeper layers (5–30 cm). More precisely, the NT topsoil, as opposed to CT, exhibited enhancements in two categories: (1) an increase in plant-derived compounds such as long-chain lipids (≥ C20) and steroids by 51%, lignin phenols by 106%, and plant-derived neutral sugars by 61%, and (2) an augmentation in microbial-derived biomolecules, which includes microbial-derived neutral sugars by 49% and microbial necromass carbon (MNC) by 94%. Furthermore, NT, relative to CT, increased the contribution of MNC to soil organic carbon in topsoil (up to 64%, mainly fungal necromass). This highlighted the predominant role of microbial-derived biomolecules in SOM formation. Similarly, RT treatments enhanced the microbial-derived neutral sugars by 18%, plant-derived neutral sugars by 14%, and lignin phenols by 43% relative to CT in the topsoil. Collectively, our study suggests that NT practice could be considered an effective strategy for enhancing SOM accumulation and persistence by fostering the accrual of plant- and microbial-derived biomolecules in the topsoil.

Thermodegradation of organic matter in soils of different mineral composition in Brazil

Thermodegradation is an analytical tool that quantifies the loss of organic carbon under increasing temperatures. The degradation process is related to chemical functions and provides data on the stability of soil organic matter (SOM). There needs to be more information about SOM stability in different Brazilian ecosystems. This study aimed to evaluate SOM stability under three tropical environments: Dry Forest, Brazilian Savanna, and Atlantic Forest. Thus, we related thermal stability to soil mineral composition and SOM quality and quantity. We analyzed samples of the surface horizon of four illitic soil profiles sampled in Dry Forest (P1 to P4), three soil profiles in Cerrado (Brazilian Savanna), with predominant 2:1 clay (P5), iron oxide (P6), and gibbsite (P7), and a kaolinite profile from Atlantic Forest (Rainforest) (P8). Mineralogical, chemical, and physical analyses of the soils were studied to better comprehend soils' behavior under thermodegradation. The thermic treatment was performed by heating air-dried fine earth (ADFE) samples at 100, 200, 300, 400, and 500 °C for 2 h. The results were submitted to sigmoidal regression with four points and presented a good curve fit (r2 > 0.99). Dry Forest soils with the most humified SOM presented greater resistance to loss of SOM by the thermic process. However, P7 (gibbsitic clayey soil) stood out and was the most resistant to thermodegradation. P4 was the sandiest soil and showed the curve most affected by the thermal process, indicating the importance of clay content to SOM stability. The results suggest that the TOC (content and quality), calcium content, and clay (content and type) play essential roles in determining the thermostability of SOM in different ecosystems.

Effect of temperature, water availability, and soil properties on soil CO2 efflux in beech-fir forests along the Carpathian Mts

Carpathian Mountain beech-fir forests are exposed to severe pressures related to climate change. Alterations in soil respiration, the main source of CO2 emissions from forest ecosystems into the atmosphere, could potentially impact the future carbon balance of these ecosystems. We performed soil CO2 efflux measurements along the whole Carpathian arc on eight selected locations during two separate campaigns in 2022 and 2023. The correlations between soil CO2 efflux and various climatic, micrometeorological, and soil conditions were evaluated as well as the impact of canopy gaps at each site.Soil CO2 efflux varied between the two campaigns primarily due to fluctuations in soil water content, while differences among the sites were more influenced by soil carbon and nitrogen content. The presence of a canopy gap did not consistently affect soil CO2 efflux; in some sites, it was lower in the gap than under a closed canopy, while in others, the opposite trend was observed. In conclusion, soil CO2 efflux measured at eight sites was closely associated with soil properties rather than with climate or micrometeorological parameters. Therefore, the potential influence of climate on the input of new organic matter through forest productivity and species composition will play a significant role in the decomposition and storage of soil organic matter.

Contribution of the Soil Macro- and Microstructure to Organic Matter Stabilisation in Natural and Post-Mining/Industrial Soils under Temperate Climatic Conditions

The soil organic matter (SOM) content and stability in natural gypsum soils and mining/industrial soils were compared to check the effects of selected soil properties (mainly macro- and microstructure) on SOM stability and determine whether the old brownfield soils regardless of being highly polluted with trace metals could store high amounts of SOM. The mining/industrial soils were 50–400 years old and so had been left sufficiently long for full self-restoration. Despite having very different origins, both natural and brownfield soils stored similar amounts of SOM and had similar pH values, calcium carbonate contents, and textures but differed in SOM stabilisation, which was expressed by higher C/N ratios, lower aggregate water resistance index, lower dehydrogenase activity, and greater areas of undecomposed or slightly decomposed plant residues in mining/industrial soils compared to gypsum ones. However, the differences diminished with time, and in the oldest (150–400 years) brownfield soils, these parameters were similar to those in natural soils. Multiple regression analysis indicated that under the study conditions, SOM amounts, besides CaCO3 contents and dehydrogenase activity were also strongly affected by porosity and microaggregation. In the research we showed the role of degraded soils in the sustainable use of the environment.

Moisture and temperature effects on the radiocarbon signature of respired carbon dioxide to assess stability of soil carbon in the Tibetan Plateau

Abstract. Microbial release of CO2 from soils to the atmosphere reflects how environmental conditions affect the stability of soil organic matter (SOM), especially in massive organic-rich ecosystems like the peatlands and grasslands of the Qinghai–Tibetan Plateau (QTP). Radiocarbon (14C) is an important tracer of the global carbon cycle and can be used to understand SOM dynamics through the estimation of time lags between carbon fixation and respiration, often assessed with metrics such as age and transit time. In this study, we incubated peatland and grassland soils at four temperature (5, 10, 15 and 20 °C) and two water-filled pore space (WFPS) levels (60 % and 95 %) and measured the 14C signature of bulk soil and heterotrophic respired CO2. We compared the relation between the Δ14C of the bulk soil and the Δ14CO2 of respired carbon as a function of temperature and WFPS for the two soils. To better interpret our results, we used a mathematical model to analyse how the calculated number of pools, decomposition rates of carbon (k), transfer (α) and partitioning (γ) coefficients affect the Δ14C bulk and Δ14CO2 relation, with their respective mean age and mean transit time. From our incubations, we found that 14C values in bulk and CO2 from peatland were significantly more depleted (old) than from grassland soil. Our results showed that changes in temperature did not affect the Δ14C values of heterotrophic respired CO2 in either soil. However, changes in WFPS had a small effect on the 14CO2 in grassland soils and a significant influence in peatland soils, where higher WFPS levels led to more depleted Δ14CO2. In our models, the correspondence between Δ14C, age and transit time highly depended on the internal dynamics of the soil (k, α, γ and number of pools) as well as on model structure. We observed large differences between slow and fast cycling systems, where low values of decomposition rates modified the Δ14C values in a non-linear pattern due to the incorporation of modern carbon (14C bomb) in the soil. We concluded that the stability of carbon in the peatland and grassland soils of the QTP depends strongly on the direction of change in moisture and how it affects the rates of SOM decomposition, while temperature regulates the number of fluxes. Current land cover modification (desiccation) in Zoigê peatlands and climate change occurring on the QTP might largely increase CO2 fluxes along with the release of old carbon to the atmosphere potentially shifting carbon sinks into sources.

Litter and root sources of soil organic matter in a temperate forest: Thirty years in the DIRT

AbstractManagers increasingly seek to increase forest soil carbon but long‐term controls on soil organic matter (SOM) sources and stability are weakly understood. We used a 30‐year detrital input/removal treatment experiment in a deciduous forest to evaluate the importance of root and leaf litter to SOM. Inputs were assessed by excluding roots and leaves (litter) or by doubling litter inputs. In mineral soil, %SOM differed only at 0–10 cm (p = 0.11), with concentrations in the no litter and no roots treatments of 26% and 9%, respectively, lower than the controls. Cessation of litter inputs had a stronger effect on SOM than cessation of root inputs, but root litter inputs may contribute stable SOM. Doubled litter increased mineral SOM.

Long-term maize-soybean rotation in Northeast China: impact on soil organic matter stability and microbial decomposition

Long-term maize-soybean rotation in Northeast China: impact on soil organic matter stability and microbial decomposition

Early production of switchgrass (Panicum virgatum L.) and willow (Salix spp.) indicates carbon accumulation potential in Appalachian reclaimed mine and agriculture soil

AbstractThe production of bioproduct feedstocks such as switchgrass (Panicum virgatum L.) and willow (Salix spp.) on degraded lands provides an opportunity to grow dedicated bioenergy crops with the potential to capture and store carbon in the soil while reducing competition with land for food production. However, how the production of these crops alters plant–soil–microbe interactions that govern soil C accumulation in highly degraded soil is underexplored. The objectives of this study were to examine select biological and chemical properties related to stable soil organic matter (SOM) production from the growth of switchgrass and willow on marginal soil over two growing seasons and whether biochar amendment can positively affect these parameters. To address our objectives, paired former surface mined lands and non‐mine impacted marginal agriculture sites were selected across West Virginia, USA, and biochar and unamended control treatments were imposed. Through the first two growing seasons, microbial activity and demand for carbon (C) increased and was accompanied by a shift in extracellular enzyme investment for decomposition‐associated enzymes. Mineral‐associated organic matter C increased over the two growing seasons, and this increase was greater in the mine sites compared to the agriculture sites. Compared to each site's previous land use, C losses were observed under bioproduct systems in the agriculture, but not the mine sites. Biochar amendments did not impact microbial activity but did increase the C:N of SOM. Overall, our results suggest that the early growth of switchgrass and willow can result in C accumulation in marginal and highly degraded lands.

Permafrost degradation and its consequences for carbon storage in soils of Interior Alaska

Permafrost soils in the northern hemisphere are known to harbor large amounts of soil organic matter (SOM). Global climate warming endangers this stable soil organic carbon (SOC) pool by triggering permafrost thaw and deepening the active layer, while at the same time progressing soil formation. But depending, e.g., on ice content or drainage, conditions in the degraded permafrost can range from water-saturated/anoxic to dry/oxic, with concomitant shifts in SOM stabilizing mechanisms. In this field study in Interior Alaska, we investigated two sites featuring degraded permafrost, one water-saturated and the other well-drained, alongside a third site with intact permafrost. Soil aggregate- and density fractions highlighted that permafrost thaw promoted macroaggregate formation, amplified by the incorporation of particulate organic matter, in topsoils of both degradation sites, thus potentially counteracting a decrease in topsoil SOC induced by the permafrost thawing. However, the subsoils were found to store notably less SOC than the intact permafrost in all fractions of both degradation sites. Our investigations revealed up to net 75% smaller SOC storage in the upper 100 cm of degraded permafrost soils as compared to the intact one, predominantly related to the subsoils, while differences between soils of wet and dry degraded landscapes were minor. This study provides evidence that the consideration of different permafrost degradation landscapes and the employment of soil fractionation techniques is a useful combination to investigate soil development and SOM stabilization processes in this sensitive ecosystem.

Soybean inclusion reduces soil organic matter mineralization despite increasing its temperature sensitivity

Legume-based cropping increased the diversity of residues and rhizodeposition input into the soil, thus affecting soil organic matter (SOM) stabilization. Despite this, a comprehensive understanding of the mechanisms governing SOM mineralization and its temperature sensitivity across bulk soil and aggregate scales concerning legume inclusion remains incomplete. Here, a 6-year field experiment was conducted to investigate the effects of three cropping systems (i.e., winter wheat/summer maize, winter wheat/summer maize-soybean, and nature fallow) on SOM mineralization, its temperature sensitivity, and the main drivers in both topsoil (0–20 cm) and subsoil (20–40 cm). Soybean inclusion decreased the SOM mineralization by 17%–24%, while concurrently increasing the majority of soil biochemical properties, such as carbon (C) acquisition enzyme activities (5%–22%) and microbial biomass C (5%–9%), within the topsoil regardless of temperature. This is attributed to the increased substrate availability (e.g., dissolved organic C) facilitating microbial utilization, thus devoting less energy to mining nutrients under diversified cropping. In addition, SOM mineralization was lower within macroaggregates (∼12%), largely driven by substrate availability irrespective of aggregate sizes. In contrast, diversified cropping amplified the Q10 of SOM mineralization in mesoaggregates (+6%) and microaggregates (+5%) rather than in macroaggregates. This underscores the pivotal role of mesoaggregates and microaggregates in dominating the Q10 of SOM mineralization under soybean-based cropping. In conclusion, legume-based cropping diminishes soil organic matter mineralization despite increasing its temperature sensitivity, which proposes a potential strategy for C-neutral agriculture and climate warming mitigation.

Chemical characterization of soil organic matter in differents management practices in the Cerrado-Pantanal ecotone

Different land uses and occupations have a direct impact on edaphic attributes, which can be identified by soil quality indicators, with organic matter being an excellent edaphic quality indicator. The objective of the work was to chemically characterize soil organic matter (SOM) in different management systems in the municipality of Aquidauana, Mato Grosso do Sul, Brazil. Soil samples were collected in five areas with different management practices, in addition to an area of native Cerrado forest (NF), and the managed areas were: exposed soil (ES), conventional tillage system (CTS), no-tillage (NTS), permanent pasture (PP) and sugarcane (SC). In addition to the determination of bulk density (Bd), total organic carbon (TOC) contents and stocks were quantified, as well as carbon (C) contents and stocks of the humified fractions: humic acid (AH), fulvic acid (AF) and humin (HUM), with subsequent calculation of HA/FA and alkaline extract/humin (AE/HUM) and TOC stratification index (SI). There were no significant changes for Ds after six years of implementation of management systems. TOC levels and stocks were higher in PP and NTS areas. The HUM fraction was predominant, with higher contents and stocks of all fractions for the PP and NTS areas, demonstrating the greater intensity of the humification process in these management systems, with greater stability of C in the soil after six years of experiment. The PCA indicated a difference between the areas that present conservation practices in the ES and CTS areas. The NF, NTS, PP and SC areas contribute to SOM stabilization.

Elevated atmospheric CO2 drives decreases in stable soil organic carbon in arid ecosystems: Evidence from a physical fractionation and organic compound analysis.

The increasing concentration of CO2 in the atmosphere is perturbing the global carbon (C) cycle, altering stocks of organic C, including soil organic matter (SOM). The effect of this disturbance on soils in arid ecosystems may differ from other ecosystems due to water limitation. In this study, we conducted a density fractionation on soils previously harvested from the Nevada Desert FACE Facility (NDFF) to understand how elevated atmospheric CO2 (eCO2 ) affects SOM stability. Soils from beneath the perennial shrub, Larrea tridentata, and from unvegetated interspace were subjected to a sodium polytungstate density fractionation to separate light, particulate organic matter (POM, <1.85 g/cm3 ) from heavier, mineral associated organic matter (MAOM, >1.85 g/cm3 ). These fractions were analyzed for organic C, total N, δ13 C and δ15 N, to understand the mechanisms behind changes. The heavy fraction was further analyzed by pyrolysis GC/MS to assess changes in organic compound composition. Elevated CO2 decreased POM-C and MAOM-C in soils beneath L. tridentata while interspace soils exhibited only a small increase in MAOM-N. Analysis of δ13 C revealed incorporation of new C into both POM and MAOM pools indicating eCO2 stimulated rapid turnover of both POM and MAOM. The largest losses of POM-C and MAOM-C observed under eCO2 occurred in soils 20-40 cm in depth, highlighting that belowground C inputs may be a significant driver of SOM decomposition in this ecosystem. Pyrolysis GC/MS analysis revealed a decrease in organic compound diversity in the MAOM fraction of L. tridentata soils, becoming more similar to interspace soils under eCO2 . These results provide further evidence that MAOM stability may be compromised under disturbance and that SOC stocks in arid ecosystems are vulnerable under continued climate change.

Functional organic matter components in mangrove soils revealed by density fractionation

ABSTRACT The mechanisms underlying stabilization of soil organic matter (SOM) in vegetated coastal ecosystems, including mangrove forests, are poorly understood, limiting our ability to predict the consequences of disturbances. Here, we introduce density fractionation to mangrove soils to identify the distribution and properties of the functional components of SOM regarding degradation state, stability, and origin, namely, the free low-density fraction (f-LF), mineral-associated LF (m-LF), and high-density fraction (HF). Three soil cores (1 m) were collected in a mangrove forest on Ishigaki Island, Japan, and cut into 10 cm intervals and analyzed. Although HF was the most abundant, the massive production of mangrove fine roots resulted in a high abundance of LFs throughout the cores, which markedly differed from terrestrial soils. Relative abundance of LFs together accounted for 38–66% of total soil C. The m-LF was as abundant as f-LF and 1.6 times higher in relative abundance than the global average of terrestrial soils. The C/N ratios and δ13C values clearly increased with depth in all fractions, which was attributed to the increased contribution from roots. We found a consistent pattern in Δ14C values of density fractions. HF was the oldest with Δ14C between −149‰ and −97‰ followed by m-LF (between −130‰ and −87‰) and then f-LF (between −89‰ and 78‰), suggesting that mineral association may be pivotal in long-term carbon storage in the mangrove mineral soil. Our analysis successfully identified meaningful functional components of mangrove SOM, yet several questions remained unanswered, including a large variability in Δ14C in different cores. Future studies would benefit from a coupled analysis of the quantity and quality of density fractions and geochemical factors in mangrove soils.

Stabilized microbial necromass in soil is more strongly coupled with microbial diversity than the bioavailability of plant inputs

Microbial necromass carbon (C) can substantially contribute to stabilized soil organic matter (SOM), and effective management of this C may help mitigate climate change. However, factors important to the formation of microbial necromass are only partly understood. While bioavailable plant inputs may induce necromass formation by boosting microbial growth and C use efficiency, other microbial traits, such as those related to secretion systems or adhesion and motility, may also be relevant. These traits may be independent of the bioavailability of plant inputs and modulated by environmental factors such as soil depth or site age. Such links, however, have hardly been studied.Here, we used replicated plots of European alder (more bioavailable inputs) and Scots pine (less bioavailable inputs) to investigate links among plant inputs, soil depth, site age, microbial community composition, and microbial necromass C in stabilized SOM, i.e., particulate organic matter occluded within aggregates (oPOM) and mineral-associated organic matter (MAOM).We did not find evidence that bioavailable plant inputs, nor soil depth and site age, were major drivers of microbial necromass formation. Instead, certain microbial taxa, and microbial diversity in particular, were most tightly related to microbial necromass C in MAOM. Microbial necromass C also substantially contributed to oPOM (up to ∼57% of the C stored in that fraction), a C pool considered to largely derive from plant biomolecules. Combined, however, microbial necromass C in oPOM and MAOM only accounted for ∼23% of bulk C contents. Our results imply that effective C-focused research and management have to consider constraints on microbial community composition and diversity, microbial necromass in pools other than MAOM, and formation of plant-derived SOM.