Soil Organic Carbon Persistence Research Articles

Shining a Light on How Soil Organic Carbon Behaves at Fine Scales under Long-Term Elevated CO2: An 8 Year Free-Air Carbon Dioxide Enrichment Study.

Building and protecting soil organic carbon (SOC) are critical to agricultural productivity, soil health, and climate change mitigation. We aim to understand how mechanisms at the organo-mineral interfaces influence SOC persistence in three contrasting soils (Luvisol, Vertisol, and Calcisol) under long-term free air CO2 enrichment conditions. A continuous wheat-field pea-canola rotation was maintained. For the first time, we provided evidence to a novel notion that persistent SOC is molecularly simple even under elevated CO2 conditions. We found that the elevated CO2 condition did not change the total SOC content or C forms compared with the soils under ambient CO2 as identified by synchrotron-based soft X-ray analyses. Furthermore, synchrotron-based infrared microspectroscopy confirmed a two-dimensional microscale distribution of similar and less diverse C forms in intact microaggregates under long-term elevated CO2 conditions. Strong correlations between the distribution of C forms and O-H groups of clays can explain the steady state of the total SOC content. However, the correlations between C forms and clay minerals were weakened in the coarse-textured Calcisol under long-term elevated CO2. Our findings suggested that we should emphasize identifying management practices that increase the physical protection of SOC instead of increasing complexity of C. Such information is valuable in developing more accurate C prediction models under elevated CO2 conditions and shift our thinking in developing management practices for maintaining and building SOC for better soil fertility and future environmental sustainability.

Controls and relationships of soil organic carbon abundance and persistence vary across pedo-climatic regions.

One of the largest uncertainties in the terrestrial carbon cycle is the timing and magnitude of soil organic carbon (SOC) response to climate and vegetation change. This uncertainty prevents models from adequately capturing SOC dynamics and challenges the assessment of management and climate change effects on soils. Reducing these uncertainties requires simultaneous investigation of factors controlling the amount (SOC abundance) and duration (SOC persistence) of stored C. We present a global synthesis of SOC and radiocarbon profiles (nProfile = 597) to assess the timescales of SOC storage. We use a combination of statistical and depth-resolved compartment models to explore key factors controlling the relationships between SOC abundance and persistence across pedo-climatic regions and with soil depth. This allows us to better understand (i) how SOC abundance and persistence covary across pedo-climatic regions and (ii) how the depth dependence of SOC dynamics relates to climatic and mineralogical controls on SOC abundance and persistence. We show that SOC abundance and persistence are differently related; the controls on these relationships differ substantially between major pedo-climatic regions and soil depth. For example, large amounts of persistent SOC can reflect climatic constraints on soils (e.g., in tundra/polar regions) or mineral absorption, reflected in slower decomposition and vertical transport rates. In contrast, lower SOC abundance can be found with lower SOC persistence (e.g., in highly weathered tropical soils) or higher SOC persistence (e.g., in drier and less productive regions). We relate variable patterns of SOC abundance and persistence to differences in the processes constraining plant C input, microbial decomposition, vertical C transport and mineral SOC stabilization potential. This process-oriented grouping of SOC abundance and persistence provides a valuable benchmark for global C models, highlighting that pedo-climatic boundary conditions are crucial for predicting the effects of climate change and soil management on future C abundance and persistence.

Microbially mediated mechanisms underlie soil carbon accrual by conservation agriculture under decade-long warming

Increasing soil organic carbon (SOC) in croplands by switching from conventional to conservation management may be hampered by stimulated microbial decomposition under warming. Here, we test the interactive effects of agricultural management and warming on SOC persistence and underlying microbial mechanisms in a decade-long controlled experiment on a wheat-maize cropping system. Warming increased SOC content and accelerated fungal community temporal turnover under conservation agriculture (no tillage, chopped crop residue), but not under conventional agriculture (annual tillage, crop residue removed). Microbial carbon use efficiency (CUE) and growth increased linearly over time, with stronger positive warming effects after 5 years under conservation agriculture. According to structural equation models, these increases arose from greater carbon inputs from the crops, which indirectly controlled microbial CUE via changes in fungal communities. As a result, fungal necromass increased from 28 to 53%, emerging as the strongest predictor of SOC content. Collectively, our results demonstrate how management and climatic factors can interact to alter microbial community composition, physiology and functions and, in turn, SOC formation and accrual in croplands.

Chemical composition of soil carbon is governed by microbial diversity during understory fern removal in subtropical pine forests

Understory vegetation has an important impact on soil organic carbon (SOC) accumulation. However, little is known about how understory vegetation alters soil microbial community composition and how microbial diversity contributes to SOC chemical composition and persistence during subtropical forest restoration. In this study, removal treatments of an understory fern (Dicranopteris dichotoma) were carried out within pine (Pinus massoniana) plantations restored in different years in subtropical China. Soil microbial community composition and microbial diversity were measured using phospholipid fatty acids (PLFAs) biomarkers and high-throughput sequencing, respectively. The chemical composition of SOC was also measured via solid-state 13C nuclear magnetic resonance (13C NMR). Our results showed that fern removal decreased alkyl C by 4.2 % but increased O-alkyl C by 15.6 % on average, leading to a decline of alkyl C/O-alkyl C ratio, suggesting altered chemical composition of SOC and lowered SOC recalcitrance without fern. Fern removal significantly lowered the fungi-to-bacteria ratio, and it also reduced fungal and bacterial diversity. Partial correlation analysis revealed that soil nitrogen availability was a key factor influencing microbial diversity. Bacterial diversity showed a close relationship with the Alkyl C/O-alkyl C ratio following fern removal. Furthermore, the microbial community structure and bacterial diversity were responsible for 18 % and 55 % of the explained variance in the chemical composition of SOC, respectively. Taken together, these analyses jointly suggest that bacterial diversity exerts a greater role than microbial community structure in supporting SOC persistence during understory fern removal. Our study emphasizes the significance of understory ferns in supporting microbial abundance and diversity as a means of altering SOC persistence during subtropical forest restoration.

Calcium promotes persistent soil organic matter by altering microbial transformation of plant litter

Calcium (Ca) can contribute to soil organic carbon (SOC) persistence by mediating physico-chemical interactions between organic compounds and minerals. Yet, Ca is also crucial for microbial adhesion, potentially affecting colonization of plant and mineral surfaces. The importance of Ca as a mediator of microbe-mineral-organic matter interactions and resulting SOC transformation has been largely overlooked. We incubated 44Ca labeled soils with 13C15N labeled leaf litter to study how Ca affects microbial transformation of litter and formation of mineral associated organic matter. Here we show that Ca additions promote hyphae-forming bacteria, which often specialize in colonizing surfaces, and increase incorporation of litter into microbial biomass and carbon use efficiency by approximately 45% each. Ca additions reduce cumulative CO2 production by 4%, while promoting associations between minerals and microbial byproducts of plant litter. These findings expand the role of Ca in SOC persistence from solely a driver of physico-chemical reactions to a mediator of coupled abiotic-biotic cycling of SOC.

Nutrient availability and stoichiometry mediate microbial effects on soil carbon sequestration in tropical forests

The persistence of soil organic carbon (SOC) is primarily driven by microbial metabolic activities; however, how microbial effects on SOC sequestration are affected by soil nutrient status remains unclear. Here, we conducted a one-year-long in situ soil incubation experiment using mesh bags (with a mesh size of 38 μm, allowing bacterial colonization and fungal hyphal penetration while preventing root penetration). This experiment involved incubating fertile sugarcane soil and infertile sand across an elevational gradient, characterized by diverse climatic and biotic conditions within a tropical forest. Biomarkers, such as phospholipid fatty acids, carbon-, nitrogen-, and phosphorus-acquiring hydrolases, glomalin-related proteins, and amino sugars, were measured to characterize the production and accumulation of microbial biomass, exo-enzymes, extracellular glycoproteins, and microbial necromass. These measurements aimed to elucidate their respective contribution to the sequestration of SOC. We found that Gram-negative bacteria dominated the microbial community composition in fertile soil, and the higher nutrient availability was related to the production and accumulation of microbial necromass via promoting microbial biomass turnover, thus enhancing the accumulation of SOC in fertile soil. This process was negatively associated with phosphorus availability and carbon- and phosphorus-acquiring enzyme activities in fertile soil. In contrast, the SOC accumulation was positively correlated with nitrogen availability and stoichiometry (including C:N and C:P), as well as moisture content in infertile sand. However, more resources were preferentially allocated to stress-tolerant fungi and Gram-positive bacteria under nutrient deficiency in infertile sand used for microbial biomass maintenance, nutrient acquisition, and environmental adaption which further aggravated the consumption of SOC, resulting in SOC loss after one year of field incubation. Our results suggest that microbial effects on SOC persistence are highly context-dependent and nutrient availability-induced changes in microbial communities and microbial resource-allocation strategies are key processes for understanding and predicting the fate of carbon in tropical forest soils.

Persistent soil organic carbon deficits from converting primary forests to plantations and secondary forests in subtropical China

Soil organic carbon (SOC) plays a critical role in supporting key ecosystem functions and services for humanity. Despite the widespread conversion of primary forests to plantations and secondary forests, it remains uncertain how it affects the content and distribution of SOC. We evaluated SOC content, its components in two fractions, particulate organic carbon (a mean residence time shorter than decades) and mineral-associated organic carbon (a mean residence time of multiple decades to centuries), and the fine root characteristics (biomass, necromass, and C: N of dead fine roots) across Chinese fir plantations and secondary forests during stand development (5–41 years old) in subtropical China, employing primary forests as a reference. We found that SOC content differed significantly between forest types and soil depths, but not with stand age within plantations or secondary forests. In the topsoil (0–10 cm), the SOC content of primary and secondary forests was 47 % and 25 % higher than that of Chinese fir plantations, respectively. The SOC content in the subsoil (40–60 cm) was approximately 6 ± 1.7 mg C g−1 and did not change significantly with forest types or stand ages. The higher SOC contents in primary forests and secondary forests in topsoil resulted primarily from particulate organic carbon accumulation caused by higher fine root biomass and dead root C: N. The fraction of mineral-associated organic carbon to SOC increased with soil depth and was higher in Chinese fir plantations than in secondary forests, suggesting that more SOC is composed of mineral-associated organic carbon that was associated with a higher amount of fine root necromass. Contrary to our expectations, we found SOC content or its fractions did not increase with 40 years of plantation and secondary forest development. Our results reveal persistent SOC deficits from converting primary forests to plantations and secondary forests and highlight the urgent need to conserve the remaining primary forests and protect secondary forests and plantations from disturbances, allowing them to evolve towards older-aged primary forests.

Combination of mineral protection and molecular characteristics rather than alone to govern soil organic carbon stability in Qinghai-Tibetan plateau wetlands

Wetlands in the Yarlung Tsangpo River Basin (YTR) on the Qinghai-Tibet Plateau provide immense soil organic carbon (SOC) storage, which is highly susceptible to climate warming and requires urgent deciphering SOC stabilization mechanisms of long-term protection of SOC against decomposition. Conflicting views exist regarding whether persistent SOC is controlled by molecular features or by mineral protection. As such, this study quantified SOC stability using two thermal indices (TG-T50, and DSC), described molecular features of SOC using pyrolysis-gas chromatography-mass spectrometry, and measured SOC protection by minerals using a chemical extraction method. Results indicated SOC of topsoils had higher thermal stability, with TG-T50 and DSC-T50 of 337.61 °C and 384.58 °C, than that of subsoils with TG-T50 and DSC-T50 of 337.32 and 382.67 °C, respectively. We found subsoils had significantly higher proportions of aliphatic and aromatic compounds, while existed higher SOC associated with minerals. It seemed SOC stabilization differed with soil depths, in which mineral protection dictated SOC thermal stability in topsoils while molecular features posed a more important constraint on SOC stabilization in subsoils. Overall, our findings support the hypothesis of physical and chemical protection but emphasized that SOC thermal stability largely depended on to extent of the combination between molecular features and mineral protection, which explained 55% in topsoils and 73% in subsoils, respectively.

Soil aggregation and aggregate‐associated organic carbon flows are affected by different restoration patterns on reclaimed mine soil in the Loess Plateau of China

AbstractVegetation restoration of land post‐mining can strengthen the development of soil aggregates and promote the accumulation of soil organic carbon (SOC), and Microbial restoration such as Arbuscular mycorrhizal fungi (AMF) can significantly improve the effect of vegetation restoration. However, the effects of different recovery modes (microbial and natural restoration) on soil macroaggregates development and the carbon flows of the organic carbon (OC) have been less studied on reclaimed mine soil. A 4‐year field experiment was conducted on opencast coal reclaimed areas in Inner Mongolia grassland, and three recovery modes were set up: artificial restoration, artificial restoration (inoculated with AMF) and natural restoration. Results showed that compared with the natural restoration, artificial restoration and AMF inoculation increased the OC, total glomalin‐related soil proteins (TGRSP) contents and the value of δ13C in macroaggregates (>0.25 mm) in deep soil (30–50 cm). The shallow roots were the most likely starting points of carbon flow in the natural recovery area surface soil, and artificial restoration inoculation with AMF increased the carbon flows between all levels of aggregates in deep soil, showing a carbon flow pattern from macroaggregates to microaggregates (<0.25 mm). Moreover, significant correlations among the TGRSP, SOC content, and δ13C value were recorded, indicating the SOC sequestration and stability of reclaimed mine soil were potentially associated with TGRSP. In summary, both natural restoration and artificial restoration had advantages and disadvantages on reclaimed mine soil, but the artificial restoration inoculation with AMF showed great potential in promoting the aggregate carbon flow and persistent SOC accumulation.

Abundance of microbial community genes encoding terminal proteins of the aerobic and denitrification respiratory chains as indicators of ecohydrologic gradients in a coastal temperate rainforest

The persistence of soil organic carbon (SOC) is closely linked to the capacity of the soil microbial community to catalyze oxidative degradation of organic matter. This study uses a landscape-scale metagenomics approach to examine the relative abundances of genes encoding different terminal electron acceptors used for aerobic respiration and denitrification between soil microbial communities across an ecohydrologic gradient. The perhumid zone of the North Pacific coastal temperate rainforest serves as a template to test the associations between these differing respiratory pathways, soil conditions, and tree species biomass. We used the occurrence of reads in soil metagenomes as a proxy for the relative abundance of genes across the ecohydrologic gradient and identified distinct microbial fingerprints differentiating the driest (upland forest) and wettest (palustrine scrub-shrub and palustrine emergent wetland) ecosystem types. There was little gene-based differentiation between the ecosystem types at the wettest end of the ecohydrologic gradient - scrub-shrub wetland and emergent wetlands – while forested wetlands, which occur as gradient sites between upland forests and emergent wetlands on the landscape, were characterized by substantial metagenomic variability and, consequently, overlapped in their soil metagenome with the other ecosystem classes. Hydrology, as measured by groundwater depth was a predictor of O2 and N-oxide reductase abundances. High affinity O2 reductase genes were associated with wetter topographic positions, which has not been shown at the ecosystem scale, or in soils in-situ previously. N-oxide reductase genes were more associated with plant communities found at wetland sites but were not related to soil inorganic nitrogen, suggesting that nitrate supply may be limiting to denitrifiers ubiquitously across this gradient. Two plant species, Tsuga heterophylla (TSHE) and Pinus contorta spp. contorta (PICO), were more strongly correlated with O2-reductase and N-oxide reductase genes than site hydrology, respectively. This research represents a step toward fingerprinting the ecosystem microbiome using genes coding for specific terminal electron acceptors that control the fate of SOC and their ecohydrologic controls.

Litter quality controls tradeoffs in soil carbon decomposition and replenishment in a subtropical forest

Abstract Species‐rich forests can produce litter of varying carbon (C) and nitrogen (N) composition (i.e. quality), which can affect decomposition and play a central role in long‐term soil organic carbon (SOC) accumulation. However, how differences in litter quality affect SOC decomposition and formation remains unclear over the full litter decomposition trajectory. We followed the in situ complete decomposition of added 13C‐labelled high‐ (low C:N) and low‐quality (high C:N) leaf‐litter and its effect on particulate organic matter (POM) and mineral‐associated organic matter (MAOM) fractions over 2 years in a natural subtropical forest. We found that during early stages of decomposition, low‐quality litter inputs decreased SOC via a positive priming effect (i.e. new C inputs favoured decomposition of native SOC), but these SOC losses were offset by SOC gains observed via a negative priming effect during decomposition of high‐quality litter. In contrast, this pattern reversed during late stages of decomposition—SOC losses via a positive priming effect induced by high‐quality litter were offset by SOC gains via a negative priming effect induced by low‐quality litter. Over the full decomposition of litter, both high‐ and low‐quality litter stimulated microbial breakdown of SOC tied to POM, but also replenished more persistent SOC that associated with soil minerals (MAOM). Altogether, we observed that low‐quality litter formed twice as much new SOC as high‐quality litter (24% vs. 12% of added litter‐C). We extend the notion of the priming effect from primarily a negative role promoting losses of native SOC, to a functional role that can replenish persistent SOC. Synthesis. Our measurements raise the possibility that, in species‐rich forests, high‐ and low‐quality litter decomposition play opposite but dynamically complementary roles in renewing POM—both by inducing its decomposition and formation—while exclusively favouring MAOM formation, which can help explain how differences in litter quality favour SOC accumulation and persistence. Global change factors that shift plant community composition may ultimately affect the fate of soil C, as changes in litter quality may force soil transitions from sinks to sources or sources to sinks of atmospheric CO2.

Macrofungi promote SOC decomposition and weaken sequestration by modulating soil microbial function in temperate steppe

Soil organic carbon (SOC) sequestration is a key grassland ecosystem function, and the magnitude of SOC reservoirs depends on microbial involvement, especially that of fungi. Mycelia developed by macrofungi potentially influence carbon (C) fixation and decomposition; however, the mechanisms underlying their effects on SOC storage in grassland ecosystems remain poorly understood. The fairy rings formed by macrofungi in grasslands are natural platform for exploring macrofungal effects on SOC. In this study, we collected topsoil (0–10 cm) from four different fairy ring zones in a temperate steppe to reveal the macrofungal effects on SOC fractions, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), and the SOC storage microbial mechanism using metagenomic sequencing technology. Both POC and MAOC decreased after macrofungal passage, resulting in a 7.37 % reduction in SOC. Macrofungal presence reduced microbial biomass carbon (MBC), but significantly enhanced the β-1,4-glucosidase (BG) activity, which increased dissolved organic carbon (DOC). In addition, the abundance of copiotrophs (Proteobacteria and Bacteroidetes) with lower C metabolic rates increased, and that of oligotrophs (Actinobacteria, Acidobacteria, Chloroflexi, and Verrucomicrobia) with higher substrate utilization efficiency decreased in the presence of macrofungi. This may further promote SOC decomposition. Correspondingly, there was a lower abundance of C-fixation genes but more C-degradation genes (especially hemicellulosic degradation genes) during macrofungal passage. Our results indicate that the presence of macrofungi can modulate the soil microbial community and functional genes to reduce SOC storage by inhibiting microbial C sequestration while promoting C decomposition in grassland ecosystems. These findings refine our mechanistic understanding of SOC persistence through the interactions between macrofungi and other microbes.

Organic Carbon and Lignin Protection by Metal Oxides Versus Silicate Clay: Comparative Study Based on Wetland and Upland Soils

AbstractMineral binding of organic carbon (OC) is one of the key mechanisms promoting soil organic carbon (SOC) persistence, which is vital for the long‐term sequestration of atmospheric carbon dioxide into soils. However, the relative importance of two main types of soil minerals ‐ metal oxides and silicate clay—in SOC protection remains unclear, hampering our ability to predict and protect this important pool of persistent SOC. Here, we analyzed OC bound to metal oxides (OCmetal) versus silicate clay (OCclay) in soils from contrasting environments (i.e., wetlands and uplands) using sequential dissolution by dithionite and hydrofluoric acid. We found that OCmetal% was higher than OCclay% in both wetlands and uplands, indicating the dominance of metal oxides in SOC protection. The sum of OCmetal% and OCclay% constituted a higher fraction of SOC in wetlands than uplands, suggesting an underappreciated role of mineral (in particular, metal oxide) protection in wetland SOC preservation. Furthermore, using lignin phenol analysis in tandem, ∼23% of lignin phenols were additionally released after the removal of silicate clay, providing a potential means to assess “hidden” lignin in mineral matrices. Given the wide use of lignin phenols as tracers for terrestrial plant‐derived OC, our results suggest that the contribution of terrestrial or plant‐derived carbon to sedimentary carbon stocks or mineral‐protected SOC deserves reassessment. These findings highlight the differential role of various soil minerals in the protection of SOC and its components in contrasting terrestrial environments, which warrants better recognition for understanding the distribution and dynamics of mineral‐protected SOC.

Microbial carbon use efficiency promotes global soil carbon storage

Soils store more carbon than other terrestrial ecosystems1,2. How soil organic carbon (SOC) forms and persists remains uncertain1,3, which makes it challenging to understand how it will respond to climatic change3,4. It has been suggested that soil microorganisms play an important role in SOC formation, preservation and loss5–7. Although microorganisms affect the accumulation and loss of soil organic matter through many pathways4,6,8–11, microbial carbon use efficiency (CUE) is an integrative metric that can capture the balance of these processes12,13. Although CUE has the potential to act as a predictor of variation in SOC storage, the role of CUE in SOC persistence remains unresolved7,14,15. Here we examine the relationship between CUE and the preservation of SOC, and interactions with climate, vegetation and edaphic properties, using a combination of global-scale datasets, a microbial-process explicit model, data assimilation, deep learning and meta-analysis. We find that CUE is at least four times as important as other evaluated factors, such as carbon input, decomposition or vertical transport, in determining SOC storage and its spatial variation across the globe. In addition, CUE shows a positive correlation with SOC content. Our findings point to microbial CUE as a major determinant of global SOC storage. Understanding the microbial processes underlying CUE and their environmental dependence may help the prediction of SOC feedback to a changing climate.

Land-use induced soil carbon stabilization at the expense of rock derived nutrients: insights from pristine Andean soils

Soils contain significantly more carbon than the atmosphere, hence we should understand how best to stabilize it. Unfortunately, the role of human interventions on soil organic carbon (SOC) persistence in the Anthropocene remains vague, lacking adequate sites that allow unbiased direct comparisons of pristine and human influenced soils. Here we present data from a unique study system in the High Andes that guarantees pristineness of the reference sites by physical inaccessibility through vertical cliffs. By comparing the isotopic signatures of SOC, mineral related carbon stabilization, and soil nutrient status across grazed versus pristine soils, we provide counterintuitive evidence that thousands of years of pastoralism increased soil C persistence. Mineral associated organic carbon (MAOC) was significantly higher in pastures. Land use increased poorly crystalline minerals (PCM’s), of which aluminum correlated best with MAOC. On the other hand, human’s acceleration of weathering led to acidification and higher losses of cations. This highlights a dilemma of lower soil quality but higher persistence of SOC due to millennia of pastoralism. The dynamics of soil genesis in the Anthropocene needs better understanding, but if human-induced weathering proves generally to promote soil carbon persistence it will need to be included in climate—soil feedback projections.

Five-year warming does not change soil organic carbon stock but alters its chemical composition in an alpine peatland

Climate warming may promote soil organic carbon (SOC) decomposition and alter SOC stocks in terrestrial ecosystems, which would in turn affect climate warming. We manipulated a warming experiment using open-top chambers to investigate the effect of warming on SOC stock and chemical composition in an alpine peatland in Zoigê on the eastern Tibetan Plateau, China. Results showed that 5 years of warming soil temperatures enhanced ecosystem respiration during the growing season, promoted above- and belowground plant biomass, but did not alter the SOC stock. However, labile O-alkyl C and relatively recalcitrant aromatic C contents decreased, and alkyl C content increased. Warming also increased the amount of SOC stored in the silt-clay fraction (< 0.053 mm), but this was offset by warming-induced decreases in the SOC stored within micro- and macroaggregates (0.053–0.25 and > 0.25 mm, respectively). These changes in labile and recalcitrant C were largely associated with warming-induced increases in soil microbial biomass C, fungal diversity, enzyme activity, and functional gene abundance related to the decomposition of labile and recalcitrant C compounds. The warming-induced accumulation of SOC stored in the silt-clay fraction could increase SOC persistence in alpine peatland ecosystems. Our findings suggest that mechanisms mediated by soil microbes account for the changes in SOC chemical composition and SOC in different aggregate size fractions, which is of great significance when evaluating SOC stability under climate warming conditions.

Evenness of soil organic carbon chemical components changes with tree species richness, composition and functional diversity across forests in China.

Higher tree species richness generally increases the storage of soil organic carbon (SOC). However, less attention is paid to the influence of varied tree species composition on SOC storage. Recently, the perspectives for the stronger persistence of SOC caused by the higher molecular diversity of organic compounds were proposed. Therefore, the influences of tree species richness and composition on the molecular diversity of SOC need to be explored. In this study, an index of the evenness of diverse SOC chemical components was proposed to represent the potential resistance of SOC to decomposition under disturbances. Six natural forest types were selected encompassing a diversity gradient, ranging from cold temperate to tropical forests. We examined the correlations of tree species richness, composition, and functional diversity, with the evenness of SOC chemical components at a molecular level by 13 C nuclear magnetic resonance. Across the range, tree species richness correlated to the evenness of SOC chemical components through tree species composition. The negative correlation of evenness of SOC chemical components with tree species composition, and the positive correlation of evenness of SOC chemical components with tree functional diversity were found. These indicate the larger difference in tree species composition and the lower community functional diversity resulted in the higher heterogeneity of SOC chemical components among the communities. The positive correlation of the evenness of SOC chemical components with the important value of indicator tree species, further revealed the specific tree species contributing to the higher evenness of SOC chemical components in each forest type. Soil fungal and bacterial α-diversity had effect on the evenness of SOC chemical components. These findings suggest that the indicator tree species conservation might be preferrable to simply increasing tree species richness, for enhancing the potential resistance of SOC to decomposition.

Soil organic carbon associated with iron oxides in terrestrial ecosystems: Content, distribution and control

<p indent="0mm">Soils are the largest terrestrial organic carbon pool, containing more organic carbon than vegetation and the atmosphere combined. Therefore, a small change in soil organic carbon (SOC) can have a dramatic influence on the concentration of atmospheric carbon dioxide. Mineral-association (e.g., interaction with iron- (Fe-) oxides) has been demonstrated to be a key mechanism underlying long-term SOC persistence. Moreover, increasing evidence suggests that Fe oxides are more widely distributed than aluminum oxides (particularly in tropical and subtropical regions) in all kinds of soils worldwide and have the advantages of a higher surface area and a larger adsorption capacity. However, the contribution of Fe-bound organic carbon to the total SOC pool (<italic>f</italic>_Fe-OC) in terrestrial ecosystems and its geographical pattern and underlying mechanisms are not fully understood. We hypothesized that (1) soil depth was the primary driver of the <italic>f</italic>_Fe-OC in terrestrial ecosystems, with higher values observed in subsoil than in topsoil, and that (2) the soils in oxygen-deficient ecosystems would have a higher <italic>f</italic>_Fe-OC than those in aerobic ecosystems. Here, we compiled 351 observations of <italic>f</italic>_Fe-OC across terrestrial ecosystems from 24 published articles to assess the <italic>f</italic>_Fe-OC content across different soil depths, ecosystem types and climatic zones using Kruskal-Wallis and Wilcoxon rank-sum tests. We also used a partial least squares (PLS) model to distinguish the relative importance of climatic, pedological, and mineral factors in explaining the geographical distribution of <italic>f</italic>_Fe-OC in bulk soil, topsoil, and subsoil, respectively. We found that the <italic>f</italic>_Fe-OC was, on average, 21.9% across terrestrial ecosystems but varied greatly at different soil depths, with average values of 15.4% in topsoil and 37.5% in subsoil. The <italic>f</italic>_Fe-OC varied greatly in different terrestrial ecosystems, following the order of wetlands (24.5%) &gt; grasslands (16.2%) &gt; forests (14.9%) &gt; croplands (14.8%), with higher values observed in oxygen-deficient ecosystems (24.2%) than in aerobic ecosystems (15.7%). Moreover, the <italic>f</italic>_Fe-OC across climatic zones showed the order of tropical and subtropical (23.7%) &gt; boreal (21.9%) &gt; temperate (20.2%) &gt; plateau (16.6%). Soil depth was a primary control of the <italic>f</italic>_Fe-OC in terrestrial ecosystems, and the underlying mechanisms were much different: The <italic>f</italic>_Fe-OC in topsoil was strongly controlled by the concentrations of amorphous and complex Fe oxides, but that in subsoil was primarily controlled by soil pH, texture (clay, silt and sand contents) and bulk density. Therefore, the <italic>f</italic>_Fe-OC in topsoil could be the result of the direct accumulation of Fe oxides in different forms, but that in subsoil was mainly derived from the complexation and coprecipitation of Fe oxides and SOC. Our findings provide fundamental support for the mineral protection of Fe oxides in long-term SOC persistence in terrestrial ecosystems and distinguish the differential mechanisms underlying Fe oxide-associated SOC between topsoil and subsoil. Future research should focus on the coupling of Fe oxides and other metals (e.g., aluminum, calcium and manganese) in controlling SOC stabilization under the fluctuation of environmental change.

Microspectroscopic visualization of how biochar lifts the soil organic carbon ceiling

The soil carbon (C) saturation concept suggests an upper limit to the storage of soil organic carbon (SOC). It is set by the mechanisms that protect soil organic matter from mineralization. Biochar has the capacity to protect new C, including rhizodeposits and microbial necromass. However, the decadal-scale mechanisms by which biochar influences the molecular diversity, spatial heterogeneity, and temporal changes in SOC persistence, remain unresolved. Here we show that the soil C storage ceiling of a Ferralsol under subtropical pasture was raised by a second application of Eucalyptus saligna biochar 8.2 years after the first application—the first application raised the soil C storage ceiling by 9.3 Mg new C ha−1 and the second application raised this by another 2.3 Mg new C ha−1. Linking direct visual evidence from one-, two-, and three-dimensional analyses with SOC quantification, we found high spatial heterogeneity of C functional groups that resulted in the retention of rhizodeposits and microbial necromass in microaggregates (53–250 µm) and the mineral fraction (<53 µm). Microbial C-use efficiency was concomitantly increased by lowering specific enzyme activities, contributing to the decreased mineralization of native SOC by 18%. We suggest that the SOC ceiling can be lifted using biochar in (sub)tropical grasslands globally.

Lignin and fungal abundance modify manganese effects on soil organic carbon persistence at the continental scale

Manganese (Mn) may play an outsized role in soil biogeochemical cycles relative to its abundance. The role of Mn-facilitated oxidation of biomacromolecules during litter decomposition is well-established, but the balance between Mn-promoted soil organic carbon (SOC) oxidation and long-term SOC protection in mineral soils is unknown, especially in subsoils. In this study, we used soils collected across the US National Ecological Observatory Network (NEON) to assess the distribution of Mn and relationships between Mn abundance and SOC concentration, potential mineralization, and persistence at a continental spatial scale. Total reducible Mn was not spatially correlated to site moisture (Spearman’s Rho = 0.24), highlighting that Mn abundance may influence SOC cycling independently from other moisture-driven soil chemical properties (e.g., reactive iron and aluminum). However, Mn effects on SOC cycling depended on depth, soil, or site-level properties. In particular, fungal:bacterial biomass ratio, proportion of SOC in the free light fraction, lignin abundance, and/or proportion of undegraded organic matter mediated the effect of Mn on SOC cycling metrics. For example, the effect of Mn on SOC concentration in subsoils shifted from positive (approximately +270 % relative to mean subsoil SOC) to negative (−125 %) with increasing fungal:bacterial ratio. We propose that convergence of high Mn, lignin-rich substrates, and fungal:bacterial ratio amplifies lignin mineralization in surface soils, but does not result in higher net SOC turnover due to fungal biomass stabilization. In contrast, we suggest that Mn abundance promotes smaller, but more persistent SOC stocks in subsoils by accelerating SOC transformation from particulate to microbial biomass pools.