Chemical Composition Of Soil Organic Carbon Research Articles

Low Carbon Loss from Long-Term Manure-Applied Soil during Abrupt Warming Is Realized through Soil and Microbiome Interplay.

Manure application is a global approach for enhancing soil organic carbon (SOC) sequestration. However, the response of SOC decomposition in manure-applied soil to abrupt warming, often occurring during diurnal temperature fluctuations, remains poorly understood. We examined the effects of long-term (23 years) continuous application of manure on SOC chemical composition, soil respiration, and microbial communities under temperature shifts (15 vs 25 °C) in the presence of plant residues. Compared to soil without fertilizer, manure application reduced SOC recalcitrance indexes (i.e., aliphaticity and aromaticity) by 17.45 and 21.77%, and also reduced temperature sensitivity (Q10) of native SOC decomposition, plant residue decomposition, and priming effect by 12.98, 15.98, and 52.83%, respectively. The relative abundances of warm-stimulated chemoheterotrophic bacteria and fungi were lower in the manure-applied soil, whereas those of chemoautotrophic Thaumarchaeota were higher. In addition, the microbial network of the manure-applied soil was more interconnected, with more negative connections with the warm-stimulated taxa than soils without fertilizer or with chemical fertilizer applied. In conclusion, our study demonstrated that the reduced loss of SOC to abrupt warming by manure application arises from C chemistry modification, less warm-stimulated microorganisms, a more complex microbial community, and the higher CO2 intercepting capability by Thaumarchaeota.

The path from root input to mineral-associated soil carbon is dictated by habitat-specific microbial traits and soil moisture

Soil microorganisms help transform plant inputs into mineral-associated soil organic carbon (SOC) – the largest and slowest-cycling pool of organic carbon on land. However, the microbial traits that influence this process are widely debated. While current theory and biogeochemical models have settled on carbon-use efficiency (CUE) and growth rate as positive predictors of mineral-associated SOC, empirical tests are sparse, with contradictory observations. Using 13C-labeling of an annual grass (Avena barbata) under two moisture regimes, we found that microbial traits associated with formation of 13C-mineral-associated SOC varied by soil habitat, as did active microbial taxa and SOC chemical composition. In the rhizosphere, bacterial-dominated communities with fast growth, high biomass, and high extracellular polymeric substance (EPS) production were positively associated with 13C-mineral-associated SOC. In contrast, the detritusphere held communities dominated by fungi and more filamentous bacteria, and with greater exoenzyme activity; there, 13C-mineral-associated SOC was associated with slower microbial growth and lower microbial biomass. CUE was a negative predictor of 13C-mineral-associated SOC in both habitats. Using 13C-quantitative stable isotope probing, we found that the majority of 13C assimilation in the rhizosphere and detritusphere at week 12 of the experiment was performed by very few bacterial and fungal taxa (3–5% of the total taxa that assimilated 13C). Several complementary chemical analyses (13C-NMR, FTICR-MS, and STXM-NEXAFS) suggested that SOC in the rhizosphere had a more oxidized chemical signature, while SOC in the detritusphere had a less oxidized, more lignin-like chemical signature. Our findings challenge current theory by demonstrating that microbial traits linked with mineral-associated SOC are not universal, but vary with soil habitat and moisture conditions, and are shaped by a small number of active taxa. Emerging SOC models that explicitly reflect these interactions may better predict SOC storage, since climate change causes shifts in soil moisture regimes and the ratio of living versus decaying roots.

The hidden depths of forest soil organic carbon chemistry in a pumice soil

Forest ecosystems can store massive amounts of organic carbon (OC) deep in their soils. Soil OC (SOC) is composed of several complex organic compounds of which groupings of these organic compounds can provide an insight into evaluating the formation and fate of SOC. However, there is little information available regarding the chemical composition of deep forest SOC and how they compare to upper soil depths. The objective of this study was to explore the SOC chemistry changes with increasing soil depth on a fine spatial scale in a planted forest soil derived from young volcanic erupted airfall deposits. Soil core samples to 1 m depth were collected from Puruki Experimental Forest (New Zealand) and examined for incremental depth changes in SOC functional groups, or chemical shift regions, using solid-state 13C cross polarisation magic angle spinning nuclear magnetic resonance (13C CPMAS-NMR) spectroscopy. The results showed that soil depth was a driver of change in soil chemical shift regions. The presence of a coarse-textured soil horizon, lapilli layer, at depth was a driver of significant (p < 0.05) differences between SOC functional groups. In the upper soil profile (0–10 cm), the chemical shift distribution was dominated by O-alkyl C (39%) followed by alkyl C (32%), aromatic C (23%), and carboxyl C (6%). In the deep soil layers, we found alkyl C increased below the lapilli layer (varying depth > 50 cm) compared to above the lapilli layer. In the deep soil layers the alkyl C was the dominant functional group (55%), followed by O-alkyl C (28%). Furthermore, there was a decrease at depth in aromatic (12%) and carboxyl C (5%). The A/O-A ratio increased with depth from soil above the lapilli layer compared to soils below the lapilli layer indicating a greater degree of organic matter decomposition and a corresponding decrease in the labile OC fractions. Within the lapilli layer the SOC functional groups and A/O-A ratio were highly variable. These results highlight that changes in SOC chemical composition occur with increasing soil depth, a property of deep soil that is frequently understudied, and are highly influenced by the multi layered nature of soil derived from volcanic airfall deposits.

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.

Organic substitutions enhanced soil carbon stabilization and reduced carbon footprint in a vegetable farm

The strategies of substituting inorganic fertilizers with organic fertilizers have been shown to significantly impact the dynamics of soil organic carbon (SOC) combating soil degradation and climate changes in intensive vegetable production. However, the underlying mechanisms and comprehensive assessment of environmental consequences are not fully understood. We conducted a five-year field experiment in the Yangtze River Delta Alluvial Plain, China, four fertilization treatments were established at equal nitrogen (N) levels: sole mineral fertilizers (SN); organic substituting 20 % (M1N4), 50 % (M1N1), and 100 % (SM) of mineral N fertilizer with organic fertilizer. Soil carbon (C) pools, physical and chemical composition of SOC, and C footprint were measured based on soil aggregates, 13C nuclear magnetic resonance spectroscopy, and life cycle assessment to investigate the effects of organic substitution on SOC quantity, quality, and environmental implications. Our results showed a significant increase in SOC content, particularly in the M1N1 treatment. This increase was attributed to changes in soil properties, an elevation in dissolved organic C content, and improved SOC stability. Specifically, organic substitution increased the proportion of macroaggregates, content of lignin phenols, and enrichment of alkyl C groups. Based on life cycle assessment incorporating field measurements, the average C footprint ranged from 1.82 to 2.58 CO2 kg eq kg−1 vegetable in the five-year experiment, primarily influenced by transportation and fertilizer manufacturing. Partial organic substitution reduced the C footprint by 6.2–10.1 %, while the SM treatment increased it by 0.54 CO2 eq kg−1 vegetable, and the M1N1 had the lowest C footprint at 1.82 CO2 eq kg−1 vegetable. As a whole, this approach of substituting 50 % of mineral N with organic fertilizers serves as a viable strategy to bolster soil C stabilization, promote C accumulation, and mitigate the C footprint linked to intensive vegetable production in subtropical regions.

Chemical Composition of Organic Carbon in Tropical Soils Under Agricultural Cultivation and Peat-Swamp Forest

ABSTRACT Soil organic carbon (SOC) is important for agricultural production and from an environmental perspective. This research determined the chemical composition of SOC and studied factors related to its distribution for various crops and peat-swamp forests in the tropical region. In total, 21 topsoil samples were collected from areas of paddy rice, sugarcane, cassava, pineapple, oil palm, para rubber, and peat-swamp forest in Thailand. Four groups of SOC chemical composition were investigated (carbonyl C, aromatic C, O-alkyl C, and alkyl C) using solid-state 13C nuclear magnetic resonance spectroscopy (NMR). The dominant peak of the NMR spectra indicated that SOC originated from a variety of lignin compound units. The results revealed the dominant SOC chemical component was O-alkyl C (36.57–50.90%), followed by alkyl C (22.32–32.60%), aromatic C (16.65–23.41%), and carbonyl C (7.99–10.95%), respectively. Most of the SOC chemical components were derived from plant debris, particularly biodegraded lignin compounds. There were slightly differences between the distributions of the SOC functional groups in each vegetation type, perhaps partially as a result of the specific management activities for each crop, including tillage. Furthermore, alkyl C was related to some soil properties (silt and clay fractions), while O-alkyl C was related to available P in the soil. The hydrophobicity index, aromaticity index, and degree of humification were in the ranges 0.69–1.12, 0.17–0.23, and 0.46–0.92, respectively. These findings provided SOC chemical composition data of several economic crops to further support soil management for crop production and carbon stock preservation in tropical soils.

Effects of maize straw and its biochar application on soil organic carbon chemical composition and carbon degradation genes in a Moso bamboo forest.

We investigated the effects of maize straw and its biochar application on soil organic carbon chemical composition, the abundance of carbon degradation genes (cbhI) and the composition of cbhI gene community in a Moso bamboo forest, to provide the theoretical and scientific basis for enhancing carbon sequestration. We conducted a one-year field experiment in a subtropical Moso bamboo forest with three treatments: control (0 t C·hm-2), maize straw (5 t C·hm-2), and maize straw biochar (5 t C·hm-2). Soil samples were collected at the 3rd and 12th months after the treatment. Soil organic carbon chemical composition, the abundance and community composition of cbhI gene were determined by solid-state 13C NMR, real-time fluorescence quantitative PCR, and high-throughput sequencing, respectively. The results showed that compared with the control, maize straw treatment significantly increased the content of O-alkyl C and decreased aromatic C content, while maize straw biochar treatment showed an opposite effect. Maize straw treatment significantly increased the abundance of cbhI gene and the relative abundance of Penicillium, Gaeumannomyces and Marasmius. However, maize straw biochar treatment reduced the abundance of this gene. The relative abundance of dominant cbhI in soils was positively correlated with the content of O-alkyl C and negatively correlated with the content of aromatic C. Results of redundancy analysis showed that maize straw treatment had a significant effect on the microbial community composition of cbhI gene by changing soil O-alkyl C content, while maize straw biochar affected the microbial community composition of cbhI gene by changing soil pH, organic carbon, and aromatic C content. Maize straw biochar treatment was more effective in increasing soil organic carbon stability and reducing microbial activity associated with carbon degradation in the subtropical Moso bamboo forest ecosystem compared with maize straw treatment. Therefore, the application of biochar has positive significance for maintaining soil carbon storage in subtropical forest ecosystems.

Response Characteristics of Soil Organic Carbon Pool and Its Chemical Composition During Secondary Forest Succession in the Loess Plateau

In order to explore the characteristics of the soil organic carbon(SOC)pool and its chemical composition during the succession of secondary forests in the Loess Plateau, samples of the primary stage (Populus davidiana forest), transition stage (Populus davidiana and Quercus wutaishansea mixed forest), and top stage (Quercus wutaishansea forest) of secondary forest succession in the Huanglong Mountain forest area of the Loess Plateau in Northern Shaanxi were selected as the research object. The variation characteristics of SOC content, storage, and its chemical composition at different soil depths (0-10, 10-20, 20-30, 30-50, and 50-100 cm) were analyzed. The results showed that:① the contents and storage of SOC increased significantly with the secondary forest succession process (P<0.05). The content of SOC decreased significantly with the increase in soil depth, and the storage of SOC increased from 64.8 Mg·hm-2 in the primary stage to 129.2 Mg·hm-2 in the top stage, with an increase of 99%. ② During the succession of secondary forests, in the surface (0-30 cm) soil organic carbon, the relative content of aliphatic carbon components that have a simple structure and can be decomposed more easily decreased, and the relative content of aromatic carbon components that have a complex structure and cannot be decomposed easily increased, indicating that the chemical composition of organic carbon stability of surface-layer soil increased significantly with the process of secondary forest succession. However, the stability of the chemical composition of SOC in the deep layer (30-100 cm) first increased and then decreased, that is, the transition stage>the top stage>the primary stage. ③In the process of secondary forest succession, the stability of SOC chemical composition in the primary stage and transition stage increased significantly with the increase in soil depth. The top stage tended to be stable, and the deep soil carbon stability decreased slightly. ④ Pearson correlation analysis showed that during the secondary forest succession process, SOC storage and chemical composition stability were significantly negatively correlated with soil total phosphorus content. In general, the content and storage of SOC in the 0-100 cm soil increased significantly during the secondary forest succession, playing the role of a "carbon sink." The stability of the chemical composition of SOC in the surface layer (0-30 cm) increased significantly, but in the deep layer (30-100 cm), it increased first and then decreased.

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.

Fertilization makes strong associations between organic carbon composition and microbial properties in paddy soil

Fertilization changes the soil organic carbon (SOC) composition, affecting the carbon cycle of paddy soil. Understanding the mechanisms of physical fraction and chemical composition of SOC responding to fertilization can help regulate the nutrient release and carbon sequestration. However, it is unclear whether these changes in SOC composition to fertilization are consistent and how these are regulated by biotic and abiotic properties. Therefore, a positioning experiment in a rice field was conducted with a total of nine treatments. Chemical fertilizers (0, 337.5, and 675 kg ha−1; C0, C50, and C100, respectively) and fungal residue (0, 10,000, and 20,000 kg ha−1; F0, F50, and F100, respectively) were applied to evaluated (i) changes in the physical fraction and chemical composition of SOC, (ii) changes in soil properties, microbial biomass and community, and (iii) establish relationships among soil properties, microbial community, microbial biomass, and SOC composition. Our results showed that the application of fungal residue exhibited more significant effects on SOC physical fractions than those with the chemical fertilizers. Furthermore, the chemical composition of SOC was more respond to the application of chemical fertilizers than fungal residue. The partial least squares path model indicated that soil properties mainly affected the mineral-associated organic carbon (MAOC) by microbial biomass. In addition, bacterial diversity played an important role in improving the accumulation of MAOC. The SOC chemical composition was mediated by fungal community composition and bacterial diversity. In conclusion, fungal residue application affected SOC physical fraction by increasing soil properties, microbial biomass, and bacterial diversity. Chemical fertilizers application mainly mediated the chemical composition of SOC by altering fungal community composition and decreasing bacterial diversity.

Forest conversion effects on soil organic carbon are regulated by soil aggregate stability and not by recalcitrance: Evidence from a reforestation experiment

Preservation of biochemically recalcitrant organic matter and protection within soil aggregates are two important mechanisms of soil organic carbon (SOC) preservation, but their relative importance remains unclear. In this study, we converted part of a mature secondary forest to two young plantation forests (Castanopsis carlesii and Chinese fir) and an assisted naturally regenerated (ANR) forest in 2012. The three young forests (five years old at the time of study) and the remaining mature secondary forest constitute a soil disturbance intensity gradient ideal for exploring effects of forest conversion on soil aggregate size classes and stability as well as SOC concentration and chemical composition. Taking advantage of the disturbance gradient, we examined the relative importance of physical protection versus recalcitrance in SOC preservations. We found that the young ANR forest conserved a similar amount of SOC to that of the mature secondary forest, which was 18 %–21 % higher than those retained in the two young plantations. Compared with the mature secondary forest, the young ANR forest retained 87 % of >2 mm aggregates, whereas the two young forest plantations retained only 45 %–53 %. Compared to the mature secondary forest, aggregate mean weight diameter (MWD) decreased by 10 % in the young ANR forest and 37 %–43 % in the two young plantations. Moreover, the young Castanopsis carlesii plantation had proportionally more recalcitrant aromatic carbon (C) and less labile O-alkyl C compared to the mature secondary forest. Total SOC was positively related to >2 mm aggregates mass percent, soil aggregate MWD, and labile O-alkyl C, but not with recalcitrant alkyl and aromatic C. Our results indicate that SOC preservation is more related to soil aggregate stability than recalcitrant C abundance, and advocate that reducing soil disturbance is key to future forest management aiming to maximize C sequestration.

Effect of Shrub Encroachment on Alpine Grass Soil Microbial Community Assembly

Shrub encroachment is a common phenomenon in grasslands all over the world. However, little is known about the consequences of shrub encroachment on soil microbial community structure in different layers. We investigated the effects of three common shrub encroachment (Potentilla fruticosa, Spiraea alpina, and Caragana microphylla) on grassland soil bacterial communities at the surface and deep layers in Qinghai–Tibetan Plateau. 16S rRNA gene sequencing was used to investigate the bacterial communities, and Fourier translation infrared spectroscopy (FTIR) was conducted to assess the soil organic carbon (SOC) chemical composition in surface and deep layers of shrub-encroached alpine grassland. Shrub encroachment has significantly increased SOC degradation in deep layer. After shrub invasion, the bacterial alpha-diversity in the surface and deep soil was higher than in grassland soil (except for the surface layer of C. microphylla). Factors driving bacterial community changes in soil surface and deep layer were different. Among the soil properties that were measured, SOC content was the primary factor that altered soil bacterial community composition in surface soil, while SOC chemical composition (aromatic and polysaccharides) was the main driver in the deep layer. A total of 39 and 42 biomarkers were found by linear discriminant analysis (LDA) effect size (LEfSe) in the surface and deep soil layer among the four sampling groups, respectively. Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) showed that the most abundant predicted functional genes belonged to categories of metabolism (52.83%) in the primary metabolic pathway. Redundancy analysis (RDA) results also showed that the key factors affecting bacterial metabolic function appear to be SOC, pH, and aromatics, which are largely consistent with those affecting community composition. We suggest that shrub encroachment affect the structure, diversity, and predicted functions of bacterial communities, thus affecting the C cycle in this region.

Climate-induced shifts in composition and protection regulate temperature sensitivity of carbon decomposition through soil profile

Through soil profile, both chemical composition of soil organic carbon (SOC) and edaphic physiochemical properties present a vertical gradient, likely resulting in depth-specific SOC dynamics in response to climate change (e.g., global warming). We assessed temperature sensitivity of SOC decomposition (Q10) by incubating (128 days) soils sampled across five sequential layer depths (i.e., 0–10, 10–20, 20–30, 30–50, and 50–100 cm) at ten sites along a ∼2500 m elevational transect (from ∼2100 m to ∼4600 m) covering various vegetation types (from evergreen broadleaved forest to alpine meadow) in southeast Tibet, China. The Q10 of SOC decomposition was significantly affected by both soil depth and elevation. However, depth-induced variation of Q10 was much smaller than that induced by the elevation gradient. Across the ten sites and five soil depths, chemical composition of SOC and its physiochemical protection against decomposition contributed >80% to the explained variance of Q10 values. Path analysis suggested that climate indirectly affected Q10 via its regulation on chemical composition of SOC and their physiochemical stabilization. The results from a carbon model constrained by the collected data further revealed that fast, slow and passive SOC pools exhibited significant difference in their Q10, resulted from different involvement of chemical composition and physicochemical protection in their decomposition. Our findings demonstrate similar temperature sensitivity of SOC decomposition across soil depths, but spatially heterogeneous temperature sensitivity due to climate-induced variability of both chemical recalcitrance of SOC and its physiochemical protection against decomposition.

Straw amendment and soil tillage alter soil organic carbon chemical composition and are associated with microbial community structure

Soil organic carbon (SOC) accumulation and its components are influenced by agricultural practices, which are interfered by soil microorganisms. A field experiment was conducted to investigate how straw amendment and soil tillage influenced SOC fractions and chemical components, and their interrelations with microbial communities evaluated through phospholipid fatty acids (PLFAs) in Mollisols. Density fractionation and Fourier-transform Infrared Spectroscopy were combined to characterize OC properties, including free and occluded light fractions (fLFC, oLFC), heavy fraction (HFC), aliphatic-C and aromatic-C functional groups. Compared with control soil, straw amendment caused surface SOC increased by 6.7%–12.4% (P < 0.05). The dissolved OC, microbial biomass carbon (SMBC), fLFC, oLFC, aliphatic-C, and total PLFAs also increased after straw amendment. Whereas the aromatic-C proportions in bulk soil, fLFC, and oLFC decreased. The OC components and microbial properties didn't obviously change in soil with straw ash amendment. Straw amendment to the subsurface layer only increased subsurface SOC and OC fractions (P < 0.05). Principal component analysis revealed that soil microbial community structure changed with the amounts of straw amendment, soil tillage further underpinned the separation. The separation via PC1 was associated with labile OC components, including SMBC, fLFC, oLFC, and aliphatic-C pointing to soils with higher amounts of straw amendment, and with recalcitrant aromatic-C and HFC pointing to soils without straw or straw ash. Higher amounts of straw amendment plus soil tillage affected soil microbial community structure, and consequently influenced OC sequestration by altering labile OC fractions and components in Mollisols.

Soil organic carbon characteristics affected by peanut shell biochar in saline-sodic paddy field

Biochar exhibits a profound impact on soil organic carbon (SOC) turnover and dynamics, but the underlying mechanism under field conditions is still unclear. A three-year field experiment was performed to evaluate the impact of peanut shell biochar applied at rates of 0, 33.75, 67.5, and 101.25 t/ha (referred to as B0, B1, B2, and B3, respectively) on SOC content and chemical composition in a saline-sodic paddy field using stable carbon isotope composition and &lt;sup&gt;13&lt;/sup&gt;C nuclear magnetic resonance technology. With increasing rates of biochar, SOC and aromatic carbon contents and alkyl carbon/oxygen-alkyl carbon and hydrophobic carbon/hydrophilic carbon ratios increased, while alkyl carbon and oxygen-alkyl carbon contents and aliphatic carbon/aromatic carbon ratio decreased. The new carbon from biochar and rice residues accounted for 26.5% of SOC under B0 and increased to above 80.0% under B2 and B3. The decay rate of old carbon was faster in biochar-amended than in unamended soil. SOC content was positively correlated with alkyl carbon/oxygen-alkyl carbon and hydrophobic carbon/hydrophilic carbon ratios but negatively correlated with aliphatic carbon/aromatic carbon ratio. The results suggest that biochar can increase SOC content by increasing its humification, aromaticity, and hydrophobicity. However, negative priming is not the main mechanism for SOC accumulation during the short-term period.

Dynamics and composition of soil organic carbon in response to 15 years of straw return in a Mollisol

Crop straw return is being widely applied as a sustainable soil management practice to improve soil organic carbon (SOC) storage in intensive agricultural ecosystems. However, the dynamics and chemical composition of SOC under long-term straw return are not fully understood. Based on a 15-year field experiment with soybean (Glycine max (L.) Merrill.)–maize (Zea mays L.) cropping system, we studied SOC temporal dynamics under no fertilization (NF), mineral fertilizers (NPK) and mineral fertilizers with straw return (NPKS). Meanwhile, we determined five labile carbon (C) pools, i.e., microbial biomass C (MBC), water-soluble organic C (WSOC), light fraction C (LFC), readily oxidizable organic C (ROC), and particulate organic C (POC), as well as δ13C values and organic functional groups of soil organic matter (SOM). After 15-year application of continuous straw return with mineral fertilizer application, SOC content increased by 14.2%. The δ13C values of SOC increased with time under all treatments, and its positive relationship with cumulative maize C input indicated a larger C contribution to SOC from maize than soybean residues. The NPKS treatment significantly increased the contents of MBC, WSOC, LFC, ROC, and POC and their proportion in SOC, compared with the NF treatment. The aliphatic and aromatic relative peak areas were significantly affected by straw return, with an increasing relative peak area at 2930 cm−1 and decreasing at 1620 cm−1, respectively. Our results demonstrate that straw return could continuously increase SOC in an inter-annual rotation of soybean and maize cropping system in Mollisol. Meanwhile, long-term fertilization can alter SOM chemical composition, with lower humification degree under the combined application of straw and mineral NPK fertilizers.

Grass waste utilization to alter aggregate-related carbon chemical composition and fungal community structure in apple orchard

The grass-waste management model affects soil organic carbon (SOC) and the microorganism community structure; however, studies on the relationship between the fungal community structure and the SOC chemical component at the aggregate level are poor. Solid-state 13C NMR and 18 S rDNA methods were used to evaluate the relationship between the SOC chemical composition and fungal community abundance at the aggregate level. Grass mulching significantly increased the percentage of labile carbon O-alkyl C (5.19%–11.79%) and decreased the instability of SOC (1.38–0.69). Microaggregates contained higher alkyl C (33.77%) and lower aromatic C (18.31%), and the A/O-A ratio (1.03) was higher than that of macroaggregates (0.89–0.96). Ascomycota, Basidiomycota and Mortierellomycota dominated the fungal community at the phylum level, and their abundance increased after grass mulching. Microaggregates supported more microbial diversity and richness and were rich in the Ascomycota (36.69%–67.49%) phylum, while LM aggregates were rich in Basidiomycota (5.62%–39.84%). We proved that changes in the O-alkyl C, carbonyl C, aromatic C and alkyl C of SOC chemical components were closely connected to fungal community composition, which together explained the change in fungal composition by 63.81%–71.99% among aggregates. We concluded that alterations in the chemical form of organic carbon were closely related to a change in the soil fungal community. This connection has a positive impact on soil nutrient utilization and SOC conversion in fruit-grass composite ecosystems and promotes the understanding of the relationship between the soil microbial community and nutrient cycling during long-term grass waste utilization.

Chemical composition of soil organic carbon and aggregate stability along an elevation gradient in Helan Mountains, northwest China

Exploring the composition of soil organic carbon (SOC) and aggregate stability over a range of elevations is essential to evaluate the stability of SOC and the conversion of carbon source or sink of mountain ecosystems. However, information of those changes in mountain ecosystems in arid and semi-arid regions is still scarce to date. In this study, we estimated the response of SOC stability along elevation gradient (1,380–2,438 m) by combining physical protection of aggregates with chemical protection of SOC using a redundancy analysis method in the Helan Mountains, northwest China. Results showed that the dominated aggregates of Helan Mountains changed from microaggregates to macroaggregates with increasing elevation, and the mean weight diameter (MWD) in mid and high elevations (2,139–2,248 m) was significantly higher than that in low elevations (1,380–1,650 m). Solid-state 13C NMR spectra showed that O-alkyl C was the main chemical composition of SOC in the different elevations. Regression analysis indicated that with the increase of elevation, the proportion of aromatic C and aromaticity gradually increased, peaked at the mid elevation and subsequently decreased. These findings indicated that the soil structure and chemical composition of SOC are more stable in the mid elevation. Macroaggregates were positively correlated with O-alkyl C and aromatic C, while silt and clay were positively correlated with alkyl C, indicative of a lower decomposition rate of macroaggregates. The RDA analysis confirmed that aggregate stability were significant associated with SOC content and chemical composition such as O-alkyl C, indicating that the soil labile carbon pool was most related to affect aggregate stability. Our findings suggest that the Helan Mountains had a better soil structure and a more stable SOC chemical composition at mid elevation ranges. The soil structure at low elevations is sensitive to environment change; therefore, a reduction in anthropogenic disturbance is suggested for improved sustainability of the ecosystem.

Coastal soil texture controls soil organic carbon distribution and storage of mangroves in China

Mangroves are among the most carbon-rich forests in the tropics, but the spatial variation in soil organic carbon (SOC) storage and the mechanisms controlling their stability remain contentious. Here, we proposed a new framework based on distinct coastal soil textures (muddy, sandy, and mud-sand mixed soil) and found that coastal soil texture exhibits regional-scale variations in mangrove soil C:N:P stoichiometry and SOC storage. Using this soil texture classification framework, we identified the important role of mud-sand mixed soil, which functions as a blue carbon hotspot in coastal wetlands. We showed that mangrove SOC storage was underestimated by approximately 37% in mud-sand mixed soil and overestimated by approximately 54% in muddy soil. We further revealed that the chemical composition of SOC and clay mineralogy are responsible for SOC stability by combining physical and chemical characterizations. As a microbial-derived C, glomalin-related soil protein (GRSP) contained higher alkyl C (∼58%) and lower O-alkyl C (∼17%) than mangrove soils, indicating that GRSP contributes to the stable SOC pool by its recalcitrant structure. We further found that the C:N:P stoichiometric signature of GRSP can capture variations in mangrove SOC storage compatible with distinct coastal soil textures. These results highlight the role of GRSP in regulating SOC stabilization mechanisms, potentially attenuating coastal blue carbon-climate feedback. Regional-scale variation in SOC linked to coastal soil textures can provide more robust estimates of the contribution of mangrove SOC to global C dynamics.

Linking soil carbon availability, microbial community composition and enzyme activities to organic carbon mineralization of a bamboo forest soil amended with pyrogenic and fresh organic matter

Despite fresh and pyrogenic organic matter have been widely used as amendments to improve soil organic carbon (SOC) storage, mineralization that links to C quality and soil temperature, microbial community composition and enzyme activity remain poorly understood. This study aims to explore the effects of amendments (bamboo leaves and its biochar) and incubation temperature on mineralization, and disentangle the relationships of SOC mineralization with chemical composition of SOC, labile organic C, microbial community composition, and activities of enzymes in a subtropical bamboo forest soil. Results showed that cumulative soil CO2 emissions ranked as bamboo leaf (Leaf) > bamboo leaf biochar (Biochar) > Control, regardless of the incubation temperature. Compared to the control, the Leaf treatment markedly increased, whereas the Biochar treatment decreased, the temperature sensitivity of SOC mineralization (P < 0.05). The cumulative soil CO2 emission was positively correlated (P < 0.05) with water-soluble organic C (WSOC), microbial biomass C (MBC), O-alkyl C and alkyl C contents, and activities of β-glucosidase and dehydrogenase, but negatively correlated (P < 0.01) with aromatic C content, regardless of the incubation temperature. This indicated that the lower SOC mineralization rate and lower temperature sensitivity in the Biochar (cf. Leaf) treatment were intimately associated with the lower WSOC, MBC, O-alkyl C content, and β-glucosidase and dehydrogenase activities, and higher aromatic C content in the Biochar. The high relative abundance of bacteria relating SOC mineralization included Rhizobiales, Sphingobacteriales and JG30-KF-AS9, whereas that of fungi included Eurotiales, Sordariales, Agaricales and Helotiales. Our results revealed that the application of pyrogenic organic matter, as compared to the application of fresh organic matter, can reduce SOC mineralization and its temperature sensitivity in a subtropical forest soil by limiting the availability of C and microbial activity, and thus has a great potential for maintaining soil carbon stock in subtropical forest ecosystems.