Soil δ13C Research Articles

Effects of maize residue and biochar applications on soil δ13C and organic carbon sources in a subtropical paddy rice ecosystem

AbstractThis study investigates the utility of plant δ¹3C natural labeling in predicting the impacts of environmental shifts on carbon cycling within ecosystems, particularly focusing on paddy fields treated with maize (Zea mays L.) residues and biochar. Specifically, it examines how soil δ¹3C and the sources of soil organic carbon (SOC), respond in paddy fields (which cultivate C3 plants like rice) when amended with maize residues, maize biochar, and silica‐enriched biochar (derived from C4 plants). Conducted in the Fuzhou paddy fields, the experiment included control groups and treatment groups with maize residue (4 t ha⁻¹), maize biochar (4 t ha⁻¹), and silicon‐modified maize biochar (4 t ha⁻¹) during both the early and late rice growth periods. The results indicate that all soil treatments increased soil δ¹3C. The application of maize residues notably affected the δ¹3C of the upper soil profile (0–15 cm) differently from the deeper layers (15–30 cm), and it increased soil organic C more than biochar or silicon‐modified maize biochar. Soil available P (AP) and pH emerged as significant factors linking δ¹3C, influencing rice yield through changes in soil physicochemical properties. Unlike maize residues, which reduced rice yields, applications of biochar and silicon‐modified maize biochar increased rice yields. The latter, which was particularly effective in lowering SOC decomposition rates and addressing rice's silica needs, emerged as the preferred option. The study highlights maize biochar and silicon‐modified maize biochar as sustainable alternatives to maize residues for rice cultivation, enhancing soil fertility, carbon pool stability, and yields.

Vegetation restoration changed the soil aggregate stability and aggregate carbon stabilization pathway according to δ13C signatures

Vegetation restoration can increase soil organic carbon (SOC) sequestration through the physical protection of soil aggregates. However, the soil aggregate stability and C flow pathway associated with long-term plantation restoration have not yet been fully characterized. Here, we conducted a study on Robinia pseudoacacia plantations at different recovery stages, studied the distribution and stability of aggregates, analysed the aggregate-associated organic carbon (OC) content and δ13C value, and quantified the aggregate C flow pathway. The results revealed that vegetation restoration increased the proportion of large macroaggregates (LMAs) and decreased the proportion of small macroaggregates (SMAs), with no changes observed in the proportion of microaggregates (MIAs) or silt + clay (SC) at 0–20 cm. The indices of aggregate stability, namely, the mean weight diameter (MWD), geometric mean diameter (GMD) and structural stability index (SSI), increased under vegetation restoration at 0–20 cm, with maximum values of 3.83 mm, 2.88 mm, and 2.00 %, respectively, at 35 years of age (35Y). The OC content of the LMAs increased from 10.96 to 21.64 g kg−1 and from 7.27 to 10.05 g kg−1 in the 0–20 cm and 20–40 cm layers, respectively. LMAs and SMAs had the greatest contributions to SOC accumulation in the 0–20 cm and 20–40 cm layers, respectively. The δ13C value increased with decreasing aggregate size. The C flow pathway was from macroaggregates to MIAs or SC. Compared with abandoned farmland, vegetation restoration decreased the aggregate C flow intensity in the 0–20 cm layer. The soil aggregate stability and aggregate-associated OC content decreased with increasing soil depth, but the soil δ13C value exhibited the opposite trend. Vegetation restoration regulated soil aggregate stability by influencing the fine root biomass (FRB) and SOC content. In summary, our analysis offers a valuable reference for the controlling effect of aggregation on C stability influenced by vegetation restoration.

Lipid biomarkers and stable isotopes uncover paleovegetation changes in extremely species-rich forest-steppe ecosystems, Central Europe

The historical development of the vegetation of semi-dry grasslands in Central Europe is not satisfactorily understood. Long-term continuity of open vegetation or, conversely, deep-past forest phases are considered possible sources of the current extreme species diversity of these ecosystems. We aimed to reveal the trajectory of paleovegetation development in these ecosystems through detailed analysis of terrestrial in-situ soil geoarchives. We measured the bulk soil carbon and nitrogen contents, lipid molecular distribution, and compound-specific stable carbon and hydrogen isotopic signatures of mid- and long-chain n-alkanes extracted from soil and modern plant material tissues (i.e., deciduous and Pinus leaves and grass/herbaceous species).The C23–C33 n-alkane homologues were identified in soils with different abundances. Normally, C27 and C29 n-alkanes were the most abundant homologues in tree-leaf samples, while grass-derived n-alkanes were mostly C31 and C33 homologues. Soils were largely dominated by C29 and C31 n-alkanes. Odd-numbered C27–C33 soil n-alkane δ13C values ranged from −36.2‰ to −23.2‰, whereas their δ2H values showed a wider range of variability that fluctuated from −224‰ to −172‰. Molecular distribution in combination with radiocarbon analysis of soil organic matter (SOM) and δ13C and δ2H values of n-alkanes revealed a large contribution of C3 trees (both deciduous and coniferous trees/pine trees) as the main source of n-alkanes between the late Pleistocene and early Holocene (ca 15,000–8200 calibrated year before present/cal year BP). A clear shift toward more grassy/herbaceous vegetation was observed from the early Holocene (ca 11,700–8200 cal year BP) onwards. Distribution patterns of lipids and soil geochemical parameters showed that plants are the main source of SOM and that biodegradation and kinetic isotope fractionation are not the main reasons for 13C enrichment in soil profiles. Past C3 vegetation shifts as well as paleoclimate changes (i.e., aridity) can have played a role in the observed 13C depth profiles.

Afforestation improves soil organic carbon and total nitrogen stocks mainly through increasing > 2 mm aggregate fractions and stimulating carbon and nitrogen transformations within aggregates in subtropical karst region

Vegetation restoration can significantly increase soil organic carbon (SOC) and total nitrogen (TN) stocks, however, the response of SOC and TN stocks, as well as their distribution, transformation, and stability within aggregates, to afforestation in fragile karst ecosystems remains unclear. In this study, soil samples were collected from orchards, 10-, 20-, and 40-year Dodonaea viscosa plantations after afforestation following orchard abandonment in a karst rocky desertification area. A natural succession shrubland soil served as the control. Aggregate size distribution, SOC and TN contents, and δ13C and δ15N values in bulk soils and aggregate fractions were determined. The results showed that 0.25–2 mm aggregates were the main contributors to SOC and TN stocks, amounting to 47.7 %-55.6 % and 43.0 %-51.7 %, respectively. Compared to orchards, D. viscosa afforestation significantly increased SOC and TN stocks by 160 %-393 % and 184 %-416 %, respectively, more obviously with prolonged afforestation. This effect was mainly attributed to the increases in > 2 mm aggregate fractions and their associated SOC and TN contents. In addition, δ13C and δ15N values decreased with decreasing aggregate size in afforested and shrubland soils. The C flow pathways followed the trend from < 0.053 mm to 4–8 mm following afforestation, but this was not the case for orchard soils exhibiting pathways from 2-4 mm to < 0.053 mm progressively. These results indicated that large aggregates showed a faster turnover rate and contributed to higher SOC and TN contents following afforestation. The significant and positive relationships between SOC and TN contents and magnesium, aluminium and iron oxides suggested the stimulatory effect on SOC and TN accumulation. Noticeably, although SOC and TN stocks in bulk soils following 40-year afforestation reached the level of shrubland, the proportion of > 2 mm aggregates and the geometric mean diameter remained significantly lower than those of shrubland soils. Soil aggregate stability following D. viscosa afforestation may take longer to reach natural restoration levels.

Newly plant-derived carbon in the deeper vadose zone of a sandy agricultural soil does not stimulate denitrification

Analysis of stable carbon (C) isotopic signatures (δ13C) in various soil C pools provides useful information on soil C sources, transport, and availability. Understanding the extent of deeper soil (below 40 cm) sequestration and transport of plant derived C is of particular interest as this provides a source for microorganisms to drive biological denitrification (as indicated by denitrification enzyme activity, DEA) and hence mitigate the nitrate leaching to groundwater. Meanwhile, studies on deeper soil C sequestration are rare due to methodological constraints. This study was done in deeper vadose zone (0–160 cm) of a sandy agricultural soil in a humid and temperature zone after a C3-C4 vegetation change taking advantage of the marked isotopic differences between C3 and C4 plants. It took place in a site previously grown with C3 crops (beet, barley, grass), but where C4 crops (maize) were grown continuously for the last 20 years. The other site where C3 crops were continuously grown was used for comparison. Specifically, the δ13C signature in top and deeper soil layers was used to distinguish between old C3– and newly C4-plant derived C in four C pools, i.e., bulk soil C, hot- and cold-water extractable C, and respired CO2-C. The δ13C signature between C3 soils and C3-C4 shifted soils was similar for bulk soil C but significantly different for water extractable and respired C pools. Hence, we estimated that the contribution of newly derived C to bulk soil C was negligible, whereas the contributions to the other C pools amounted up to 28.4 % along the soil profile. This emphasizes the importance of simultaneously analysing δ13C signature in various soil C pools to accurately assess C vertical transport and distribution. The concentrations of cold-DOC and values of specific ultraviolet visible absorbance of the wavelengths 254 and 280 nm decreased from 50 to 130 cm soil depths, while they increased below these depths. However, this suggested rise in C chemical quality at the deepest soil depths did not cause an increase in soil respiration activity or DEA, which was attributed to the protective effects of iron and aluminium oxides on C decomposition. Upon the application of labile C and N substrates, the deepest soil layers displayed a significantly increased DEA, suggesting the presence of a relatively abundant population of active denitrifying organisms. Overall, this study documents the presence of plant-derived C in the deeper vadose zone. Meanwhile, this particular C pool might not be an important substrate to drive deep-soil denitrification due to constraints imposed by the protection by metal oxides.

Vegetation Types Can Affect Soil Organic Carbon and δ13C by Influencing Plant Inputs in Topsoil and Microbial Residue Carbon Composition in Subsoil

Vegetation Types Can Affect Soil Organic Carbon and δ13C by Influencing Plant Inputs in Topsoil and Microbial Residue Carbon Composition in Subsoil

The spatial variation and drivers of soil δ13C in an alpine-gorge region on the eastern Qinghai-Tibetan Plateau

Soil stable carbon isotope (soil δ13C) can reflect soil carbon metabolic processes and record environmental and vegetation change information, so mapping the spatial distribution of soil δ13C (isoscapes of soil δ13C) can help us better understand the spatial variability of ecosystem carbon cycles. However, efficient approaches for obtaining the isoscapes of soil δ13C remain challenging, especially in mountainous areas with complex terrain. A total of 150 soil samples were collected and their soil δ13C was measured to investigate the spatial variation of soil δ13C in an alpine-gorge region on the eastern Qinghai-Tibetan Plateau which is characterized by complex terrain. Four prediction methods included ordinary kriging interpolation, multiple linear regression, random forest regression, and geographically weighted regression (GWR) were selected and compared to find the best prediction model for mapping the isoscapes of soil δ13C. The pathways of vegetation, topography, soil, and spatial factors influencing the spatial variability of soil δ13C were explored using variance partitioning analysis and structural equation model. The soil δ13C was significantly different among vegetation types, and ranged from −27.155‰ to −9.647‰. The spatial heterogeneity of soil δ13C showed moderate variation, and was dominated by spatial structural factors. The GWR model had higher prediction accuracy in the modeling soil δ13C in comparison to other models. Soil carbon to nitrogen ratio and normalized difference vegetation index were the main factors determining the spatial variability of soil δ13C, and other topographic and soil factors indirectly regulated the spatial distribution of soil δ13C by influencing these two factors. Our results provided a useful tool (GWR model) for mapping the isoscapes of soil δ13C and explored the factors controlling the spatial variability of soil δ13C in the alpine-gorge region on the eastern Qinghai-Tibetan Plateau, which are important for an in-depth understanding of soil carbon cycling in mountain ecosystems.

Contrasting stocks and origins of particulate and mineral-associated soil organic carbon in a mangrove-salt marsh ecotone

The global warming-driven poleward expansion of mangrove habitats (e.g., Avicennia germinans and Rhizophora mangle) into temperate salt marshes (e.g., Spartina alterniflora and Juncus roemerianus) has been shown to alter coastal soil organic carbon (SOC) storage. However, the taxa-specific consequences of this vegetation shift on the origin and size of SOC sub-fractions (particulate OC (POC); mineral-associated OC (MAOC); and reactive iron-associated OC (FeR-MAOC)) remain largely unexplored. In this study, we used a particle size-based SOC fractionation method to compare quantity and δ13C composition of bulk and each SOC sub-fractions in soil cores collected from Apalachicola Bay barrier islands in Florida, USA, the highest latitude where monospecific communities of all four aforementioned plants co-occur. Depth-dependent variation of bulk soil δ13C clearly showed the global warming-driven replacement of S. alterniflora by mangroves, as well as reciprocal substitutions of S. alterniflora and J. roemerianus, probably driven by changes in wetland elevation. Higher OC burial rates in mangrove habitats suggested that mangrove soils were principally developed by particle deposition. In contrast, comparatively lower OC burial rates but higher OC stocks in salt marsh habitats illustrated subsurface OC input from salt marsh roots. POC was primarily derived from contemporary plant detritus; its concentration was higher in salt marsh habitats (58.8 ± 9.0 % of SOC) relative to mangroves (38.4 ± 6.0 % of SOC). In contrast, MAOC content did not vary across plant habitats (53.5 ± 10.9 % of SOC), and principally originated from microbially-transformed OC and pre-existing plants. FeR-MAOC was essentially absent in R. mangle soils (2.9 ± 3.6 % of SOC) while representing a minor fraction of MAOC in three other plant habitats (7.8 ± 7.0 % of SOC). The δ13C of FeR-MAOC was more like the present-day surface plants, highlighting the in situ FeR-MAOC formation in their active oxidizing rhizospheres.

Natural δ13C and δ15N Abundance of Plants and Soils Under Long-term N Addition in a Temperate Secondary Forest

Natural δ13C and δ15N Abundance of Plants and Soils Under Long-term N Addition in a Temperate Secondary Forest

Land use changes and edaphic properties control contents and isotopic compositions of soil organic carbon and nitrogen in wetlands

Land use change in wetlands leads to significant losses of soil organic matter (SOM). Stable carbon (C) and nitrogen (N) isotopes offer insights into changes in C3/C4 vegetation, SOM sources, and decomposition processes. Yet, predicting the spatial–temporal dynamics of SOM contents and isotopes under land use changes remains challenging. This study delves into the effects of land use changes on soil organic carbon (SOC), total nitrogen (TN), δ13C and δ15N values, and soil physico-chemical properties and lignin phenols. Our results highlight the significance of soil water content (SWC) in determining the outcomes of land use changes. The conversion of wetland to cropland, forestland and construction land, led to notable reductions in SOC contents (8.71–56.33 %), and TN contents (7.87–37.12 %). Wetland conversion resulted in an enrichment of 13C and 15N abundance, with wetlands exhibiting the lowest δ13C (−25.57 to –22.89 ‰) and δ15N (2.66 to 6.67 ‰) values. A significant correlation occurred between δ13C and δ15N values in wetlands, but underwent considerable changes after wetland conversion. Key parameters, including bulk density (BD), C:N, the acid-to-aldehyde of vanillyl ((Ad/Al)v), lignin content (Λ8), and total phosphorus (TP), were identified as influencing factors for both SOC and TN contents. When evaluating δ13C values, the most influential factors included silt, C:N, SOC, sand, and BD. These indicate the importance of soil chemical group (from 41 % to 21 %) in elucidating δ13C values declined, while lignin group’s (from 9 % to 28 %) importance increased from topsoil to subsoil. The acid-to-aldehyde of syringyl ((Ad/Al)s), Λ8, C:N, BD and the cinnamyl-to-vanillyl ratio (C/V) were identified as the primary factors influencing δ15N values, with chemical group accounting for 36 % and lignin group for 48 % in topsoil, while physical group dominated 42 % in subsoil. Our findings underscore the shifts in SOM sources and distinct mechanisms of degradation/preservation of SOM following land use changes.

Using Multi‐Homolog Plant‐Wax Carbon Isotope Compositions to Reconstruct Tropical Vegetation Types

AbstractThe stable carbon isotope composition (δ13C) of plant components such as plant wax biomarkers is an important tool for reconstructing past vegetation. Plant wax δ13C is mainly controlled by photosynthetic pathways, allowing for the differentiation of C4 tropical grasses and C3 forests. Proxy interpretations are however complicated by additional factors such as aridity, vegetation density, elevation, and the considerable δ13C variability found among C3 plant species. Moreover, studies on plant wax δ13C in tropical soils and plants have focused on Africa, while structurally different South American savannas, shrublands and rainforests remain understudied. Here, we analyze the δ13C composition of long‐chain n‐alkanes and fatty acids from tropical South American soils and leaf litter to assess the isotopic variability in each vegetation type and to investigate the influence of climatic features on δ13C. Rainforests and open vegetation types show distinct values, with rainforests having a narrow range of low δ13C values (n‐C29 alkane: ‰ ; Suess‐effect corrected). This allows for the detection of even minor incursions of savanna (δ13C n‐C29 alkane ‰) into rainforests. While Cerrado savannas and semi‐arid Caatinga shrublands grow under distinctly different climates, they can yield indistinct δ13C values for most compounds. Cerrado soils and litter show pronounced isotopic spreads between the n‐C33 and n‐C29 alkanes, while Caatinga shrublands show consistent values across the two homologs, thereby enabling the differentiation of these vegetation types. The same multi‐homolog isotope analysis can be extended to differentiate African shrublands from savannas.

Elevated CO2 enhanced the incorporation of 13C-residue into plant but depressed it in the microbe in the tomato (Solanum lycopersicum L.) rhizosphere soils

The translation of plant residue carbon in plant–soil–microbe systems under atmospheric CO2 (aCO2) and elevated CO2 (eCO2) is key to understanding the response of ecosystems to climatic change. Using stable isotope probing technology, we conducted an experiment on 3.5 g·kg−1 tomato 13C-residue (1050 ‰) added to tomato soils to investigate the decomposition and incorporation of 13C-residue under varied CO2 concentrations (aCO2 and eCO2: 400 and 800 μmol·mol−1, respectively). The results showed that the 13C-residue decomposition rate was stimulated by approximately 7.2 % and the 13C amount in plant stems was stimulated by 15.6 % (22.2 vs. 19.2 μg/pot), while the total 13C amount in soil microbial phospholipid fatty acids (PLFAs) was significantly decreased by 16.3 % (541 vs. 646 nmol·g−1), the DNA δ13C in soil microbes decreased by 5.1 % (−27.629 ‰ vs. −26.289 ‰), and soil δ13C decreased by 7.1 % (49.9 ‰ vs. 53.7 ‰) under eCO2. The microbial community structure involved in 13C-residue incorporation was significantly affected by the CO2 concentration. Firmicutes (Caldalkalibacillus 20.4 % and Bacillus 20.4 %) and Actinobacteria (Streptomyces 6.6 %) were the dominant phyla under aCO2 and eCO2, respectively. Proteobacteria (Dyella) were the most depleted microbes under eCO2, whereas almost no genera were found under aCO2. The 13C-residue decomposition rate was positively correlated with soil NO3−-N (r = 0.998, P < 0.05) at t1 (17 days of cultivation under varied CO2 concentrations), and DNA δ13C was positively related with soil pH (r = 0.978, P < 0.05) at t2 (35 days of cultivation under varied CO2 concentrations). Mantel tests showed that the microbial community composition involved in 13C incorporation was significantly correlated with the soil pH under various CO2 conditions (r = 0.879, P < 0.05). These results indicate that eCO2 stimulated the decomposition of 13C-residue and the transportation of 13C to the plant stem, but suppressed the incorporation of 13C into microbes (PLFA and DNA), which were adjusted by the rhizosphere N status and soil pH.

Soil carbon, nitrogen, and phosphorus storage in juniper–oak savanna: role of vegetation and geology

Abstract. Woody-plant encroachment into grasslands and savannas has been globally widespread during the past century, likely driven by interactions between grazing, fire suppression, rising atmospheric CO2, and climate change. In the southernmost US Great Plains, Ashe juniper and live oak have increased in abundance. To evaluate potential interactions between this vegetation change and the underlying soil parent material on ecosystem biogeochemistry, we quantified soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and δ13C of SOC in soils obtained from trenches passing through grassland, juniper, and oak patches on soils lying atop the respective Edwards and Buda limestone formations in central Texas. Soils on the Edwards formation are more shallow and have more rock outcropping than those on Buda. The δ13C values of SOC under grasslands was −19 ‰, whereas those under woody patches were −21 ‰ to −24 ‰, indicating that wooded areas were relatively recent components of the landscape. Compared with grasslands, areas now dominated by juniper or oak had elevated SOC, TN, and TP storage in soils lying atop Edwards limestone. In Buda soils, only oak patches had increased SOC, TN, and TP storage compared with grasslands. Woody encroachment effects on soil nutrients were higher in soils on the Edwards formation, perhaps because root and litter inputs were more concentrated in the relatively shallow layer of soil atop the Edwards bedrock. Our findings suggest that geological factors should be considered when predicting nutrient store responses in savannas following vegetation change. Given that woody encroachment is occurring globally, our results have important implications for the management and conservation of these ecosystems. The potential interactive effects between vegetation change and soil parent material on C, N, and P storage warrant attention in future studies aimed at understanding and modeling the global consequences of woody encroachment.

Modelling Soil δ13C across the Tibetan Plateau Using Deep-Learning

Soil carbon isotopes (δ13C) provide reliable insights for studying soil carbon turnover at a long-term scale. The Tibetan Plateau (TP), often referred as “the third pole of the earth”, is highly sensitive to global climate change, and exhibits an early warning signal of global warming. Although many studies detected soil δ13C variability at site scales, there is still a knowledge gap existing in the spatial pattern of soil δ13C across the TP. In this study, we compiled a database of 198 topsoil δ13C observations from published literatures and used a modified multi-layer perceptron (MLP) neural network algorithm to predict the spatial pattern of topsoil δ13C and β (indicating the decomposition rate of soil organic carbon (SOC), calculated as δ13C divided by logarithmically converted SOC) at 500m resolution. Results showed that MLP model effectively predicted topsoil δ13C with a model efficiency of 0.72 and a root mean square error of 1.16‰. Topsoil δ13C varied significantly across different ecosystem types (p &lt; 0.001) with a mean δ13C of –25.89 ± 1.15‰ (mean ± standard deviation) for forests, –24.91 ± 1.03‰ for shrublands, –22.95 ± 1.44‰ for grasslands, and –18.88 ± 2.37‰ for deserts. Furthermore, there was an increasing trend of predicted δ13C from the southeastern to the northwestern TP, likely linked to vegetation type and climatic conditions. β values were low in the eastern TP and higher in the northern and northwestern TP, indicating faster SOC turnover rate in the east TP compared to the north and northwest. This study represents the first effort to develop a fine resolution product of topsoil δ13C and β across the TP, which could provide an independent, data-driven benchmark for biogeochemical cycling models to study SOC turnover and terrestrial carbon-climate feedback over the TP under climate change.

Effects of planting basins and farmyard manure addition on soil carbon and nitrogen pools under on‐farm conditions in Makueni county of Kenya

AbstractClimate change, land degradation and inadequate soil nutrients pose significant threats to food security and agricultural sustainability. This study aims to examine the effects of planting basins with farmyard manure on soil total carbon (C), nitrogen (N), isotopic C (δ13C) and N (δ15N) compositions within smallholder‐managed farms in Makueni County, Kenya. The study involved two management practices: planting basins with manure (PM) and conventional farming practices (FP) in 12 experimental sites. Soil samples were taken at three depths (0–10, 10–20 and 20–40 cm), with three replicates for each treatment. Significant interactions were observed between land management practices and sites as well as land management practices and soil depth on soil total C and N. At each of the 12 sites, soil total C was higher under PM (ranging from 0.44% to 1.86%, p &lt; .05) than FP management (ranging from 0.35% to 1.37%), across all soil depths. Soil total N concentrations ranged from 0.027% to 0.100% under FP and (0.060% to 0.190%, p &lt; .05) under PM management. Across soil depths, higher (less negative) soil δ13C values were observed under conventional farmer practice (range − 22.5‰ to −17.1‰) compared with PM management range (−24.3‰ to −18.1‰). Soil δ15N was significantly enriched under PM management (range: 7.4‰ to 12.6‰, p &lt; .05) compared with the conventional farmer practices (range: 6.1‰ to 9.8‰, p &lt; .05). The findings show that planting basins with farmyard manure offers both climate mitigation and adaptation benefits by increasing soil C contents and improving soil fertility. The study provides insights into the real‐world implications of these practices, emphasizing the potential of planting basins with manure in enhancing soil quality and climate resilience.

Five-year vegetation conversion from pasture to C3 and C4 plants affects dynamics of SOC and TN and their natural stable C and N isotopes via mediating C input and N leaching

Understanding the effects of land-use change on stock and composition of soil organic carbon (SOC) and nitrogen (N) is pivotal for sustainable agriculture and climate change adaption. However, previous studies have often overlooked the specific vegetation type in land-use changes. Therefore, a five-year lysimeter block experiment was conducted, involving non-vegetation, eulalia (C4 plant), and clover (C3 plant) to investigate the impacts of vegetation conversion from pasture on SOC and N dynamics and their natural stable isotopes. Non-vegetation caused 26.21 % and 25.88 % decreases in SOC and total N (TN) contents. Five-year eulalia and clover cultivation maintained stable SOC content, with clover exhibiting higher soil TN content. Eulalia-derived soil C was 1.64–7.58 g C kg−1 and SOC loss in eulalia treatment was 1.86–7.90 g C kg−1. Soil δ13C in eulalia increased at a rate of 0.90 ‰ year−1, significantly surpassing clover and non-vegetation treatments. Conversely, soil δ15N decreased over time, showing insignificant difference among all treatments. Eulalia exhibited significantly higher dry weight and δ13C but lower TN content compared with clover. However, no significant differences were observed in total C and δ15N between the two vegetation treatments. Non-vegetation exhibited higher dissolved organic C concentration than two vegetation treatments in 2017, decreasing over time. Dissolved TN and nitrate concentrations in leachate followed the order clover> non-vegetation> eulalia, with nitrate being the predominant form of N leaching from leachate. Our findings reveal that vegetation conversion affects soil C and N contents, and alters their natural isotopes as well as the leaching of labile soluble nutrients. Notably, non-vegetation consistently reduced SOC and TN contents, whereas eulalia cultivation maintained SOC content, improved C/N ratio and δ13C, and reduced N leaching compared with clover cultivation. These results highlight the potential of eulalia as a candidate plant for enhancing C sequestration and reducing N leaching in cold regions of Japan.

Source and stability of soil organic carbon jointly regulate soil carbon pool, but source alteration is more effective in mangrove ecosystem following Spartina alterniflora invasion

Spartina alterniflora invasion seriously affects blue carbon function in mangrove ecosystems. However, the mechanisms by which S. alterniflora invasion affects the sources and stability of soil organic carbon (SOC) in mangrove ecosystems have not been fully elucidated. In this study, stable isotopes and nuclear magnetic resonance spectroscopy techniques were used to investigate the impacts of S. alterniflora invasion on SOC sources, decomposition characteristics, and SOC pool stability in the Zhangjiang Estuary, Fujian Province, China. The results showed that SOC content in the mangrove was 8.25–51.46 g kg−1, which was 21.3 % and 93.4 % higher than those in the S. alterniflora and tidal flat, respectively. Additionally, the SOC storage at depths of 0–30 cm was 26.0 % and 96.9 % higher in mangrove than in S. alterniflora and tidal flats, respectively. The values of soil δ13C and δ15N were lower, whereas the C/N ratio was higher in mangrove than in S. alterniflora. A Bayesian stable isotope mixing model revealed that mangrove plant tissues were the highest contributor to SOC (40.8 ± 18.7 %) in mangrove topsoil because of litter effects. However, in the S. alterniflora and tidal flat ecosystems, SOC was mainly derived from allochthonous sources with particulate organic matter (POM) proportions of 72.1 ± 3.3 % and 56.4 ± 5.3 %, respectively. Alkyl C (16.3–41.4 %) was the dominant functional group in all soils, followed by O-alkyl C and aromatic C. Alkyl C, A/O-A, and hydrophobicity index (HI) were significantly lower in S. alterniflora soil than in mangrove and tidal flat soils. Therefore, the SOC sources and stability, including fractions and chemical composition, were substantially altered in mangrove ecosystems after S. alterniflora invasion, leading to decreases in SOC pools. Nevertheless, source alteration was more effective in mangrove wetland invaded by S. alterniflora. These findings may provide new perspectives regarding blue carbon sources, decomposition, stabilization, and sequestration in mangrove wetlands invaded by S. alterniflora.

Dietary habits of pastoralists on the Tibetan plateau are influenced by remoteness and economic status

In general, dietary habits of pastoralists are livestock-derived, but are also influenced by external food sources under globalization. We hypothesized that dietary habits of pastoralists would be influenced by their remoteness, and that changes from the traditional diet would result in deviations in the local ecological chain. To test this hypothesis, we determined the δ13C and δ15N values of soil, plants, and hair of animals and pastoralists (n = 885). The δ13C value in human hair reflects the proportions of protein originating from C3 and C4 plants; whereas, the δ15N value reflects the proportions of protein derived from plants and animals, with higher values indicating a greater consumption of meat. The isotopic signatures enabled us to estimate the variation in dietary habits of pastoralists across a socio-economic gradient of easily accessible to remote areas on the Tibetan plateau, and to determine the trophic transfer of the isotopes along an ecological chain. The trophic magnification factor (TMF) evaluated the trophic transfer of δ15N in the soil-plants-animals-pastoralists ecological chain. The high δ15N values in soil and plants were not recovered in animals and pastoralists in easily accessible and developed areas, indicating the use of external feed and food resources, and that they deviated from the ecological chain. The mean δ13C (−22.0 ‰) and δ15N values (6.9 ‰) of pastoralists indicated diets consisting mainly of local C3 plants and animal products. However, pastoralists in remote areas relied more on meat protein and on the local ecological chain than pastoralists in easily accessible areas, as their δ15N values and trophic magnification factor of δ15N in the ecological chain were greater. In addition to remoteness, per capita GDP influenced dietary changes in pastoralists, with richer pastoralists consuming more external food. We concluded that dietary changes of pastoralists in the easily accessible areas were due to external food resources and alterations in the local ecological chain of animals and plant-based foods available to the pastoralists.

Spatiotemporal Variability in Soil Properties and Composition in Mangrove Forests in Baía de Todos os Santos (NE Brazil)

Soil properties and components in mangrove ecosystems influence their geochemical processes and services. Despite the extensive mangrove areas present in Brazil, few studies focusing on these themes are under development. In this sense, this work aimed to investigate the spatial variability in soil attributes and composition, the geochemistry of Fe, and the isotopic characteristics of organic matter in mangroves in Baía de Todos os Santos (Cacha Prego, Ponta Grossa, Ilha de Maré, Pitinga), which constitutes Brazil’s second largest bay. The soils investigated showed spatial and temporal changes affecting their properties (pH, Eh) and composition (TOC, Fe fractions), as well as clear spatial changes in the redox potential values (+30–+188 mV), with higher values in PT. Soil textures ranged from predominantly sandy (CP, PT, PG: sand, &gt;70%) to a finer granulometry (IM: sand, 33–64%). These characteristics influenced Fe partitioning and organic matter content, with higher TOC and pyrite values observed in IM (FeS2: 2720–9233 mg kg−1; TOC: 4.4–6.6%) and lower sulfide values found in PT, mainly in the dry season (FeS2: 85–235 mg kg−1). The soil δ13C and N/C ratios seem to suggest a mixed origin of organic matter.

Conversion of native grassland to coniferous forests decreased stocks of soil organic carbon and microbial biomass

AimsEncroachment of woody species into grasslands is a global phenomenon that affects ecosystem services, including soil organic carbon (SOC) storage and microbial community structure. We determined stocks of SOC and soil microbial biomass as affected by conversion of grasslands to coniferous forests.MethodsWe examined SOC and soil δ13C signatures under three vegetation covers: native grasslands, eastern redcedar (Juniperus virginiana), and ponderosa pine (Pinus ponderosa), at six soil depths (0 − 10, 10 − 30, 30 − 100, 100 − 170, 170 − 240, 240 − 300 cm). Fatty acid methyl esters (FAMEs) were used to quantify soil microbial biomass (MB) and as biomarkers for bacteria, arbuscular mycorrhizal fungi (AMF), and saprophytic fungi (SF) which includes ectomycorrhizal fungi (ECM).ResultsNative grasslands had higher overall SOC and MB stocks in the top 300 cm soil depth (7.39 kg SOC m− 2 and 65.6 mmol MB m− 2) than converted cedar (6.80 kg SOC m− 2 and 38.4 mmol MB m− 2) or pine (5.87 kg SOC m− 2 and 39.9 mmol MB m− 2) forest. Losses of AMF biomass occurred on conversion of native grasslands to pine forest. Stocks of SF biomass only differed with vegetation type in topsoil.ConclusionsConversion of native grasslands to cedar or pine forest increased topsoil (0 − 10 cm) SOC storage due to needle litter accumulation. Conversion of native grasslands to forests decreased subsoil (10 − 300 cm) SOC stocks and had replaced, by increasing depth, 30 to 19% and 45 to 26% of grassland C with cedar and pine C, respectively. Changes in soil pH, nutrients, and soil microbial communities accelerated SOC turnover.