δ13C Values Of Soil Organic Matter Research Articles

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

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

13C-14C relations reveal that soil 13C-depth gradient is linked to historical changes in vegetation 13C

The understanding of the dynamics of subsoil (>30 cm) soil organic matter (SOM) is critical to predict the future evolution of the carbon cycle. Stable carbon isotopes ratios (13C/12C) are helpful to study the dynamics of SOM, but their variations with depth are still speculative. Several studies indicated that the 13C/12C ratio of C3 vegetation decreased over time more than that of atmospheric CO2 did. From these studies, we modelled the average variation of δ13C values of vegetation from 20,000 years Before Present (BP) to today. Then, we conducted a meta-analysis of the δ13C vs ∆14C values relations in forty-five soil profiles sampled all around the world. We first found evidence of the change in SOM δ13C values with the sampling year of the profile. Then, by converting ∆14C values into mean calendar age of SOM, we showed that 40% of the change in SOM δ13C values was explained by the historical change in plant δ13C values. We conclude that the average increase of SOM δ13C values with depth was mostly linked to the change in vegetation δ13C values over the last 20,000 years. The variance around the trend was attributed to the contribution of root derived carbon and to soil processes such as interaction of SOM with minerals or to microbial processes.

Stable carbon analysis of alluvial paleosols in the Mixteca Alta, Oaxaca, Mexico

We examine δ13C values from soil organic matter (SOM) from the Culebra, Inguiro and Verde watersheds (Verde data are from Mueller et al., 2012) located in the Mixteca Alta from the terminal Pleistocene to the present. The SOM δ13C values were compared against local paleosol data and other paleoenvironmental proxies from central and southwestern Mexico. The paleovegetation implications of the SOM reflect overall changes in paleoclimate from the terminal Pleistocene through the middle Holocene. The late Holocene stable carbon isotope values varied widely, suggesting that human activities such as agriculture affected the paleoenvironment and paleovegetation in the study area, similar to findings in central Mexico. Values of δ13C show a significant rise between 3500 cal BP and 1250 cal BP, with one notable cluster of enriched values corresponding to the Post-Classic cultural period (local Natividad phase, 1250–500 cal BP), when the local population is thought to have peaked over 100,000. Since the late Formative period (local late Cruz phase, starting ∼3500 cal BP), lama-bordo (i.e., sediment check-dams) and agricultural terrace constructions were important for land use management and agriculture, which became widespread by the Post-Classic period. The highly enriched values associated with widespread agricultural structures suggest the increased importance of maize cultivation, and potentially the increased significance of CAM plant management, such as maguey and nopal during the late Holocene.

Nitrogen fertilization increases rhizodeposit incorporation into microbial biomass and reduces soil organic matter losses

Agricultural soils receive large amounts of anthropogenic nitrogen (N), which directly and indirectly affect soil organic matter (SOM) stocks and CO2 fluxes. However, our current understanding of mechanisms on how N fertilization affects SOM pools of various ages and turnover remains poor. The δ13C values of SOM after wheat (C3)-maize (C4) vegetation change were used to calculate the contribution of C4-derived rhizodeposited C (rhizo-C) and C3-derived SOM pools, i.e., rhizo-C and SOM. Soil (Ap from Haplic Luvisol) sampled from maize rhizosphere was incubated over 56 days with increasing N fertilization (four levels up to 300 kg N ha−1), and CO2 efflux and its δ13C were measured. Nitrogen fertilization decreased CO2 efflux by 27–42% as compared to unfertilized soil. This CO2 decrease was mainly caused by the retardation of SOM (C3) mineralization. Microbial availability of rhizo-C (released by maize roots within 4 weeks) was about 10 times higher than that of SOM (older than 4 weeks). Microbial biomass and dissolved organic C remained at the same level with increasing N. However, N fertilization increased the relative contribution of rhizo-C to microbial biomass by two to five times and to CO2 for about two times. This increased contribution of rhizo-C reflects strongly accelerated microbial biomass turnover by N addition. The decomposition rate of rhizo-C was 3.7 times faster than that of SOM, and it increased additionally by 6.5 times under 300 kg N ha−1 N fertilization. This is the first report estimating the turnover and incorporation of very recent rhizo-C (4 weeks old) into soil C pools and shows that the turnover of rhizo-C was much faster than that of SOM. We conclude that the contribution of rhizo-C to CO2 and to microbial biomass is highly dependent on N fertilization. Despite acceleration of rhizo-C turnover, the increased N fertilization facilitates C sequestration by decreasing SOM decomposition.

Autotrophic fixation of geogenic CO<sub>2</sub> by microorganisms contributes to soil organic matter formation and alters isotope signatures in a wetland mofette

Abstract. To quantify the contribution of autotrophic microorganisms to organic matter (OM) formation in soils, we investigated natural CO2 vents (mofettes) situated in a wetland in northwest Bohemia (Czech Republic). Mofette soils had higher soil organic matter (SOM) concentrations than reference soils due to restricted decomposition under high CO2 levels. We used radiocarbon (Δ14C) and stable carbon (δ13C) isotope ratios to characterize SOM and its sources in two mofettes and compared it with respective reference soils, which were not influenced by geogenic CO2. The geogenic CO2 emitted at these sites is free of radiocarbon and enriched in 13C compared to atmospheric CO2. Together, these isotopic signals allow us to distinguish C fixed by plants from C fixed by autotrophic microorganisms using their differences in 13C discrimination. We can then estimate that up to 27 % of soil organic matter in the 0–10 cm layer of these soils was derived from microbially assimilated CO2. Isotope values of bulk SOM were shifted towards more positive δ13C and more negative Δ14C values in mofettes compared to reference soils, suggesting that geogenic CO2 emitted from the soil atmosphere is incorporated into SOM. To distinguish whether geogenic CO2 was fixed by plants or by CO2 assimilating microorganisms, we first used the proportional differences in radiocarbon and δ13C values to indicate the magnitude of discrimination of the stable isotopes in living plants. Deviation from this relationship was taken to indicate the presence of microbial CO2 fixation, as microbial discrimination should differ from that of plants. 13CO2-labelling experiments confirmed high activity of CO2 assimilating microbes in the top 10 cm, where δ13C values of SOM were shifted up to 2 ‰ towards more negative values. Uptake rates of microbial CO2 fixation ranged up to 1.59 ± 0.16 μg gdw−1 d−1. We inferred that the negative δ13C shift was caused by the activity of autotrophic microorganisms using the Calvin–Benson–Bassham (CBB) cycle, as indicated from quantification of cbbL/cbbM marker genes encoding for RubisCO by quantitative polymerase chain reaction (qPCR) and by acetogenic and methanogenic microorganisms, shown present in the mofettes by previous studies. Combined Δ14C and δ13C isotope mass balances indicated that microbially derived carbon accounted for 8–27 % of bulk SOM in this soil layer. The findings imply that autotrophic microorganisms can recycle significant amounts of carbon in wetland soils and might contribute to observed radiocarbon reservoir effects influencing Δ14C signatures in peat deposits.

Minor stable carbon isotope fractionation between respired carbon dioxide and bulk soil organic matter during laboratory incubation of topsoil

A common assumption in paleoenvironmental reconstructions using soils is that the carbon isotope composition of soil-respired CO2 is equivalent to the carbon isotope composition of bulk soil organic matter (SOM). However, the occurrence of a non-zero per mil carbon isotope enrichment factor between CO2 and SOM (\(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\)) during soil respiration is the most widely accepted explanation for the down-profile increase in SOM δ13C values commonly observed in well-drained soils. In order to shed light on this apparent discrepancy, we incubated soil samples collected from the top 2 cm of soils with pure C3 vegetation and compared the δ13C values of soil-respired CO2 to the δ13C values of bulk SOM. Our results show near-zero \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values (−0.3 to 0.4 ‰), supporting the use of paleosol organic matter as a proxy for paleo soil-respired CO2. Significantly more negative \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values are required to explain the typical δ13C profiles of SOM in well-drained soils. Therefore our results also suggest that typical SOM δ13C profiles result from either (1) a process other than carbon isotope fractionation between CO2 and SOM during soil respiration or (2) \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values that become increasingly negative as SOM matures.

Temperature and precipitation effects on δ13C depth profiles in SOM under temperate beech forests

Enrichment of 13C in SOM with soil depth is related to interacting processes influenced by temperature and precipitation. Our objectives were to derive climate effects on patterns of vertical δ13C values of soil organic matter (SOM) while minimizing the effect of confounding variables.We investigated vertical changes in δ13C values of SOM in 1-cm depth intervals in silvicultural mature beech (Fagus sylvatica L.) forest ecosystems in northern Rhineland-Palatinate across gradients of MAT (7.9 to 9.7°C mean annual temperature) and MAP (607 to 1085mm mean annual precipitation) in winter 2011. Forest stands (n=10) were chosen based on data sets provided by the Rhineland-Palatinate Forest Administration so that variations in these gradients occurred while other environmental factors like physico-chemical soil properties, tree species, stand age, exposition and precipitation (for the temperature gradient) or temperature (for the precipitation gradient) did not differ among study sites.From litter down to the mineral soil at 10cm depth, soil organic carbon (SOC) content decreased (47.5±SE 0.1% to 2.5±0.1%) while the δ13C values increased (−29.4±0.1‰ to −26.1±0.1‰). Litter of sites under higher MAP/lower MAT had lower δ13C values which was in line with literature data on climate driven plant physiological process. To compare the dimension of the vertical 13C enrichment, δ13C values were regressed linearly against log-transformed carbon contents yielding absolute values of these slopes (beta). Beta values ranged between 0.6 and 4.5 (range of r from −0.7 to −1.0; p<0.01). Due to an assumed decay continuum and similar variations of δ13C values in litter and in 10cm depth, we conclude that effects on isotope composition in the Oi layer continue vertically and therefore, δ13C values in litter do not solely control beta values. Beta values decreased with increasing MAT (r=−0.83; p<0.05). Reduced soil moisture and therefore both, reduced microbial activity and reduced downward transport of microbial cycled DOM (=13C enriched) might be responsible for less pronounced δ13C depth profiles in case of high temperatures. Greater C:N ratios (lower degradability) of the litter under higher temperatures likely contributed to these depth trends. Beta values increased with increasing MAP (r=0.73; p<0.05). We found decreasing C:N ratios in the mineral soil that possibly indicates higher decomposition under higher precipitation. Exclusion of the organic layers from linear regressions indicated a stronger impact of MAP on the development of δ13C depth profiles.Our results confirm temperature and precipitation effects on δ13C depth profiles and indicate stronger 13C enrichment under lower MAT/higher MAP. Therefore, time series of vertical δ13C depth profiles might provide insights into climate change effects.

Soil organic carbon decomposition from recently added and older sources estimated by δ13C values of CO2 and organic matter

The production of CO2 in soil strongly depends on the availability of organic carbon (C) for microorganisms. It is obvious, that C that entered the soil recently is more easily available for microorganisms in comparison to older C. However, only very few approaches allow for a quantitative estimation of the availability of C in relation to the time it is entering the soil. We hypothesized that δ13C values of CO2 and of soil organic matter (SOM) after a C3 to C4 vegetation change will enable to calculate the relative availability of younger (C4-derived) and older C (C3-derived) sources for microorganisms. Soil CO2 was sampled over one vegetation period at depths of 10, 40–50 and 60–70 cm at three treatments: a C3 reference (wheat), a C4/fallow (fallow after one year of maize cropping), and a C4/C4 (two years of maize cropping). Based on the δ13C of CO2 purified from the admixture of atmospheric CO2 by the Miller/Tans model and on the δ13C values of SOM, the contributions of younger and older C sources to CO2 and SOM were assessed. Depending on the soil depth and the presence of living roots, the contribution of younger C to soil CO2 ranged from 16 to 50%, but that to SOM was less than 5%. By comparing the contributions of older and younger C to CO2 and SOM, we found that the relative availability of organics recently introduced into the soil (C4-derived) was about 7 times higher than the availability of C stabilized in soil for longer than one year (C3-derived). We concluded that simultaneous analysis of the δ13C values of both SOM and of CO2 allows not only for the quantification of the CO2 sources, but also for the estimation of the availability of soil C pools of different age for microorganisms.

Stable carbon isotopic composition of soil organic matter in the karst areas of Southwest China

This study dealt with the distribution characteristics of soil organic carbon (SOC) and the variation of stable carbon isotopic composition (δ13C values) with depth in six soil profiles, including two soil types and three vegetation forms in the karst areas of Southwest China. The δ13C values of plant-dominant species, leaf litter and soils were measured using the sealed-tube high-temperature combustion method. Soil organic carbon contents of the limestone soil profiles are all above 11.4 g/kg, with the highest value of 71.1 g/kg in the surface soil. However, the contents vary between 2.9 g/kg and 46.0 g/kg in three yellow soil profiles. The difference between the maximum and minimum δ13C values of soil organic matter (SOM) changes from 2.2‰ to 2.9‰ for the three yellow soil profiles. But it changes from 0.8‰ to 1.6‰ for the limestone soil profiles. The contrast research indicated that there existed significant difference in vertical patterns of organic carbon and δ13C values of SOM between yellow soil and limestone soil. This difference may reflect site-specific factors, such as soil type, vegetation form, soil pH value, and clay content, etc., which control the contents of different organic components comprising SOM and soil carbon turnover rates in the profiles. The vertical variation patterns of stable carbon isotope in SOM have a distinct regional character in the karst areas.

STABLE CARBON ISOTOPE STUDY OF THE PRAIRIE-FOREST TRANSITION SOIL IN LOUISIANA 1

The calcareous prairies of Louisiana are among the most endangered and the least understood ecosystems in North America. The major threat to this ecosystem is the invasion of woody plants, primarily eastern red cedar (Juniperus virginiana L.). We studied the vertical profile of soil organic C (SOC) and the stable carbon isotope (δ13C) from soil organic matter (SOM) to infer the vegetation history and to understand the C dynamics of these prairies. SOC decreased with depth (0–100 cm), but variation due to vegetation type (prairie, transition, forest) and landscape position were not statistically significant. The mean total SOC stock within the 0–30 cm depth from three sites, three vegetation types, and two landscape positions was 7.6 ± 0.4 kg/m2 (mean ± SE, n = 18). The SOM δ13C values within the 0–10 cm depth showed that although the forest vegetation was exclusively C3, the transition and the prairie vegetation were composed of a mixture of C3 and C4 plant functional forms. Statistical comparison of SOM δ13C values from the forest, transition, and prairie with depth showed that the SOM δ13C from the prairie and transition are significantly different from that of the forest up to the 30–40 cm depth. SOM δ13C values of the prairie and transition were not significantly different at all depths, suggesting that the invasion of C3 trees and shrubs observed within the transition may be a recent phenomenon coinciding with fire suppression. SOM δ13C below 30–40 cm indicated that in the past, a C4 community might have dominated the entire site.

Changes of the forest-savanna boundary in Brazilian Amazonia during the Holocene revealed by stable isotope ratios of soil organic carbon.

The possibility of ecosystem boundary changes in northern Brazilian Amazonia during the Holocene period was investigated using soil organic carbon isotope ratios. Determination of past and present fluctuations of the forest-savanna boundary involved the measurement of natural 13C isotope abundance, expressed as δ13C, in soil organic matter (SOM). SOM 13C analyses and radiocarbon dating of charcoal fragments were carried out on samples derived from soil profiles taken along transects perpendicular to the ecotonal boundary. SOM δ13C values in the upper soil horizons appeared to be in equilibrium with the overlying vegetation types and did not point to a movement of the boundary during the last decades. However, δ13C values obtained from deeper savanna and forest soil layers indicated that the vegetation type has changed in the past. In current savanna soil profiles, we observed the presence of mid-Holocene charcoals derived from forest species: fire frequency at that time was probably greater, and more extensive savanna may have resulted. Isotope data and the presence of these charcoals thus suggest that the forest-savanna boundary has shifted significantly in the recent Holocene period, forest being more extensive during the early Holocene than today. During the middle Holocene, the forest could have strongly regressed, and fires appeared, with a maximum development of the savanna vegetation. At the beginning of the late Holocene, the forest may have invaded a part of this savanna, and fires occurred again.

Forest savanna ecotone dynamics in India as revealed by carbon isotope ratios of soil organic matter.

In India, the stability of the forest savanna boundary (ecotone) has been questioned. To investigate the possibility of vegetation change at this boundary, we used the natural difference in the stable carbon isotope ratio (13C/12C, expressed as δ13C) of C3 (forest) and C4 (savanna) plants, which is preserved in the isotopic composition of soil organic carbon. Past changes in the position of the ecotone can thus be documented by 13C/12C analysis of soil organic matter (SOM). Measurements were made on soil samples taken along a transect across the forest savanna boundary. In both ecosystems, δ13C values of SOM in upper soil layers appeared to be in equilibrium with the current plant community: SOM was near-14‰ under savanna cover, and near-28‰ under forest. By contrast, δ13C values of SOM from deep layers of soil profiles under forest cover near the ecotone, were less negative than would be expected for a C3-dominated community. These results indicate that a change in ecosystem dominance occurred recently, and that forest has invaded an area occupied previously by a C4 plant community (savanna and cultivated grassland).