δ13C Of Soil Organic Matter Research Articles

The linkage of 13C and 15N soil depth gradients with C:N and O:C stoichiometry reveals tree species effects on organic matter turnover in soil

The knowledge of tree species dependent turnover of soil organic matter (SOM) is limited, yet required to understand the carbon sequestration function of forest soil. We combined investigations of 13C and 15N and its relationship to elemental stoichiometry along soil depth gradients in 35-year old monocultural stands of Douglas fir (Pseudotsuga menziesii), black pine (Pinus nigra), European beech (Fagus sylvatica) and red oak (Quercus rubra) growing on a uniform post-mining soil. We investigated the natural abundance of 13C and 15N and the carbon:nitrogen (C:N) and oxygen:carbon (O:C) stoichiometry of litterfall and fine roots as well as SOM in the forest floor and mineral soil. Tree species had a significant effect on SOM δ13C and δ15N reflecting significantly different signatures of litterfall and root inputs. Throughout the soil profile, δ13C and δ15N were significantly related to the C:N and O:C ratio which indicates that isotope enrichment with soil depth is linked to the turnover of organic matter (OM). Significantly higher turnover of OM in soils under deciduous tree species depended to 46% on the quality of litterfall and root inputs (N content, C:N, O:C ratio), and the initial isotopic signatures of litterfall. Hence, SOM composition and turnover also depends on additional—presumably microbial driven—factors. The enrichment of 15N with soil depth was generally linked to 13C. In soils under pine, however, with limited N and C availability, the enrichment of 15N was decoupled from 13C. This suggests that transformation pathways depend on litter quality of tree species.

Dynamics of Organic Matter of Soil Profiles with Different Vegetation Conditions from the Chinese Loess Plateau: δ13C and δ15N Approaches

To understand the biogeochemical processes associated with soil organic matter (SOM) decomposition, we analyzed the SOM contents, the δ13C and δ15N values of the dominant species foliage, litter and SOM from soil samples for five soil profiles with different vegetation conditions in the Loess Plateau, Northwestern China. Results showed that the amounts of soil organic carbon (SOC) and total nitrogen (TN) mainly concentrated on the surface soil and differentiated according to the vegetation conditions in the following order: broad-leaved forest > coniferous woodland > shrub forest > grassland > wasteland. SOC and TN contents decreased with depth and varied in the ranges of 1.1–31.2 g/kg and 0.3–3.7 g/kg, respectively. Compared with the other regions, the 13C and 15N were enriched and the δ13C and δ15N values of topsoil SOM respectively increased in the ranges of 0.5%o–3.2%o and 0.7%o–4.6%o during litter degradation to SOM on the surface soil, which was controlled by SOM turnover rates. This result indicates that the effect of isotopic fractionation was obvious during the transformation of SOM from plant debris to SOM in topsoil, which resulted in great increments of SOM δ13C and δ15N. Litter inputs lowered the surface soil δ13C and δ15N values while decomposition increased δ13C and δ15N values in deeper soil. Foliage and litter inputs averaged 1.0% and 1.3% δ15N and -28.3% and -27.0% δ13C, respectively. The five soil profiles with different vegetation conditions had similar characteristics in variations of SOM δ13C and δ15N and increased with depth, respectively. However, the patterns of δ13C in our sites were less pronounced than the patterns of δ15N primarily because the discrimination against 13C during organic matter decomposition is weaker than the discrimination against 15N. Except for the shrub profiles, significant correlations were found between the two stable isotopes, 15N and 13C. Combined with information on SOM contents, the variations of the isotopic values of SOM showed a mixing process of litter inputs between different soil profiles. Two controls of soil isotopic compositions were established: new litter inputs and overall isotopic fractionation during decomposition. In conclusion, the overall isotopic fractionation during decomposition left residual soil N and C enriched in 15N and 13C, explaining the high δ15N and δ13C values observed in deeper soil.

Organic matter stabilization in aggregates and density fractions in paddy soil depending on long-term fertilization: Tracing of pathways by 13C natural abundance

Previous studies on upland soils showed that 13C natural abundance can successfully reveal C stabilization pathways between aggregates and soil organic matter (SOM) density fractions. The direction of C stabilization in paddies can, however, deviate from that in upland soils owing to i) periodic drying–rewetting cycles, with oxygen pulses under oxic conditions, and thus, shifts in microbial processing of organic residues, and ii) intensive organic and mineral fertilization. To trace C stabilization in paddies, soil was sampled from a long-term field experiment under an unfertilized Control and NPK, NPK + straw, and NPK + manure fertilizer regimes. Soil was analyzed for total C, microbial biomass (MB), and dissolved organic C, and separated into three classes based on aggregate size (>250 μm, 53–250 μm, and <53 μm) followed by density fractionation of each class to obtain free and occluded light fractions as well as dense and mineral heavy fractions. Pathways of C were determined based on C content and δ13C in all pools. The highest increase in total C (69%) was in NPK + manure, whereas the MBC increased with fertilization by at least 29% compared with the Control. All fertilizers increased macro-aggregation by at least 111% compared with the Control. The highest C content in the aggregates was in the mineral fractions of macroaggregates. Fertilization decreased the δ13C of total SOM compared with the Control, indicating suppressed decomposition of organic compounds. Aggregate size classes showed a typical δ13C enrichment trend from macro-to microaggregates, reflecting similarities between paddy and upland soils. A detailed scheme of C flows within aggregates and SOM fractions based on the δ13C natural abundance revealed the following general sequence: mineral → dense → free light → occluded light fractions. This trend, which is partly opposite to that observed in upland soils, reflects the anoxic and variable redox conditions of paddies. This facilitates the predominant stabilization of recent C input in the mineral fraction with Fe oxides due to Fe2+/Fe3+ dynamics, whereas light fractions are processed by microorganisms mainly in periods without overflooding. The C pathways in the two heavy fractions were separate from those in the two light fractions, which also indicates differences in the C stabilization processes between paddy and upland soils. Thus, the present study provides further detailed insights into the C stabilizing mechanisms in paddy soils which depend on management.

Aridity-driven decoupling of δ13C between pedogenic carbonate and soil organic matter

Abstract Pedogenic carbonate is an invaluable archive for reconstructing continental paleoclimate and paleoecology. The δ13C of pedogenic carbonate (δ13Cc) has been widely used to document the rise and expansion of C4 plants over the Cenozoic. This application requires a fundamental presumption that in soil pores, soil-respired CO2 dominates over atmospheric CO2 during the formation of pedogenic carbonates. However, the decoupling between δ13Cc and δ13C of soil organic matter (δ13CSOM) have been observed, particularly in arid regions, suggesting that this presumption is not always valid. To evaluate the influence of atmospheric CO2 on soil δ13Cc, here we performed systematic δ13C analyses of paleosols across the Chinese Loess Plateau, with the sample ages spanning three intervals: the Holocene, the Late Pleistocene, and the mid-Pliocene warm period. Our paired δ13Cc and δ13CSOM data reveal broadly divergent trending patterns. Using a two-component CO2-mixing model, we show substantial incorporations of atmospheric CO2 (up to 60%) into soil pore space during carbonate precipitation. This result readily explains the enrichment of δ13Cc and its divergence from δ13CSOM. As a consequence, δ13C of pedogenic carbonates formed under semiarid and/or arid conditions are largely driven by regional aridity through its control on soil CO2 composition, and thus cannot be used to evaluate the relative abundance of C3 versus C4 plants. Nonetheless, these carbonates can be applied for atmospheric CO2 reconstructions, even for periods with low CO2 levels.

Carbon-nitrogen isotope coupling of soil organic matter in a karst region under land use change, Southwest China

The soil stable carbon (C) and nitrogen (N) isotopes are widely used to indicate C3/C4 vegetation history, N sources and transformation processes, respectively. However, land use change, particularly converting forest into farm land, alters soil organic matter (SOM) sources and processes in soils, resulting in a hard understanding of soil C and N fate. In the present study, soil organic carbon (SOC) and soil organic nitrogen (SON) contents, and their stable isotope compositions (δ13C and δ15N) were determined in the five soil profiles under land use change (i.e., conversion of native forest land into shrub land, grass land, maize field, and paddy land) in Lobo county, Guizhou province, Southwest China. A coupling of 13C and 15N isotope in SOM under land use change was verified whether it could provide more accurate indications of sources and transformation processes.The SOC and SON contents of native forest land at the 0∼20 cm depth were significantly larger than these under other transformed lands. The SOC and SON contents decreased exponentially with increasing soil depth under all land use types, and showed opposite trends with soil pH. The C/N ratios of SOM in the soils under undisturbed native forest decreased from 10 to 7 with increasing soil depth, while an irregular fluctuation along soil profile was shown in other transformed lands. Similarly to the most study in the soils under C3 forest, the δ13C and δ15N values of SOM in the soils under native forest at the 0∼50 cm depth increased with increasing soil depth, with the range of −27.7‰∼−25.7‰ and 6.5‰∼10.0‰, respectively. While decreasing trends of them in the soils below 50 cm depth were attributed to the mixing of 13C and 15N-depleted organic matters from bedrocks. However, the δ13C and δ15N values of SOM along the soil profiles under other transformed lands were intensively irregularly fluctuated between −29.1‰ and −19.0‰, 1.2‰ and 7.9‰, respectively. The single δ13C and δ15N signals in the soil profiles of transformed lands indeed revealed the alterations of historical C3/C4 composition and N transformation processes after land use change, but these indications were not specific. The result of the coupling of 13C and 15N isotope under native forest land reveals a positive relationship between them, which associated with full plant-absorption against 15N-depleted inorganic nitrogen derived from SOM mineralization. This study suggests that the coupling of CN isotope fractionation more likely occurs in the C3 forest ecosystem with high N utilization efficiency. However, the replacement of native forest by farm land or grass land will reduce soil N utilization efficiency.

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.

Biochar mineralization and priming effect in a poplar short rotation coppice from a 3-year field experiment

The present study assesses the stability of biochar, its effect on original soil organic matter (SOM) decomposition, and the interactions with plant roots over a 3-year period in a short rotation coppice plantation in Northern Italy. Biochar produced from gasification of maize silage (δ13C of biochar ≈ − 13.8‰) was applied into the soil of the plantation (δ13C of SOM ≈ −23.5‰). Total and heterotrophic respirations were measured in control and biochar-treated plots, and the amount of biochar-derived carbon dioxide (CO2) was calculated using an isotopic mass balance method. Biochar mean residence time (MRT) was assessed using exponential decay models. The remaining amount of biochar at the end of the experiment was estimated by a soil isotopic mass balance. In the absence of plant roots, MRT of the more recalcitrant biochar fraction (96% of the total) was 24.3 years, significantly lower than expected from previous shorter-term studies or laboratory incubations. In the presence of plant roots, MRT decreased to 12.6 years, confirming the previously observed positive effect of roots on biochar decomposition. However, in the absence of roots, the biochar decreased the decomposition of original SOM by 16%, indicating long-lasting protective effect on SOM (negative priming effect). The soil isotopic mass balance suggested that part of the applied biochar could have been lost by downward migration. This study provides the first estimation of the biochar stability and priming effect on SOM in field conditions in the medium term.

Soil microbial anaplerotic CO2 fixation in temperate soils

In soils both δ13C and δ15N values of soil organic matter (SOM) tend to increase with soil depth. This change in soil C and N isotopic composition is at least partially linked to soil microbial activity that has been suggested to drive fractionation during decomposition, preferential microbial decomposition of components of soil organic matter and mixing of diverse sources of C. Soil microbes are capable of fixing N2 and CO2 from the soil atmosphere. Apart from surface photosynthetic CO2 fixation and chemoautotrophic fixation, dark anaplerotic (i.e. non-photosynthetic) fixation of CO2 is especially important for provision of C-skeletons for amino acid synthesis. We hypothesized that these N2 and CO2 fixing processes may contribute to determining SOM δ13C and δ15N values. Soils from 10 sites across South Africa differing in soil properties were incubated in the dark for 3 d under continuous exposure to 13CO2- and 15N2-enriched atmospheres with varying soil moisture (10, 50 and 100% of field capacity) and temperature (4, 25, 40 °C). We did not detect significant N2 fixation in any treatment. Significant soil microbial anaplerotic CO2 fixation, however, occurred in all soils. Highest rates of anaplerotic CO2 fixation occurred in soils at 50% field capacity and 25 °C, suggesting a link with microbial biotic activity. Soils with low C and N concentrations and low C:N ratios exhibited the highest rates of CO2 fixation, indicating a possible link between anaplerotic CO2 fixation rates and soil nutrient status. The higher rates of CO2 fixation in soils with low nutrients may indicate that soil microbes rely increasingly on anaplerotic fixation as SOM-N declines, forcing greater reliance on de novo amino acid synthesis, and thus anaplerotic CO2 fixation. Diffusion of bulk atmospheric CO2 (δ13C ca. −10‰) into the soil atmosphere (δ13C ≪ −10‰) drives soil atmospheric CO2 δ13C values up towards those of the bulk atmosphere. Anaplerotic CO2 fixation in this CO2 may contribute to determining soil δ13C values.

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.

Sources of dissolved organic carbon in small volcanic mountainous tropical rivers, examples from Guadeloupe (French West Indies)

In the tropical zone, small watersheds are affected by intense meteorological events. These events play an important role in the erosion of soils and therefore on the sources of organic carbon in small tropical rivers. We studied the geochemistry of two soils on Basse-Terre Island (French West Indies, FWI): ferralitic soil and Andosol. The two studied soils are very similar in terms of soil organic matter (SOM) and soil solution parameters. The total organic carbon (TOC) and total nitrogen (TN) contents vary between 1.7 and 92g kg‐1 and between 0.1 and 5.5g kg‐1, respectively, with the highest concentrations observed in the topsoil. The C/N ratios are relatively constant throughout the soil profiles (ca. 12). The carbon isotopic composition of SOM varies between ‐27.3‰ and ‐22.7‰ and presents an enrichment with increasing depth of soil profiles. Dissolved organic carbon (DOC) concentrations in soil solutions, varying from 3.2 to 91.3mg L‐1, are similar for the both extraction used in lab (with milliQ water and Ca(NO3)2) but are higher than those measured in soil solutions sampled from lysimeters (0.65–1.46mg L‐1). The isotopic compositions of DOC obtained by extractions and SOM are comparable, with δ13C values ranging from ‐28.6‰ to ‐25.8‰. The DOC sampled from lysimeters is systematically depleted in 13C compared to DOC obtained by extractions, with δ13C values of ‐33.8‰ to ‐30.6‰. The enrichment of δ13C of SOM through the soil profiles is either consistent with the carbon isotopic fractionation of SOM by decomposing organisms, or the differential mineralization of both labile and stable carbon stocks in soils. DOC concentrations in stream waters vary between 0.46 and 5.75mg L‐1, and are generally lower during low water level than floods. The isotopic compositions of DOC in the rivers range from ‐38.9‰ to ‐27.2‰, with δ13C values, which are more depleted in 13C during low water level than flood events. The δ13CDOC of water river samples and soil solutions obtained by extraction and collected with lysimeters demonstrates that the DOC in rivers derives essentially from both the lixiviation of the soil surface layers during floods and groundwater flow during low water levels. Lixiviation of soil surface layers can be boosted by significant increases of intensity and duration of meteorological events and can strongly favor the release of surface soil organic matter in rivers and the impoverishment in nutrients of soil surface layers.

Suppression of soil organic matter decomposition by gasoline and diesel as assessed by 13C natural abundance

Petroleum products are common contaminants in soils due to human activities. They are toxic for microorganisms and threat their functions, including decomposition of soil organic matter (SOM). The direct estimation of altered SOM decomposition – based on the CO2 efflux – is impossible after petroleum contamination because petroleum decomposition also contributes to these CO2 fluxes. We used the natural differences in the isotopic signature (δ13C) of SOM and of petroleum products to partition the total CO2 for both sources and to analyse the suppression of SOM decomposition. The dynamics of 13C fractionation during the mineralization of gasoline and diesel was measured during 42 days. The 13C fractionation varied between −8.8‰ and +3.6‰ within the first 10 days, and stabilized thereafter at about −5.3‰ for gasoline and +3.2‰ for diesel. These 13C fractionations and δ13C values of CO2 emitted from the soil were used to partition the total CO2. Contamination with gasoline reduced the CO2 efflux from SOM by a factor of 25 (from 151 to 6.1 mg C-CO2 kg−1 soil during 42 days). The negative effect of diesel was much lower: the CO2 efflux from SOM was decreased by less than a factor of 2. The strong effect of gasoline versus diesel reflects the lower absorption of gasoline to mineral particles and the development of a thin film on water surfaces, leading to toxicity for microorganisms. We conclude that the soil contamination by gasoline and diesel strongly decreased microbial functions and so, the degradation of native SOM.

Distinct fungal and bacterial δ13C signatures as potential drivers of increasing δ13C of soil organic matter with depth

Soil microbial biomass is a key source of soil organic carbon (SOC), and the increasing propor- tion of microbially derived SOC is thought to drive the enrichment of 13 C during SOC decomposition. Yet, little is known about how the d 13 C of soil microbial biomass differs across space or time, or with the composition of the microbial community. Variation in soil microbial d 13 C may occur due to variation in substrates used by soil microorganisms, and variation in how different microorganisms synthesize biomass. Understanding these variations in soil microbial d 13 C would enable more accurate interpretation of patterns in the d 13 C of SOC. Here, we report the variation in d 13 C values of individual phospholipid fatty acids

Holocene wildfire history and human activity from high-resolution charcoal and elemental black carbon records in the Guanzhong Basin of the Loess Plateau, China

High-resolution sedimentary charcoal and black carbon (BC, including char and soot) records from a loess–soil profile, combined with magnetic susceptibility, δ13C of soil organic matter of analyses, pollen counts and other paleoenvironmental proxies reveal past fire patterns and landscape evolution over the past 12,000 years. Results from the analyses of charcoal and BC influx show that regional fire activity was high in the early and late Holocene, whereas fire was less frequent and pervasive in the middle Holocene. Locally, fires were infrequent near the study site until the Late Holocene. Soot and char analyses do not parallel changes in charcoal variability, and thus appear to reflect either a different aspect of fire activity or else these data are registering aspects of particle transportation and deposition in addition to fire characteristics.The patterns in fire activity observed during the Holocene are consistent with variations in vegetation inferred from δ13C values in soil organic matter, pollen counts, and paleoclimate proxies. Drier and colder-than-present conditions on the Loess Plateau occurred during the Lateglacial and early Holocene (12,000–8500 years BP), which likely enhanced regional fire activity across the Artemisia and Gramineae-dominated steppe landscape in the south of the Loess Plateau. Wetter and warmer-than-present conditions during the mid-Holocene (8500–3100 years BP), reduced fire episodes and promoted the development of mixed forest and forest-steppe. The distribution of C4 plants and woodland expanded at this time from 40% to 60% cover. The subsequent increase in fire-episode frequency during the past 3100 years is consistent with cooler and drier conditions in the late Holocene and also with changes in the spatial and temporal distributions of Neolithic burning practices, such as land reclamation and crop cultivation, during those periods (e.g., expansion of C3 plants).The close association between millennial-scale variations in fire and monsoon activity on the Loess Plateau suggests that future shifts in monsoon-related climate variability could have important consequences for fire and human activities as both respond to regional climate change.

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.

Test stable carbon isotopic composition of soil organic matters as a proxy indicator of past precipitation: Study of the sand fields in northern China

Understanding the stable carbon isotopic composition (δ13C) of surface soil organic matter under various climates is crucial for quantitative reconstruction of past climatic changes. Although the relationship between precipitation and plant δ13C is well understood at global and regional scale, studies relating δ13C of surface soil organic matter to precipitation in northern China remain controversial. In this study, 90 surface soil samples were collected from the desert/loess transition zone of Northern China, where vegetation is sensitive to climatic changes. The stable carbon isotopic composition of organic matter in these soil samples was analyzed in order to examine its palaeoclimatic implications. Our results show that the δ13C varied from −26.56 to −19.41‰ among the investigated sites, with an average of −23.54‰. There was a strong negative correlation between the δ13C of surface soils and the mean annual precipitation (MAP) (R2 = 0.52, p < 0.05). Variation in the δ13C of C3 plants was the main factor that determined the δ13C of soil organic matter. The C4/C3 biomass ratio may also contribute to the δ13C changes, but this effect was relatively minor in our study area. Our findings suggest that a general transform function between the proxy index δ13C value and climates may not be applicable at one specific site, and that a comprehensive investigation is needed when reconstructing the past climate change using the δ13C of buried soils.

Pathways of litter C by formation of aggregates and SOM density fractions: Implications from 13C natural abundance

Aggregate formation is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of particulate organic matter (POM) and its interactions with mineral particles. C stabilization processes lead to 13C fractionation and consequently to various δ13C values of soil organic matter (SOM) fractions. Differences in δ13C within the aggregates and fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy δ13C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to heavier δ13C in remaining C. We hypothesized that: 1) 13C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2) 13C fractionation (but not the δ13C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, minimal differences between Δ13C of SOM fractions between various ecosystems correspond to maximal probability of the SOM formation pathways.We tested these hypotheses on three soils formed from cover loam during 45 years of growth of coniferous or deciduous forests or arable crops. Organic C pools in large macroaggregates, small macroaggregates, and microaggregates were fractionated sequentially for four density fractions to obtain free POM with ρ < 1.6 g cm−3, occluded POM with two densities (ρ < 1.6 and 1.6–2.0 g cm−3), and mineral fraction (ρ > 2.0 g cm−3).The density fractions were 13C enriched in the order: free POM < light occluded POM < heavy occluded POM < mineral fraction. This, as well as their C/N ratios confirmed that free POM was close to initial plant material, whereas the mineral fraction was the most microbially processed. To evaluate the successive steps of SOM formation, the Δ13C values between δ13C of SOM fractions and δ13C of bulk SOM were calculated. The Δ13C indicated that, parallel with biochemical transformations, the physical disintegration strongly contributed to the formation of free and occluded light POM. In contrast, biochemical transformations were more important than physical disintegration for formation of heavy occluded POM from light occluded POM. This was confirmed by review of 70 Δ13C values from other studies analyzed Δ13C depending on the density of SOM fractions. Accordingly, the successive steps of SOM formation were: fLF<1.6 = oLF<1.6 → oDF1.6–2.0 = MF>2.0. The obtained steps of C stabilization were independent on the initial precursors (litter of coniferous forest, deciduous forest or grasses).To test the second hypothesis, we proposed an extended scheme of C flows between the 3 aggregate size classes and 4 SOM fractions. Δ13C enrichment of the SOM fractions showed that the main direction of C flows within the aggregates and SOM fractions was from the macroaggregate-free POM to the mineral microaggregate fraction. This confirmed the earlier concept of SOM turnover in aggregates, but for the first time quantified the C flows within the aggregates and SOM density fractions based on δ13C values. So, the new 13C natural abundance approach is suitable for analysis of C pathways by SOM formation under steady state without 13C or 14C labeling.

Implications for Late Holocene climate from stable carbon and oxygen isotopic variability in soil and land snail shells from archaeological site 41KM69 in Texas, USA

Well-preserved fossil land snail shell collected in a 115 cm soil profile from archaeological site 41KM69 in Texas, USA, minimally span the past 2200 years and provide an opportunity to explore the paleoclimate implications of isotopic variability in shell carbonates, bulk soil carbonates, and soil organic matter. Carbon isotopic compositions (δ13C values) of modern and archaeological adult snail shell carbonates suggest significant (>60%) dietary input of C3 plants during all periods. Oxygen isotopic compositions of shells suggest that the isotopic composition of environmental water available at the time of shell growth has changed little over the period of the profile. A linearly decreasing trend in δ13C of soil organic matter from −22.8‰ at the bottom of the profile (115 cm below surface) to −25.6‰ for modern surface samples indicate an overall 20% increase in abundance of C3 plants. This implies a progressively wetter climate and less severe water stress conditions or decreasing summer rainfall that is in agreement with other studies on Late Holocene climate change in the southern Great Plains, USA. These results strongly suggest a stable climate that favored C3 vegetation in this portion of Texas during this period.

Reconstructing savanna tree cover from pollen, phytoliths and stable carbon isotopes

AbstractAimTo calibrate a model of the relationship between bio‐proxies (pollen, phytoliths and δ13C of soil organic matter) and woody cover, measured as the leaf area index (LAI). This relationship, applied in palaeosequences, enables reconstruction of past savanna tree cover.LocationThe samples are from tropical Africa. Modern soil samples are from the Central African Republic and past samples are from sediments of lakes in Senegal and Congo.MethodsWe analysed the pollen and phytolith content and stable carbon isotope values of 17 soil samples taken from three short transects in the Central African Republic; LAI was measured on the same transects. The indices used were the AP/NAP ratio of arboreal (AP) to non‐arboreal (NAP) pollen, the D/P ratio of ligneous dicotyledons (D) to Poaceae (P) phytoliths, and the δ13C of soil organic matter, i.e. the 13C/12C ratio.ResultsA multi‐proxy model was calibrated. The best model included only a combination of pollen and phytolith as proxies, excluding organic matter δ13C because of its long mean residence time in the soil. The model was then applied to two palaeosequences in Africa, and a time series of relative LAI changes was obtained, providing new information about vegetation changes.ConclusionThis model can be applied in palaeosequences to reconstruct relative time series of LAI in African savannas and can help interpret vegetation changes quantitatively. This approach is complementary to the description of pollen and phytolith assemblages.

The ancient agricultural landscape of the satellite settlement of Ramonal near Tikal, Guatemala

Soil science methodologies often enrich archaeological reconstructions. In the Maya Lowlands of Mesoamerica, stable carbon (C) isotope ratio (δ13C) analysis of soil organic matter (SOM) from profiles near ancient archaeological sites has been used to interpret the vegetative histories related to ancient maize agriculture. Due to distinct photosynthetic processes, the decomposed organic matter from the cultivation of maize can be distinguished from SOM derived from native forest detritus. The recalcitrant nature of humic substances allows for this distinction to be preserved through time. This study evaluates SOM δ13C from 98 soil profiles near Tikal, Guatemala to identify areas of ancient Maya agriculture and the staple crops used. Ancillary physical and chemical properties of the soil profiles were examined to facilitate and to supplement the interpretation of the isotope data. Most of the soils analyzed in this study are shallow Haprendolls with limited C isotopic evidence of prehistoric vegetation changes. The deeper, well-developed Argiudolls and Hapludolls contained strong evidence of vegetation changes associated with ancient maize agriculture. Areas with strong δ13C signatures of ancient C4 vegetation corresponded with foot- and toeslope locations, high clay content and low phosphorus (P) levels. The shallow backslope soils lacked significant evidence of ancient agriculture. The connections between profile location, settlement, isotope data, and P levels enable the identification of both agriculture production zones and enhanced understanding of stable C isotope dynamics in soils within the unique physiographic and archaeological surroundings of the Maya Lowlands.

Determination of organic and inorganic carbon, δ13C, and nitrogen in soils containing carbonates after acid fumigation with HCl

AbstractIn carbonate‐containing soils a reliable determination of organic C requires a method that effectively separates organic and inorganic C without altering the organic matter. This study was conducted to determine whether HCl vapor completely removes carbonates even in dolomite‐rich soils and to what extent a widely used acid‐fumigation method has to be modified for humus‐rich soils. Furthermore, it was tested whether HCl fumigation alters organic‐C content. Since C and N parameters are often analyzed simultaneously we also tested the influence of acid‐vapor treatment on N content and on δ13C of soil organic matter. We applied fumigation with 37% HCl for 8 and 32 h using 9 carbonate‐containing soil samples. Inorganic C ranged from 7 to 124 and organic C from 9 to 267 g kg–1. The maximum contents of dolomite and calcite were 940 and 640 g kg–1, respectively. A time of 8 h was enough to completely remove all carbonates. Neither the content nor the δ13C of organic C were significantly affected by fumigation. In contrast, N contents were altered by acid treatment. Based on these results and on our experience in analyzing more than 1000 soil samples, a recommended procedure for acid fumigation of carbonate‐containing soils with a wide range of organic‐ and inorganic‐C contents was derived. Samples pretreated in this way can be analyzed reliably for their organic‐C content and δ13C. Furthermore, N and inorganic‐C contents can be determined with a quality sufficient for many purposes.