Aims
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 (
13
C/
12
C) are ...helpful to study the dynamics of SOM, but their variations with depth are still speculative.
Methods
Several studies indicated that the
13
C/
12
C ratio of C
3
vegetation decreased over time more than that of atmospheric CO
2
did. From these studies, we modelled the average variation of δ
13
C values of vegetation from 20,000 years Before Present (BP) to today. Then, we conducted a meta-analysis of the δ
13
C vs ∆
14
C values relations in forty-five soil profiles sampled all around the world.
Results
We first found evidence of the change in SOM δ
13
C values with the sampling year of the profile. Then, by converting ∆
14
C values into mean calendar age of SOM, we showed that 40% of the change in SOM δ
13
C values was explained by the historical change in plant δ
13
C values.
Conclusion
We conclude that the average increase of SOM δ
13
C values with depth was mostly linked to the change in vegetation δ
13
C 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.
Understanding hydrogen dynamics in soil organic matter is important to predict the fate of 3H in terrestrial environments. One way to determine hydrogen fate and to point out processes is to examine ...the isotopic signature of the element in soil. However, the non-exchangeable hydrogen isotopic signal in soil is complex and depends on the fate of organic compounds and microbial biosyntheses that incorporate water-derived hydrogen. To decipher this complex system and to understand the close link between hydrogen and carbon cycles, we followed labeled hydrogen and labeled carbon throughout near-natural soil incubations. We performed incubation experiments with three labeling conditions: 1 - 13C2H double-labeled molecules in the presence of 1H2O; 2 - 13C-labeled molecules in the presence of 2H2O; 3 - no molecule addition in the presence of 2H2O. The preservation of substrate-derived hydrogen after 1 year of incubation (ca. 5% in most cases) was lower than the preservation of substrate-derived carbon (30% in average). We highlighted that 70% of the C-H bonds are broken during the degradation of the molecule, which permits the exchange with water hydrogen. Added molecules are used more for trophic resources. The isotopic composition of the non-exchangeable hydrogen was mainly driven by the incorporation of water hydrogen during microbial biosynthesis. It is linearly correlated with the amount of carbon that is degraded in the soil. The quantitative incorporation of water hydrogen in bulk material and lipids demonstrates that non-exchangeable hydrogen exists in both organic and mineral-bound forms. The proportion of the latter depends on soil type and minerals. This experiment quantified the processes affecting the isotopic composition of non-exchangeable hydrogen, and the results can be used to predict the fate of tritium in the ecosystem or the water deuterium signature in organic matter.
Aims 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 (.sup.13C/.sup.12C) ...are helpful to study the dynamics of SOM, but their variations with depth are still speculative. Methods Several studies indicated that the .sup.13C/.sup.12C ratio of C.sub.3 vegetation decreased over time more than that of atmospheric CO.sub.2 did. From these studies, we modelled the average variation of delta.sup.13C values of vegetation from 20,000 years Before Present (BP) to today. Then, we conducted a meta-analysis of the delta.sup.13C vs â.sup.14C values relations in forty-five soil profiles sampled all around the world. Results We first found evidence of the change in SOM delta.sup.13C values with the sampling year of the profile. Then, by converting â.sup.14C values into mean calendar age of SOM, we showed that 40% of the change in SOM delta.sup.13C values was explained by the historical change in plant delta.sup.13C values. Conclusion We conclude that the average increase of SOM delta.sup.13C values with depth was mostly linked to the change in vegetation delta.sup.13C 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.
Ecosystem reconstruction after mining disturbance is a challenge considering the multitude of factors that affect soil formation and revegetation. In the boreal forest of western Canada, peat ...material is often used as the organic amendment for land reclamation to upland forest. Carbon and water dynamics of peat‐dominated ecosystems differ from natural upland forest soils. The objective of this work was to evaluate the evolution of soils reconstructed after mining disturbance using 13C and 2H analyses of n‐alkane tracers. Ten soils from natural ecosystems were sampled (0–10 cm) and compared with 11 soils from novel ecosystems ranging in age from 0 to 30 yr, as well as a fresh peat sample. Soils supported different vegetation, including pine (Pinus spp.), aspen (Populus spp.), and white spruce Picea glauca (Moench) Voss. Despite overlaps for some individual n‐alkanes, we found a dominance of n‐C25 in reconstructed soils, also dominant in the peat material, and a dominance of n‐C27 in natural soils, one of the dominant n‐alkanes in natural forest vegetation. In addition, there was a significant difference in odd n‐alkane δ2H and δ13C values between natural and reconstructed soils (p < .05). Differences in δ2H values, more negative for reconstructed soils than for natural soils, were attributed to changes in soil moisture, from wetter peat‐dominated soils to drier upland forests; among forest types, δ2H values were most negative under pine vegetation. The δ13C composition of odd n‐alkanes, in particular n‐C27, was significantly related to tree age (p < .05). Overall, both 2H and 13C isotopic signatures of odd n‐alkanes exhibited differences between natural and reconstructed soils. However, within the reconstructed soils, neither isotopic signature showed a clear evolution with age since reclamation.
Purpose
Douglas-fir occupies a growing role in the European timber industries but its effects on soils and the environment are still not well understood. We monitored the biogeochemical cycle of ...major elements in 11 representative mature stands throughout France over the 2013 to 2020 period to better understand repercussions on soil processes.
Methods
The present study is focused on throughfall and soil solution chemistry: concentrations were measured monthly at 15 cm and 60 cm depth and fluxes of major anions and cations were computed.
Results
Our results show that nitrate strongly dominated the soil solution chemistry: concentrations were elevated (>20 mg L
−1
) for 8 out of 11 sites with mean annual NO
3
−
values ranging from 2 to 253 mg L
−1
. Fluxes of nitrate at 60 cm of depth, ranging from 23 to 309 kg ha
−1
yr
−1
, were much higher than those in throughfall for nine studied sites. Nitrate, which was the dominant anion in soil solution, associated either to Ca
2+
, Mg
2+
and K
+
, or to Al
3+
was lost out of the soil-plant system through leaching.
Conclusions
These results suggest a production of NO
3
−
in the soil exceeding tree uptake and microbial immobilization resulting in strong ongoing soil acidification at many sites and potential degradation of water quality. Significant negative correlations were found between nitrate concentration in soil solution and sandy texture, soil C and N, site elevation and, annual rainfall, but further research is necessary to clarify the role of these factors in reducing the negative impacts on terrestrial and aquatic ecosystems.
In rural areas, nitrate concentrations in surface waters most often originate from the leaching of excess N fertilizer in agricultural lands, whereas forested catchments often have good water ...quality. However, Douglas-fir plantations may induce nitrogen cycle unbalances which may lead to an excess of nitrate production in the soil. We hypothesize that the excess of production of nitrate in the soil and nitrate leaching to streamwater is greater in catchments planted with Douglas fir. We used paired catchments in both France and Luxembourg with different land covers (Douglas-fir, Spruce, Deciduous, Grassland and clearcut) which were monitored over a 3–5 year period in order to assess the effect of Douglas-fir plantations on the chemical composition of surface water. Nitrate concentration in the soil and groundwater were also monitored. The results show that nitrate concentrations in streams draining Douglas-fir catchments were two to ten times higher than in streams draining other land covers, but were similar to the clearcut catchment. Nitrate concentrations under Douglas-fir in groundwater (up to 50 mg L−1) and in the soil were also higher than under all other land covers. Soil nitrate concentration was related to stream nitrate concentration. This suggests that soil processes, through excessive nitrate production under Douglas-fir, are driving the nitrate concentration in the stream water and our hypothesis of a transfer of a fairly large proportion of this excessive production from the soil to the stream is supported. This study also shows that nitrate concentrations in surface and ground waters in rural areas could also originate from Douglas fir forested catchments. The impact of Douglas-fir is nevertheless reduced downstream through a dilution effect: mixing tree species at the catchment scale could thus be a solution to mitigate the effect of Douglas-fir on nitrate concentration in surface waters.
Display omitted
•Higher nitrate concentration in streams draining Douglas-fir than other covers•NO3- in surface waters can derived from forested catchments draining Douglas fir•Imbalance between production and consumption of NO3 revealed by a seasonal dynamics•Nitrate concentration in streams is linked to excessive soil nitrate production
Measurements of δ15N–NO3– were taken in a highly flood-responsive agricultural catchment in the southwest of France to trace the sources and transfer pathways of nitrates during flood events. From ...January to March 2013, surface water samples were collected every week at the outlet, and four floods were sampled with a high resolution. Sampling was also performed in surface waters and sand lenses from the rest of the basin to trace nitrate sources and processes spatially. Nitrate extractions were performed using a method based on the solubility difference between inorganic salts and organic solutions. The δ15N values were in the range of surface water contaminated by N-fertilisers. Depending on the hydroclimatic event, nitrates resulted from a combination of sources and processes. At the start of the floods, the values of δ15N–NO3– and nitrate concentrations were low, demonstrating the dilution of water with rainwater. During a second phase, the nitrate concentration and the δ15N were higher. Deeper waters and soil solutions were the second source of nitrates. When the water level was low, both nitrate concentration and isotopic composition were high. These values reflected the denitrification processes that occurred in the soil under anaerobic conditions. An analysis of δ15N–NO3– in stream water in a small agricultural catchment was efficient at determining the origin of nitrates during flood events using a simple method.
Understanding hydrogen dynamics in soil organic matter is important to predict the fate of 3H in terrestrial environments. One way to determine hydrogen fate and to point out processes is to examine ...the isotopic signature of the element in soil. However, the non-exchangeable hydrogen isotopic signal in soil is complex and depends on the fate of organic compounds and microbial biosyntheses that incorporate water-derived hydrogen. To decipher this complex system and to understand the close link between hydrogen and carbon cycles, we followed labeled hydrogen and labeled carbon throughout near-natural soil incubations. We performed incubation experiments with three labeling conditions: 1 – 13C2H double-labeled molecules in the presence of 1H2O; 2 – 13C-labeled molecules in the presence of 2H2O; 3 – no molecule addition in the presence of 2H2O. The preservation of substrate-derived hydrogen after 1 year of incubation (ca. 5 % in most cases) was lower than the preservation of substrate-derived carbon (30 % in average). We highlighted that 70 % of the C–H bonds are broken during the degradation of the molecule, which permits the exchange with water hydrogen. Added molecules are used more for trophic resources. The isotopic composition of the non-exchangeable hydrogen was mainly driven by the incorporation of water hydrogen during microbial biosynthesis. It is linearly correlated with the amount of carbon that is degraded in the soil. The quantitative incorporation of water hydrogen in bulk material and lipids demonstrates that non-exchangeable hydrogen exists in both organic and mineral-bound forms. The proportion of the latter depends on soil type and minerals. This experiment quantified the processes affecting the isotopic composition of non-exchangeable hydrogen, and the results can be used to predict the fate of tritium in the ecosystem or the water deuterium signature in organic matter.
Understanding hydrogen dynamics in soil organic matter is important to predict the fate of .sup.3 H in terrestrial environments. One way to determine hydrogen fate and to point out processes is to ...examine the isotopic signature of the element in soil. However, the non-exchangeable hydrogen isotopic signal in soil is complex and depends on the fate of organic compounds and microbial biosyntheses that incorporate water-derived hydrogen. To decipher this complex system and to understand the close link between hydrogen and carbon cycles, we followed labeled hydrogen and labeled carbon throughout near-natural soil incubations. We performed incubation experiments with three labeling conditions: 1 - .sup.13 C.sup.2 H double-labeled molecules in the presence of .sup.1 H.sub.2 O; 2 - .sup.13 C-labeled molecules in the presence of .sup.2 H.sub.2 O; 3 - no molecule addition in the presence of .sup.2 H.sub.2 O. The preservation of substrate-derived hydrogen after 1 year of incubation (ca. 5â¯% in most cases) was lower than the preservation of substrate-derived carbon (30â¯% in average). We highlighted that 70â¯% of the C-H bonds are broken during the degradation of the molecule, which permits the exchange with water hydrogen. Added molecules are used more for trophic resources. The isotopic composition of the non-exchangeable hydrogen was mainly driven by the incorporation of water hydrogen during microbial biosynthesis. It is linearly correlated with the amount of carbon that is degraded in the soil. The quantitative incorporation of water hydrogen in bulk material and lipids demonstrates that non-exchangeable hydrogen exists in both organic and mineral-bound forms. The proportion of the latter depends on soil type and minerals. This experiment quantified the processes affecting the isotopic composition of non-exchangeable hydrogen, and the results can be used to predict the fate of tritium in the ecosystem or the water deuterium signature in organic matter.
Le radiocarbone (14C) et le tritium (3H) sont libérés dans l'environnement de manière naturelle et par les activités nucléaires. Les rejets devant perdurer pendant les prochaines décennies, il est ...primordial de prévoir leur devenir et leur temps de résidence dans les sols. L'objectif de cette thèse est de proposer une prévision quantitative et une modélisation simple du devenir de 14C et 3H dans les matières organiques du sol (MOS). L'originalité de ce travail est double: nous faisons l'hypothèse que l'incorporation et le devenir des atomes d'hydrogène non-échangeable (HNE) dans le sol sont couplés à la dynamique du carbone; l'approche choisie est le traçage naturel ou artificiel par les isotopes stables 13C et 2H.A travers le traçage naturel par le 13C in situ, nous avons quantifié le carbone récemment incorporé par la végétation sur quelques décennies. Nous avons alors adapté le modèle RothC à la dynamique du C profond des sols. Cela nous a permis de prévoir que 10% du C persisteront pendant plusieurs siècles dans les couches profondes. Les expériences croisées d'incubation de composés marqués en 13C et 2H nous a permis de montrer que l’activité microbienne est le moteur de l’incorporation d’hydrogène de l’eau dans les MOS et nous a permis d'établir la stœchiométrie CH des biotransformations. Ces expérimentations ont permis de proposer un modèle de la dynamique couplée de C et H des MOS à court et moyen terme (décennies). Une méta-analyse des corrélations entre les teneurs en 13C et 14C de sols mondiaux nous a permis de démontrer que l'enrichissement en 13C des MOS peut être expliqué en totalité par le rapport 13C/12C de la végétation dont elles sont issues.
Radiocarbon (14C) and tritium (3H) are naturally released into the environment but also through nuclear activities. The releases are expected to persist for the next decades, it is important to predict their fate and their residence time in soils. The objective of this thesis is to propose a quantitative prediction and a simple modeling of the fate of 14C and 3H in soil organic matter (SOM). The originality of this work is twofold: first, we hypothesize that the incorporation and fate of NEH atoms in the soil are coupled to the carbon dynamics. Second, we chose to trace carbon and hydrogen by natural or artificial 13C and 2H tracing.Through natural in situ 13C tracing, we quantified the carbon recently incorporated by vegetation in few decades. Deep horizons contain a large part of this carbon (typically 20 to 30%). We adapted the RothC model to the deep soil C dynamics. This allowed us to predict that 10% of C will persist for several centuries in the deeper layers. The labelling experiments showed that the microbial activity is driving the incorporation of hydrogen from water into SOM, and allowed us to establish the CH stoichiometry of biotransformations. These experiments were a mean to propose a model of the coupled C and H dynamics of the SOM in the short and medium term (decades). The results of this thesis contribute as well to the improvement of the interpretation of natural abundances in 13C and 2H stable isotopes. A meta-analysis of the correlations between the 13C and 14C concentrations of global soils has demonstrated that the 13C enrichment of deep organic matter can be fully explained by the 13C/12C ratio of the vegetation from which they are derived.
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