Nitrogen nutrition effects on δ13C of plant respired CO2 are mostly caused by concurrent changes in organic acid utilisation and remobilisation.

Nitrogen (N) nutrition impacts on primary carbon metabolism and can lead to changes in δ13C of respired CO2. However, uncertainty remains as to whether (1) the effect of N nutrition is observed in all species, (2) N source also impacts on respired CO2 in roots and (3) a metabolic model can be constructed to predict δ13C of respired CO2 under different N sources. Here, we carried out isotopic measurements of respired CO2 and various metabolites using two species (spinach, French bean) grown under different NH4 +:NO3 - ratios. Both species showed a similar pattern, with a progressive 13C-depletion in leaf-respired CO2 as the ammonium proportion increased, while δ13C in root-respired CO2 showed little change. Supervised multivariate analysis showed that δ13C of respired CO2 was mostly determined by organic acid (malate, citrate) metabolism, in both leaves and roots. We then took advantage of nonstationary, two-pool modelling that explained 73% of variance in δ13C in respired CO2. It demonstrates the critical role of the balance between the utilisation of respiratory intermediates and the remobilisation of stored organic acids, regardless of anaplerotic bicarbonate fixation by phosphoenolpyruvate carboxylase and the organ considered.

Agronomic characterization of anaerobic digestates with near-infrared spectroscopy.

Anaerobic digestion is an increasingly widespread process for organic waste treatment and renewable energy production due to the methane content of the biogas. This biological process also produces a digestate (i.e., the remaining content of the waste after treatment) with a high fertilizing potential. The digestate composition is highly variable due to the various organic wastes used as feedstock, the different plant configurations, and the post-treatment processes used. In order to optimize digestate spreading on agricultural soils by optimizing the fertilizer dose and, thus, reducing environmental impacts associated to digestate application, the agronomic characterization of digestate is essential. This study investigates the use of near infrared spectroscopy for predicting the most important agronomic parameters from freeze-dried digestates. A data set of 193 digestates was created to calibrate partial least squares regression models predicting organic matter, total organic carbon, organic nitrogen, phosphorus, and potassium contents. The calibration range of the models were between 249.8 and 878.6 gOM.kgDM-1, 171.9 and 499.5 gC.kgDM-1, 5.3 and 74.1 gN.kgDM-1, 2.7 and 44.9 gP.kgDM-1 and between 0.5 and 171.8 gK.kgDM-1, respectively. The calibrated models reliably predicted organic matter, total organic carbon, and phosphorus contents for the whole diversity of digestates with root mean square errors of prediction of 70.51 gOM.kgDM-1, 34.84 gC.kgDM-1 and 4.08 gP.kgDM-1, respectively. On the other hand, the model prediction of the organic nitrogen content had a root mean square error of 7.55 gN.kgDM-1 and was considered as acceptable. Lastly, the results did not demonstrate the feasibility of predicting the potassium content in digestates with near infrared spectroscopy. These results show that near infrared spectroscopy is a very promising analytical method for the characterization of the fertilizing value of digestates, which could provide large benefits in terms of analysis time and cost.

Effect of in-situ aged and fresh biochar on soil hydraulic conditions and microbial C use under drought conditions.

Biochar (BC) amendments may be suitable to increase the ecosystems resistance to drought due to their positive effects on soil water retention and availability. We investigated the effect of BC in situ ageing on water availability and microbial parameters of a grassland soil. We used soil containing 13C labeled BC and determined its water holding capacity, microbial biomass and activity during a 3 months incubation under optimum and drought conditions. Our incubation experiment comprised three treatments: soil without BC (Control), soil containing aged BC (BCaged) and soil containing fresh BC (BCfresh), under optimum soil water (pF 1.8) and drought conditions (pF 3.5). Under optimum water as well as drought conditions, soils containing BC showed higher soil organic carbon (SOC) mineralization as compared to control soil. Moreover, BC effects on the soil water regime increase upon in situ aging. Native SOC mineralization increased most for soils containing BCaged. The BCaged led to improved C use under drought as compared to the other treatments. We conclude that BC addition to soils can ameliorate their water regime, especially under drought conditions. This beneficial effect of BC increases upon its aging, which also improved native substrate availability.

Changes and their possible causes in δ13C of dark-respired CO2 and its putative bulk and soluble sources during maize ontogeny.

The issues of whether, where, and to what extent carbon isotopic fractionations occur during respiration affect interpretations of plant functions that are important to many disciplines across the natural sciences. Studies of carbon isotopic fractionation during dark respiration in C3 plants have repeatedly shown respired CO2 to be (13)C enriched relative to its bulk leaf sources and (13)C depleted relative to its bulk root sources. Furthermore, two studies showed respired CO2 to become progressively (13)C enriched during leaf ontogeny and (13)C depleted during root ontogeny in C3 legumes. As such data on C4 plants are scarce and contradictory, we investigated apparent respiratory fractionations of carbon and their possible causes in different organs of maize plants during early ontogeny. As in the C3 plants, leaf-respired CO2 was (13)C enriched whereas root-respired CO2 was (13)C depleted relative to their putative sources. In contrast to the findings for C3 plants, however, not only root- but also leaf-respired CO2 became more (13)C depleted during ontogeny. Leaf-respired CO2 was highly (13)C enriched just after light-dark transition but the enrichment rapidly decreased over time in darkness. We conclude that (i) although carbon isotopic fractionations in C4 maize and leguminous C3 crop roots are similar, increasing phosphoenolpyruvate-carboxylase activity during maize ontogeny could have produced the contrast between the progressive (13)C depletion of maize leaf-respired CO2 and (13)C enrichment of C3 leaf-respired CO2 over time, and (ii) in both maize and C3 leaves, highly (13)C enriched leaf-respired CO2 at light-to-dark transition and its rapid decrease during darkness, together with the observed decrease in leaf malate content, may be the result of a transient effect of light-enhanced dark respiration.

Changes in δ(13)C of dark respired CO2 and organic matter of different organs during early ontogeny in peanut plants.

Carbon isotope composition in respired CO2 and organic matter of individual organs were measured on peanut seedlings during early ontogeny in order to compare fractionation during heterotrophic growth and transition to autotrophy in a species with lipid seed reserves with earlier results obtained on beans. Despite a high lipid content in peanut seeds (48%) compared with bean seeds (1.5%), the isotope composition of leaf- and root-respired CO2 as well as its changes during ontogeny were similar to already published data on bean seedlings: leaf-respired CO2 became (13)C-enriched reaching -21.5‰, while root-respired CO2 became (13)C-depleted reaching around -31‰ at the four-leaf stage. The opposite respiratory fractionation in leaves vs. roots already reported for C3 herbs was thus confirmed for peanuts. However, contrarily to beans, the peanut cotyledon-respired CO2 was markedly (13)C-enriched, and its (13)C-depletion was noted from the two-leaf stage onwards only. Carbohydrate amounts being very low in peanut seeds, this cannot be attributed solely to their use as respiratory substrate. The potential role of isotope fractionation during glyoxylate cycle and/or gluconeogenesis on the (13)C-enriched cotyledon-respired CO2 is discussed.

Higher temperature sensitivity for stable than for labile soil organic carbon--evidence from incubations of long-term bare fallow soils.

The impact of climate change on the stability of soil organic carbon (SOC) remains a major source of uncertainty in predicting future changes in atmospheric CO2 levels. One unsettled issue is whether the mineralization response to temperature depends on SOC mineralization rate. Long-term (>25 years) bare fallow experiments (LTBF) in which the soil is kept free of any vegetation and organic inputs, and their associated archives of soil samples represent a unique research platform to examine this issue as with increasing duration of fallow, the lability of remaining total SOC decreases. We retrieved soils from LTBF experiments situated at Askov (Denmark), Grignon (France), Ultuna (Sweden), and Versailles (France) and sampled at the start of the experiments and after 25, 50, 52, and 79 years of bare fallow, respectively. Soils were incubated at 4, 12, 20, and 35 °C and the evolved CO2 monitored. The apparent activation energy (Ea) of SOC was then calculated for similar loss of CO2 at the different temperatures. The Ea was always higher for samples taken at the end of the bare-fallow period, implying a higher temperature sensitivity of stable C than of labile C. Our results provide strong evidence for a general relationship between temperature sensitivity and SOC stability upon which significant improvements in predictive models could be based.

Variability of 13C-labeling in plant leaves.

RATIONALE: Plant tissues artificially labeled with (13)C are increasingly used in environmental studies to unravel biogeochemical and ecophysiological processes. However, the variability of (13)C-content in labeled tissues has never been carefully investigated. Hence, this study aimed at documenting the variability of (13)C-content in artificially labeled leaves. METHODS: European beech and Italian ryegrass were subjected to long-term (13)C-labeling in a controlled-environment growth chamber. The (13)C-content of the leaves obtained after several months labeling was determined by isotope ratio mass spectrometry. RESULTS: The (13)C-content of the labeled leaves exhibited inter- and intra-leaf variability much higher than those naturally occurring in unlabeled plants, which do not exceed a few per mil. This variability was correlated with labeling intensity: the isotope composition of leaves varied in ranges of ca 60‰ and 90‰ for experiments that led to average leaf (13)C-content of ca +15‰ and +450‰, respectively. CONCLUSIONS: The reported variability of isotope composition in (13)C-enriched leaves is critical, and should be taken into account in subsequent experimental investigations of environmental processes using (13)C-labeled plant tissues.

A method for (13)C-labeling of metabolic carbohydrates within French bean leaves (Phaseolus vulgaris L.) for decomposition studies in soils.

The molecular composition of plant residues is suspected to largely govern the fate of their constitutive carbon (C) in soils. Labile compounds, such as metabolic carbohydrates, are affected differently from recalcitrant and structural compounds by soil-C stabilisation mechanisms. Producing (13)C-enriched plant residues with specifically labeled fractions would help us to investigate the fate in soils of the constitutive C of these compounds. The objective of the present research was to test (13)C pulse chase labeling as a method for specifically enriching the metabolic carbohydrate components of plant residues, i.e. soluble sugars and starch. Bean plants were exposed to a (13)CO(2)-enriched atmosphere for 0.5, 1, 2, 3 and 21 h. The major soluble sugars were then determined on water-soluble extracts, and starch on HCl-hydrolysable extracts. The results show a quick differential labeling between water-soluble and water-insoluble compounds. For both groups, (13)C-labeling increased linearly with time. The difference in delta(13)C signature between water-soluble and insoluble fractions was 7 per thousand after 0.5 h and 70 per thousand after 21 h. However, this clear isotopic contrast masked a substantial labeling variability within each fraction. By contrast, metabolic carbohydrates on the one hand (i.e. soluble sugars + starch) and other fractions (essentially cell wall components) on the other hand displayed quite homogeneous signatures within fractions, and a significant difference in labeling between fractions: delta(13)C = 414 +/- 3.7 per thousand and 56 +/- 5.5 per thousand, respectively. Thus, the technique generates labeled plant residues displaying contrasting (13)C-isotopic signatures between metabolic carbohydrates and other compounds, with homogenous signatures within each group. Metabolic carbohydrates being labile compounds, our findings suggest that the technique is particularly appropriate for investigating the effect of compound lability on the long-term storage of their constitutive C in soils.