Elevated CO2 Improves Growth and Phosphorus Utilization Efficiency in Cereal Species Under Sub-Optimal Phosphorus Supply

Cultivars of Triticum aestivum, T. durum, and Secale cereale were grown at low (2 μM) and sufficient (500 μM) phosphorus (P) under ambient carbon dioxide (380 μmol mol^?1; aCO₂) and elevated CO₂ (700 μmol mol^?1, eCO₂) to study responses of cereal species in terms of growth and P utilization efficiency (PUE) under P x CO₂ interaction. Dry matter accumulation increased under eCO₂ with sufficient P. Nevertheless, dry matter accumulated at eCO₂ with low-P was similar to that obtained at aCO₂ with sufficient P. Leaf area was 43% higher under eCO₂ with sufficient P. Significant increase in lateral root density, length and surface area were noted at low-P under eCO₂. Phosphorus use efficience (PUE) increased by 59% in response to eCO_2­ in low-P plants. Thus, eCO₂ can partly compensate effect of low-P supply because of improved utilization efficiency. Among cereals, durum wheat was more suitable in terms of PUE under high CO₂ and limiting P supply.     

水稻叶际细菌群对二氧化碳升高的不同响应特征

A vast number of microorganisms colonize the leaf surface of terrestrial plants, known as the phyllosphere, and these microorganisms are thought to be of critical importance in plant growth and health. However, the taxonomic identities and ecological functions of the microorganisms inhabiting the rice phyllosphere remain poorly understood. Using a massive, parallel pyrosequencing technique, we identified the phyllosphere bacterial taxa of four different rice varieties and investigated the microbial response to elevated CO2 （eCO2） in a rice field of a free-air CO2 enrichment （FACE） facility located in Jiangsu Province, China. The results showed that the dominant phylotype, the Enterobacteriaceae family of Gammaproteobacteria~ accounted for 70.6%-93.8% of the total bacterial communities in the rice phyllosphere. The dominant phylotype was stimulated by eCO2, with its relative abundance increasing from 70.6%-75.2% at ambient CO2 （aCO2） to 86.5%-93.8% at eCO2 in the phyllosphere of rice varieties IIYou084 （TY-084）, YangLiangYou6 （YLY-6）, and ZhenXian96 （ZX-96）. The rare phylotypes, including the bacterial taxa of Sphingobacteriaceae, Xanthomonadaceae, Oxalobacteraceae, Clostridiaceae, and Pseudomonadaceae, were suppressed and their relative abundance decreased from 13.4%-23.0% at aCO2 to 1.47% 6.11% at eGO2. Furthermore, the bacterial diversity indices decreased at eCO2 in the phyllosphere of the rice varieties TY-084, YLY-6, and ZX-96. In contrast, an opposite response pattern was observed for the rice variety of YangDao8 （YD-8）. In the phyllosphere of this variety, the relative abundance of the dominant phylotype, Enterobacteriaceae, decreased from 94.1% at aCO2 to 81.4% at eCO2, while that of the rare phylotypes increased from 3.37% to 6.59%. In addition, eCO2 appeared to stimulate bacterial diversity in the rice variety YD-8. Our results suggest that the phyllosphere microbial response to eCO2 might be relative abundance-dependent in paddy fields.     

Elevated CO2 alters the structure of the bacterial community assimilating plant‐derived carbon in the rhizosphere of soya bean

Elevated CO₂ (eCO₂) increases rhizodeposits, which in turn alters the soil microbial community. However, it is not really known how the microbial community metabolizes plant‐derived carbon (C) in the rhizosphere under eCO₂, especially in agricultural soils. This study used a ¹³CO₂ labelling technique combined with DNA‐stable isotope probing (SIP) to fractionate the ¹³C‐DNA and ¹²C‐DNA from the rhizosphere of soya bean plants (Glycine max (L.) Merr. cv. Suinong 14) grown for 54 days under ambient CO₂ (aCO₂) (390 ppm) or eCO₂ (550 ppm). The DNA fractions were then subjected to Illumina Miseq sequencing. The results showed that eCO₂ decreased the richness and diversity of the ¹³C‐assimilating bacterial community compared to aCO₂ (p < 0.05). Elevated CO₂ decreased the abundances of genera, including Pseudarthrobacter, Gaiellales_uncultured, Microlunatus, Gemmatimonas, Gemmatimonadaceae_uncultured, Ramlibacter, Massilia, Luteimonas, Acidobacteriaceae_uncultured, Bryobacter and Candidatus_Solibacter. These genera were probably fast‐growing bacteria and sensitive to labile C. In contrast, eCO₂ stimulated the growth of genera Novosphingobium, Acidimicrobiales_uncultured, Bacillus, Flavisolibacter and Schlesneria, which were able to assimilate complex C compounds. Moreover, the increased population of Novosphingobium under eCO₂ might have accelerated electron flow from the oxidation of organic C. Correspondingly, eCO₂ did not affect the concentration of the dissolved organic C but increased the plant‐derived ¹³C in the rhizosphere. These results indicated that an eCO₂‐induced increase in non‐labile C in rhizodeposits contributed to the increase in population size of a number of the plant‐C‐metabolizing genera that might become the mechanism for the turnover of fresh C in the rhizosphere, modifying the soil C cycle under eCO₂ environments.     

Elevated atmospheric CO2 reduces CH4 and N2O emissions under two contrasting rice cultivars from a subtropical paddy field in China

Elevated CO₂(eCO₂) and rice cultivars can strongly alter CH₄ and N₂ O emissions from paddy fields.However,detailed information on how their interaction affects greenhouse gas fluxes in the field is still lacking.In this study,we investigated CH4 and N₂ O emissions and rice growth under two contrasting rice cultivars(the strongly and weakly responsive cultivars) in response to eCO₂,200 μmol mol^-1 higher than the ambient ...     

Elevated CO<Subscript>2</Subscript> alters the abundance but not the structure of diazotrophic community in the rhizosphere of soybean grown in a Mollisol

The effect of elevated CO₂ (eCO₂) on rhizospheric diazotrophic community in cropland has little been studied, although eCO₂ facilitates nodulation and N₂ fixation in legumes. In this study, four soybean cultivars (Xiaohuangjin, Suinong 8, Suinong 14, and Heinong 45) were grown in Mollisols for 65 days under ambient CO₂ (aCO₂) (390 ppm) or eCO₂ (550 ppm). Quantitative PCR and Illumina MiSeq sequencing targeting the nifH gene that reflects the composition of diazotrophic community were determined. Elevated CO₂ significantly increased the abundance of nifH gene copies in the rhizospheres of the Suinong 8 and Heinong 45 cultivars, but not in the Suinong 14 and Xiaohuangjin cultivars. The nifH abundance correlated negatively with nodule density (p?≤?0.01) but positively with nodule size (p?≤?0.01). Elevated CO₂ did not significantly alter the composition of diazotrophic community, nor shift dominant bacterial operational taxonomic units (OTUs). These results indicated that eCO₂ stimulated the growth but did not alter the community composition of diazotrophs in the rhizosphere of soybean, which depended on cultivar and might contribute to nodulation responses to eCO₂.     

Atmospheric CO2 enrichment induces life strategy- and species-specific responses of collembolans in the rhizosphere of sugar beet and winter wheat

We studied atmospheric CO₂ enrichment effects on life form types, species composition, dominance structure and individual density of collembolans under cultivation of sugar beet and winter wheat. The study was part of a long-term CO₂ enrichment field experiment (FACE: Free Air CO₂ Enrichment) at the Federal Agricultural Research Centre (FAL) in Braunschweig (Germany), using isotopically labelled CO₂. The stable C-isotopic signature (δ¹³C) of collembolan species, plant material, and soil indicated CO₂ impacts on C translocation. The δ¹³C values of both crops significantly increased from above-ground to below-ground plant parts and significantly decreased under FACE conditions. The δ¹³C values of collembolan species differed significantly depending on CO₂ treatment and crop and showed a distinct tendency depending on plant growth stage. The extent, to which δ¹³C values of collembolans decreased under FACE conditions, was species- and life strategy-dependent. The stable C-isotopic signatures of euedaphic and hemiedaphic species were similar in the control, but, depending on crop, differently affected by atmospheric CO₂ enrichment. Under winter wheat cultivation, hemiedaphic species showed more negative δ¹³C values than euedaphic ones under FACE conditions. CO₂ enrichment effects on occurrence, density and dominance distribution of the collembolan species differed strongly between crops and their developmental stages, which reveal crop-specific below-ground effects due to different food qualities in the rhizosphere. CO₂ impacts were stronger under sugar beet compared to winter wheat cultivation. Independent of crop, CO₂ enrichment enhanced the diversity of collembolans before harvest and increased the proportion of hemiedaphic in relation to euedaphic species in a community. Our results on collembolan communities imply CO₂-induced changes in the root-derived carbon resources used by the soil food web. The present study reveals atmospheric CO₂ enrichment impacts to specifically affect collembolan species according to their food preferences.     

Impact of plant species and atmospheric CO2 concentration on rhizodeposition and soil microbial activity and community composition

Both plant species and CO₂ concentration can potentially affect rhizodeposition and consequently soil microbial activity and community composition. However, the effect differs based on plant developmental stage. We focused on the effect of three plant species (forbs, grasses, and N₂‐fixers) at an early stage of development on root C deposition and fate, soil organic matter (SOM) mineralization and soil microbial community composition at ambient (aCO₂) and elevated (eCO₂) CO₂ levels. Plants were grown from seed, under continuous ¹³C‐labelling atmospheres (400 and 800 µmol mol^?1 CO₂), in grassland soil for three weeks. At the end of the growth period, soil respiration, dissolved organic C (DOC) and phospholipid fatty acid (PLFA) profiles were quantified and isotopically partitioned into root‐ and soil‐derived components. Root‐derived DOC (0.53 ± 0.34 and 0.26 ± 0.29 µg mL soil solution^?1) and soil‐derived CO₂ (6.14 ± 0.55 and 5.04 ± 0.44 µg CO₂‐C h^?1) were on average two times and 22% higher at eCO₂ than at aCO₂, respectively. Plant species differed in exudate production at aCO₂ (0.11 ± 0.11, 0.10 ± 0.18, and 0.58 ± 0.58 µg mL soil solution^?1 for Plantago, Festuca, and Lotus, respectively) but not at eCO₂ (0.20 ± 0.28, 0.66 ± 0.32, and 0.75 ± 0.15 µg mL soil solution^?1 for Plantago, Festuca, and Lotus, respectively). However, no differences among plant species or CO₂ levels were apparent when DOC was expressed per gram of roots. Relative abundance of PLFAs did not differ between the two CO₂ levels. A higher abundance of actinobacteria and G‐positive bacteria occurred in unplanted (8.07 ± 0.48 and 24.36 ± 1.18 mol%) and Festuca‐affected (7.63 ± 0.31 and 23.62 ± 0.69 mol%) soil than in Plantago‐ (7.04 ± 0.36 and 23.41 ± 1.13 mol%) and Lotus‐affected (7.24 ± 0.17 and 23.13 ± 0.52 mol%) soil. In conclusion, the differences in root exudate production and soil respiration are mainly caused by differences in root biomass at an early stage of development. However, plant species evidently produce root exudates of varying quality affecting associated microbial community composition.     

用树轮δ13C重建过去300年大气CO2浓度

The annual series of δ13C were measured in tree rings of three Cryptomeria fortunei disks （OF-1, OF-2, and OF- 3） collected from West Tianmu Mountain, Zhejiang Province, China, according to cross-dating tree ring ages. There was no obvious decreasing trend of the δ13C annual time series of CF-2 before 1835. However, from 1835 to 1982 the three tree ring δ13C annual series exhibited similar decreasing trends that were significantly （P ≤ 0.001） correlated. The distribution characteristics of a scatter diagram between estimated δ13C series of CF-2 from modeling and the atmospheric CO2 concentration extracted from the Law Dome ice core from 1840 to 1978 were analyzed and a curvilinear regression equation for reconstructing atmospheric CO2 concentration was established with R2 = 0.98. Also, a test of independent samples indicated that between 1685 and 1839 the reconstructed atmospheric CO2 concentration .using the δ13C series of CF-2 had a close relationship with the Law Dome and Siple ice cores, with a standard deviation of 1.98. The general increasing trend of the reconstructed atmospheric CO2 concentration closely reflected the 10ng-term variation of atmospheric CO2 concentration recorded both before and after the Industrial Revolution. Between 1685 and 1840 the evaluated atmospheric CO2 concentration was stable, but after 1840 it exhibited a rapid increase. Given a longer δ13C annual time series of tree rings, it was feasible to rebuild a representative time series to describe the atmospheric CO2 concentration for an earlier period and for years that were not in the ice core record.     

The effect of diet on isotopic turnover in Collembola examined using the stable carbon isotopic compositions of lipids

Combined compound-specific stable carbon isotopic methods and fatty acid abundance determinations have been used to examine feeding preferences and C allocation in organisms where direct observation of feeding is difficult. In order to examine the effect of differing diets on the δ¹³C values of fatty acids and sterols of Collembola, the diets of two collembolan species, Folsomia candida and Proisotoma minuta, were switched from a yeast diet to one of four isotopically distinct diets, and the δ¹³C values of the lipids monitored over the next 39 d. Cholesterol remained the only sterol detected in both collembolan species, despite the diets containing widely differing sterol compositions. The δ¹³C values of collembolan lipids recorded after long term feeding were often different to those of the same components in the diet, indicating that fractionation or partitioning occurs during digestion, assimilation and biosynthesis within the Collembola, thereby shifting consumer lipid δ¹³C values away from those of the corresponding dietary components. The rates of change of δ¹³C values differed among compounds, with half-lives ranging between 29 min and 14 d. Some of these differences appear to be related to the abundance of dietary components, such that fatty acids present in high abundance in the diet (e.g. 18:2(n−6)) were rapidly assimilated in high proportions into collembolan lipids, leading to a rapid change in δ¹³C values. Similarly, isotopic turnover in the 16:1(n−7) fatty acid, present in the newly presented diets in only low abundances, was significantly correlated to the rate of removal of this component from the consumer fatty acid pool. The rates of change of δ¹³C values in P. minuta lipids did not vary significantly with diet, whilst the rates of change of δ¹³C values of lipids in F. candida were affected by the diets the Collembola consumed. Results of an experiment providing F. candida and P. minuta with two diets of different quality demonstrated that F. candida responded to the high quality diet with increased growth and fecundity, whilst P. minuta responded with increased fecundity only. Thus, the abilities of the two species to respond to diets of varying quality, amongst other factors, is concluded to lead to differences in the rates of change of δ¹³C values reflecting differences in lipid turnover.     

Turnover of soil organic matter and of microbial biomass under C3-C4 vegetation change: Consideration of C fractionation and preferential substrate utilization

Two processes contribute to changes of the δ¹³C signature in soil pools: ¹³C fractionation per se and preferential microbial utilization of various substrates with different δ¹³C signature. These two processes were disentangled by simultaneously tracking δ¹³C in three pools - soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) - and in CO₂ efflux during incubation of 1) soil after C₃-C₄ vegetation change, and 2) the reference C₃ soil.The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C₃ vegetation. Miscanthus giganteus - a perennial C₄ plant - was grown for 12 years, and the δ¹³C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ¹³C signature of the three C pools and of CO₂ in the reference C₃ soil were less than 1‰, and only δ¹³C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ¹³C of all pools in the soil after C₃-C₄ vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO₂. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ¹³C changes in DOC, CO₂ and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ¹³C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ¹³C values) and not ¹³C fractionation per se.Based on the δ¹³C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C.Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ¹³C signature of linked pools helps calculate the relative turnover of old and recent pools.     

A new method for continuously measuring the δ13C of soil CO2 concentrations at different depths by laser spectrometry

Soil carbon dioxide (CO₂) efflux is an important component of the carbon (C) cycle but the biological and physical processes involved in soil CO₂ production and transport are not fully understood. To improve our knowledge, we present a new approach to measure simultaneously soil CO₂ concentrations and efflux, and their respective isotopic signatures (δ¹³C‐CO₂). To quantify soil air ¹³CO₂ and ¹²CO₂ concentrations, we adapted a method based on CO₂ diffusion from soil pores into tubes with a highly gas‐permeable membrane wall. These tubes were placed horizontally at different depths in the soil. Air was sampled automatically from the tubes and injected through a diluting system into a tuneable diode laser absorption spectrometer. The CO₂ and δ¹³C‐CO₂ vertical profiles were thus obtained at hourly intervals. Our tests demonstrated the absence of fractionation in the membrane tubes for δ¹³C‐CO_2. Subsequently, we set up field experiments for two forest soils, which showed that natural soil CO₂ concentrations and δ¹³C‐CO₂ were not affected significantly by the measurement system. While δ¹³C‐CO₂ in air‐filled pores below 5 cm was constant over 3 days, we observed large diurnal variations in δ¹³C‐CO₂ efflux. However, the average difference between the two measurements was close to ?4.4‰, which supports steady‐state diffusion over this 3‐day period. This new method seems to be a very effective way to measure the δ¹³C‐CO₂ profile of the soil atmosphere, and demonstrates that the fractionation that occurs during diffusion is the main transport process that affects the δ¹³C‐CO₂ of the soil CO₂ efflux on a daily timescale while advection may account for within‐day variations.     

Combined effects of phosphorus nutrition and elevated carbon dioxide concentration on chlorophyll fluorescence,photosynthesis, and nutrient efficiency of cotton

To examine the combined effects of phosphorus (P) nutrition and CO₂ on photosynthesis, chlorophyll fluorescence (CF), and nutrient utilization and uptake, two controlled‐environment experiments were conducted using 0.01, 0.05 and 0.20 mM external phosphate each at ambient and elevated CO₂ (aCO₂: 400 and eCO₂: 800 µmol mol^?1, respectively). The CF parameters were affected more by P nutrition than by CO₂ treatment. Photoinhibition of photosystem II (PSII) was due to increased minimal CF (Fo′) and decreased maximal CF (Fm′), and efficiency of energy harvesting (Fv′/Fm′). In addition, reduced electron transport rate (ETR), the quantum yield of PSII (Φ_PSII) and CO₂ assimilation ( ), and overall photochemical quenching in the P‐deficient leaves led to reduction in the efficiency of energy transfer to the PSII reaction center. Stimulation in the Φ_PSII/ and photorespiration (ETR/P_net) was found under P deficiency, whereas the opposite was the case under CO₂ enrichment. On average, photosynthetic rate (P_net) and stomatal conductance declined by 50–53% at 0.05 mM P and by 70–72% at 0.01 mM P as compared to the 0.20 mM P treatment. However, P deficiency, especially at eCO₂, tended to increase the intrinsic water‐use efficiency. In the P‐deficient plants, the decline in the P and N utilization efficiency (up to 91%) of biomass production was mainly associated with greater reduction in the biomass relative to the tissue P concentration as the P supply was reduced. However, it was significantly stimulated by eCO₂ especially at higher P supply. The CO₂ × P interaction was observed for some parameters such as Fo′, Fm′, P utilization efficiencies of photosynthesis and biomass production that might be attributed to the irresponsiveness of these parameters to eCO₂ under low P treatment. Thus, P deficiency limited the beneficial effect of eCO₂. A close relationship between total biomass and photosynthesis with the P and N utilization or uptake efficiencies was found. The P utilization efficiency of P_net appeared to be stable across a range of leaf P concentrations, whereas the N‐utilization efficiency markedly increased with leaf P and differed between CO₂ levels. An apparent effect of both the treatments (P and CO₂) on N‐uptake and utilization efficiency also indicated the alteration in N acquisition and assimilation in cotton plants.     

Application of a quantum cascade laser-based spectrometer in a closed chamber system for real-time δC and δO measurements of soil-respired CO2

Laser spectroscopy is an emerging technique to analyze the stable isotopic composition of soil-respired CO₂ (δ¹³C_resp, δ¹⁸O_resp) in situ and at high temporal resolution. Here we present the first application of a quantum cascade laser-based spectrometer (QCLS) in a closed soil-chamber system to determine simultaneously δ¹³C_resp and δ¹⁸O_resp. In a Swiss beech forest, a total of 90 chamber measurements with 20 min sampling time each were performed. The instrument measured the δ¹³C and δ¹⁸O of the CO₂ in the chamber headspace at every second with a precision of 0.25‰, resulting in Keeling plots with 1200 data points. In addition, we calculated δ¹³C_resp directly from the flux ratio of ¹³CO₂ and ¹²CO₂. The flux-ratio values were 0.8‰ lower than the Keeling plot intercepts when the flux rates were derived from quadratic curve fits of the CO₂ increase. The δ¹⁸O-Keeling plots showed a significant bending very likely due to the equilibration of chamber CO₂ with the ¹⁸O of surface soil water. Therefore, we used a quadratic curve fit of the Keeling plots to estimate δ¹⁸O_resp. Our results also revealed that δ¹³C_resp was not constant throughout the CO₂ accumulation in the closed soil chambers: there were significant but non-systematic variations in δ¹³C_resp for the first 10 min, and systematic shifts in δ¹³C_resp of on average 1.9 ‰ in the second part of the 20-min measurements. These biases were probably caused by non-steady-state conditions in the soil-chamber system. Our study illustrates that the high temporal resolution of QCLS measurements allows the detection of non-linearities in the isotopic effluxes of CO₂ from the soil due to soil-chamber feedbacks. This information can be used to improve the estimates for δ¹³C_resp and δ¹⁸O_resp.     

Translocation of surface litter carbon into soil by Collembola

Soil invertebrates are important in nutrient cycling in soils, but the degree to which mesofauna such as Collembola are responsible for the direct movement of carbon (C) from the litter layer into soil has not yet been ascertained. We used naturally occurring stable C isotopic differences between a C₄ soil and alder leaves (C₃) to examine the effect of the collembolan Folsomia candida on C translocation into soil in laboratory microcosms. Collembolan numbers greatly increased in the presence of alder, but despite large collembolan populations there were no changes in decomposition rate (measured as litter mass loss, cumulative respired CO₂ and alder C:N ratios). Small changes in the δ¹³C values of bulk soil organic matter were detected, but could not be assigned to collembolan activity. However, mean δ¹³C values of soil microbial phospholipid fatty acids (PLFAs) were significantly lower in the presence of alder and Collembola together, demonstrating that collembolan activities resulted in greater availability of litter-derived C to the soil microbial community. Additionally, the presence of Collembola resulted in the translocation of alder-derived compounds (chlorophyll and its breakdown product pheophytin) into soil, demonstrating that Collembola modify soil organic matter at the molecular level. These results are consistent with deposition of collembolan faeces in underlying soil and demonstrate that despite their small size, Collembola contribute directly to C transport in the litter-soil environment.     

Use of C abundance to study short-term pig slurry decomposition in the field

Applying pig slurry (PS) on agricultural soils is a common practice. However, its impact on soil organic C dynamics is not clear. This experiment investigated the use of natural ¹³C abundance to study the short-term C mineralization of anaerobically stored PS under field conditions. Measurements of δ¹³C-CO₂ were made on soil air samples obtained from a bare sandy loam during 22 d following incorporation of either PS alone, PS+barley straw, or barley straw alone; an unamended treatment was used as a control. Slurry C was enriched in ¹³C (−20.0‰) because of the high corn (Zea mays L.) content of the animal diet. This value contrasted with δ¹³C of −28.4‰ for the soil organic matter and of −29.0‰ for the barley straw. A peak of high δ¹³CO₂ values (average of −9.2‰) was observed on the day of PS application and was attributed to the dissociation of PS carbonates when mixed with the relatively acidic soil. After this initial burst, 36% of the evolved CO₂ originated from the decomposing PS. After 22 d of incubation, approx. 20% of the PS-C had been lost as CO₂. This short-term field study did not show any priming effect of PS on the mineralization of straw or native soil C. Due to its heterogeneity, the use of the isotopic composition of the evolved CO₂ for estimating PS decomposition requires precaution either through the use of a specific experimental design involving comparable C3 and C4 treatments, or calculations to account for the presence of ¹³C-enriched inorganic C in the PS.     

Dual-chamber measurements of δ13C of soil-respired CO2 partitioned using a field-based three end-member model

The contribution of old soil C (SOM) to total soil respiration (R_S) in forest has been a crucial topic in global change research, but remains uncertain. Isotopic methods, such as natural variations in carbon isotope composition (δ¹³C) of soil respiration, are more frequently being applied, and show promise in separating heterotrophic and autotrophic contributions to R_S. However, natural and artificial modification of δ¹³C_Rs can cause isotopic disequilibria in the soil-atmosphere system generating a mismatch between what is usually measured and what process-based models will predict. Here we report the partitioning of the soil surface CO₂ flux in a warm Mediterranean forest into components derived from root, litter/humus, and SOM sources using a new, three end-member mixing model, and compare this with the conventional partitioning into autotrophic and heterotrophic components. The three end-member mixing model takes into account both the discrimination during CO₂ respiration/decomposition of the three components, as well as the fractions of their CO₂ fluxes integrated over the total soil profile mass. In addition, we used a novel dual-chamber technique to ensure that δ¹³C_Rs was subjected to minimal artefacts during measurement.We observed that by using measured soil surface CO₂ concentrations as a baseline level for the dual-chamber operation, it was possible to achieve and monitor the necessary conservation of the soil CO₂ steady-state diffusion conditions during the measurements, without using permanent collars inserted deeply into the soil. When R_S (8.64 g CO₂ m² d⁻¹) was partitioned into two components, the mean autotrophic and heterotrophic respiration was 56 and 44%, respectively. When R_S was partitioned using the three-way model, however, roots, litter/humus, and SOM contributed 30, 33, and 37% of the total flux. Our results confirm that to improve the estimates of the partitioning method, it is important to distinguish the fractional contribution of the long-term SOM-derived flux from younger and more labile sources.     

Glomalin-related soil protein responses to elevated CO2 and nitrogen addition in a subtropical forest: Potential consequences for soil carbon accumulation

According to the economy theory, plants should preferentially allocate photosynthate to acquire below-ground resources under elevated atmospheric carbon dioxide (eCO₂) but decrease below-ground C allocation when nitrogen (N) is sufficient for plant growth. Arbuscular mycorrhizae (AM) represent a critical mechanism of below-ground nutrient acquisition for plants. The dynamics of arbuscular mycorrhizal fungi (AMF) could therefore reflect the response of plant C allocation under eCO₂ and N addition. We examined the responses of glomalin-related soil protein (GRSP) to eCO₂ (approximately 700 μmol mol⁻¹ CO₂) and/or N addition (100 kg N ha⁻¹ yr⁻¹ as NH₄NO₃) in a modeled subtropical forest to better understand its potential influence on soil C storage. We hypothesized that GRSP would increase under eCO₂ and decrease under N addition. Furthermore, the positive effects of eCO₂ on GRSP would be offset by extra N addition, and GRSP would remain unchanged under combined eCO₂ and N addition. Our results showed that the mean concentrations of easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP) were 0.35 ± 0.05 and 0.72 ± 0.13 mg C cm⁻³, respectively, which accounted for 2.76 ± 0.53% and 5.67 ± 0.92% of soil organic carbon (SOC) in the 0–10 cm soil layer. Elevated CO₂ significantly increased T-GRSP by 35.02% but decreased EE-GRSP by 5.09% in the top 10 cm soil layer. The opposite responses of T-GRSP and EE-GRSP to eCO₂ might result from an unchanged photosynthate investment to AMF with possible changes in their decomposition rates. The effect of N on GRSP was contrary to our hypothesis, i.e., there was a 1.72%–48.49% increase in T-GRSP and a slightly increase in EE-GRSP. Both EE-GRSP and T-GRSP concentrations increased under the combination of eCO₂ and N addition, which was inconsistent with our hypothesis. The significant increase of EE-GRSP under the combination of eCO₂ and N addition was partly caused by more rapid plant growth and reduced microbial diversity, and the marginal increase of T-GRSP indicated that the interaction between eCO₂ and N addition offset their independent effects. In addition, the relatively higher accumulation ratios of GRSP (22.6 ± 13.6%) compared with SOC (15.9 ± 9.4%) indicated that more rapid GRSP deposition in the soil might accelerate SOC accumulation under eCO₂ and N addition. Our results will improve the understanding of the functioning of GRSP in soil C sequestration under global environmental change scenarios.     

Effluxed CO2-C from sterilized and unsterilized treatments of a calcareous soil

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO₂-¹³C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO₂. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl₂ addition for Oklahoma soil and unbuffered HgCl₂ addition, buffered HgCl₂ addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO₂ flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO₂ production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO₂-¹³C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO₂-¹³C values in Mojave Desert soil HgCl₂ sterilization treatments (both buffered and non-buffered). Plots of 1/CO₂ versus CO₂-δ¹³C (similar to Keeling plots) indicated that the source CO₂-δ¹³C value of the Oklahoma Prairie soil treatments was similar to the δ¹³C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl₂ sterilized Mojave Desert soil was similar to the δ¹³C value of carbonates (−0.93‰ VPDB). The source CO₂-δ¹³C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.     

Collembolan trophic preferences determined using fatty acid distributions and compound-specific stable carbon isotope values

The trophic preferences of soil invertebrates such as Collembola are often determined by the analysis of gut contents, or through visual observations of the location of individuals. As an alternative approach, two species of Collembola, Folsomia candida and Proisotoma minuta, were offered a choice of the soil fungus Cladosporium cladosporioides or the bacterial feeding nematode Panagrellus redivivus; each exhibited distinct fatty acid profiles and stable carbon isotopic compositions. Over 21 days, the fatty acids i15:0, i17:0, 18:1(n-7) and 18:2(n-6) all increased in abundance in both collembolan species consistent with direct routing from the nematode dietary choice which contained a high concentration of these components. Collembolan fatty acid δ¹³C values increased by between 5.7 and 21.6‰ over 21 days reflecting those of the nematode diet. Therefore, both fatty acid profiles and δ¹³C values were consistent with a strong feeding preference of F. candida and P. minuta for the nematodes over the offered fungi. In fact, neither collembolan species consumed any detectable amount of C. cladosporioides. Comparison of the δ¹³C values of the 16:0 and 18:0 fatty acids (which are biosynthesised by the Collembola as well as directly incorporated from the diet) and the 16:1(n-7) and 18:2(n-6) components (which are not biosynthesised by the Collembola) demonstrated that the input of distinct pools of C can lead to large shifts in δ¹³C values between diet and consumer. The fatty acids that were not biosynthesised by Collembola better reflected the δ¹³C values of the diet helping to differentiate between biosynthesised and directly incorporated compounds; an important prerequisite in the interpretation of compound-specific δ¹³C values in trophic behaviour tests. The combination of fatty acid distributions and δ¹³C values is a significant improvement on traditional methods of examining feeding preferences, since it determines directly the assimilated dietary carbon rather than relying on indirect observations, such as the proximity of individuals to a defined food source.     

Origin of positive δC of emitted CO2 from soils after application of biogas residues

Bioenergy production from renewable organic material is known to be a clean energy source and therefore its use is currently much promoted in many countries. Biogas by-products also called biogas residues (BGR) are rich in partially stable organic carbon and can be used as an organic fertilizer for crop production. However so far, many environmental issues relevant when BGR are applied to agricultural land (soil C sequestration, increased denitrification and nutrient leaching) still have to be studied. Therefore a field experiment was set up to investigate the degradation of BGR and its impact on the decomposition of native soil organic matter based on a natural abundance stable isotope approach. Maize, a C₄ plant has been used as bioenergy crop, therefore the δ¹³C of total C in BGR was −16.0‰_PDB and soil organic matter was mostly derived from C₃ plant based detritus, SOM thus showed a δ¹³C of −28.4‰_PDB. Immediately after BGR application, soil-emitted CO₂ showed unexpectedly high δ¹³C of up to +23.6‰_PDB, which has never been reported earlier. A subsequent laboratory scale experiment confirmed the positive δ¹³C of soil-emitted CO₂ after BGR addition and showed that obviously, the added BGR led to a consumption of dissolved inorganic C in soils. Additionally, it was observed that the δ¹³C of CO₂ driven from inorganic C of BGR (BGR-IC) by acid treatment was +35.6‰_PDB. Therefore, we suggest that also under field conditions the transformation of BGR-IC into CO₂ contributed largely to CO₂ emissions in addition to the decomposition of organic matter, which affected both the amount and the carbon isotope signature of emitted CO₂ in the initial period after BGR application. Positive δ¹³C of inorganic C contained in BGR was attributed to processes with strong fractionation of C isotopes during anaerobic fermentation in the biogas formation process.