Rice root-derived carbon input and its effect on decomposition of old soil carbon pool under elevated CO2

Rice (Oryza sativa) was grown in sunlit, semi-closed growth chambers (4×3×2 m, L×W×H) at 650 μl l⁻¹ CO₂ (elevated CO₂) to determine: (1) rice root-derived carbon (C) input into the soil under elevated CO₂ in one growing season, and (2) the effect of the newly input C on decomposition of the more recalcitrant native soil organic C. The initial δ¹³C value of the experimental soil was −25.8‰, which was 6‰ less depleted in ¹³C than the plants grown under elevated CO₂. Significant changes in δ¹³C of the soil organic C were detected after one growing season. The amount of new soil C input was estimated to be 0.9 t ha⁻¹ (or 2.1%) at 30 kg N ha⁻¹ and 1.8 t ha⁻¹ (4.1%) at 90 kg N ha⁻¹. Changes in soil δ¹³C suggested that the surface 5 cm of soil received more C input from plants than soils below. Laboratory incubation (25 °C) of soils from different horizons indicated that increased availability of the labile plant-derived C in the soil reduced decomposition of the native soil organic C. Provided the retardant effect of the new C on old soil organic C holds in the field in the longer-term, paddy soils will likely sequester more C from the atmosphere if more plant C enters the soil under elevated atmospheric CO₂.     

Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2

Elevated atmospheric CO₂ tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO₂ concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO₂. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO₂ plots at 0-10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO₂ stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO₂ increased net N mineralization in the 10-30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO₂. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO₂ studies. Increased mineralization of stable soil organic carbon by a CO₂-induced priming effect may diminish the terrestrial carbon sink globally.     

Effects of elevated CO2 and O3 on soil respiration under ponderosa pine

Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system. The study used a 2×2 factorial design with two levels of CO₂ and two levels of O₃ and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO₂ and O₃, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO₂ will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O₃ will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO₂ and O₃ have opposing effects on C assimilation and allocation, elevated CO₂ will eliminate or reduce the negative effects of elevated O₃ on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO₂ and O₃ on soil respiration. The covariance analysis showed that elevated CO₂ significantly reduced the soil respiration while elevated O₃ had no significant effect. Despite the lack of a direct CO₂ stimulation of soil respiration, there were significant interactions between CO₂ and soil temperature, soil moisture and days from planting indicating that elevated CO₂ altered soil respiration indirectly. In elevated CO₂, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO₂. Soil respiration increased more with days from planting in elevated than in ambient CO₂. Elevated CO₂ had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO₂ had no significant effect on microarthropod and nematode abundance. Elevated O₃ had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.     

Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO₂) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O₃) or vary based on species-specific growth responses to elevated CO₂. To understand how projected increases in atmospheric CO₂ and O₃ alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO₂-O₃ Enrichment (FACE) experiment. Tracer amounts of ¹⁵NH₄⁺ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO₂ and O₃ treatments. The ¹⁵N tracer and strongly depleted ¹³C-CO₂ were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO₂ relative to ambient CO₂. As main effects, neither CO₂ nor O₃ significantly altered ¹⁵N recovery in SOM. Elevated CO₂ significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO₂ has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O₃ had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO₂ can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.     

Long term CO2 enrichment stimulates N-mineralisation and enzyme activities in calcareous grassland

Elevated concentration of atmospheric carbon dioxide will affect carbon cycling in terrestrial ecosystems. Possible effects include increased carbon input into the soil through the rhizosphere, altered nutrient concentrations of plant litter and altered soil moisture. Consequently, the ongoing rise in atmospheric carbon dioxide might indirectly influence soil biota, decomposition and nutrient transformations.N-mineralisation and activities of the enzymes invertase, xylanase, urease, protease, arylsulfatase, and alkaline phosphatase were investigated in spring and summer in calcareous grassland, which had been exposed to ambient and elevated CO₂ concentrations (365 and 600 μl l⁻¹) for six growing seasons.In spring, N-mineralisation increased significantly by 30% at elevated CO₂, while there was no significant difference between treatments in summer (+3%). The response of soil enzymes to CO₂ enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly by 32 and 21%. In summer, differences of activities between CO₂ treatments were greatest in the case of urease and protease (+21 and +17% at elevated CO₂).The stimulation of N-mineralisation and enzyme activities at elevated CO₂ was probably caused by higher soil moisture and/or increased root biomass. We conclude that elevated CO₂ will enhance below-ground C- and N-cycling in grasslands.     

Seasonality of soil biological properties in a poplar plantation growing under elevated atmospheric CO2

Microorganisms are the regulators of decomposition processes occurring in soil, they also constitute a labile fraction of potentially available N. Microbial mineralization and nutrient cycling could be affected through altered plant inputs at elevated CO_2. An understanding of microbial biomass and microbial activity in response to belowground processes induced by elevated CO₂ is thus crucial in order to predict the long-term response of ecosystems to climatic changes. Microbial biomass, microbial respiration, inorganic N, extractable P and six enzymatic activities related to C, N, P and S cycling (β-glucosidase, cellulase, chitinase, protease, acid phosphatase and arylsulphatase) were investigated in soils of a poplar plantation exposed to elevated CO_2. Clones of Populus alba, Populus nigra and Populus x euramericana were grown in six 314 m² plots treated either with atmospheric (control) or enriched (550 μmol mol⁻¹ CO₂) CO₂ concentration with FACE technology (free-air CO₂ enrichment). Chemical and biochemical parameters were monitored throughout a year in soil samples collected at five sampling dates starting from Autumn 2000 to Autumn 2001.

The aim of the present work was: (1) to determine if CO₂ enrichment induces modifications to soil microbial pool size and metabolism, (2) to test how the seasonal fluctuations of soil biochemical properties and CO₂ level interact, (3) to evaluate if microbial nutrient acquisition activity is changed under elevated CO₂.

CO₂ enrichment significantly affected soil nutrient content and three enzyme activities: acid phosphatase, chitinase and arylsulphatase, indicators of nutrient acquisition activity. Microbial biomass increased by a 16% under elevated CO₂. All soil biochemical properties were significantly affected by the temporal variability and the interaction between time and CO₂ level significantly influenced β-glucosidase activity and microbial respiration. Data on arylsulphatase and chitinase activity suggest a possible shift of microbial population in favour of fungi induced by the FACE treatment.     

Nitrogen fixation by biological soil crusts and heterotrophic bacteria in an intact Mojave Desert ecosystem with elevated CO2 and added soil carbon

Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO₂ may affect N₂-fixation in aridland ecosystems, we measured C₂H₂ reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO₂. We also measured crust and soil δ¹⁵N and total N to assess changes in N sources, and δ¹³C of crusts to determine a functional shift in crust species, with elevated CO₂. The mean rate of C₂H₂ reduction by biological soil crusts was 76.9±5.6 μmol C₂H₄ m⁻² h⁻¹. There was no significant CO₂ effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO₂. Additions of dextrose-C had a positive effect on rates of C₂H₂ reduction in soil. There was no elevated CO₂ effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C₂H₂ reduction in soils either with or without C additions were 8.5±3.6 μmol C₂H₄ m⁻² h⁻¹ and 4.8±2.1 μmol m⁻² h⁻¹, respectively. Crust and soil δ¹⁵N and δ¹³C values were not affected by CO₂ treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ¹⁵N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO₂ and concomitant changes in litter composition likely will overwhelm any physiological changes in N₂-fixation.     

Effects of root and litter exclusion on soil CO2 efflux and microbial biomass in wet tropical forests

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass at a soil depth of 25 cm in a secondary forest (dominated by Tabebuia heterophylla) and a pine (Pinus caribaea) plantation in the Luquillo Experimental Forest in Puerto Rico. The experimental plots were initially established in 1990, when root, forest floor mass and new litterfall were excluded for 7 y since then. Soil respiration was significantly reduced in the litter and root exclusion plots in both the secondary forest and the pine plantation compared with the control. Root exclusion had a greater effect on soil CO₂ efflux than the litter exclusion in the plantation, whereas a reversed pattern was observed in the secondary forest. The reduction of microbial biomass in the root exclusion plot was greater in the secondary forest (59%) than in the plantation (31%), while there was no difference of the reduction in the litter exclusion plots between these forests. Our results suggest that above-ground input and roots (root litter and exudates) differentially affect soil CO₂ efflux under different vegetation types.     

Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO₂ effluxes and soil CO₂ concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO₂ sensors for long-term continuous real-time measurement of CO₂ concentrations at different depths, and measured half-hourly soil CO₂ effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO₂ diffusivity in undisturbed soil cores. The method accounts for the CO₂ production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO₂ concentration at all depths increased with increase in soil temperature, likely due to a rise in CO₂ production, and with increase in soil water content due to decreased diffusivity or increased CO₂ production or both. It also increased with soil depth reaching almost 10 mmol mol⁻¹ at the 50-cm depth. Annually, soil CO₂ efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C (F₁₀) of 2.6 μmol m⁻² s⁻¹ and the Q₁₀ of 3.7. No evidence of displacement of CO₂-rich soil air with rain was observed.Effluxes calculated from soil CO₂ concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO₂ efflux originated in the top 20 cm soil. Calculated CO₂ production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO₂ efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO₂ storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO₂ efflux was proportional to CO₂ concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO₂ in the soil. The latter proved successful in calculating effluxes during 2004.     

A new technique to measure soil CO2 efflux at constant CO2 concentration

A new principle for measuring soil CO₂ efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO₂ within the system to achieve a constant CO₂ concentration inside the soil chamber at ambient level, thus balancing the amount of CO₂ entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO₂ efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO₂ within the soil, while allowing for continuous capture of the CO₂ released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO₂ efflux and its δ¹³C, since both variables show sensitivity to a distortion of the soil CO₂ profile commonly found in conventional chamber techniques.     

Carbon dynamics in a temperate grassland soil after 9 years exposure to elevated CO2 (Swiss FACE)

Elevated pCO₂ increases the net primary production, C/N ratio, and C input to the soil and hence provides opportunities to sequester CO₂-C in soils to mitigate anthropogenic CO₂. The Swiss 9 y grassland FACE (free air carbon-dioxide enrichment) experiment enabled us to explore the potential of elevated pCO₂ (60 Pa), plant species (Lolium perenne L. and Trifolium repens L.) and nitrogen fertilization (140 and 540 kg ha⁻¹ y⁻¹) on carbon sequestration and mineralization by a temperate grassland soil. Use of ¹³C in combination with respired CO₂ enabled the identification of the origins of active fractions of soil organic carbon. Elevated pCO₂ had no significant effect on total soil carbon, and total soil carbon was also independent of plant species and nitrogen fertilization. However, new (FACE-derived depleted ¹³C) input of carbon into the soil in the elevated pCO₂ treatments was dependent on nitrogen fertilization and plant species. New carbon input into the top 15 cm of soil from L. perennne high nitrogen (LPH), L. perenne low nitrogen (LPL) and T. repens low nitrogen (TRL) treatments during the 9 y elevated pCO₂ experiment was 9.3±2.0, 12.1±1.8 and 6.8±2.7 Mg C ha⁻¹, respectively. Fractions of FACE-derived carbon in less protected soil particles >53 μm in size were higher than in <53 μm particles. In addition, elevated pCO₂ increased CO₂ emission over the 118 d incubation by 55, 61 and 13% from undisturbed soil from LPH, LPL and TRL treatments, respectively; but only by 13, 36, and 18%, respectively, from disturbed soil (without roots). Higher input of new carbon led to increased decomposition of older soil organic matter (priming effect), which was driven by the quantity (mainly roots) of newly input carbon (L. perenne) as well as the quality of old soil carbon (e.g. higher recalcitrance in T. repens). Based on these results, the potential of well managed and established temperate grassland soils to sequester carbon under continued increasing concentrations of atmospheric CO₂ appears to be rather limited.     

Denitrification in grass swards is increased under elevated atmospheric CO2

Emissions of N₂O and N₂ were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO₂ at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m⁻² as ¹⁵NH₄¹⁵NO₃ or ¹⁴NH₄¹⁵NO₃ (1 at.% excess ¹⁵N). Total denitrification (N₂O+N₂) was increased under elevated pCO₂ with emissions of 6.2 and 19.5 mg ¹⁵N m⁻² measured over 22 d from ambient and elevated pCO₂ swards, respectively, supporting the hypothesis that increased belowground C allocation under elevated pCO₂ provides the energy for denitrification. Nitrification was the predominant N₂O producing process under ambient pCO₂ whereas denitrification was predominant under elevated pCO₂. The N₂-to-N₂O ratio was often higher under elevated pCO₂ suggesting that previous estimates of gaseous N losses based only on N₂O emissions have greatly underestimated the loss of N by denitrification.     

Elevated CO2 does not favor a fungal decomposition pathway

We examined the effect of prolonged elevated CO₂ on the concentration of fungal- and bacterial-derived compounds by quantifying the soil contents of the amino sugars glucosamine, galactosamine and muramic acid. Soil samples were collected from three different terrestrial ecosystems (grassland, an aspen forest and a soybean/corn agroecosystem) that were exposed to elevated CO₂ under FACE conditions for 3-10 years. Amino sugars were extracted from bulk soil and analyzed by gas chromatography. Elevated CO₂ did not affect the size or composition of the amino sugar pool in any of the systems. However, high rates of fertilizer N applications decreased the amount of fungal-derived residues in the grassland system. We suggest that these results are caused by a decrease in saprophytic fungi following high N additions. Furthermore, our findings imply that the contribution of saprophytic fungi and bacteria to SOM in the studied ecosystems is largely unaffected by elevated CO₂.     

Effects of elevated CO2 concentration on rhizodeposition from Lolium perenne grown on soil exposed to 9 years of CO2 enrichment

The effects of enriched CO₂ atmosphere on partitioning of recently assimilated carbon were investigated in a plant-soil-microorganism system in which Lolium perenne seedlings were planted into cores inserted into the resident soil within a sward that had been treated with elevated CO₂ for 9 consecutive years, under two N fertilisation levels (Swiss FACE experiment). The planted cores were excavated from the ambient (35 Pa pCO₂) and enriched (60 Pa pCO₂) rings at two dates, in spring and autumn, during the growing season. The cores were brought back to the laboratory for ¹⁴C labelling of shoots in order to trace the transfer of recently assimilated C both within the plant and to the soil and microbial biomass. At the spring sampling, high N supply stimulated shoot and total dry matter production. Consistently, high N enhanced the allocation of recently fixed C to shoots, and reduced it to belowground compartments. Elevated CO₂ had no consequences for DM or the pattern of C allocation. At the autumn sampling, at high N plot, yield of L. perenne was stimulated by elevated CO₂. Consistently, ¹⁴C was preferentially allocated aboveground and, consequently belowground recent C allocation was depressed and rhizodeposition reduced. At both experimental periods, total soil C content was similar in all treatments, providing no evidence for soil carbon sequestration in the Swiss Free Air CO₂ Enrichment experiment (FACE) after 9 years of enrichment. Recently assimilated C and soil C were mineralised faster in soils from enriched rings, suggesting a CO₂-induced shift in the microbial biomass characteristics (structure, diversity, activity) and/or in the quality of the root-released organic compounds.     

Impact of atmospheric CO2 and plant life forms on soil microbial activities

From the global change perspective, increase of atmospheric CO₂ and land cover transformation are among the major impacts caused by human activities. In this study, we are addressing the combined issues of the effect of CO₂ concentration increase and plant type on soil microbial activities by asking how annual and perennial plant groups affect soil microbial processes under elevated CO₂. The experimental design used a mix of species of different growth forms for both annuals and perennials. Our objective was: (1) to determine how two years of annual or perennial plant cover and CO₂ enrichment could affect Mediterranean soil microbial processes; (2) to test the resistance and the resilience of these soil functional processes after a natural perturbation. We determined the effects of 2 years atmospheric CO₂ enrichment on soil potential respiration (SIR), denitrification (DEA) and nitrification (NEA) activities. We could not find any significant effect of CO₂ increase on SIR, DEA and NEA. However, we found a strong effect of the plant cover type, i.e. annuals versus perennials, on the potential microbial activity related to N cycling. DEA and NEA were significantly higher in soil under annual plants while SIR was not significantly different. To determine whether these changes would survive a natural perturbation, we carried out a rain event experiment once the experimental treatments (i.e. different plant cover and atmospheric CO₂ concentration) were stopped. The soil potential respiration, as expressed by the SIR, was not affected and remained stable. DEA rates converged rapidly under annuals and perennials after the rain event. Under both annuals and perennials NEA increased significantly after the rain event but remained significantly higher in the soil with annual plants. The relative change of the soil microbial processes induced by annual and perennial plants was inversely related to the density and the diversity of the corresponding microbial functional groups.     

Litter- and ecosystem-driven decomposition under elevated CO2 and enhanced N deposition in a Sphagnum peatland

Peatlands represent massive global C pools and sinks. Carbon accumulation depends on the ratio between net primary production and decomposition, both of which can change under projected increases of atmospheric CO₂ and N deposition. The decomposition of litter is influenced by 1) the quality of the litter, and 2) the microenvironmental conditions in which the litter decomposes. This study aims at experimentally testing the effects of these two drivers in the context of global change. We studied the in situ litter decomposition from three common peatland species (Eriophorum vaginatum, Polytrichum strictum and Sphagnum fallax) collected after one year of litter production under pre-treatment conditions (elevated CO₂: 560 ppm or enhanced N: 3 g m⁻² y⁻¹ NH₄NO₃) and decomposed the following year under treatment conditions (same as pre-treatment). By considering the cross-effects between pre-treatments and treatments, we distinguished between the effects on mass loss of 1) the pre-treatment-induced litter quality and 2) the treatment conditions under which the litters were decomposing. The combination between CO₂ pre-treatment and CO₂ treatment reduced Polytrichum decomposition by −24% and this can be explained by litter quality-driven decomposition changes brought by the pre-treatment. CO₂ pre-treatment reduced Eriophorum litter quality, although this was not sufficient to predict decomposition. The N addition pre-treatment reduced the decomposition of Eriophorum, due to enhanced lignin and soluble phenols concentrations in the initial litter, and reduced litter-driven losses of starch and enhanced litter-driven losses of soluble phenols. While decomposition indices based on initial litter quality provide a broad explanation of quantitative and qualitative decomposition, they can only be taken as first approximations. Indeed, the microbial ATP activity, the litter N loss and resulting litter quality, were strongly altered irrespective of the compounds' initial concentration and by means of processes that occurred independently of the initial litter-qualitative changes. The experimental design was valuable to assess litter- and ecosystem-driven decomposition pathways simultaneously or independently. The ability to separate these two drivers makes it possible to attest the presence of litter-qualitative changes even without any litter biochemical determinations, and shows the screening potential of this approach for future experiments dealing with multiple plant species.     

Palatability trials on hardwood leaf litter grown under elevated CO2: a stable carbon isotope study

The palatability to isopods and microbes of a broad range of hardwood leaf litter, derived from three field CO₂-enrichment experiments in the USA, was investigated, using δ¹³C, to trace the C flow from litter to isopods and to CO₂ respired by microbial decomposition. Leaf litter grown under elevated CO₂ had δ¹³C values ranging from −39 to −45‰, which were significantly different from ambient litter δ¹³C values of around −30‰. Litter palatability to isopods of the Porcellio sp. was tested by incubating ambient- and elevated-CO₂ litter, and a mixture of the two, in the presence of isopods for 14 days, under environmentally controlled conditions; δ¹³C was measured on litter and isopods' body before and after incubation. In an additional experiment, litter was incubated in the absence of fauna for 30 days, and on five occasions the δ¹³C of the CO₂ respired from litter was measured. The ¹³C label was clearly carried from the litter source to the isopods' bodies, and their faeces. For microbial-respired CO₂, δ¹³C was significantly higher than that of the litter source, suggesting preferential degradation of substrates enriched in ¹³C as compared to those in the overall litter. With the exception of Quercus myrtifolia leaf litter, elevated CO₂ did not affect the palatability to isopods nor the microbial degradation of any of the litters, possibly as a result of unaltered litter N concentration. However, significant differences in litter palatability and decay rates were observed among the different species. With this study, the use of isotopically labelled litter material was confirmed as a key methodology that can significantly contribute to the advancement of the understanding of litter decomposition and of the quantification of C fluxes in the process.     

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2

Elevated CO₂ may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to ¹³C labeling at ambient (392 μmol mol⁻¹) and elevated (792 μmol mol⁻¹) CO₂ concentrations, in soil containing ¹⁵N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-¹³C was enhanced by 50% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N uptake increased (+7%) under elevated CO₂ while ¹⁵N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO₂ can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO₂.     

Effect of elevated CO2 on soil N dynamics in a temperate grassland soil

The response of terrestrial ecosystems to elevated atmospheric CO₂ is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO₂ Enrichment (FACE) for six years. A ¹⁵N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO₂ on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a ¹⁵N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH₄⁺ and from organic-N to NO₃⁻ and analysed the rate of dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO₂. At the same time the gross rate of NH₄⁺ immobilization increased by 20%, while NH₄⁺ oxidation to NO₃⁻ was reduced by 25% under elevated CO₂. The NO₃⁻ dynamics under elevated CO₂ were characterized by a 52% increase in NO₃⁻ immobilization and a 141% increase in the DNRA rate, while NO₃⁻ production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH₄⁺ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH₄⁺ under elevated CO₂. The advantage of such a shift is that NH₄⁺ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO₂.     

Elevated CO2 and plant species diversity interact to slow root decomposition

Changes in plant species diversity can result in synergistic increases in decomposition rates, while elevated atmospheric CO₂ can slow the decomposition rates; yet it remains unclear how diversity and changes in atmospheric CO₂ may interact to alter root decomposition. To investigate how elevated CO₂ interacts with changes in root-litter diversity to alter decomposition rates, we conducted a 120-day laboratory incubation. Roots from three species (Trifolium repens, Lespedeza cuneata, and Festuca pratense) grown under ambient or elevated CO₂ were incubated individually or in combination in soils that were exposed to ambient or elevated CO₂ for five years. Our experiment resulted in two main findings: (1) Roots from T. repens and L. cuneata, both nitrogen (N) fixers, grown under elevated CO₂ treatments had significantly slower decomposition rates than similar roots grown under ambient CO₂ treatments; but the decomposition rate of F. pratense roots (a non-N-fixing species) was similar regardless of CO₂ treatment. (2) Roots of the three species grown under ambient CO₂ and decomposed in combination with each other had faster decomposition rates than when they were decomposed as single species. However, roots of the three species grown under elevated CO₂ had similar decomposition rates when they were incubated alone or in combination with other species. These data suggest that if elevated CO₂ reduces the root decomposition rate of even a few species in the community, it may slow root decomposition of the entire plant community.