Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].     

Arbuscular mycorrhiza improve growth,nitrogen uptake,and nitrogen use efficiency in wheat grown under elevated CO<Subscript>2</Subscript>

Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning was investigated in Triticum aestivum L. plants grown under elevated CO₂ in a pot experiment. Wheat plants inoculated or not inoculated with the AM fungus were grown in two glasshouse cells with different CO₂ concentrations (400 and 700 ppm) for 10 weeks. A ¹⁵N isotope labeling technique was used to trace plant N uptake. Results showed that elevated CO₂ increased AM fungal colonization. Under CO₂ elevation, AM plants had higher C concentration and higher plant biomass than the non-AM plants. CO₂ elevation did not affect C and N partitioning in plant organs, while AM symbiosis increased C and N allocation into the roots. In addition, plant C and N accumulation, ¹⁵N recovery rate, and N use efficiency (NUE) were significantly higher in AM plants than in non-AM controls under CO₂ enrichment. It is concluded that AM symbiosis favors C and N partitioning in roots, increases C accumulation and N uptake, and leads to greater NUE in wheat plants grown at elevated CO₂.     

Carbon and nitrogen allocation and partitioning in traditional and modern wheat genotypes under pre‐industrial and future CO2 conditions

The results of a simultaneous ¹³C and ¹⁵N labelling experiment with two different durum wheat cultivars, Blanqueta (a traditional wheat) and Sula (modern), are presented. Plants were grown from the seedling stage in three fully controllable plant growth chambers for one growing season and at three different CO₂ levels (i.e. 260, 400 and 700 ppm). Short‐term isotopic labelling (ca. 3 days) was performed at the anthesis stage using ¹³CO₂ supplied with the chamber air and ¹⁵NH₄‐¹⁵NO₃ applied with the nutrient solution, thereby making it possible to track the allocation and partitioning of ¹³C and ¹⁵N in the different plant organs. We found that photosynthesis was up‐regulated at pre‐industrial CO₂ levels, whereas down‐regulation occurred under future CO₂ conditions. ¹³C labelling revealed that at pre‐industrial CO₂ carbon investment by plants was higher in shoots, whereas at future CO₂ levels more C was invested in roots. Furthermore, the modern genotype invested more C in spikes than did the traditional genotype, which in turn invested more in non‐reproductive shoot tissue. ¹⁵N labelling revealed that the modern genotype was better adapted to assimilating N at higher CO₂ levels, whereas the traditional genotype was able to assimilate N more efficiently at lower CO₂ levels.     

C and N allocation in soil under ryegrass and alfalfa estimated by 13C and 15N labelling

Background and Aims

Below-ground translocated carbon (C) released as rhizodeposits is an important driver for microbial mobilization of nitrogen (N) for plants. We investigated how a limited substrate supply due to reduced photoassimilation alters the allocation of recently assimilated C in plant and soil pools under legume and non-legume species.

Methods

A non-legume (Lolium perenne) and a legume (Medicago sativa) were labelled with ¹⁵N before the plants were clipped or shaded, and labelled twice with ¹³CO₂ thereafter. Ten days after clipping and shading, the ¹⁵N and ¹³C in shoots, roots, soil, dissolved organic nitrogen (DON) and carbon (DOC) and in microbial biomass, as well as the ¹³C in soil CO₂ were analyzed.

Results

After clipping, about 50 % more ¹³C was allocated to regrowing shoots, resulting in a lower translocation to roots compared to the unclipped control. Clipping also reduced the total soil CO₂ efflux under both species and the ¹³C recovery of soil CO₂ under L. perenne. The ¹⁵N recovery increased in the shoots of M. sativa after clipping, because storage compounds were remobilized from the roots and/or the N uptake from the soil increased. After shading, the assimilated ¹³C was preferentially retained in the shoots of both species. This caused a decreased ¹³C recovery in the roots of M. sativa. Similarly, the total soil CO₂ efflux under M. sativa decreased more than 50 % after shading. The ¹⁵N recovery in plant and soil pools showed that shading has no effect on the N uptake and N remobilization for L. perenne, but, the ¹⁵N recovery increased in the shoot of M. sativa.

Conclusions

The experiment showed that the dominating effect on C and N allocation after clipping is the need of C and N for shoot regrowth, whereas the dominating effect after shading is the reduced substrate supply for growth and respiration. Only slight differences could be observed between L. perenne and M. sativa in the C and N distribution after clipping or shading.     

Effects of elevated atmospheric carbon dioxide on amino acid and NH4+-N cycling in a temperate pine ecosystem

Rising atmospheric carbon dioxide (CO₂) is expected to increase forest productivity, resulting in greater carbon (C) storage in forest ecosystems. Because elevated atmospheric CO₂ does not increase nitrogen (N) use efficiency in many forest tree species, additional N inputs will be required to sustain increased net primary productivity (NPP) under elevated atmospheric CO₂. We investigated the importance of free amino acids (AAs) as a source for forest N uptake at the Duke Forest Free Air CO₂ Enrichment (FACE) site, comparing its importance with that of better‐studied inorganic N sources. Potential proteolytic enzyme activity was monitored seasonally, and individual AA concentrations were measured in organic horizon extracts. Potential free AA production in soils ranged from 190 to 690 nmol N g⁻¹ h⁻¹ and was greater than potential rates of soil NH₄⁺ production. Because of this high potential rate of organic N production, we determined (1) whether intact AA uptake occurs by Pinus taeda L., the dominant tree species at the FACE site, (2) if the rate of cycling of AAs is comparable with that of ammonium (NH₄⁺), and (3) if atmospheric CO₂ concentration alters the aforementioned N cycling processes. A field experiment using universally labeled ammonium (¹⁵NH₄⁺) and alanine (¹³C₃H₇¹⁵NO₂) demonstrated that ¹⁵N is more readily taken up by plants and heterotrophic microorganisms as NH₄⁺. Pine roots and microbes take up on average 2.4 and two times as much NH₄^{+ 15}N compared with alanine ¹⁵N 1 week after tracer application. N cycling through soil pools was similar for alanine and NH₄⁺, with the greatest ¹⁵N tracer recovery in soil organic matter, followed by microbial biomass, dissolved organic N, extractable NH₄⁺, and fine roots. Stoichiometric analyses of ¹³C and ¹⁵N uptake demonstrated that both plants and soil microorganisms take up alanine directly, with a ¹³C : ¹⁵N ratio of 3.3 : 1 in fine roots and 1.5 : 1 in microbial biomass. Our results suggest that intact AA (alanine) uptake contributes substantially to plant N uptake in loblolly pine forests. However, we found no evidence supporting increased recovery of free AAs in fine roots under elevated CO₂, suggesting plants will need to acquire additional N via other mechanisms, such as increased root exploration or increased N use efficiency.     

Estimation of Carbon and Nitrogen Allocation during Stalk Elongation by C and N Tracing in Zea mays L

Zea mays L. (cv Dea) plants grown to the stage of stalk elongation, were allowed to assimilate ¹³CO₂ and ¹⁵N-nitrates from 45 to 53 days after sowing. Isotopic abundances in labeled nutrients were slightly enriched compared to natural abundances. The new C in plant was acropetally distributed and the new N was preferentially accumulated in the sheath and stalk in the medium region. C input was 25-fold higher than N input. The new C in total plant C was 20%, whereas it was 10% for N. The stalk acted as a major sink because it accumulated, respectively, 27.5 and 47.5% of the C and N inputs. The new C in soluble carbohydrates was 76% in growing organs (upper stalk) and only 39% in source leaves, whereas it was 43% and 13% in starch, respectively. New N in nitrates+amino-acids spanned in the range from 20% (leaf) to 50% (stalk). New C and N in soluble proteins were, respectively, 13.4 and 3.8% in leaves, 8.8 and 9.6% in stalk, and 8.7 and 14.3% in roots. In the middle stalk and leaves, the proteins and carbohydrates represent an equivalent C and N source for remobilization.     

Fine Roots vs. Needles: A Comparison of 13C and 15N Dynamics in a Ponderosa Pine Forest Soil

Plant allocation patterns may affect soil C and N storage due to differences in litter quality and the depth of plant C and N inputs into the soil. We studied the dynamics of dual-labeled (¹³C/¹⁵N) Pinus ponderosa needles and fine roots placed at two soil depths (O and A horizon) in a temperate conifer forest soil during 2 y. Input of C as fine roots resulted in much more C retained in soil (70.5 ± 2.2% of applied) compared with needle C (42.9 ± 1.3% of applied) after 1.5 y. Needles showed faster mass loss, rates of soil ¹³CO₂ efflux, and more ¹⁵N immobilized into microbial biomass than did fine roots. The larger proportion of labile C compounds initially present in needles (17% more needle C was water soluble than in fine roots) likely contributed to its shorter C residence time and greater degree of transformation in the soil. A double exponential decay function best described the rate of ¹³C loss, with a smaller initial pulse of C loss from fine roots (S₁k₁) and a slower decay rate of the recalcitrant C pool for fine roots (0.03 y⁻¹) compared with (0.19 y⁻¹) for needles. Soil ¹³C respiration, representing heterotrophic respiration of litter C, was much more seasonal from the O horizon than from the A. However, offsetting seasonal patterns in ¹³C dynamics in the O horizon resulted in no net effect of soil depth on total ¹³C retention in the soil after 1.5 y for either litter. Almost 90% of applied litter N was retained in the soil after 1.5 y, independent of litter quality or soil depth. Very small amounts of ¹³C or ¹⁵N (<3% of applied) moved to the horizon above or below the placement depth (i.e., O to A or A to O). Our results suggest that plant allocation belowground to fine roots results in more C retained and less N mineralized compared with allocation aboveground to needles, primarily due to litter quality differences.     

Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [CO₂], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO₂ enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN, USA, greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ¹³C) into root‐derived particulate organic matter (POM) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [CO₂] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [CO₂] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO₂‐enriched plots. Furthermore, the δ¹³CO₂ of the C mineralized from the incubated soil closely tracked the δ¹³C of the labile POM pool in the elevated [CO₂] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter (SOM). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [CO₂] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [CO₂].     

Carbon and nitrogen fluxes between beech and their ectomycorrhizal assemblage

To determine the exchange of nitrogen and carbon between ectomycorrhiza and host plant, young beech (Fagus sylvatica) trees from natural regeneration in intact soil cores were labelled for one growing season in a greenhouse with ¹³CO₂ and ¹⁵NO₃ ¹⁵NH₄. The specific enrichments of ¹⁵N and ¹³C were higher in ectomycorrhizas (EMs) than in any other tissue. The enrichments of ¹³C and ¹⁵N were also higher in the fine-root segments directly connected with the EM (mainly second-order roots) than that in bulk fine or coarse roots. A strict, positive correlation was found between the specific ¹⁵N enrichment in EM and the attached second-order roots. This finding indicates that strong N accumulators provide more N to their host than low N accumulators. A significant correlation was also found for the specific ¹³C enrichment in EM and the attached second-order roots. However, the specific enrichments for ¹⁵N and ¹³C in EM were unrelated showing that under long-term conditions, C and N exchange between host and EMs are uncoupled. These findings suggest that EM-mediated N flux to the plant is not the main control on carbon flux to the fungus, probably because EMs provide many different services to their hosts in addition to N provision in their natural assemblages.     

Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l^-1 CO₂) and temperature increase (+3 °C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser supplies (160 and 530 kg N ha^-1 y^-1) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water. During the growing season, swards from the control climate (350 μl l^-1 [CO₂] at outdoor air temperature) were pulse labelled by the addition of ¹³CO₂. The elevated [CO₂] treatments were continuously labelled by the addition of fossil-fuel derived CO₂ (^{13 C} of -40 to -50 ‰). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the soil macro-organic matter (‘old’ C) was at −32‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was at −14 and −54‰ in the ambient and elevated [CO₂] treatments, respectively. During the experiment, the ¹³C isotopic mass balance method was used to calculate, for the top soil (0–15 cm), the carbon turnover in the stubble and roots and in the soil macro-organic matter above 200 μ (MOM). Elevated [CO₂] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N−. At the high N supply, the mean replacement time of ‘old’ C by ‘new’ C declined in elevated, compared to ambient [CO₂], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates of ‘new’ C accumulation and of ‘old’ C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation of ‘new’ C, the soil C pools did not turnover faster in elevated [CO₂], as the rate of ‘old’ C decomposition was reduced. A 3 °C temperature increase in elevated [CO₂] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the ‘old’ C decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble and in the MOM, in ambient but not in elevated [CO₂]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic N availability and of a possible priming effect between C-substrates are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of shoot removal on remobilization of carbon and nitrogen during regrowth of nitrogen‐fixing alfalfa

The contribution of carbon and nitrogen reserves to regrowth following shoot removal has been studied in the past. However, important gaps remain in understanding the effect of shoot cutting on nodule performance and its relevance during regrowth. In this study, isotopic labelling was conducted at root and canopy levels with both ¹⁵N₂ and ¹³C‐depleted CO₂ on exclusively nitrogen‐fixing alfalfa plants. As expected, our results indicate that the roots were the main sink organs before shoots were removed. Seven days after regrowth the carbon and nitrogen stored in the roots was invested in shoot biomass formation and partitioned to the nodules. The large depletion in nodule carbohydrate availability suggests that root‐derived carbon compounds were delivered towards nodules in order to sustain respiratory activity. In addition to the limited carbohydrate availability, the upregulation of nodule peroxidases showed that oxidative stress was also involved during poor nodule performance. Fourteen days after cutting, and as a consequence of the stimulated photosynthetic and N₂‐fixing machinery, availability of C_new and N_new strongly diminished in the plants due to their replacement by C and N assimilated during the post‐labelling period. In summary, our study indicated that during the first week of regrowth, root‐derived C and N remobilization did not overcome C‐ and N‐limitation in nodules and leaves. However, 14 days after cutting, leaf and nodule performance were re‐established.     

Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota

In annual crops, the partitioning of photosynthates to support root growth, respiration and rhizodeposition should be greater during early development than in later reproductive stages due to source/sink relationships in the plant. Therefore, seasonal fluctuations in carbon dioxide (CO₂) and nitrous oxide (N₂O) production from roots and root-associated soil may be related to resource partitioning by the crop. Greenhouse studies used ¹³C and ¹⁵N stable isotopes to evaluate the carbon (C) partitioning and nitrogen (N) uptake by corn and soybean. We also measured the CO₂ and N₂O production from planted pots as affected by crop phenology and N fertilization. Specific root-derived respiration was related to the ¹³C allocated to roots and was greatest during early vegetative growth. Root-derived respiration and rhizodeposition were greater for corn than soybean. The ¹⁵N uptake by corn increased between vegetative growth, tasseling and milk stages, but the ¹⁵N content in soybean was not affected by phenology. A peak in N₂O production was observed with corn at the milk stage, suggesting that the corn rhizosphere supported microbial communities that produced N₂O. Most of the ¹⁵N-NO₃ applied to soybean was not taken up by the plant and negative N₂O production during vegetative growth and floral initiation stages suggests that soybean roots supported the reduction of N₂O to dinitrogen (N₂). We conclude that crop phenology and soil N availability exert important controls on rhizosphere processes, leading to temporal variation in CO₂ and N₂O production.     

Linking sequestration of 13C and 15N in aggregates in a pasture soil following 8 years of elevated atmospheric CO2

The influence of N availability on C sequestration under prolonged elevated CO₂ in terrestrial ecosystems remains unclear. We studied the relationships between C and N dynamics in a pasture seeded to Lolium perenne after 8 years of elevated atmospheric CO₂ concentration (FACE) conditions. Fertilizer‐¹⁵N was applied at a rate of 140 and 560 kg N ha^2?1 y^2?1 and depleted ¹³C‐CO₂ was used to increase the CO₂ concentration to 60 Pa pCO₂. The ¹³C–¹⁵N dual isotopic tracer enabled us to study the dynamics of newly sequestered C and N in the soil by aggregate size and fractions of particulate organic matter (POM), made up by intra‐aggregate POM (iPOM) and free light fraction (LF). Eight years of elevated CO₂ did not increase total C content in any of the aggregate classes or POM fractions at both rates of N application. The fraction of new C in the POM fractions also remained largely unaffected by N fertilization. Changes in the fractions of new C and new N (fertilizer‐N) under elevated CO₂ were more pronounced between POM classes than between aggregate size classes. Hence, changes in the dynamics of soil C and N cycling are easier to detect in the POM fractions than in the whole aggregates. Within N treatments, fractions of new C and N in POM classes were highly correlated with more new C and N in large POM fractions and less in the smaller POM fractions. Isotopic data show that the microaggregates were derived from the macro‐aggregates and that the C and N associated with the microaggregates turned over slower than the C and N associated with the macroaggregates. There was also isotopic evidence that N immobilized by soil microorganisms was an important source of N in the iPOM fractions. Under low N availability, 3.04 units of new C per unit of fertilizer N were sequestered in the POM fractions. Under high N availability, the ratio of new C sequestered per unit of fertilizer N was reduced to 1.47. Elevated and ambient CO₂ concentrations lead to similar ¹⁵N enrichments in the iPOM fractions under both low and high N additions, clearly showing that the SOM‐N dynamics were unaffected by prolonged elevated CO₂ concentrations.     

The decomposition of tylosin fermentation waste in soil at four temperatures

Summary Aerobic decomposition of tylosin fermentation waste was studied by O₂ uptake and CO₂, NH₄ ⁺ and NO₃ ^– release over 10 weeks in a light compost-soil at 3 concentrations and 4 temperatures. Comparisons of O₂ uptake and CO₂ release at each temperature showed that aerobic conditions were maintained in the system. Maximal rates of respiration (C mineralization) increased with temperature. At 23°C 50% of the substrate C had been mineralized in 10 weeks. At 10–15°C and at 4°C C mineralization was approximately 38% and 22% respectively. Except at 4°C mineralization had almost ceased within 10 weeks. There was evidence of a permanent inhibition of C mineralization at 10–15°C compared with 23°C, and a temporary inhibition at 10°C compared with 15°C.At 10 weeks 25% of the N had been mineralized at 23, 15 and 10°C, while 14% had been mineralized at 4°C. The time taken to reach maximum N mineralization was reduced by increase in temperature and by 10 weeks mineralization had almost ceased at 15 and 23°C. In terms of the fertilizing effect of tylosin fermentation, 25% of the total N was available within 10 weeks at 10–23°C. Nitrification was strongly inhibited at 4 and 10°C. Both C and N mineralization were in direct proportion to the concentration of tylosin fermentation waste added to the soil.     

Immobilization, stabilization and remobilization of nitrogen in forest soils at elevated CO2: a 15N and 13C tracer study

The fate of immobilized N in soils is one of the great uncertainties in predicting C sequestration at increased CO₂ and N deposition. In a dual isotope tracer experiment (¹³C, ¹⁵N) within a 4‐year CO₂ enrichment (+200 ppm_v) study with forest model ecosystems, we (i) quantified the effects of elevated CO₂ on the partitioning of N; (ii) traced immobilized N into physically separated pools of soil organic matter (SOM) with turnover rates known from their ¹³C signals; and (iii) estimated the remobilization and thus, the bio‐availability of newly sequestered C and N. (1) CO₂ enrichment significantly decreased NO₃^? concentrations in soil waters and export from 1.5 m deep lysimeters by 30–80%. Consequently, elevated CO₂ increased the overall retention of N in the model ecosystems. (2) About 60–80% of added ¹⁵NH₄¹⁵NO₃ were retained in soils. The clay fraction was the greatest sink for the immobilized ¹⁵N sequestering 50–60% of the total new soil N. SOM associated with clay contained only 25% of the total new soil C pool and had small C/N ratios (<13), indicating that it consists of humified organic matter with a relatively slow turn over rate. This implies that added ¹⁵N was mainly immobilized in stable mineral‐bound SOM pools. (3) Incubation of soils for 1 year showed that the remobilization of newly sequestered N was three to nine times smaller than that of newly sequestered C. Thus, inorganic inputs of N were stabilized more effectively in soils than C. Significantly less newly sequestered N was remobilized from soils previously exposed to elevated CO₂. In summary, our results show firstly that a large fraction of inorganic N inputs becomes effectively immobilized in relative stable SOM pools and secondly that elevated CO₂ can increase N retention in soils and hence it may tighten N cycling and diminish the risk of nitrate leaching to groundwater.     

Effects of elevated atmospheric CO2 in agro-ecosystems on soil carbon storage

Increasing global atmospheric CO₂ concentration has led to concerns regarding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large shortfall in C accounting (≈1.4 × 10¹⁵ g C y^–1) and this has led to the hypothesis that C is being stored in the soil of terrestrial ecosystems. This study examined the effects of CO₂ enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain sorghum (Sorghum bicolor L.) Moench. agro-ecosystems established on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO₂ concentration (ambient and twice ambient) as subplots using open top field chambers. Carbon isotopic techniques using δ¹³C were used to track the input of new C into the soil system. At the end of two years, shifts in δ¹³C content of soil organic matter carbon were observed to a depth of 30 cm. Calculated new C in soil organic matter with grain sorghum was greater for elevated CO₂ vs. ambient CO₂ (162 and 29 g m^–2, respectively), but with soybean the new C in soil organic matter was less for elevated CO₂ vs. ambient CO₂ (120 and 291 g m^–2, respectively). A significant increase in mineral associated organic C was observed in 1993 which may result in increased soil C storage over the long-term, however, little change in total soil organic C was observed under either plant species. These data indicate that elevated atmospheric CO₂ resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent.     

Total soil C and N sequestration in a grassland following 10 years of free air CO2 enrichment

Soil C sequestration may mitigate rising levels of atmospheric CO_2. However, it has yet to be determined whether net soil C sequestration occurs in N‐rich grasslands exposed to long‐term elevated CO₂. This study examined whether N‐fertilized grasslands exposed to elevated CO₂ sequestered additional C. For 10 years, Lolium perenne, Trifolium repens, and the mixture of L. perenne/T. repens grasslands were exposed to ambient and elevated CO₂ concentrations (35 and 60 Pa pCO₂). The applied CO₂ was depleted in δ¹³C and the grasslands received low (140 kg ha^?1) and high (560 kg ha^?1) rates of ¹⁵N‐labeled fertilizer. Annually collected soil samples from the top 10 cm of the grassland soils allowed us to follow the sequestration of new C in the surface soil layer. For the first time, we were able to collect dual‐labeled soil samples to a depth of 75 cm after 10 years of elevated CO₂ and determine the total amount of new soil C and N sequestered in the whole soil profile. Elevated CO₂, N‐fertilization rate, and species had no significant effect on total soil C. On average 9.4 Mg new C ha^?1 was sequestered, which corresponds to 26.5% of the total C. The mean residence time of the C present in the 0–10 cm soil depth was calculated at 4.6±1.5 and 3.1±1.1 years for L. perenne and T. repens soil, respectively. After 10 years, total soil N and C in the 0–75 cm soil depth was unaffected by CO₂ concentration, N‐fertilization rate and plant species. The total amount of ¹⁵N‐fertilizer sequestered in the 0–75 cm soil depth was also unaffected by CO₂ concentration, but significantly more ¹⁵N was sequestered in the L. perenne compared with the T. repens swards: 620 vs. 452 kg ha^?1 at the high rate and 234 vs. 133 kg ha^?1 at the low rate of N fertilization. Intermediate values of ¹⁵N recovery were found in the mixture. The fertilizer derived N amounted to 2.8% of total N for the low rate and increased to 8.6% for the high rate of N application. On average, 13.9% of the applied ¹⁵N‐fertilizer was recovered in the 0–75 cm soil depth in soil organic matter in the L. perenne sward, whereas 8.8% was recovered under the T. repens swards, indicating that the N₂‐fixing T. repens system was less effective in sequestering applied N than the non‐N₂‐fixing L. perenne system. Prolonged elevated CO₂ did not lead to an increase in whole soil profile C and N in these fertilized pastures. The potential use of fertilized and regular cut pastures as a net soil C sink under long‐term elevated CO₂ appears to be limited and will likely not significantly contribute to the mitigation of anthropogenic C emissions.     

Effects of six years atmospheric CO2 enrichment on plant,soil, and soil microbial C of a calcareous grassland

Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO₂ raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO₂ for 6 years. At elevated CO₂, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO₂. Incorporation of C fixed during the experiment (C_new) was tracked by C isotopic analysis of soil fractions which were labelled due to ¹³C depletion of the commercial CO₂ used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. C_new entered soils at an initial rate of 210±42 g C m^–2 year^–1, but only 554±39 g C_new m^–2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant–soil C fluxes at elevated CO₂ in the same system (¹³C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO₂ occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO₂, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO₂ because C storage may be limited by stabilisation of C_new in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se.     

Combined effects of atmospheric CO<Subscript>2</Subscript> and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms

It is uncertain whether elevated atmospheric CO₂ will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO₂ may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO₂ to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO₂ (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 μg N g^–1). Ambient and twice-ambient CO₂ concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with ¹⁴CO₂ and measured soil ¹⁴CO₂ respiration and the ¹⁴C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used ¹⁵N pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO₂ significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO₂. CO₂ enrichment had no significant effect on the allometric distribution of biomass or ¹⁴C among plant components, total ¹⁴C allocation belowground, or cumulative (6-day) ¹⁴CO₂ soil respiration. Elevated CO₂ significantly increased microbial ¹⁴C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial ¹⁴C contents at elevated CO₂ was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO₂, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO₂. Despite significantly greater rates of N immobilization under elevated CO₂, standing pools of microbial N were not affected by elevated CO₂, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO₂. Received: 12 February 1999 / Accepted: 2 March 2000     

温度增高、CO2浓度升高、施氮对蒙古栎幼苗非结构碳水化合物积累及其分配的综合影响

蒙古栎(Quercus mongolica)是中国东北地区天然次生林重要组成树种, 研究该树种幼苗有机碳积累及碳库容对未来气候变化的响应, 可为预测未来气候变暖情景下蒙古栎林的天然更新及幼苗的培育提供科学参考。该文旨在探讨CO₂浓度和温度升高综合作用对蒙古栎幼苗非结构性碳水化合物(NSC)积累及其分配的影响。实验环境条件用人工气候箱控制, 控制条件如下: 1) CO₂浓度倍增(700 μmol·mol^-1), 温度升高4 ℃处理(HCHT); 2) CO₂浓度正常(400 μmol·mol^-1), 温度升高4 ℃处理(HT); 3) CO₂浓度和温度均正常, 即对照组(CK); 每个气候箱幼苗分别在3种氮素水平下生长: N2 (15 mmol·L^-1, 高氮), N1 (7.5 mmol·L^-1, 正常供氮)和N0 (不施氮), 一共为9个处理。研究结果表明, 1) HCHT共同作用对NSC积累无促进作用, 但改变了植物各器官中NSC的分配比例, 叶片中可溶性糖和淀粉的积累明显增加, HCHT下N2水平有利于NSC的积累。2) HT明显影响了蒙古栎一年生幼苗NCS的积累和分配。在N2水平下, HT明显促进NSC的积累, 并增加了在主根中的分配比例。3)植株各器官可溶性糖含量的动态变化因处理不同而异。主根淀粉含量随时间逐渐增加, 而细根淀粉含量随时间逐渐减少。在未来气候变暖的情况下, 土壤中大量的氮供给, 可能将促进蒙古栎幼苗的生长、增加其碳库容和抵御不良环境的能力, 进而提高其天然更新潜力。