Foliage application of nitrogen has less influence on soil microbial biomass and community composition than soil application of nitrogen

Purpose

Many studies have shown the simulated effects of nitrogen (N) deposition on soil microbial community composition by adding N directly to the forest floor but have ignored the N retention process by the canopy. This study was conducted to compare the responses of soil microbial biomass and community composition between soil application of N (SAN) and foliage application of N (FAN).

Materials and methods

A pot experiment was designed with (1) two N application methods (SAN and FAN), (2) three N application levels (5.6, 15.6 and 20.6 g N m^?2 year^?1), and (3) two tree species (Schima superba Gardn. et Champ. and Pinus massoniana Lamb.) following a nested factorial design. Soil microbial biomass and community composition were determined using phospholipid fatty acids (PLFAs) techniques after 1 and 1.5 years of treatments.

Results and discussion

Nitrogen addition increased (P?<?0.05) soil NH₄⁺-N content and soil NO₃^?-N content and decreased (P?<?0.05) soil pH and soil microbial (bacterial, fungal, and actinomycete) biomass for both N application methods. Compared with the SAN treatment, the FAN treatment had higher (P?<?0.05) pH and lower (P?<?0.05) contents of soil NH₄⁺-N and soil NO₃^?-N. Soil microbial biomass and community composition were significantly different between the different N addition levels under the SAN treatment, but they showed no significant difference (P?<?0.05) between the different N addition levels under the FAN treatment. The soil microbial biomass in the S. superba soil was higher (P?<?0.05) than that in the P. massoniana soil for the FAN treatment, with the opposite trend observed under the SAN treatment. Moreover, redundancy analysis showed that soil microorganisms were significantly correlated with soil pH, soil water content, NH₄⁺-N, and NO₃^?-N.

Conclusions

The results showed that N addition affected soil properties, microbial biomass, and the composition of microbial communities; however, the FAN treatment had less influence on soil properties and soil microorganisms than did the SAN treatment over short time scales, and the extent of this effect was different between coniferous and broadleaf trees.

     

Elevated CO2, but not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland

Elevated CO₂ and defoliation effects on nitrogen (N) cycling in rangeland soils remain poorly understood. Here we tested whether effects of elevated CO₂ (720 μl L⁻¹) and defoliation (clipping to 2.5 cm height) on N cycling depended on soil N availability (addition of 1 vs. 11 g N m⁻²) in intact mesocosms extracted from a semiarid grassland. Mesocosms were kept inside growth chambers for one growing season, and the experiment was repeated the next year. We added ¹⁵N (1 g m⁻²) to all mesocosms at the start of the growing season. We measured total N and ¹⁵N in plant, soil inorganic, microbial and soil organic pools at different times of the growing season. We combined the plant, soil inorganic, and microbial N pools into one pool (PIM-N pool) to separate biotic + inorganic from abiotic N residing in soil organic matter (SOM). With the ¹⁵N measurements we were then able to calculate transfer rates of N from the active PIM-N pool into SOM (soil N immobilization) and vice versa (soil N mobilization) throughout the growing season. We observed significant interactive effects of elevated CO₂ with N addition and defoliation with N addition on soil N mobilization and immobilization. However, no interactive effects were observed for net transfer rates. Net N transfer from the PIM-N pool into SOM increased under elevated CO₂, but was unaffected by defoliation. Elevated CO₂ and defoliation effects on the net transfer of N into SOM may not depend on soil N availability in semiarid grasslands, but may depend on the balance of root litter production affecting soil N immobilization and root exudation affecting soil N mobilization. We observed no interactive effects of elevated CO₂ with defoliation. We conclude that elevated CO₂, but not defoliation, may limit plant productivity in the long-term through increased soil N immobilization.     

Atmospheric CO2 enrichment and nutrient additions to planted soil increase mineralisation of soil organic matter, but do not alter microbial utilisation of plant- and soil C-sources

Plants link atmospheric and soil carbon pools through CO₂ fixation, carbon translocation, respiration and rhizodeposition. Within soil, microbial communities both mediate carbon-sequestration and return to the atmosphere through respiration. The balance of microbial use of plant-derived and soil organic matter (SOM) carbon sources and the influence of plant-derived inputs on microbial activity are key determinants of soil carbon-balance, but are difficult to quantify. In this study we applied continuous ¹³C-labelling to soil-grown Lolium perenne, imposing atmospheric CO₂ concentrations and nutrient additions as experimental treatments. The relative use of plant- and SOM-carbon by microbial communities was quantified by compound-specific ¹³C-analysis of phospholipid fatty acids (PLFAs). An isotopic mass-balance approach was applied to partition the substrate sources to soil respiration (i.e. plant- and SOM-derived), allowing direct quantification of SOM-mineralisation. Increased CO₂ concentration and nutrient amendment each increased plant growth and rhizodeposition, but did not greatly alter microbial substrate use in soil. However, the increased root growth and rhizosphere volume with elevated CO₂ and nutrient amendment resulted in increased rates of SOM-mineralisation per experimental unit. As rhizosphere microbial communities utilise both plant- and SOM C-sources, the results demonstrate that plant-induced priming of SOM-mineralisation can be driven by factors increasing plant growth. That the balance of microbial C-use was not affected on a specific basis may suggest that the treatments did not affect soil C-balance in this study.     

Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO₂) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO₂ impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO₂ and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l⁻¹ (ambient) CO₂. Within each of three replicate rooms per CO₂ treatment, the plant communities were grown under high (30 mg l⁻¹) or low (5 mg l⁻¹) N. In contrast to what is often predicted under N limitation, we found that elevated CO₂ increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO₂, despite it being a fast-growing C3 species. Although elevated CO₂ had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO_2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO₂ level for the varying N and invasion treatments. This study demonstrates that even when elevated CO₂ does not have visible effects on aboveground plant biomass, it can have large impacts belowground.     

Long-term nitrogen fertilization alters microbial community structure and denitrifier abundance in the deep vadose zone

Purpose

The excessive use of nitrogen (N) fertilizer in intensive agriculture has increased nitrate leaching into groundwater, but its impacts on N transformation processes and the associated microbial communities in the deep vadose zone remain unclear.

Materials and methods

Soil samples from 0–1050 cm depth were collected from a 20-year field experiment with two N fertilization treatments: 0 (N0) and 600 kg N ha^?1 year^?1 (N600). Amplicon sequencing and quantitative PCR analyses were performed to profile the vertical distribution of soil microbial communities and denitrification genes.

Results and discussion

The soil microbial community structure and diversity were strongly influenced by soil depth and N fertilization. The 250 cm depth was identified as a threshold depth, as dramatically different microbial communities were found below and above this depth. Quantitative PCR results showed that the absolute abundance of denitrification genes decreased with increasing soil depth.

Conclusion

This study elucidated the profound effects of long-term N input on the composition and diversity of the microbial communities and the abundance of denitrifiers in the deep vadose zone. Our results provide basic information for use in mitigating nitrate leaching by enhancing microbial denitrification in deep vadose zones in intensive agricultural areas.

     

Responses of Bougainvillea spectabilis to elevated atmospheric CO2 under galaxolide (HHCB) pollution and the mechanisms of its rhizosphere metabolism

Purpose

Due to the discovery of synthetic musks in soil and the gradual increase in atmospheric carbon dioxide (CO₂), it is important to reveal the potential implications of these compounds for bioremediation systems. Hence, this study was conducted to investigate the combined influence of galaxolide (HHCB) and elevated CO₂ on an ornamental remediation plant.

Materials and methods

We conducted pot experiments with Bougainvillea spectabilis, an ornamental remediation plant, in which the biomass, HHCB and chlorophyll contents, and rhizosphere metabolism of the plants were analyzed.

Results and discussion

We showed that B. spectabilis exhibited high tolerance under combined HHCB and elevated CO₂ stresses. The addition of HHCB alone to the soil did not significantly reduce the biomass components of B. spectabilis, whereas the presence of elevated CO₂ (750 μL L^?1) alone showed a relatively strong ability to increase plant biomass, especially that of the leaves. An elevated CO₂ concentration stimulated the absorption of low doses of HHCB by the roots. Regarding the root metabolites of B. spectabilis, carbohydrates and organic acids were highly correlated with HHCB concentration, and amino acids were well correlated with CO₂ concentration.

Conclusions

Our study indicates that B. spectabilis may be well suited to remove HHCB from contaminated soil under elevated CO₂ levels, and the root metabolism of this plant provides information about HHCB contamination and elevated CO₂ conditions.

     

Distinct microbial and faunal communities and translocated carbon in Lumbricus terrestris drilospheres

Lumbricus terrestris is a deep-burrowing anecic earthworm that builds permanent, vertical burrows with linings (e.g., drilosphere) that are stable and long-lived microhabitats for bacteria, fungi, micro- and mesofauna. We conducted the first non-culture based field study to assess simultaneously the drilosphere (here sampled as 0–2 mm burrow lining) composition of microbial and micro/mesofaunal communities relative to bulk soil. Our study also included a treatment of surface-applied ¹³C- and ¹⁵N-labeled plant residue to trace the short-term (40 d) translocation of residue C and N into the drilosphere, and potentially the assimilation of residue C into drilosphere microbial phospholipid fatty acids (PLFAs). Total C concentration was 23%, microbial PLFA biomass was 58%, and PLFAs associated with protozoa, nematodes, Collembola and other fauna were 200-to-300% greater in the drilosphere than in nearby bulk soil. Principal components analysis of community PLFAs revealed that distributions of Gram-negative bacteria and actinomycetes and other Gram-positive bacteria were highly variable among drilosphere samples, and that drilosphere communities were distinct from bulk soil communities due to the atypical distribution of PLFA biomarkers for micro- and mesofauna. The degree of microbial PLFA ¹³C enrichment in drilosphere soils receiving ¹³C-labeled residue was highly variable, and only one PLFA, 18:1ω9c, was significantly enriched. In contrast, 11 PLFAs from diverse microbial groups where enriched in response to residue amendment in bulk soil 0–5 cm deep. Among control soils, however, a significant δ¹³C shift between drilosphere and bulk soil at the same depth (5–15 cm) revealed the importance of L. terrestris for translocating perennial ryegrass-derived C into the soil at depth, where we estimated the contribution of the recent grass C (8 years) to be at least 26% of the drilosphere soil C. We conclude that L. terrestris facilitates the translocation of plant C into soil at depth and promotes the maintenance of distinct soil microbial and faunal communities that are unlike those found in the bulk soil.     

Transient elevation of carbon dioxide modifies the microbial community composition in a semi-arid grassland

Using open-top chambers (OTC) on the shortgrass steppe in northern Colorado, changes of microbial community composition were followed over the latter 3 years of a 5-year study of elevated atmospheric CO₂ as well as during 12 months after CO₂ amendment ended. The experiment was composed of nine experimental plots: three chambered plots maintained at ambient CO₂ levels of 360±20 μmol mol^?1 (ambient treatment), three chambered plots maintained at 720±20 μmol mol^?1 CO₂ (elevated treatment) and three unchambered plots. The abundance of fungal phospholipid fatty acids (PLFAs) shifted in the shortgrass steppe under the influence of elevation of CO₂ over the period of 3 years. Whereas the content of the fungal signature molecule (18:2ω6) was similar in soils of the ambient and elevated treatments in the third year of the experiment, CO₂ treatment increased the content of 18:2ω6 by around 60% during the two subsequent years. The shift of microbial community composition towards a more fungal dominated community was likely due to slowly changing substrate quality; plant community forage quality declined under elevated CO₂ because of a decline of N in all tested species as well as shift in species composition towards greater abundance of the low forage quality species (Stipa comata). In the year after which CO₂ enrichment had ceased, abundances of fungal and bacterial PLFAs in the post-CO₂ treatment plots shifted slowly back towards the control plots. Therefore, quantity and quality of available substrates had not changed sufficiently to shift the microbial community permanently to a fungal dominated community. We conclude from PLFA composition of soil microorganisms during the CO₂ elevation experiment and during the subsequent year after cessation of CO₂ treatment that a shift towards a fungal dominated system under higher CO₂ concentrations may slow down C cycling in soils and therefore enhance C sequestration in the shortgrass steppe in future CO₂-enriched atmospheres.     

Effects of temperature,glucose and inorganic nitrogen inputs on carbon mineralization in a Tibetan alpine meadow soil

High levels of available nitrogen (N) and carbon (C) have the potential to increase soil N and C mineralization. We hypothesized that with an external labile C or N supply alpine meadow soil will have a significantly higher C mineralization potential, and that temperature sensitivity of C mineralization will increase. To test the hypotheses an incubation experiment was conducted with two doses of N or C supply at temperature of 5, 15 and 25 °C. Results showed external N supply had no significant effect on CO₂ emission. However, external C supply increased CO₂ emission. Temperature coefficient (Q₁₀) ranged from 1.13 to 1.29. Significantly higher values were measured with C than with N addition and control treatment. Temperature dependence of C mineralization was well-represented by exponential functions. Under the control, CO₂ efflux rate was 425 g CO₂–C m^?2 year^?1, comparable to the in situ measurement of 422 g CO₂–C m^?2 year^?1. We demonstrated if N is disregarded, microbial decomposition is primarily limited by lack of labile C. It is predicted that labile C supply would further increase CO₂ efflux from the alpine meadow soil.     

Residue retention mitigated short-term adverse effect of clear-cutting on soil carbon and nitrogen dynamics in subtropical Australia

Purpose

This study aimed to investigate the benefits of retaining harvest residues on the dynamics of soil C and N pools following clear-cut harvesting of a slash pine plantation in South East Queensland of subtropical Australia.

Materials and methods

Immediately following clear-cut harvesting, macro-plots (10?×?10 m) were established on a section of the plantation in a randomised complete block design with four blocks and three treatments: (1) residue removal (RR₀), (2) single level of residue retention (RR₁) and (3) double level of residue retention (RR₂). Soils were sampled at 0, 6, 12, 18 and 24 months following clear-cutting and analysed for total C and N, microbial biomass C (MBC) and N (MBN), hot water–extractable organic C (HWEOC), hot water–extractable organic N (HWEON), NH₄⁺–N and NO_x^?–N.

Results and discussion

The study showed that although soil total C decreased in the first 12 months following clear-cutting, harvest residue retention increased soil total C and N by 45% (p?<?0.001) and 32% (p?<?0.001), respectively, over the 12–24 months. NH₄⁺–N, HWEOC, HWEON and MBC showed initial surges in the first 6 months irrespective of residue management, which declined after the 6th month. However, residue retention significantly increased HWEOC and HWEON over the 12–24 months (p?<?0.001).

Conclusions

This study demonstrated that harvest residue retention during the inter-rotation period can minimise large changes in C and nutrient pools, and can even increase soil C and nutrient pools for the next plantation rotation.

     

Biochar amendment of chromium-polluted paddy soil suppresses greenhouse gas emissions and decreases chromium uptake by rice grain

Purpose

Soil chromium (Cr) pollution has received substantial attention owing to related food chain health risks and possible promotion of greenhouse gas (GHG) emissions. The aim of the present study was to develop a promising remediation technology to alleviate Cr bioavailability and decrease GHG emissions in Cr-polluted paddy soil.

Materials and methods

We investigated the potential role of biochar amendment in decreasing soil CO₂, CH₄, and N₂O emissions, as well in reducing Cr uptake by rice grains at application rates of 0 t ha^?1 (CK), 20 t ha^?1 (BC20), and 40 t ha^?1 (BC40) in Cr-polluted paddy soil in southeastern China. In addition, the soil aggregate size distribution, soil organic carbon (SOC) concentration of soil aggregates, soil available Cr concentration, and rice yield were analyzed after harvesting.

Results and discussion

Biochar amendment significantly reduced CO₂, CH₄, and N₂O emission fluxes. Compared to CK, total C emissions in the BC20 and BC40 treatments decreased by 9.94% and 17.13% for CO₂-C, by 30.46% and 37.10% for CH₄-C, and by 34.24% and 37.49% for N₂O-N, respectively. Biochar amendment increased the proportion of both the 2000–200 μm and 200–20 μm size fractions in the soil aggregate distribution. Accordingly, the organic carbon concentration of these fractions increased, which increased the total SOC. Moreover, biochar amendment significantly decreased soil available Cr concentration and total Cr content of the rice grains by 33.6% and 14.81% in BC20 and 48.1% and 33.33% in BC40, respectively. Rice yield did not differ significantly between biochar amendment treatment and that of CK.

Conclusions

Biochar application reduced GHG emissions in paddy soil, which was attributed to its comprehensive effect on the soil properties, soil microbial community, and soil aggregates, as well as on the mobility of Cr. Overall, the present study demonstrates that biochar has a great potential to enhance soil carbon sequestration while reducing Cr accumulation in rice grains from Cr-polluted rice paddies.

     

Biochar application as a win-win strategy to mitigate soil nitrate pollution without compromising crop yields: a case study in a Mediterranean calcareous soil

Purpose

The environmental benefits of biochar application, ranging from improvements in crop yield to global change mitigation, have been extensively studied in the last decade. However, such benefits have not been profusely demonstrated under a Mediterranean climate and still less in combination with high pH soils. In our study, the short to medium effects of biochar application on a soil-plant system under Mediterranean conditions in an alkaline soil were assessed.

Material and methods

Barley plants were grown in field mesocosms during three agronomical years at three biochar addition rates (0, 5, and 30 t ha^?1). Related to soil, different physicochemical parameters were analyzed as well as microbial respiration, biomass, and functional diversity. In the plant domain, in vivo ecophysiology variables such as leaf transpiration rate, stomatal conductance, and photosynthesis rate were determined while photosynthetic pigment content and soluble protein concentrations were measured in the laboratory. Additionally, crop yield and nutrient composition were also analyzed. The soil-plant connection was investigated by the N content ratio in both fractions establishing the nitrogen efficiency in the system.

Results and discussion

The highest rate of biochar amendment enhanced soil moisture and electrical conductivity combined with an increase of SO₄^2?, Cl^?, Mg²⁺, and K⁺, and decrease of NO₃^? and HPO₄^?. Notable variations regarding nutrition and moisture were induced in this Mediterranean alkaline soil after biochar addition although pH remained stable. Contrastingly, there were no major effects on microbial activity, but a lower abundance of the nosZ functional gene was found. Similarly, plant parameters were unaffected regarding chemical composition and ecophysiology although biochar induced a higher efficiency in the plant nitrogen uptake without increasing crop yield.

Conclusions

Biochar addition at the highest rate (30 t ha^?1) reduced soil-soluble nitrate although N uptake by the plant remained invariable, in turn coupled to no effects on crop productivity. Our study showed that, in a Mediterranean agroecosystem, a wood biochar produced by gasification was unable to increase crop yield, but enhanced soil water retention, decreased the need for N fertilization, and decreased soil-soluble nitrate concentrations, something that could help to mitigate the excessive nitrate levels associated with over-fertilization.

     

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.     

Effects of simulated nitrogen deposition on soil respiration components and their temperature sensitivities in a semiarid grassland

Nitrogen (N) deposition to semiarid ecosystems is increasing globally, yet few studies have investigated the ecological consequences of N enrichment in these ecosystems. Furthermore, soil CO₂ flux – including plant root and microbial respiration – is a key feedback to ecosystem carbon (C) cycling that links ecosystem processes to climate, yet few studies have investigated the effects of N enrichment on belowground processes in water-limited ecosystems. In this study, we conducted two-level N addition experiments to investigate the effects of N enrichment on microbial and root respiration in a grassland ecosystem on the Loess Plateau in northwestern China. Two years of high N additions (9.2 g N m⁻² y⁻¹) significantly increased soil CO₂ flux, including both microbial and root respiration, particularly during the warm growing season. Low N additions (2.3 g N m⁻² y⁻¹) increased microbial respiration during the growing season only, but had no significant effects on root respiration. The annual temperature coefficients (Q₁₀) of soil respiration and microbial respiration ranged from 1.86 to 3.00 and 1.86 to 2.72 respectively, and there was a significant decrease in Q₁₀ between the control and the N treatments during the non-growing season but no difference was found during the growing season. Following nitrogen additions, elevated rates of root respiration were significantly and positively related to root N concentrations and biomass, while elevated rates of microbial respiration were related to soil microbial biomass C (SMBC). The microbial respiration tended to respond more sensitively to N addition, while the root respiration did not have similar response. The different mechanisms of N addition impacts on soil respiration and its components and their sensitivity to temperature identified in this study may facilitate the simulation and prediction of C cycling and storage in semiarid grasslands under future scenarios of global change.     

Structural and functional diversity of soil microbes is affected by elevated [CO<Subscript>2</Subscript>] and N addition in a poplar plantation

Background, Aims, and Scope  The genetic structure and the functionality of soil microbes are both important when studying the role of soil in the C cycle in elevated CO₂ scenarios. The aim of this work was to investigate the genetic composition of the fungal community by means of PCR-DGGE and the functional diversity of soil micro-organisms in general with MicroResp-based community level physiological profiling (CLPP) in a poplar plantation (POPFACE) grown under elevated [CO₂] with and without nitrogen fertilization. Materials and Methods  The POPFACE experimental plantation and FACE facility are located in central Italy, Tuscania (VT). Clones of Populus alba, Populus nigra and Populus x euramericana were grown, from 1999 to 2004, in six 314 m² plots treated either with atmospheric (control) or enriched (550 μmol mol⁻¹) CO₂ with FACE (Free Air CO₂ Enrichment) technology in each growing season. Each plot is divided into six triangular sectors, with two sectors per poplar genotype: three species × two nitrogen levels. After removal of the litter layer one soil core per genotype (10 cm wide, 20 cm depth) was taken inside each of the three sectors in each plot, for a total of 36 soil cores (3 replicates × 2 [CO₂] × 2 fertilization × 3 species) in October 2004 and in July 2005. DNA was extracted with a bead beating procedure. 18S rDNA gene fragments were amplified with PCR using fungal primers (FR1 GC and FF390). Analysis of CLPP was performed using the MicroResp method. Carbon substrates were selected depending on their ecological relevance to soil and their solubility in water. In particular rhizospheric C sources (carboxylic acids and carbohydrates) were chosen considering the importance of root inputs for microbial metabolism. Results  The fertilization treatment differentiated the fungal community composition regardless of elevated [CO₂] or the poplar species; moreover the number of fungal species was lower in fertilized soil. The effect of elevated [CO₂] on the fungal community composition was evident only as interaction with the fertilization treatment as, in N-sufficient soils, the elevated [CO₂] selected a different microbial community. For CLPP, the differ ent poplar species were the main factors of variation. The FACE treatment, on average, resulted in lower C utilization rates in un-fertilized soils and higher in fertilized soils. Discussion  Fungal biomass and fungal composition depend on different factors: from previous studies we know that the greater quantity and the higher C/N ratio of organic inputs under elevated [CO₂] influenced positively the fungal biomass both in fertilized and in un-fertilized soil, whereas nitrogen availability resulted to be the main determinant of fungal community composition in this work. Whole active microbial community was directly influenced by the soil nutrient availability and the poplar species. Under elevated CO₂ the competition for N with plants strongly affected the microbial communities, which were not able to benefit from added rhizospheric substrates. Under Nsufficient conditions, the increase of microbial activity due to [CO₂] enrichment was related to a more active microbial community, favoured by the current availability of C and N. Conclusions  Different factors influenced the microbial community at different levels: poplar species and root exudates affected the functional properties of the microbial community, while the fungal specific composition (as seen with DGGE) remained unaffected. On the other hand, factors such as N and C availability had a strong impact on the community functionality and composition. Fungal community structure reflected the availability of N in soils and the effect of elevated [CO₂] on community structure and function was evident only in N-sufficient soils. The simultaneous availability of C and N was therefore the main driving force for microbial structure and function in this plantation. Recommendations and Perspectives  Using the soil instead of soil extracts for CLPP determination provides a direct measurement of substrate catabolism by microbial communities and reflects activity rather than growth because more immediate responses to substrates are measured. Further applications of this approach could include selective inhibition of different microbial functional groups to investigate specific CLPPs. To combine the structural analysis and the catabolic responses of specific microbial communities (i.e. fungi or bacteria) could provide new outlooks on the role of microbes on SOM decomposition. ESS-Submission Editor: Dr. Kirk Semple (k.semple@lancaster.ac.uk)     

稻麦轮作系统中大气CO2浓度升高对冬小麦生长季土壤呼吸的影响

Studies on the effect of elevated CO2 on C dynamics in cultivated croplands are critical to a better understanding of the C cycling in response to climate change in agroecosystems. To evaluate the effects of elevated CO2 and different N fertilizer application levels on soil respiration, winter wheat （Triticum aestivum L. cv. Yangmai 14） plants were exposed to either ambient CO2 or elevated CO2 （ambient [CO2] ＋ 200 μmol mol-1）, under N fertilizer application levels of 112.5 and 225 kg N ha-1 （as low N and normal N subtreatments, respectively）, for two growing seasons （2006-2007 and 2007-2008） in a rice-winter wheat rotation system typical in China. A split-plot design was adopted. A root exclusion method was used to partition soil respiration （RS） into heterotrophic respiration （RH） and autotrophic respiration （RA）. Atmospheric CO2 enrichment increased seasonal cumulative RS by 11.8% at low N and 5.6% at normal N when averaged over two growing seasons. Elevated CO2 significantly enhanced （P 〈 0.05） RS （12.7%）, mainly due to the increase in RH （caused by decomposition of larger amounts of rice residue under elevated CO2） during a relative dry season in 2007-2008. Higher N supply also enhanced RS under ambient and elevated CO2. In the 2007-2008 season, normal N treatment had a significant positive effect （P 〈 0.01） on seasonal cumulative RS relative to low N treatment when averaged across CO2 levels （16.3%）. A significant increase in RA was mainly responsible for the enhanced RS under higher N supply. The correlation （r2） between RH and soil temperature was stronger （P 〈 0.001） than that between RS and soil temperature when averaged across all treatments in both seasons. Seasonal patterns of RA may be more closely related to the plant phenology than soil temperature. The Q10 （the multiplier to the respiration rate for a 10 ℃ increase in soil temperature） values of RS and RH were not affected by elevated CO2 or higher N supply. These results mainly suggested that the increase in RS at elevated CO2 depended on the input of rice residue, and the increase in RS at higher N supply was due to stimulated root growth and concomitant increase in RA during the wheat growing portion of a rice-winter wheat rotation system.     

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.     

Natural N abundances in a tallgrass prairie ecosystem exposed to 8-y of elevated atmospheric CO2

After 8-y of elevated CO₂, we previously detected greater amounts of total soil nitrogen, suggesting that rates of ecosystem N flux into or out of tallgrass prairie had been altered. Denitrification and associative N fixation rates are the two primary biological processes that are known to control N loss and accumulation in tallgrass prairie soil. Therefore, our objective was to assess the natural abundance of plant and soil ¹⁵N isotopes as a cumulative index of potential change in efflux or influx of N into and out of the tallgrass prairie after 8-y of exposure to elevated CO₂. Aboveground plant delta ¹⁵N values of Andropogon gerardii were close to zero and more positive as a result of elevated CO₂, but whole-soil values at the 5-30 cm depth were significantly reduced (6.8 vs 7.3; P<0.05) under elevated CO₂-chamber (EC) relative to ambient CO₂- chamber (AC). Total, aboveground plant biomass, root-in-growth, extractable N, microbial biomass N, and soil pools collectively exhibited a range of delta ¹⁵N values from −2.8 to 7.3. Measurements of surface soil ¹⁵N indicate that a change in N inputs and outputs has occurred as a result of elevated atmospheric CO₂. In addition to possible changes in denitrification and N₂ fixation, other sources of N such as the re-translocation of N to the surface from deeper soil layers are needed to explain how soil N accrues in surface soils as a consequence of elevated CO₂. Our results support the notion that C accrual may promote N accrual, possibly driven by high plant and microbial N demand amplified by soil N limitation.     

CO2 and inorganic N supply modify competition for N between co-occurring grass plants, tree seedlings and soil microorganisms

Plant-plant and plant-soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO₂) and N treatments. Fraxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dactylis glomerata) at low (380 μmol mol⁻¹) or high (645 μmol mol⁻¹) CO₂ and at two levels of N nutrition in a mesocosm experiment. Pulse ¹⁵N labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and ¹⁵N recovery irrespective of N and CO₂ treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO₂, but the pattern of Fraxinus- and microbial-N pool response to N and CO₂ varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, ¹⁵N recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO₂ treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of ¹⁵N recovery were exacerbated by N addition. Contrary to expectations, elevated CO₂ did not increase plant-microbe competition. Nevertheless, microbial ¹⁵N recovery showed a small positive increase in the high CO₂ treatment. Overall, elevated CO₂ and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO₂ or N-induced modifications in N competition between plant and soil compartments.     

Methanotrophic communities in a landfill cover soil as revealed by [13C] PLFAs and respiratory quinones: Impact of high methane addition and landfill leachate irrigation

The soil microbial communities of a landfill cover substrate, which was treated with landfill gas (100 l CH₄ m^?2 d^?1) and landfill leachate for 1.5 years, were investigated by phospholipid fatty acid (PLFA), ergosterol and respiratory quinone analyses. The natural ¹³C depletion of methane was used to assess the activity of methanotrophs and carbon turnover in the soil system. Under methane addition, the soil microbial community was dominated by PLFAs (14:0 and 16:1 isomers) and quinones (ubiquinone-8 and 18-methylene-ubiquinone-8) related to type I methanotrophs, and 18:1 PLFAs contained in type II methanotrophs. While type I methanotrophic PLFAs were ¹³C depleted, i.e. type I methanotrophs were actively oxidising and assimilating methane, ¹³C depletion of 18:1 PLFAs was low and inconsistent with their abundance. This, possibly reflects isotopic discrimination, assimilation of carbon derived from type I methanotrophs and a high contribution of non-methanotrophic bacteria to the 18:1 isomers. Landfill leachate irrigation caused the methanotrophic community to shift closer to the soil surface. It also decreased 18:1 PLFAs, while type I methanotrophs were probably stimulated. Gram positive bacteria, but not fungi, were also ¹³C depleted and consequently involved in the secondary turnover of carbon originating from methanotrophic bacteria. Cy17:0 PLFA was ¹³C depleted in deep soil layers, indicating anaerobic methane oxidation.