Effects of land-use type and nitrogen addition on nitrous oxide and carbon dioxide production potentials in Japanese Andosols

Land-use type and nitrogen (N) addition strongly affect nitrous oxide (N₂O) and carbon dioxide (CO₂) production, but the impacts of their interaction and the controlling factors remain unclear. The aim of this study was to evaluate the effect of both factors simultaneously on N₂O and CO₂ production and associated soil chemical and biological properties. Surface soils (0–10 cm) from three adjacent lands (apple orchard, grassland and deciduous forest) in central Japan were selected and incubated aerobically for 12 weeks with addition of 0, 30 or 150 kg N ha^–1 yr^–1. Land-use type had a significant (p < 0.001) impact on the cumulative N₂O and CO₂ production. Soils from the apple orchard had higher N₂O and CO₂ production potentials than those from the grassland and forest soils. Soil net N mineralization rate had a positive correlation with both soil N₂O and CO₂ production rates. Furthermore, the N₂O production rate was positively correlated with the CO₂ production rate. In the soils with no N addition, the dominant soil properties influencing N₂O production were found to be the ammonium-N content and the ratio of soil microbial biomass carbon to nitrogen (MBC/MBN), while those for CO₂ production were the content of nitrate-N and soluble organic carbon. N₂O production increased with the increase in added N doses for the three land-use types and depended on the status of the initial soil available N. The effect of N addition on CO₂ production varied with land use type; with the increase of N addition doses, it decreased for the apple orchard and forest soils but increased for the grassland soils. This difference might be due to the differences in microbial flora as indicated by the MBC/MBN ratio. Soil N mineralization was the major process controlling N₂O and CO₂ production in the examined soils under aerobic incubation conditions.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Elevated CO2 stimulates N2O emissions in permanent grassland

To evaluate climate forcing under increasing atmospheric CO₂ concentrations, feedback effects on greenhouse gases such as nitrous oxide (N₂O) with a high global warming potential should be taken into account. This requires long-term N₂O flux measurements because responses to elevated CO₂ may vary throughout annual courses. Here, we present an almost 9 year long continuous N₂O flux data set from a free air carbon dioxide enrichment (FACE) study on an old, N-limited temperate grassland. Prior to the FACE start, N₂O emissions were not different between plots that were later under ambient (A) and elevated (E) CO₂ treatments, respectively. However, over the entire experimental period (May 1998–December 2006), N₂O emissions more than doubled under elevated CO₂ (0.90 vs. 2.07 kg N₂O-N ha⁻¹ y⁻¹ under A and E, respectively). The strongest stimulation occurred during vegetative growth periods in the summer when soil mineral N concentrations were low. This was surprising because based on literature we had expected the highest stimulation of N₂O emissions due to elevated CO₂ when mineral N concentrations were above background values (e.g. shortly after N application in spring). N₂O emissions under elevated CO₂ were moderately stimulated during late autumn–winter, including freeze–thaw cycles which occurred in the 8th winter of the experiment. Averaged over the entire experiment, the additional N₂O emissions caused by elevated CO₂ equaled 4738 kg CO₂-equivalents ha⁻¹, corresponding to more than half a ton (546 kg) of CO₂ ha⁻¹ which has to be sequestered annually to balance the CO₂-induced N₂O emissions. Without a concomitant increase in C sequestration under rising atmospheric CO₂ concentrations, temperate grasslands may be converted into greenhouse gas sources by a positive feedback on N₂O emissions. Our results underline the need to include continuous N₂O flux measurements in ecosystem-scale CO₂ enrichment experiments.     

Changes in the fungal-to-bacterial respiratory ratio and microbial biomass in agriculturally managed soils under free-air CO2 enrichment (FACE) - A six-year survey of a field study

In soil ecology, microbial parameters have been identified as sensitive indicators of changes in the soil environment. The Braunschweig FACE project provided the opportunity to study the effects of elevated CO₂ (550 μmol mol⁻¹) as compared to ambient CO₂ (370 μmol mol⁻¹) on total microbial biomass (C_mic), C_mic-to-C_org ratio and the fungal-to-bacterial respiratory ratio together with total C_org, N_t, C:N ratio and pH over a six-year period. Field management followed a typical crop rotation system of this region with either a crop-related full nitrogen supply (N100) or 50% reduced N supply (N50). The soil microbial parameters responded to the elevated CO₂ treatment in varying intensities and time spans. The fungal-to-bacterial respiratory ratio was the most sensitive parameter in responding to an elevated CO₂ treatment with highly significant differences to ambient CO₂-treated control plots in the third year of CO₂ fumigation. After six years bacterial respiratory activity had increased in ascending order to 34% in FACE-treated plots (N50 and N100) as compared to control plots. Soil microbial biomass (C_mic) responded more slowly to the FACE treatment with highly significant increases of >12% after the fourth year of CO₂ fumigation. The C_mic-to-C_org ratio responded very late in the last two years of the CO₂ treatment with a significant increase of >7.0% only in the N100 variant. Total C_org and N_t were slightly but significantly increased under FACE around 10.0% with ascending tendency over time starting with the second year of CO₂ treatment. No significant FACE effects could be recorded for the C:N ratio or pH.These results suggest that under FACE treatment changes in the soil microbial community will occur. In our study the fungal-to-bacterial respiratory ratio was superior to total C_mic as microbial bioindicators in reflecting changes in the soil organic matter composition.     

Heterotrophy-coordinated diazotrophy is associated with significant changes of rare taxa in soil microbiome

Soil heterotrophic respiration during decomposition of carbon (C)-rich organic matter plays a vital role in sustaining soil fertility. However, it remains poorly understood whether dinitrogen (N₂) fixation occurs in support of soil heterotrophic respiration. In this study, ¹⁵N₂-tracing indicated that strong N₂ fixation occurred during heterotrophic respiration of carbon-rich glucose. Soil organic ¹⁵N increased from 0.37 atom% to 2.50 atom% under aerobic conditions and to 4.23 atom% under anaerobic conditions, while the concomitant CO₂ flux increased by 12.0-fold under aerobic conditions and 5.18-fold under anaerobic conditions. Soil N₂ fixation was completely absent in soils replete with inorganic N, although soil N bioavailability did not alter soil respiration. High-throughput sequencing of the 16S rRNA gene further indicated that: i) under aerobic conditions, only 15.2% of soil microbiome responded positively to glucose addition, and these responses were significantly associated with soil respiration and N₂ fixation and ii) under anaerobic conditions, the percentage of responses was even lower at 5.70%. Intriguingly, more than 95% of these responses were originally rare with < 0.5% relative abundance in background soils, including typical N₂-fixing heterotrophs such as Azotobacter and Clostridium and well-recognized non-N₂-fixing heterotrophs such as Sporosarcina, Agromyces, and Sedimentibacter. These results suggest that only a small portion of the soil microbiome could respond quickly to the amendment of readily accessible organic C in a fluvo-aquic soil and highlighted that rare phylotypes might have played more important roles than previously appreciated in catalyzing soil C and nitrogen turnovers. Our study indicates that N₂ fixation could be closely associated with microbial turnover of soil organic C when available in excess.     

Increasing the pH of Wastewater to High Levels with Different Gases—CO2 Stripping

The potential of increasing the pH of wastewater to high levels by CO₂ stripping through air, N₂, O₂ and a gas mixture (95% N₂+ 5% CO₂) has been examined in this work. Wastewater collected after the biological treatment from a large-scale plant was monitored for pH in a continuous, computer-controlled laboratory setup. The use of air as the gaseous medium for CO₂ stripping increased the pH to a maximum value of approximately 8.53. Using pure N₂ instead of air increased the pH of wastewater to a final value of 10.3 after 24 h at a flow rate of 1.5 L min^–1. An experimental system dismantling air from its CO₂ content by precipitation as CaCO₃ in a super-saturated lime solution (in a closed circulation operation), increased the pH of wastewater to a final value of 9.4 after 24 h at a flow rate of 0.7 L min^–1. Applying the same closed circulation operation to an artificial carbonate solution instead of wastewater resulted in final pH values of 9.8 after 24 h using a flow rate of 0.7 L min^–1. The results suggest that a closed circulation operation with CO₂-freed air has the potential to increase the pH of wastewater to high levels. The process may be applied for phosphorus precipitation from domestic wastewater not requiring chemicals for a pH increase.     

Saccharidic compounds as soil redox effectors and their influence on potential N2O production

Cellulose, xylan, and glucose were compared in waterlogged soil as modifying factors of the redox potential (Eh), of the quantity of reducing equivalents, and of the soil capacity to produce N₂O and CO₂. During the study period (168 h) soils supplied with glucose and xylan showed a higher Eh decrease than the control soil and the soil treated with cellulose. In samples taken after 0, 24, 48, and 168 h, the soils supplied with C showed a higher number of reducing equivalents than the control soil did. These quantities were not correlated with Eh values, nor with N₂O production. N₂O production was increased compared with the control soil over the entire experimental period in the glucose-amended soils but only after 48 h in the xylan-amended soils and not until 168 h in the cellulose-treated soils. The CO₂:N₂O ratio was consistently higher than the theoretical value of 2, suggesting that denitrification and CO₂ production via fermentation occurred simultaneously. Moreover, this ratio was highly correlated with the Eh values. We conclude that more research is needed to explain the role of soil redox intensity (Eh) and capacity (quantity of redox species undergoing reduction) in the expression of soil denitrification-fermentation pathways.     

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.     

Microbiological characterization and nitrate reduction in subsurface soils

Summary Two borings (20 m depth) were performed in a sandy-clayey soil over a limestone bed and in a sandy soil with lumps of clay in some depths. Bacteria were found in the deeper soil layers of both profiles. The methods used to detect bacteria were those normally used for topsoil layers, plate counts of bacteria, ATP content, and direct microscopy. Measurements of CO₂ evolution showed that the potential for bacterial activity was present in all depths of the two profiles. However, the activity was strongly dependent on the presence of easily available organic C. An indication of the denitrification potential was obtained by measuring the N₂O evolution. Under aerobic incubation without the addition of glucose, N₂O was detected only in the topsoil. When glucose was added to the soil samples, N₂O was found at a low level in the deeper soil layers. Under anaerobic incubation, N₂O was detected in all deeper layers, and increased markedly when glucose was added to the soil samples.     

Fate of nitrogen and carbon in the vadose zone: in situ and laboratory measurements of seasonal variations in aerobic respiratory and denitrifying activities

In situ and laboratory measurements of aerobic respiratory and denitrifying activities were studied in the vadose zone (almost 2.5 m thick) of a fluvic hypercalcaric cambisol characterized by transitory anaerobic conditions. A field experiment was conducted in a bare soil, over a 7-month period starting just after maize harvest and incorporation of maize crop residues. Weather variables (air and soil temperature, rainfall), soil water content, soil solutes (NO₃⁻ and dissolved organic carbon) and soil gases (CO₂ and N₂O), were recorded throughout the experiment. Four soil layers were defined. Bacterial counts were performed in each layer using the most probable number (MPN) method. Aerobic respiratory and denitrifying activities were estimated from laboratory measurements. In situ microbial activity, as revealed by CO₂ and N₂O measurements in the soil atmosphere, was strongly influenced by weather. Laboratory measurements showed that potential aerobic respiratory activity (ARA) occurred throughout the soil profile, whereas semi-potential denitrifying activities SPDA (i.e. measured under organic-C limiting condition) occurred mainly in the top 30 cm soil layer. In the soil profile, the CO₂ concentration gradient was stronger than the N₂O concentration gradient. Seasonal variations in microbial activities increased with depth, whereas DOC concentrations, and variations in those concentrations, decreased with depth, suggesting that DOC quality investigations are necessary in the deep vadose zone to understand microbial activities seasonal variations. Laboratory measurements of potential activities agreed well with in situ microbial activity in natural environmental conditions. NO₃⁻ was a stronger limiting factor for SPDA than was denitrifier density in the soil profile.     

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.     

Elevation of atmospheric CO2 and N-nutritional status modify nodulation, nodule-carbon supply, and root exudation of Phaseolus vulgaris L.

Increased root exudation and a related stimulation of rhizosphere-microbial growth have been hypothesised as possible explanations for a lower nitrogen- (N-) nutritional status of plants grown under elevated atmospheric CO₂ concentrations, due to enhanced plant-microbial N competition in the rhizosphere. Leguminous plants may be able to counterbalance the enhanced N requirement by increased symbiotic N₂ fixation. Only limited information is available about the factors determining the stimulation of symbiotic N₂ fixation in response to elevated CO₂.In this study, short-term effects of elevated CO₂ on quality and quantity of root exudation, and on carbon supply to the nodules were assessed in Phaseolus vulgaris, grown in soil culture with limited (30 mg N kg⁻¹ soil) and sufficient N supply (200 mg N kg⁻¹ soil), at ambient (400 μmol mol⁻¹) and elevated (800 μmol mol⁻¹) atmospheric CO₂ concentrations.Elevated CO₂ reduced N tissue concentrations in both N treatments, accelerated the expression of N deficiency symptoms in the N-limited variant, but did not affect plant biomass production. ¹⁴CO₂ pulse-chase labelling revealed no indication for a general increase in root exudation with subsequent stimulation of rhizosphere microbial growth, resulting in increased N-competition in the rhizosphere at elevated CO₂. However, a CO₂-induced stimulation in root exudation of sugars and malate as a chemo-attractant for rhizobia was detected in 0.5-1.5 cm apical root zones as potential infection sites. Particularly in nodules, elevated CO₂ increased the accumulation of malate as a major carbon source for the microsymbiont and of malonate with essential functions for nodule development. Nodule number, biomass and the proportion of leghaemoglobin-producing nodules were also enhanced. The release of nod-gene-inducing flavonoids (genistein, daidzein and coumestrol) was stimulated under elevated CO₂, independent of the N supply, and was already detectable at early stages of seedling development at 6 days after sowing.     

The Influence of Carbon Dioxide or Ozone Concentration on Growth and Assimilate Partitioning in Seedlings of Nine Conifers

Abstract

Seedlings of nine different conifers were exposed to 355 and 730 μmol mol-1 CO₂, or low (> 15 nmol mol^?1) and elevated 0₃ concentration (70 nmol mol^?1) for 81–116 days. The experiments were conducted in growth chambers placed in a greenhouse. Increased CO₂ concentration enhanced the mean relative growth rate (RGR) and total plant dry weight by 4 and 33% in Larix leptolepis, by 4 and 38% in Larix sibirica, by 7 and 47% in Picea glauca and by 3 and 16% in Picea sitchensis, respectively. The growth rates and dry weights of Pimis contorta, Pinus mugo and Pseudotsuga menziesii were not significantly affected. Carbon dioxide enrichment enhanced RGR of two provenances of Picea abies by 4 and 6%, respectively, while a third provenance was unaffected. In Pimis sylvestris, only the RGR of one of three provenances was stimulated by CO₂ enrichment (4%).

After two growth seasons CO₂ enrichment enhanced RGR and total plant dry weight by 11 and 35% in Picea abies and by 12 and 36% in Pinus sylvestris, respectively. Elevated CO₂ decreased the shoot:root ratio in Larix leptolepis, and decreased the needlerstem ratio in Picea glauca, but increased it in Pseudotsuga menziesii.

Elevated O₃ significantly decreased the plant dry weight in Picea sitchensis, Pseudotsuga menziesii and in one of three provenances of Pinus sylvestris, while the other species and provenances were unaffected. Increased O₃ concentration increased the shoot:root dry weight ratio in one of three Picea abies provenances, in all three Pinus sylvestris provenances and in Pinus contorta. The needle:stem ratio was enhanced by O₃ in seven of the nine species. The O₃ exposure caused chlorosis of needles in all species except Pseudotsuga menziesii.     

Effects of greenhouse intensive cultivation and organic amendments on greenhouse gas emission according to a soil incubation study

Soil cultivation changes and usage of agricultural wastes can have profound impacts on greenhouse gas (GHG) emission from soil. In this study, the effects of soil cultivation and organic amendment on GHG emission were investigated using aerobic incubation. Surface soil (0–20 cm) from (1) rice–legume consecutive rotation (Rice) and (2) recently (<3 years) converted from rice field to plastic-covered intensive vegetable and flower production (VegC) were collected in Kunming, P.R. China. Rose (Rosa rugosa Thunb.) residues and cattle manure were applied at 5% by weight. Results indicated that N₂O and CO₂ fluxes were significantly influenced by soil cultivation, organic amendment, incubation time and their interaction (p <0.05). Applying cattle manure increased, while rose residue decreased, cumulative N₂O emissions from soil (84 days). Rose residue application significantly increased cumulative CO₂ emissions with peak values of 6371 (Rice) and 7481 mg kg^?1 (VegC), followed by cattle manure addition figure of 2265 (VegC) and 3581 mg kg^?1 (Rice). Both were significantly higher (p <0.05) than the un-amended Control at 709 (VegC) and 904 mg kg^?1 (Rice). Our study demonstrates that a low C/N ratio in cattle manure is better than a high C/N ratio in rose residue in regard to reducing the global warming potential of agricultural soil.     

The effect of some carbon substrates on denitrification rates and carbon utilization in soil

Summary Soil was amended with a variety of carbon sources, including four soluble compounds (glucose, sucrose, glycerol and mannitol) and two plant residues (straw and alfalfa).. Potential denitrification rates, measured both as N₂O accumulation and NO₃ ^– disappearance, were compared, and the predicted values of available C, measured as CO₂ production and water-extractable C, were assessed.The two measures of denitrification agreed well although N₂O accumulation was, found to be most sensitive. Soil treated with the four soluble C compounds resulted in the same rate of denitrification although glycerol was not as rapidly oxidized. Alfalfa-amended soil produced a significantly higher rate of denitrification than the same amount of added straw. CO₂ evolution was found to be a good predictor of denitrification over the first 2 days of sampling, but neither measure of available substrate C correlated well with denitrification rate beyond 4 days, when NO₃ ^– was depleted in most treatments. The data with alfalfa-amended soil suggested that denitrifiers used water-extractable C. materials produced by other organisms under anaerobic conditions.     

Autumn growth and cold hardening of winter wheat under simulated climate change

Abstract

Plant responses to elevated CO₂ are governed by temperature, and at low temperatures the beneficial effects of CO₂ may be lost. To document the responses of winter cereals grown under cold conditions at northern latitudes, autumn growth of winter wheat exposed to ambient and elevated levels of temperature (+2.5°C), CO₂ (+150 µmol mol^?1), and shade (?30%) was studied in open-top chambers under low light and at low temperatures. Throughout the experiment, temperature dominated plant responses, while the effects of CO₂ were marginal, except for a positive effect on root biomass. Increased temperature resulted in increased leaf area, total biomass, total root biomass, total stem biomass, and number of tillers, but also a lower content of total sugars and a weaker tolerance to frost. The loss of frost tolerance was related to the larger size of plants grown at elevated temperature. The 30% light reduction under shading did not affect the growth, sugar content, or frost tolerance of winter wheat. At the low temperatures found at high latitudes during autumn, the atmospheric CO₂ increase is unlikely to enhance autumn growth of winter wheat to any significant extent, while a temperature increase may have important and major effects on its development and growth.     

Effect of nitrogen fertilization on methane and carbon dioxide production potential in relation to labile carbon pools in tropical flooded rice soils in eastern India

In an incubation experiment with flooded rice soil fertilized with different N amounts and sampled at different rice stages, the methane (CH₄) and carbon dioxide (CO₂) production in relation to soil labile carbon (C) pools under two temperature (35°C and 45°C) and moisture (aerobic and submerged) regimes were investigated. The field treatments imposed in the wet season included unfertilized control and 40, 80 and 120 kg ha^?1 N fertilization. The production of CH₄ was significantly higher (27%) under submerged compared to aerobic conditions, whereas CO₂ production was significantly increased under aerobic by 21% compared to submerged conditions. The average labile C pools were significantly increased by 21% at the highest dose of N (120 kg ha^?1) compared to control and was found highest at rice panicle initiation stage. But the grain yield had significantly responded only up to 80 kg ha^?1 N, although soil labile C as well as gaseous C emission was noticed to be highest at 120 kg ha^?1 N. Hence, 80 kg N ha^?1 is a better option in the wet season at low land tropical flooded rice in eastern India for sustaining grain yield and minimizing potential emission of CO₂ and CH₄.     

Denitrification,N2-fixation and fermentation during anaerobic incubation of soils amended with glucose and nitrate

Nitrate and glucose additions were investigated for their role in the C and N dynamics during anaerobic incubation of soil. A gas-flow soil core method was used, in which the net production of N₂, N₂O, NO, CO₂, and CH₄ under a He atmosphere could be monitored both accurately and frequently. In all experiments clayey silt loam soil samples were incubated for 9 days at 25 °C. Addition of nitrate (50 mg KNO₃-N kg^-1 soil) had no effect on total denitrification and CO₂ production rates, while the N₂O/N₂ ratio was affected considerably. The cumulative N₂O production exceeded the cumulative N₂ production for 6 days in the treatment with nitrate addition, compared to 1.2 days in the unamended treatment. Glucose addition stimulated the microbial activity considerably. The denitrification rates were limited by the growth rate of the denitrifying population. During denitrification no significant differences were observed between the treatments with 700 mg glucose-C kg^-1 and 4200 mg glucose-C kg^-1, both in combination with 50 mg KNO₃-N kg^-1. The N₂ production rates were remarkably low, until NO _inf3 ^sup- exhaustion caused rapid reduction of N₂O to N₂ at day 2. During the denitrification period 15–18 mg N kg^-1 was immobilised in the growing biomass. After NO _inf3 ^sup- shortage, a second microbial population, capable of N₂-fixation, became increasingly important. This change was clearly reflected in the CO₂ production rates. Net volatile fatty acid (VFA) production was monitored during the net N₂-fixation period with acetate as the dominant product. N₂-fixation faded out, probably due to N₂ shortage, followed by increased VFA production. In the high C treatment butyrate became the most important VFA, while in the low C treatment acetate and butyrate were produced at equal rates. During denitrification no VFA accumulation occurred; this does not prove, however, that denitrification and fermentation appeared sequentially. The experiments illustrate clearly the interactions of C-availability, microbial population and nitrate availability as influencing factors on denitrification and fermentation.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

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

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

Dynamics of labile and recalcitrant soil carbon pools in a sorghum free-air CO2 enrichment (FACE) agroecosystem

Experimentation with dynamics of soil carbon pools as affected by elevated CO₂ can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ¹³C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa free air CO₂ enrichment (FACE) experiment, in which elevated CO₂ (FACE: 560 μmol mol⁻¹) and ambient CO₂ (Control: 360 μmol mol⁻¹) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO₂ transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO₂ was higher than that under ambient CO₂, indicating that elevated CO₂ reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO₂ enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO₂ and mitigating harmful CO₂ under future atmospheric conditions.