Dynamics of methanogenesis and methanotrophy in tropical paddy soils as influenced by elevated CO2 and temperature interaction

Response of methanogenesis and methanotrophy to elevated carbon dioxide (CO₂) could be affected by changes in soil moisture content and temperature. In soil microcosms contained in glass bottles and incubated under laboratory conditions, we assessed the impact of elevated CO₂ and temperature interactions on methanogenesis and methanotrophy in alluvial and laterite paddy soils of tropical origin. Soil samples were incubated at ambient (370 μmol mol⁻¹) and elevated (600 μmol mol⁻¹) CO₂ concentrations at 25, 35 and 45 °C under non-flooded and flooded conditions for 60 d. Under flooded condition, elevated CO₂ significantly increased methane (CH₄) production while under non-flooded condition, only marginal increase in CH₄ production was observed in both the soils studied and the increase was significantly enhanced by further rise in temperature. Increased methanogenesis as a result of elevated CO₂ and temperature interaction was mostly attributed to decreased soil redox potential, increased readily mineralizable carbon, and also noticeable stimulation of methanogenic bacterial population. In contrast to CH₄ production, CH₄ oxidation was consistently low under elevated CO₂ concentration and the decrease was significant with rise in temperature. The low affinity and high affinity CH₄ oxidation were faster under non-flooded condition as compared to flooded condition. Admittedly, decreased low and high affinity CH₄ oxidation as a result of elevated CO₂ and temperature interaction was related to unfavorable lower redox status of soil and the inhibition of CH₄-oxidizing bacterial population.     

Effects of biochar addition on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> emissions from two paddy soils

Impacts of biochar addition on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from paddy soils are not well documented. Here, we have hypothesized that N₂O emissions from paddy soils could be depressed by biochar incorporation during the upland crop season without any effect on CO₂ emissions. Therefore, we have carried out the 60-day aerobic incubation experiment to investigate the influences of rice husk biochar incorporation (50 t ha⁻¹) into two typical paddy soils with or without nitrogen (N) fertilizer on N₂O and CO₂ evolution from soil. Biochar addition significantly decreased N₂O emissions during the 60-day period by 73.1% as an average value while the inhibition ranged from 51.4% to 93.5% (P < 0.05–0.01) in terms of cumulative emissions. Significant interactions were observed between biochar, N fertilizer, and soil type indicating that the effect of biochar addition on N₂O emissions was influenced by soil type. Moreover, biochar addition did not increase CO₂ emissions from both paddy soils (P > 0.05) in terms of cumulative emissions. Therefore, biochar can be added to paddy fields during the upland crop growing season to mitigate N₂O evolution and thus global warming.     

The effect of increased atmospheric carbon dioxide concentration on emissions of nitrous oxide, carbon dioxide and methane from a wheat field in a semi-arid environment in northern China

There are no reports on the effects of elevated carbon dioxide [CO₂] on the fluxes of N₂O, CO₂ and CH₄ from semi-arid wheat cropping systems. These three soil gas fluxes were measured using closed chambers under ambient (420 ± 18 μmol mol⁻¹) and elevated (565 ± 37 μmol mol⁻¹) at the Free-Air Carbon dioxide Enrichment experimental facility in northern China. Measurements were made over five weeks on a wheat crop (Triticum aestivum L. cv. Zhongmai 175). Elevated [CO₂] increased N₂O and CO₂ emission from soil by 60% and 15%, respectively, but had no significant effect on CH₄ flux. There was no significant interaction between [CO₂] and N application rate on these gas fluxes, probably because soil N was not limiting. At least 22% increase in C storage is required to offset the observed increase in greenhouse gas emissions under 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.     

Soil N<Subscript>2</Subscript>O emissions and N<Subscript>2</Subscript>O/(N<Subscript>2</Subscript>O+N<Subscript>2</Subscript>) ratio as affected by different fertilization practices and soil moisture

The objective of this work was to evaluate the effect of the chemical nature and application frequency of N fertilizers at different moisture contents on soil N₂O emissions and N₂O/(N₂O+N₂) ratio. The research was based on five fertilization treatments: unfertilized control, a single application of 80 kg ha⁻¹ N-urea, five split applications of 16 kg ha⁻¹ N-urea, a single application of 80 kg ha⁻¹ N–KNO₃, five split applications of 16 kg ha⁻¹ N–KNO₃. Cumulative N₂O emissions for 22 days were unaffected by fertilization treatments at 32% water-filled pore space (WFPS). At 100% and 120% WFPS, cumulative N₂O emissions were highest from soil fertilized with KNO₃. The split application of N fertilizers decreased N₂O emissions compared to a single initial application only when KNO₃ was applied to a saturated soil, at 100% WFPS. Emissions of N₂O were very low after the application of urea, similar to those found at unfertilized soil. Average N₂O/(N₂O+N₂) ratio values were significantly affected by moisture levels (p = 0.015), being the lowest at 120% WFPS. The N₂O/(N₂O+N₂) ratio averaged 0.2 in unfertilized soil and 0.5 in fertilized soil, although these differences were not statistically significant.     

Relationships between the denitrification capacities of soils and total,water-soluble and readily decomposable soil organic matter

The relationships between the denitrification capacities of 17 surface soils and the amounts of total organic carbon, mineralizable carbon, and water-soluble organic carbon in these soils were investigated. The soils used differed markedly in pH, texture, and organic-matter content. Denitrification capacity was assessed by determining the N evolved as N₂ and N₂O on anaerobic incubation of nitrate-treated soil at 20°C for 7 days, and mineralizable carbon was assessed by determining the C evolved as CO₂ on aerobic incubation of soil at 20°C for 7 days. The denitrification capacities of the soils studied were significantly correlated (r = 0·7771) with total organic carbon and very highly correlated (r = 0·9971) with water-soluble organic carbon or mineralizable carbon. The amount of nitrate N lost on anaerobic incubation of nitrate-treated soils for 7 days was very closely related (r = 0·99971) to the amount of N evolved as N₂ and N₂O.The work reported indicates that denitrification in soils under anaerobic conditions is controlled largely by the supply of readily decomposable organic matter and that analysis of soils for mineralizable carbon or water-soluble organic carbon provides a good index of their capacity for denitrification of nitrate.     

Symbiotic N<Subscript>2</Subscript> fixation and nitrate utilisation in irrigated lucerne (<Emphasis Type="Italic">Medicago sativa</Emphasis>) systems

Symbiotic N₂ fixation by lucerne (Medicago sativa) has capacity to provide significant inputs of N to agro-ecosystems, and the species has also been shown to scavenge soil mineral N and thus act as a sink for excess reactive N. The balance between these two N cycle processes was investigated in an extensive irrigated lucerne growing region where nitrate contamination of groundwater has been reported. We sampled 18 permanent pure lucerne stands under irrigation for standing dry matter, total shoot N, and N₂ fixation using ¹⁵N natural abundance along with activity of the inducible enzyme nitrate reductase as indicators of use of soil NO₃⁻ by lucerne. On average 65% of lucerne N was obtained from symbiotic N₂ fixation. Converting standing dry matter estimates to annual N₂ fixation amounts we calculated average N₂ fixation of 311 kg N/ha, including N in roots and nodules. Uptake of N from soil by lucerne was calculated to be 181 kg N/ha/year. We were not able to identify the source of this soil mineral N, although nitrate reductase activity of lucerne was higher than that of non-N₂ fixing species examined.     

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.     

Elevated carbon dioxide level along with phosphorus application and cyanobacterial inoculation enhances nitrogen fixation and uptake in cowpea crop

Climate change, as a result of increase in the concentration of greenhouse gases, influences growth and productivity of leguminous crops. A study was carried out to analyse the impacts of elevated carbon dioxide (CO₂) and cyanobacterial inoculation on growth, N₂ fixation and N availability and uptake in cowpea crop, under different doses of phosphorus. Cowpea crop was grown under ambient (400 µmol mol^?1) and elevated (550 ± 20 µmol mol^?1) CO₂ levels using Free-Air Carbon dioxide Enrichment facility. Elevated CO₂ level increased chlorophyll content in leaves, improved nodulation and nitrogen fixation by the crop. Increase in P dose up to 16 mg kg^?1 soil enhanced nodule development and N₂ fixation under high CO₂ condition. Cyanobacterial inoculation increased nodule weight, leghaemoglobin content in nodules and total nitrogenase activity. Although nitrogen concentration in cowpea seeds decreased in high CO₂ treatment, higher N uptake was recorded. Under elevated CO₂ condition, cyanobacterial inoculation and higher P doses led to enhanced root growth and N₂ fixation and availability of soil nitrogen. The study illustrated the synergistic effect of high CO₂ and cyanobacterial inoculation in enhancing crop growth and availability of soil N, mediated by biological N₂ fixation in cowpea under different levels of P.     

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

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

Spatial variability and biophysicochemical controls on N<Subscript>2</Subscript>O emissions from differently tilled arable soils

Nitrous oxide (N₂O) emissions, soil microbial community structure, bulk density, total pore volume, total C and N, aggregate mean weight diameter and stability index were determined in arable soils under three different types of tillage: reduced tillage (RT), no tillage (NT) and conventional tillage (CT). Thirty intact soil cores, each in a 25 × 25-m² grid, were collected to a depth of 10 cm at the seedling stage of winter wheat in February 2008 from Maulde (50°3′ N, 3°43′ W), Belgium. Two additional soil samples adjacent to each soil core were taken to measure the spatial variance in biotic and physicochemical conditions. The microbial community structure was evaluated by means of phospholipid fatty acids analysis. Soil cores were amended with 15 kg NO₃⁻-N ha⁻¹, 15 kg NH₄⁺-N ha⁻¹ and 30 kg ha⁻¹ urea-N ha⁻¹ and then brought to 65% water-filled pore space and incubated for 21 days at 15°C, with regular monitoring of N₂O emissions. The N₂O fluxes showed a log-normal distribution with mean coefficients of variance (CV) of 122%, 78% and 90% in RT, NT and CT, respectively, indicating a high spatial variation. However, this variability of N₂O emissions did not show plot scale spatial dependence. The N₂O emissions from RT were higher (p < 0.01) than from CT and NT. Multivariate analysis of soil properties showed that PC1 of principal component analysis had highest loadings for aggregate mean weight diameter, total C and fungi/bacteria ratio. Stepwise multiple regression based on soil properties explained 72% (p < 0.01) of the variance of N₂O emissions. Spatial distributions of soil properties controlling N₂O emissions were different in three different tillages with CV ranked as RT > CT > NT.     

Nitrogen rhizodeposition of young wheat plants under elevated CO<Subscript>2</Subscript> and drought stress

The objective of this study was to determine the effect of drought stress and elevated CO₂ concentrations around the shoots on N rhizodeposition of young wheat plants. In a pot experiment, the plant N pool was labeled through ¹⁵NH₃ application to shoots at nontoxic NH₃ concentrations, and the impact of low water supply (40% field capacity), elevated CO₂ (720 μmol mol⁻¹ CO₂), and the combination of both factors on the ¹⁵N distribution was studied. Total ¹⁵N rhizodeposition ranged from 5 to 11% of the total ¹⁵N recovered in the plant/soil system. Elevated CO₂ concentration as well as drought stress increased the belowground transport of N and increased the relative portion of N rhizodeposition on total ¹⁵N in the plant/soil system. However, while the increased N rhizodeposition with elevated CO₂ was the result of increased total belowground N transport, drought stress additionally increased the portion of ¹⁵N found in rhizodeposition vs roots. Elevated CO₂ intensified the effect of drought stress. The percentage of water soluble ¹⁵N in the ¹⁵N rhizodeposition was very low under all treatments, and it was significantly decreased by the drought-stressed treatments.     

Methane production in a flooded soil in response to elevated atmospheric carbon dioxide concentrations

 CH₄ production in a flooded soil as affected by elevated atmospheric CO₂ was quantified in a laboratory incubation study. CH₄ production in the flooded soil increased by 19.6%, 28.2%, and 33.4% after a 2-week incubation and by 38.2%, 62.4%, and 43.0% after a 3-week incubation under atmospheres of 498, 820, and 1050 μl l^–1 CO₂, respectively, over that in soil under the ambient CO₂ concentration. CH₄ production in slurry under 690, 920, and 1150 μl l^–1 CO₂ increased by 2.7%, 5.5%, and 5.0%, respectively, after a 3-day incubation, and by 6.7%, 12.8%, and 5.4%, respectively, after a 6-day incubation over that in slurry under the ambient CO₂ concentration. The increase in CH₄ production in the soil slurry under elevated CO₂ concentrations in a N₂ atmosphere was more pronounced than that under elevated CO₂ concentrations in air. These data suggested that elevated atmospheric CO₂ concentrations could promote methanogenic activity in flooded soil. Received: 2 March 1998     

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)     

Increases in soil CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O emissions with warming depend on plant species in restored alpine meadows of Wugong Mountain,China

Purpose

Ecosystem restorations can impact carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions which are important greenhouse gasses. Alpine meadows are degraded worldwide, but restorations are increasing. Because their soils represent large carbon (C) and nitrogen (N) pools, they may produce significant amounts of CO₂ and N₂O depending on the plant species used in restorations. In addition, warming and N deposition may impact soil CO₂ and N₂O emissions from restored meadows.

Materials and methods

We collected soils from degraded meadows and plots restored using three different plant species at Wugong Mountain (Jiangxi, China). We measured CO₂ and N₂O emissions when soils were incubated at different temperatures (15, 25 or 35 °C) and levels of N addition (control vs. 4 g m^?2) to understand their responses to warming and N deposition.

Results and discussion

Dissolved organic C was higher in restored plots (especially with Fimbristylis dichotoma) compared to non-restored bare soils, and their soil inorganic N was lower. CO₂ emission rates were increased by vegetation restorations, decreased by N deposition, and increased by warming. CO₂ emission rates were similar for the three grass species at 15 and 25 °C, but they were lower with Miscanthus floridulus at 35 °C. Soils from F. dichotoma and Carex chinensis plots had higher N₂O emissions than degraded or M. floridulus plots, especially at 25 °C.

Conclusions

These results show that the effects of restorations on soil greenhouse gas emissions depended on plant species. In addition, these differences varied with temperature suggesting that future climate should be considered when choosing plant species in restorations to predict soil CO₂ and N₂O emissions and global warming potential.

     

Effect of the temperature and moisture on the N<Subscript>2</Subscript>O emission from some arable soils

The effect of the temperature and moisture on the emission of N₂O from arable soils was studied in model experiments with arable soils at three contrasting levels of wetting and in a wide temperature range (from −5 to +25°C), including freeze-thaw cycles. It was shown that the losses of fertilizer nitrogen from the soils with water contents corresponding to 60 and 75% of the total water capacity (TWC) did not exceed 0.01–0.09% in the entire temperature range. In the soils with an elevated water content (90% of the TWC) at 25°C, the loss of fertilizer nitrogen in the form of N₂O reached 2.35% because of the active denitrification. The extra N₂O flux initiated by the freeze-thaw processes made up 88–98% of the total nitrous oxide flux during the entire experiment.     

Influence of carbon substrates and moisture regime on nitrogen fixation in paddy soils

The influence of several carbon sources on heterotrophic N₂ fixation in four paddy soils under flooded and nonflooded conditions was investigated by ¹⁵N-tracer technique. Greater N₂ fixation occurred in submerged soils amended with cellulose and rice straw, the former being superior. Addition of sucrose, glucose and malate in that order stimulated N₂ fixation in submerged alluvial soil, while sucrose alone enhanced N₃ fixation in laterite soil. In submerged acid soils none of these C sources stimulated N₂ fixation. Nonflooded conditions favoured N₂ fixation in alluvial and acid saline soils amended with cellulose, sucrose and glucose.     

Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes

The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecology of agricultural soil. Carbon dioxide (CO₂) and nitrous oxide (N₂O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs (“organic” management) or one of low OM and inorganic fertilizer inputs (“conventional” management). In microcosms, where short-term microbial responses to wetting and drying were studied, the highest CO₂ efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient availability were elevated and a PLFA stress indicator was depressed, suggesting peak microbial activity. The highest N₂O efflux in the organically managed soil (0.94 mg N₂O-N m⁻² h⁻¹) occurred after manure and legume cover crop incorporation, and in the conventionally managed soil (2.12 mg N₂O-N m⁻² h⁻¹) after inorganic N fertilizer inputs. Elevated N₂O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0–150 mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N₂O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.     

Effect of salt stress on interaction between lentil (Lens culinaris) genotypes and Rhizobium spp. strains: symbiotic N2 fixation in normal and sodic soils

 Following screening, selection, characterization and examination of their symbiotic N₂ fixation, only two Rhizobium strains (ND-16 and TAL-1860) and four lentil genotypes (DLG-103, LC-50, LC-53 and Sehore 74-3) were found to be suited to sodic soils. Interactions between salt-tolerant lentil genotypes and Rhizobium strains were found to be significant, and resulted in greater nodulation, N₂ fixation (nitrogenase activity), total nitrogen, plant height, root length and grain yield in sodic soils under field conditions compared to uninoculated controls. Significantly more nodulation, nitrogenase activity, glutamine synthetase (GS) and NADH-dependent glutamate synthase (NADH-GOGAT) activities were found in normal soil as compared to the soil supplemented with 4% and 8% NaCl. Salt stress inhibited nitrogenase, GS and NADH-GOGAT activities. However, nitrogenase activity in nodules was more sensitive to salt stress than GS and NADH-GOGAT activities (NH₄ ⁺ assimilation). The relevance of these findings for salt-tolerant symbionts is discussed. Received: 14 November 1997     

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.