Microbial responses to fluctuation of soil aeration status and redox conditions

 The response of the microbial community to changes in aeration status, from oxic to anoxic and from anoxic to oxic, was determined in arable soil incubated in a continuous flow incubation apparatus. Soil incubated in permanently oxic (air) and/or anoxic (O₂-free N₂) conditions was used as the control. Before experiments soil was preincubated for 6 days, then aeration status was changed and glucose added. Glucose concentration, extractable C, CO₂ production, microbial biomass, pH and redox potential were determined 0, 4, 8, 12, 16, 24, 36 and 48 h after change of aeration status. If oxic conditions were changed to anoxic, the amount of glucose consumed was reduced by about 60%, and CO₂ production was 10 times lower at the end of incubation compared to the control (permanently oxic conditions). Microbial biomass increased by 114% in glucose-amended soil but did not change in unamended soil. C immobilization prevailed over C mineralization. Redox potential decreased from +627 mV to –306 mV. If anoxic conditions were changed to oxic, consumption of glucose and CO₂ evolution significantly increased, compared to permanently anoxic conditions. Microbial biomass did not change in glucose-amended soil, but decreased by 78% in unamended soil. C mineralization was accelerated. Redox potential increased from +238 to +541 mV. The rate of glucose consumption was low in anoxic conditions if soil was incubated in pure N₂ but increased significantly when incubation was carried out in a CO₂/N₂ mixture. Received: 6 January 1999     

Short-term decomposition of <Superscript>14</Superscript>C-labelled glucose in a fluvo-aquic soil as affected by lanthanum amendment

In this study, we investigated the effects of lanthanum (La), one of the rare earth elements (REEs), on microbial biomass C as well as the decomposition of ¹⁴C-labelled glucose in a fluvo-aquic soil in 28 days. The soil was collected from the field plots under maize/wheat rotation in Fengqiu Ecological Experimental Station of Chinese Academy of Sciences, Henan Province, China. Application of La decreased soil microbial biomass C during the experimental period, and there was a negative correlation (P < 0.01) between microbial biomass and application rate of La. La increased microbial biomass ¹⁴C after ¹⁴C glucose addition, and the increase was significant (P < 0.05) at the rates of more than 160 mg kg⁻¹ soil. La slightly increased ¹⁴CO₂ evolution at lower rates of application but decreased it at higher rates 1 day after ¹⁴C glucose addition, while there was no significant effect from days 2 to 28. For the cumulative ¹⁴CO₂ evolution during the incubation of 28 days, La slightly increased it at the rates of less than 120 mg kg⁻¹ soil, while significantly decreased (P < 0.05) it at the rate of 200 mg kg⁻¹ soil. The results indicated that agricultural use of REEs such as La in soil could decrease the amount of soil microbial biomass and change the pattern of microbial utilization on glucose C source in a short period.     

Carbon turnover in the rhizosphere under continuous plant labeling with <Superscript>13</Superscript>CO<Subscript>2</Subscript>: Partitioning of root,microbial, and rhizomicrobial respiration

The input of labeled C into the pool of soil organic matter, the CO₂ fluxes from the soil, and the contribution of root and microbial respiration to the CO₂ emission were studied in a greenhouse experiment with continuous labeling of oat plants with ¹³CO₂ using the method of the natural ¹³C abundance in the air. The carbon of the microbial biomass composed 56 and 39% of the total amounts of ¹³C photoassimilates in the rhizosphere and in the bulk soil, respectively. The contribution of root respiration to the CO₂ emission from the soil reached 61–92%, including 4–23% of the rhizomicrobial respiration. The contribution of the microbial respiration to the total CO₂ emission from the soil varied from 8 to 39%. The soil organic matter served as the major carbon-containing substrate for microorganisms in the bulk soil and in the rhizosphere: 81–91% of the total amount of carbon involved in the microbial metabolism was derived from the soil organic matter.     

Mineralisation of <Superscript>15</Superscript>N-labelled sheep manure in soils of different texture and water contents

In order to investigate the effect of soil water and texture on C and N mineralisation of applied organic matter, sheep manure was sandwiched between two halves of intact soil cores and incubated at 20°C. The soils contained 10.8% (L1), 22.4% (L3) and 33.7% (L5) clay, respectively, and were drained to seven different matric potentials in the range -15 to -1,500 hPa. Evolution of CO₂-C was determined during 4 weeks of incubation. Contents of NO₃^--N, ¹⁵N and microbial biomass N were determined at the end of the incubation. The net release of CO₂-C from the manure (estimated as the difference between soils with and without manure) and the total CO₂-C evolution from soils with manure was not related to soil water content. Most CO₂-C evolved from manure-amended soils in the least clayey L1 soil. The manure caused immobilisation of soil NO₃^--N but the soil matric potential had no major effects on the net NO₃^--N production. Less than 1% of the manure ¹⁵N was found as NO₃^--N at the end of the incubation. When unamended, the sandy L1 soil held the least N in microbial biomass but the largest increases in biomass N caused by manure application were found in this soil. Despite the higher increases in microbial biomass N in the L1 soil, the total content of microbial biomass N in soils with manure application peaked in the most clayey soil (L5). The recovery of manure ¹⁵N at the end of the incubation ranged from 89% to 102%. The variation in ¹⁵N recovery was not related to soil clay content nor to soil matric potential. The experimental set-up was designed to mimic field conditions where manure is left as a discrete layer surrounded by structurally intact soil. In this situation the soil clay content and the soil water level appeared to have little influence on the C and N turnover in the manure layer.     

Abbau von Huminstoffen durch Bodenmikroorganismen — eine Übersicht

Degradation of humic substances by soil microorganisms — a review Humic substances which represent differently extractable fractions of the soil organic matter exert multifarious effects on soil as a site for plant growth and a part of terrestrial environments. Among them especially humic acids and fulvic acids are subject to degradation and/or transformation by soil microorganisms. Several authors demonstrated the participation of different species of fungi, actinomycetes and also of non-mycelial aerobic or anaerobic bacteria in those processes under laboratory conditions. Indications exist that humic substances irrespective of their structure undergo degradation on cell surfaces due to the activity of exoenzymes. In this respect microbial phenoloxidases play an extraordinarily important role. The degradation rate of humic substances can be followed by optical, gravimetric and chemoanalytical methods as well as using biochemical and microbiological procedures (CO₂ release, microbial growth, biomass formation). An objective evaluation, however, can be hindered by the adsorption of humic substances on microbial biomass and sometimes also by formation of novel humic-like microbial metabolites. Therefore it is necessary to apply a multifactorial approach in the study of the degradation of humic substances which includes both quantitative and qualitative parameters. To better elucidate how these processes may occur under natural conditions, mixed populations of soil microorganisms should be predominantly involved in future studies.     

Short-term uptake of <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript> into soil microbes and seedlings of pine,spruce and birch in potted soils

Short-term competition between soil microbes and seedlings of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and silver birch (Betula pendula Roth) for N was assessed in a pot study using (¹⁵NH₄)₂SO₄ as a tracer. Seedlings were grown in organic and mineral soil, collected from a podsol soil; 3.18 mg (¹⁵NH₄)₂SO₄ per pot were injected into the soil, corresponding to 4 µg ¹⁵N g^-1 d.m. (dry matter) mineral soil and 17 µg ¹⁵N g^-1 d.m. organic soil. The amounts of N and ¹⁵N in the seedlings and in microbial biomass derived from fumigation-extraction were measured 48 h after addition of ¹⁵N. In the mineral soil, 19–30% of the added ¹⁵N was found in the plants and 14–20% in the microbial biomass. There were no statistically significant differences between the tree species. In the organic soil, 74% of the added ¹⁵N was recovered in the microbial biomass in birch soil, compared to 26% and 17% in pine and spruce soils, respectively. Correspondingly, about 70% of the ¹⁵N was recovered in pine and spruce seedlings, and only 23% in birch seedlings. In conclusion, plants generally competed more successfully for added ¹⁵NH₄ ⁺ than soil microbes did. An exception was birch growing in organic soil, where the greater amount of available C from birch root exudates perhaps enabled micro-organisms to utilise more N.     

Mid-term tracing of <Superscript>15</Superscript>N derived from urine and dung in soil microbial biomass

Large amounts of C and N are returned to pasture soils by grazing animals in the form of urine and dung. Therefore, a field trial was carried out to investigate the mid-term effects of ¹⁵N-labeled excrements, produced by feeding a cow with ¹⁵N-labeled grass silage, on the soil microbial biomass. Simulating the deposition of excrements, ¹⁵N-labeled urine and dung were applied to a 0.09-m2 area of a sandy pasture soil in October 2000 and 2001. Applied amounts of N were 1,030 and 1,052 kg ha⁻¹, respectively. Soil was sampled at 0–15 cm depth, three times over 7 months and analyzed for total C and N, and microbial biomass C and N. Recovery of urine and dung N in microbial biomass was determined by ¹⁵N analysis of K₂SO₄ extracts of pre-extracted fumigated and unfumigated soils. Under dung patches, microbial biomass C was 16% and 45% higher, and microbial biomass N was 24% and 57% higher than under the untreated soil in 2001 and 2002, respectively. Under urine patches, microbial biomass C was increased after 12 weeks and decreased after 27 weeks. Microbial biomass assimilated 7% to 17% and 10% to 21% of the ¹⁵N applied initially as urine and dung, respectively. These percentages were considerably higher than those for artificially with spiked ¹⁵N urea-created and labeled manures reported in previous experiments. An important reason may be that the naturally ¹⁵N-labeled N components behave differently in soil than urea spikes.     

Efficiency of acetylene reduction (nitrogen fixation) in soil: Effect of type and concentration of available carbohydrate

Sandy loam field soil and Acer saccharum (maple) forest soil were amended with different concentrations of glucose and mannitol and incubated at different pO₂ levels. Nitrogenase activity was determined by repeated 1-h C₂H₂ reduction assays performed at the ambient pO₂ of incubation. Calculated efficiencies of N₂ fixation increased with increasing anaerobiosis and with decreasing added carbohydrate concentration. Efficiencies up to 30 mg N₂ fixed per gram of glucose consumed were obtained under anaerobic conditions in the presence of 0.25% (w/w) glucose. Evidence suggested that low aerobic efficiencies were caused by intense competition for carbohydrate and by lower pH values attained. High concentrations (up to 3.0% w/w) of glucose under aerobic conditions suppressed the development of N₂ase activity. Mannitol supported N₂ase activity the development of which was very much delayed under aerobic conditions but little delayed under anaerobic conditions.     

Evaluating the effect of liming on N<Subscript>2</Subscript>O fluxes from denitrification in an Andosol using the acetylene inhibition and <Superscript>15</Superscript>N isotope tracer methods

We assessed the effect of liming on (1) N₂O production by denitrification under aerobic conditions using the ¹⁵N tracer method (experiment 1); and (2) the reduction of N₂O to N₂ under anaerobic conditions using the acetylene inhibition method (experiment 2). A Mollic Andosol with three lime treatments (unlimed soil, 4 and 20 mg CaCO₃ kg^?1) was incubated at 15 and 25 °C for 22 days at 50% and then 80% WFPS with or without 200 mg N kg^?1 added as ¹⁵N enriched KNO₃ in experiment 1. In experiment 2, the limed and unlimed soils were incubated under completely anaerobic conditions for 44 h (with or without 100 mg N kg^?1 as KNO₃). In experiment 1, limed treatments increased N₂O fluxes at 50% WFPS but decreased these fluxes at 80% WFPS. At 25 °C, cumulative N₂O and ¹⁵N₂O emissions in the high lime treatment were the lowest (with at least 30% less ¹⁵N₂O and total N₂O than the unlimed soil). Under anaerobic conditions, the high lime treatment showed at least 50% less N₂O than the unlimed treatment at both temperatures with or without KNO₃ addition but showed enhanced N₂ production. Our results suggest that the positive effect of liming on the mitigation of N₂O evolution from soil was influenced by soil temperature and moisture conditions.     

Effects of transient anaerobic conditions in the presence of acetylene on subsequent aerobic respiration and N2O emission by soil aggregates

Our objective was to assess the effect of anaerobic conditioning in the presence of acetylene on subsequent aerobic respiration and N₂O emission at the scale of soil aggregates. Nitrous oxide production was measured in intact soil aggregates Δ (compacted aggregates without visible porosity) and Γ (aggregates with visible porosity) incubated under oxic conditions, with or without anaerobic conditioning for 6 d. N₂O emissions were much higher in aggregates that had been submitted to anaerobic conditioning than in aggregates that did not experience this conditioning, although very little NO₃⁻ remained in soil after the anaerobic period. ¹⁵N isotope tracing technique was used to check whether N₂O came from nitrification or denitrification. The results showed that denitrification was the major process responsible for N₂O emissions. The aerobic CO₂ production rate was also measured in intact soil aggregates. It was greater in aggregates submitted to anaerobic conditioning than in those that were not, suggesting that the anaerobic conditioning lead to an accumulation of small compounds including fatty acids that are readily available for microbial decomposition in aerobic conditions. This process increases the aerobic CO₂ production and favours the N₂O emissions through denitrification.     

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.     

Effect of the herbicide glyphosate on respiration and hydrogen consumption in soil

The effect of glyphosate on soil respiration and Hz oxidation in an agricultural soil was investigated. The effects of the pure herbicide and commercial formulation, Roundup® (Monsanto Company), were compared in soil under both aerobic and anaerobic conditions. Both formulations stimulated O₂ uptake as well as aerobic and anaerobic CO₂ evolution. Roundup caused more stimulation than glyphosate under aerobic incubation conditions; the formulations had an equal effect on anaerobic CO₂ evolution. Hydrogen oxidation was inhibited by both formulations in aerobic and anaerobic soil. Aerobic H₂ oxidation was inhibited to the same extent by both formulations; Roundup had a stronger inhibitory effect on anaerobic H₂ oxidation than did glyphosate.     

Effects of carbon and phosphorus addition on microbial respiration,N<Subscript>2</Subscript>O emission,and gross nitrogen mineralization in a phosphorus-limited grassland soil

Soil microbes are frequently limited by carbon (C), but also have a high phosphorus (P) requirement. Little is known about the effect of P availability relative to the availability of C on soil microbial activity. In two separate experiments, we assessed the effect of P addition (20 mg P kg^?1 soil) with and without glucose addition (500 mg C kg^?1 soil) on gross nitrogen (N) mineralization (¹⁵N pool dilution method), microbial respiration, and nitrous oxide (N₂O) emission in a grassland soil. In the first experiment, soils were incubated for 13 days at 90% water holding capacity (WHC) with addition of NO₃^? (99 mg N kg^?1 soil) to support denitrification. Addition of C and P had no effect on gross N mineralization. Initially, N₂O emission significantly increased with glucose, but it decreased at later stages of the incubation, suggesting a shift from C to NO₃^? limitation of denitrifiers. P addition increased the N₂O/CO₂ ratio without glucose but decreased it with glucose addition. Furthermore, the ¹⁵N recovery was lowest with glucose and without P addition, suggesting a glucose by P interaction on the denitrifying community. In the second experiment, soils were incubated for 2 days at 75% WHC without N addition. Glucose addition increased soil ¹⁵N recovery, but had no effect on gross N mineralization. Possibly, glucose addition increased short-term microbial N immobilization, thereby reducing N-substrates for nitrification and denitrification under more aerobic conditions. Our results indicate that both C and P affect N transformations in this grassland soil.     

Interactive effects of biochar and polyacrylamide on decomposition of maize rhizodeposits: implications from <Superscript>14</Superscript>C labeling and microbial metabolic quotient

Purpose

The applications of biochar (BC) and polyacrylamide (PAM) may have interactive effects on carbon (C) dynamics and sequestration for improving the soil quality and achieving sustainable agriculture. Relative to BC and PAM, rhizodeposits act as C and energy source for microorganisms and may change the mineralization dynamics of soil organic matter (SOM). No attempt has been made to assess the effects of BC, anionic PAM, or their combination on the decomposition of different aged ¹⁴C-labeled rhizodeposits. The objective of this study was to investigate the effects of the treatments mentioned above on the decomposition of different aged ¹⁴C-labeled maize rhizodeposits.

Materials and methods

biochar (BC) at 10 Mg ha^?1 or anionic PAM at 80 kg ha^?1 or their combination (BC + PAM) was applied to soils with/without 2-, 4-, 8-, and 16-day-aged ¹⁴C-labeled maize rhizodeposits. After that, the soil was incubated at 22 °C for 46 days.

Results and discussion

After 2 days of incubation, the total CO₂ efflux rates from the soil with rhizodeposits were 1.4–1.8 times higher than those from the soil without rhizodeposits. The cumulative ¹⁴CO₂ efflux (32 % of the ¹⁴C input) was maximal for the soil containing 2-day-aged ¹⁴C-labeled rhizodeposits. Consequently, 2-day-aged rhizodeposits were more easily and rapidly decomposed than the older rhizodeposits. However, no differences in the total respired ¹⁴CO₂ from rhizodeposits were observed at the end of the incubation. Incorporation of ¹⁴C into microbial biomass and 66–85 % of the ¹⁴C input remained in the soil after 46 days indicated that neither the age of ¹⁴C-labeled rhizodeposits nor BC, PAM, or BC + PAM changed microbial utilization of rhizodeposits.

Conclusions

Applying BC or BC + PAM to soil exerted only minor effects on the decomposition of rhizodeposits. The contribution of rhizodeposits to CO₂ efflux from soil and MBC depends on their age as young rhizodeposits contain more labile C, which is easily available for microbial uptake and utilization.

     

Effect of soil CO2 concentration on microbial biomass

The effect of increasing soil CO₂ concentration was studied in six different soils. The soils were incubated in ambient air (0.05 vol.% CO₂) or in air enriched with CO₂ (up to 5.0 vol.% CO₂). Carbon dioxide evolution, microbial biomass, growth or death rate quotients and glucose decay rate were measured at 6, 12 and 24 h of CO₂ exposure. The decrease in soil respiration ranged from 7% to 78% and was followed by a decrease in microbial biomass by 10–60% in most cases. High CO₂ treatments did not affect glucose decay rate but the portion of C_gluc mineralized to CO₂ was lowered and a larger portion of C_gluc remained in soils. This carbon was not utilized by soil microorganisms. Received: 30 August 1996     

Effects of biochar and polyacrylamide on decomposition of soil organic matter and <Superscript>14</Superscript>C-labeled alfalfa residues

Purpose

Various soil conditioners, such as biochar (BC) and anionic polyacrylamide (PAM), improve soil fertility and susceptibility to erosion, and may alter microbial accessibility and decomposition of soil organic matter (SOM) and plant residues. To date, no attempts have been made to study the effects of BC in combination with PAM on the decomposition of soil SOM and plant residues. The objective of this study was to evaluate the effects of BC, PAM, and their combination on the decomposition of SOM and alfalfa residues.

Materials and methods

An 80-day incubation experiment was carried out to investigate the effects of oak wood biochar (BC; 10 Mg ha^?1), PAM (80 kg ha^?1), and their combination (BC?+?PAM) on decomposition of SOM and ¹⁴C-labeled alfalfa (Medicago sativa L.) residues by measuring CO₂ efflux, microbial biomass, and specific respiration activity.

Results and discussion

No conditioner exerted a significant effect on SOM decomposition over the 80 days of incubation. PAM increased cumulative CO₂ efflux at 55–80 days of incubation on average of 6.7 % compared to the soil with plant residue. This was confirmed by the increased MBN and MB¹⁴C at 80 days of incubation in PAM-treated soil with plant residue compared to the control. In contrast, BC and BC?+?PAM decreased plant residue decomposition compared to that in PAM-treated soil and the respective control soil during the 80 days. BC and BC?+?PAM decreased MBC in soil at 2 days of incubation indicated that BC suppressed soil microorganisms and, therefore, decreased the decomposition of plant residue.

Conclusions

The addition of oak wood BC alone or in combination with PAM to soil decreased the decomposition of plant residue.

     

Criteria for measurement of microbial growth and activity in soil

Changes in CO₂ evolution, phosphatase and urease activity and ATP contents were related to bacterial and fungal biomass determined microscopically during glucose mineralization at different concentrations of mineral nutrients. Similar results were obtained in a sandy loam and a clay soil except that in the clay the increase in microbial and enzyme activities were delayed. Higher initial rates of CO₂ evolution were noted after the addition of P to a glucose and N amended soil at C:P ratios greater than 30:1. Increases in phosphatase activity coincided with increases in bacterial and fungal populations only in treatments without inorganic P. Peak rates of CO₂ evolution preceded biomass production by 18–24 h, therefore, CO₂ evolution rates did not show a correlation on normal regression analysis with biomass. Soil ATP content was influenced by P concentrations and soil type. ATP was therefore not a specific indicator of biomass in the detailed studies where P concentrations and sequential growth of bacteria and fungi were major factors. Soil urease increased with bacterial and fungal populations. It did not respond to P other than through microbial biomass and was highly correlated with microbial biomass. The results show that no one measurement of microbial biomass or activity is sufficient to interpret microbial growth in the soil system. Each of the criteria measured were sensitive to specific conditions affecting biomass and activity.     

CO<Subscript>2</Subscript> emission from the surface of dark gray forest soils of the forest steppe and sandy soddy-podzolic soils of the southern taiga

Studies performed on dark gray loamy forest soils in an oak forest in the southern forest steppe and on sandy soddy-podzolic soil in a pine forest in the southern taiga showed that the annual emission of CO₂ from the soil surface in the pine forest was 16.3 t CO₂/ha, including 10.1 t CO₂/ha due to root respiration and 6.2 t CO₂/ha due to soil microbial respiration. In the southern forest steppe, the corresponding values were 17.8 t CO₂/ha due to root respiration at the optimum water content (20%) and 28.3 t CO₂/ha due to soil microbial respiration. With the insufficient soil water content (12.5%), 10.3 and 17.8 t CO₂/ha were due to root respiration and soil microbial respiration, respectively. Under strong drought conditions (water content of 10%), the emission of CO₂ decreased to 8.2 and 16.3 t/ha due to root respiration and soil microbial respiration, respectively.     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

Effect of CO<Subscript>2</Subscript> on nitrification and immobilization of NH<Subscript>4</Subscript><Superscript>+</Superscript>-N

A laboratory incubation experiment was conducted to demonstrate that reduced availability of CO₂ may be an important factor limiting nitrification. Soil samples amended with wheat straw (0%, 0.1% and 0.2%) and (¹⁵NH₄)₂SO₄ (200 mg N kg^–1 soil, 2.213 atom% ¹⁵N excess) were incubated at 30±2°C for 20 days with or without the arrangement for trapping CO₂ resulting from the decomposition of organic matter. Nitrification (as determined by the disappearance of NH₄⁺ and accumulation of NO₃^–) was found to be highly sensitive to available CO₂ decreasing significantly when CO₂ was trapped in alkali solution and increasing substantially when the amount of CO₂ in the soil atmosphere increased due to the decomposition of added wheat straw. The co-efficient of correlation between NH₄⁺-N and NO₃^–-N content of soil was highly significant (r =0.99). During incubation, 0.1–78% of the applied NH₄⁺ was recovered as NO₃^– at different incubation intervals. Amendment of soil with wheat straw significantly increased NH₄⁺ immobilization. From 1.6% to 4.5% of the applied N was unaccounted for and was due to N losses. The results of the study suggest that decreased availability of CO₂ will limit the process of nitrification during soil incubations involving trapping of CO₂ (in closed vessels) or its removal from the stream of air passing over the incubated soil (in open-ended systems).