Modeling soil metabolic processes using isotopologue pairs of position-specific C-labeled glucose and pyruvate

Most organic carbon (C) in soils eventually turns into CO₂ after passing through microbial metabolic pathways, while providing cells with energy and biosynthetic precursors. Therefore, detailed insight into these metabolic processes may help elucidate mechanisms of soil C cycling processes. Here, we describe a modeling approach to quantify the C flux through metabolic pathways by adding 1-¹³C and 2,3-¹³C pyruvate and 1-¹³C and U-¹³C glucose as metabolic tracers to intact soil microbial communities. The model calculates, assuming steady-state conditions and glucose as the only substrate, the reaction rates through glycolysis, Krebs cycle, pentose phosphate pathway, anaplerotic activity through pyruvate carboxylase, and various biosynthesis reactions. The model assumes a known and constant microbial proportional precursor demand, estimated from literature data. The model is parameterized with experimentally determined ratios of ¹³CO₂ production from pyruvate and glucose isotopologue pairs. Model sensitivity analysis shows that metabolic flux patterns are especially responsive to changes in experimentally determined ¹³CO₂ ratios from pyruvate and glucose. Calculated fluxes are far less sensitive to assumptions concerning microbial chemical and community composition. The calculated metabolic flux pattern for a young volcanic soil indicates significant pentose phosphate pathway activity in excess of pentose precursor demand and significant anaplerotic activity. These C flux patterns can be used to calculate C use efficiency, energy production and consumption for growth and maintenance purposes, substrate consumption, nitrogen demand, oxygen consumption, and microbial C isotope composition. The metabolic labeling and modeling methods may improve our ability to study the biochemistry and ecophysiology of intact and undisturbed soil microbial communities.     

Origin of positive δC of emitted CO2 from soils after application of biogas residues

Bioenergy production from renewable organic material is known to be a clean energy source and therefore its use is currently much promoted in many countries. Biogas by-products also called biogas residues (BGR) are rich in partially stable organic carbon and can be used as an organic fertilizer for crop production. However so far, many environmental issues relevant when BGR are applied to agricultural land (soil C sequestration, increased denitrification and nutrient leaching) still have to be studied. Therefore a field experiment was set up to investigate the degradation of BGR and its impact on the decomposition of native soil organic matter based on a natural abundance stable isotope approach. Maize, a C₄ plant has been used as bioenergy crop, therefore the δ¹³C of total C in BGR was −16.0‰_PDB and soil organic matter was mostly derived from C₃ plant based detritus, SOM thus showed a δ¹³C of −28.4‰_PDB. Immediately after BGR application, soil-emitted CO₂ showed unexpectedly high δ¹³C of up to +23.6‰_PDB, which has never been reported earlier. A subsequent laboratory scale experiment confirmed the positive δ¹³C of soil-emitted CO₂ after BGR addition and showed that obviously, the added BGR led to a consumption of dissolved inorganic C in soils. Additionally, it was observed that the δ¹³C of CO₂ driven from inorganic C of BGR (BGR-IC) by acid treatment was +35.6‰_PDB. Therefore, we suggest that also under field conditions the transformation of BGR-IC into CO₂ contributed largely to CO₂ emissions in addition to the decomposition of organic matter, which affected both the amount and the carbon isotope signature of emitted CO₂ in the initial period after BGR application. Positive δ¹³C of inorganic C contained in BGR was attributed to processes with strong fractionation of C isotopes during anaerobic fermentation in the biogas formation process.     

Carbon dynamics determined by natural C abundance in microcosm experiments with soils from long-term maize and rye monocultures

Understanding carbon dynamics in soil is the key to managing soil organic matter. Our objective was to quantify the carbon dynamics in microcosm experiments with soils from long-term rye and maize monocultures using natural ¹³C abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C₃-plant) since 1878 and maize (C₄-plant) since 1961 with and without NPK fertilization from the long-term experiment ‘Ewiger Roggen’ in Halle, Germany, were incubated for 230 days at 8 °C and irrigated with 2 mm 10⁻² M CaCl₂ per day. Younger, C₄-derived and older, C₃-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C_mic) and CO₂ from heterothropic respiration were determined by natural ¹³C abundance. The percentage of maize-derived carbon was highest in CO₂ (42-79%), followed by C_mic (23-46%), DOC (5-30%) and SOC (5-14%) in the surface soils and subsoils of the maize plots. The percentage of maize-derived C was higher for the NPK plot than for the unfertilized plot and higher for the surface soils than for the subsoils. Specific production rates of DOC, CO₂-C and C_mic from the maize-derived SOC were 0.06-0.08% for DOC, 1.6-2.6% for CO₂-C and 1.9-2.7% for C_mic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03-0.05% for DOC, 0.1-0.2% for CO₂-C and 0.3-0.5% for C_mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C₄-derived C_mic and C₄-derived SOC, DOC and CO₂-C (r≥0.90), whereas the relationship between C₃-derived C_mic and C₃-derived SOC, DOC and CO₂-C was not as pronounced (r≤0.67). The results stress the different importance of former (older than 40 years) and recent (younger than 40 years) litter C inputs for the formation of different C pools in the soil.     

Using metabolic tracer techniques to assess the impact of tillage and straw management on microbial carbon use efficiency in soil

Tillage practices and straw management can affect soil microbial activities with consequences for soil organic carbon (C) dynamics. Microorganisms metabolize soil organic C and in doing so gain energy and building blocks for biosynthesis, and release CO₂ to the atmosphere. Insight into the response of microbial metabolic processes and C use efficiency (CUE; microbial C produced per substrate C utilized) to management practices may therefore help to predict long term changes in soil C stocks. In this study, we assessed the effects of reduced (RT) and conventional tillage (CT) on the microbial central C metabolic network, using soil samples from a 12-year-old field experiment in an Irish winter wheat cropping system. Straw was removed from half of the RT and CT plots after harvest or incorporated into the soil in the other half, resulting in four treatment combinations. We added 1-¹³C and 2,3-¹³C pyruvate and 1-¹³C and U-¹³C glucose as metabolic tracer isotopomers to composite soil samples taken at two depths (0–15 cm and 15–30 cm) from each of the treatments and used the rate of position-specific respired ¹³CO₂ to parameterize a metabolic model. Model outcomes were then used to calculate CUE of the microbial community. Whereas the composite samples differed in CUE, the changes were small, with values ranging between 0.757 and 0.783 across treatments and soil depth. Increases in CUE were associated with a reduced tricarboxylic acid cycle and reductive pentose phosphate pathway activity and increased consumption of metabolic intermediates for biosynthesis. Our results suggest that RT and straw incorporation do not substantially affect CUE.     

Energetic eco‐physiology of the soil microbiota in two landscapes of southern and northern Germany

Interactions between microbial communities and organic matter were analyzed for soils from the project regions ’︁Ecosystem Research in the Agricultural Landscape/FAM, Munich’ in southern Germany and ’︁Ecosystem Research in the Bornhöved Lake district’ from northern Germany using ratios between microbial biomass content (C_mic), microbial metabolic quotient (qCO₂) and organic carbon content (C_org). In the agricultural soils in southern Germany, the qCO₂/C_org ratio differed significantly with respect to agricultural management in contrast to ecophysiological C_mic/C_org ratio. In addition, C_mic/C_org ratio decreased from 39 to 21 mg C_mic g^—1 C_org and qCO₂/C_org ratio increased from 72 to 180 mg CO₂‐C g^—1 C_mic h^—1 (g C_org g^—1 soil)^—1 with increasing soil depth. For the upper soil horizons from the landscape in northern Germany the two quotients differed significantly with reference to land use showing highest microbial colonization under grassland and lowest under beech forest. In contrast, C use efficiency was lowest in arable field under maize monoculture and highest in a wet grassland having a high organic C content.     

C fractionation at the root-microorganisms-soil interface: A review and outlook for partitioning studies

Natural variations of the ¹³C/¹²C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural-¹³C-labelling approach, i.e. natural δ¹³C variation after C₃-C₄ vegetation changes. In this review, we focus on ¹³C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C₃ and C₄ plants are ¹³C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO₂ released by root respiration was ¹³C depleted by about −2.1 ± 2.2‰ for C₃ plants and −1.3 ± 2.4‰ for C₄ plants compared to root tissue. However, only a very few studies investigated ¹³C fractionation by root respiration. This urgently calls for further research. In soils developed under C₃ vegetation, the microbial biomass was ¹³C enriched by +1.2 ± 2.6‰ and microbial CO₂ was also ¹³C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of ¹³C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ¹³C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO₂, however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ¹³C differences between microorganisms and CO₂ by an ‘apparent’ ¹³C discrimination. Preferential consumption of easily decomposable substrates and less negative δ¹³C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C₃ soils because, in C₄ soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (? = +1.1‰) and respired lighter C (? = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the ¹³C fractionation in these processes in C₃ soils. Increasing temperature and decreasing precipitation led, however, to increasing δ¹³C of soil C pools.Based on these ¹³C fractionations we developed a number of consequences for C partitioning studies using ¹³C natural abundance. In the framework of standard isotope mixing models, we calculated CO₂ partitioning using the natural-¹³C-labelling approach at a vegetation change from C₃ to C₄ plants assuming a root-derived fraction between 0% and 100% to total soil CO₂. Disregarding any ¹³C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for ¹³C fractionation in the standard deviations of the C₄ source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO₂ pools. Including ¹³C fractionations directly into the mass balance equations reproduced the assumed CO₂ partitioning exactly. At the end, we therefore give recommendations on how to consider ¹³C fractionations in research on carbon flows between plants, microorganisms, and soil.     

Determination of the fate of C labelled maize and wheat rhizodeposit-C in two agricultural soils in a greenhouse experiment under C-CO2-enriched atmosphere

A deeper understanding of the contribution of carbon (C) released by plant roots (rhizodeposition) to soil organic matter (SOM) can help to increase our knowledge of global C-cycling. These insights can eventually lead to sustainable management of SOM especially in agricultural systems. This study was conducted to determine the fate of ¹³C labelled rhizodeposit-C of maize and wheat plants. They were grown in a greenhouse in permeable nylon bags filled with upper soil material from two agricultural soils of the same location, but with different crop yields. The bags were placed into pots, which were also filled with soil surrounding the bags. Soil inside the bags was considered as rhizosphere soil, wheras the one outside the bags represented bulk soil. The contributions of rhizodeposits to water extractable organic carbon (WEOC), microbial biomass-C (MB-C), CO₂-C evolution, and total organic carbon (C_org) were investigated during a 7-week growing period. The WEOC, MB-C, CO₂-C, C_org contents and the respective δ¹³C values were determined regularly, and a newly developed method for determining δ¹³C values in soil extracts was applied.In both soils, regardless of crop yield potential, significant incorporation of rhizodeposition-derived C was observed in the MB-C, CO₂-C, and C_org pool, but not in the WEOC. The pattern of C incorporation into the different pools was the same for both soils with both plants, and rhizodeposit-derived C was recovered in the order MB-C<C_org<CO₂-C. This showed that rhizodeposits were mainly respired, but since C_org was the second largest pool of the overall balances, they were also stabilized in the soils at least in the short term. It is suggested that the increased SOM mineralization observed in this study (positive priming effects) was probably induced by C exchange processes between the soil matrix and soluble rhizodeposits. Moreover, soluble rhizodeposit-C was detected in MB-C and CO₂-C evolved outside the direct root zone, showing the availability of these C-components in the bulk soil.     

Carbon decomposition in broiler litter-amended soils

Organic carbon (OC) is generally low in Alabama (U.S.A.) soils and varies considerably with cropping systems. Information on decomposition rates of the added C is a prerequisite to designing strategies that improve C sequestration in farming systems. Different models including exponential models have been used to describe OC mineralization in soils as well as to describe its potential as CO₂ to be released into the environment. We investigated the decomposition of broiler litter added to ten non-calcareous soils (Appling, Troup, Cecil, Decatur, Sucarnoochee, Linker, Hartsells, Dothan, Maytag, and Colbert soils). A non-linear regression approach for N mineralization was used to estimate the potentially mineralizable OC pools (C_o) and the first-order rate constant (k) in the soil samples. Results showed that the non-amended soils have distinct differences in their ability to release their native OC as CO₂ and can be divided into four groups depending on their potentially mineralizable C (C_o) and their ability to protect stable organic matter. Sucarnoochee soil represents the first group and contains a moderate amount of OC (11.4 g C kg⁻¹) but had the highest C_o (7.30 g C kg⁻¹ soil). The second distinct group of soils has C_o varying between 5.50 and 5.00 g C kg⁻¹ soil (Decatur, Hartsells, Dothan, and Maytag). The third group has C_o between 5.00 and 4.00 (Appling, Cecil, and Linker). The fourth group has C_o less than 4.00 g C kg⁻¹ soil (Troup and Colbert). Half-life of C remaining in non-amended soils varied from 26 days in Maytag soil to 139 days in Cecil soil. The OC in these non-amended soils represents a very stable form of organic C and thus, not easily decomposed by soil microorganisms. In the broiler litter-amended soils, the C_o varied from 3.82 g C kg⁻¹ in Appling soil amended with broiler litter 1-7.04 g C kg⁻¹ soil in Maytag amended with broiler litter 2. Decomposition of the added OC proceeded in two phases with less than 31% decomposed in 43 days. Potentially mineralizable organic C (C_o) was related to soil organic C (r = 0.661**) and soil C/N ratio (r = 0.819*).     

Effluxed CO2-C from sterilized and unsterilized treatments of a calcareous soil

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO₂-¹³C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO₂. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl₂ addition for Oklahoma soil and unbuffered HgCl₂ addition, buffered HgCl₂ addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO₂ flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO₂ production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO₂-¹³C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO₂-¹³C values in Mojave Desert soil HgCl₂ sterilization treatments (both buffered and non-buffered). Plots of 1/CO₂ versus CO₂-δ¹³C (similar to Keeling plots) indicated that the source CO₂-δ¹³C value of the Oklahoma Prairie soil treatments was similar to the δ¹³C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl₂ sterilized Mojave Desert soil was similar to the δ¹³C value of carbonates (−0.93‰ VPDB). The source CO₂-δ¹³C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.     

The Origin of Soil Organic C,Dissolved Organic C and Respiration in a Long‐Term Maize Experiment in Halle,Germany, Determined by 13C Natural Abundance

For a quantitative analysis of SOC dynamics it is necessary to trace the origins of the soil organic compounds and the pathways of their transformations. We used the ¹³C isotope to determine the incorporation of maize residues into the soil organic carbon (SOC), to trace the origin of the dissolved organic carbon (DOC), and to quantify the fraction of the maize C in the soil respiration. The maize‐derived SOC was quantified in soil samples collected to a depth of 65 cm from two plots, one ’︁continuous maize’ and the other ’︁continuous rye’ (reference site) from the long‐term field experiment ’︁Ewiger Roggen’ in Halle. This field trial was established in 1878 and was partly changed to a continuous maize cropping system in 1961. Production rates and δ¹³C of DOC and CO₂ were determined for the Ap horizon in incubation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the topsoil originated from maize C. The fraction of the maize‐derived C below the ploughed horizon was only 5 to 3%. The total amount of maize C stored in the profile was 9080 kg ha⁻¹ which was equal to about 31% of the estimated total C input via maize residues (roots and stubble). Total leaching of DOC during the incubation period of 16 weeks was 1.1 g m⁻² and one third of the DOC derived from maize C. The specific DOC production rate from the maize‐derived SOC was 2.5 times higher than that from the older humus formed by C₃ plants. The total CO₂‐C emission for 16 weeks was 18 g m⁻². Fifty‐eight percent of the soil respiration originated from maize C. The specific CO₂ formation from maize‐derived SOC was 8 times higher than that from the older SOC formed by C₃ plants. The ratio of DOC production to CO₂‐C production was three times smaller for the young, maize‐derived SOC than for the older humus formed by C₃ plants.     

Nitrous oxide flux from soil amino acid mineralization

Nitrous oxide (N₂O) is a greenhouse gas produced during microbial transformation of soil N that has been implicated in global climate warming. Nitrous oxide efflux from N fertilized soils has been modeled using NO₃⁻ content with a limited success, but predicting N₂O production in non-fertilized soils has proven to be much more complex. The present study investigates the contribution of soil amino acid (AA) mineralization to N₂O flux from semi-arid soils. In laboratory incubations (−34 kPa moisture potential), soil mineralization of eleven AAs (100 μg AA-N g⁻¹ soil) promoted a wide range in the production of N₂O (156.0±79.3 ng N₂O-N g⁻¹ soil) during 12 d incubations. Comparison of the δ¹³C content (‰) of the individual AAs and the δ¹³C signature of the respired AA-CO₂-C determined that, with the exception of TYR, all of the AAs were completely mineralized during incubations, allowing for the calculation of a N₂O-N conversion rate from each AA. Next, soils from three different semi-arid vegetation ecosystems with a wide range in total N content were incubated and monitored for CO₂ and N₂O efflux. A model utilizing CO₂ respired from the three soils as a measure of organic matter C mineralization, a preincubation soil AA composition of each soil, and the N₂O-N conversion rate from the AA incubations effectively predicted the range of N₂O production by all three soils. Nitrous oxide flux did not correspond to factors shown to influence anaerobic denitrification, including soil NO₃⁻ contents, soil moisture, oxygen consumption, and CO₂ respiration, suggesting that nitrification and aerobic nitrifier denitrification could be contributing to N₂O production in these soils. Results indicate that quantification of AA mineralization may be useful for predicting N₂O production in soils.     

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.     

Microbial and enzyme properties of apple orchard soil as affected by long-term application of copper fungicide

Copper-based fungicides have been applied in apple orchards for a long time, which has resulted in increasing soil Cu concentration. However, the microbial and enzyme properties of the orchard soils remain poorly understood. This study aimed to evaluate the effect of long-term application of Cu-based fungicides on soil microbial (microbial biomass carbon (C_mic), C mineralization, and specific respiration rate) and enzyme (urease, acid phosphatase, and invertase activities) properties in apple orchards. Soil samples studied were collected from apple orchards 5, 15, 20, 30, and 45 years old, and one adjacent forest soil as for reference. The mean Cu concentrations of orchard soils significantly increased with increasing orchard ages ranging from 21.8 to 141 mg kg⁻¹, and the CaCl₂-extractable soil Cu concentrations varied from 0.00 to 4.26 mg kg⁻¹. The soil mean C_mic values varied from 43.6 to 116 mg kg⁻¹ in the orchard soils, and were lower than the value of the reference soil (144 mg kg⁻¹). The ratio of soil C_mic to total organic C (C_org) increased from 8.10 to 18.3 mg C_mic g⁻¹ C_org with decreasing orchard ages, and was 26.1 mg C_mic g⁻¹ C_org for the reference soil. A significant correlation was observed between total- or CaCl₂-extractable soil Cu and soil C_mic or C_mic/C_org, suggesting that the soil Cu was responsible for the significant reductions in C_mic and C_mic/C_org. The three enzyme activity assays also showed the similar phenomena, and declined with the increasing orchard ages. The mean soil C mineralization rates were elevated from 110 to 150 mg CO₂-C kg⁻¹ soil d⁻¹ compared with the reference soil (80 mg CO₂-C kg⁻¹ soil d⁻¹), and the mean specific respiration rate of the reference soil (0.63 mg CO₂-C mg⁻¹ biomass C d⁻¹) was significantly smaller than the orchard soils from 1.19 to 3.55 mg CO₂-C mg⁻¹ biomass C d⁻¹. The soil C mineralization rate and the specific respiration rate can be well explained by the CaCl₂-extractable soil Cu. Thus, the long-term application of copper-based fungicides has shown adverse effects on soil microbial and enzyme properties.     

Decomposition of wheat straw differing in nitrogen content in soils under conventional and organic farming management

An incubation experiment was carried out to investigate the interactions of two straw qualities differing in N content and two soils differently accustomed to straw additions. One soil under conventional farming management (CFM) regularly received straw, the other soil under organic farming management (OFM) only farmyard manure. The soils of the two sites were similar in texture, pH, cation‐exchange capacity, and glucosamine content. The soil from the OFM site had higher contents of organic C, total N, muramic acid, microbial biomass C and N (C_mic and N_mic), but a lower ergosterol content and lower ratios ergosterol to C_mic and fungal C to bacterial C. The straw from the CFM had threefold higher contents of total N, twofold higher contents of ergosterol and glucosamine, a 50% higher content of muramic acid, and a 30% higher fungal C–to–bacterial C ratio. The straw amendments led to significant net increases in C_mic, N_mic, and ergosterol. Microbial biomass C showed on average a 50% higher net increase in the organic than in the CFM soil. In contrast, the net increases in N_mic and ergosterol differed only slightly between the two soils after straw amendment. The CO₂ evolution from the CFM soil always exceeded that from the OFM, by 50% or 200 µg (g soil)^–1 in the nonamended control soil and by 55% or additional 600 µg (g soil)^–1 in the two straw treatments. In both soils, 180 µg g^–1 less was evolved as CO₂‐C from the OFM straw. The metabolic quotient qCO₂ was nearly twice as high in the control and in the straw treatments of the CFM soil compared with that of the OFM. In contrast, the difference in qCO₂ was insignificant between the two straw qualities. Differences in the fungal‐community structure may explain to a large extent the difference in the microbial use of straw in the two soils under different managements.     

The fate of soyabean photosynthetic carbon varies in Mollisols differing in organic carbon

The fate of photosynthetically‐fixed carbon (C) in the plant–soil–microbe continuum has received much interest because of its relevance to soil C and the global C cycle. However, information on the flow of this plant C below ground and its contribution to soil C sequestration in soils with contrasting organic C (C_org) is limited. In this study, soyabean (Glycine max L. Merr.) was grown in three Mollisols with low (1.04%), medium (2.90%) and high (5.05%) C_org, respectively. Plants were labelled with ¹³CO₂ to trace the photosynthetic C dynamics in the plant–soil system for up to 288 hours. The total amount of net fixed ¹³C by plants ranged from 66 to 78 mg pot^?1, and there was no difference between soils. The amount of ¹³C in soil organic matter (SOM) increased from 1.9 to 6.1 mg pot^?1 over time in the high‐C_org soil, while it showed a non‐significant change with 2.2 mg pot^?1 (on average) in the medium‐C_org soil, and decreased from 2.9 to 0.1 mg pot^?1 in the low‐C_org soil. In the low‐C_org soil, the amount of ¹³C in soil microbes decreased markedly over time, showing a fast turnover, and had a significant correlation (P ≤ 0.01) with ¹³C in the SOM pool. However, such a relationship was not significant in the soil with high or medium C_org. These results indicate that most of the root‐derived C in the low‐C_org soil is degraded quickly by microbial activity, while the greater input of the photosynthetic C to SOM in the high‐ and/or medium‐C_org soil can probably be attributed to physical sorption of root‐derived C by SOM and minerals, thus protecting it against microbial decomposition.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

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.     

The effect of young biochar on soil respiration

The low temperature pyrolysis of organic material produces biochar, a charcoal like substance. Biochar is being promoted as a soil amendment to enhance soil quality, it is also seen as a mechanism of long-term sequestration of carbon. Our experiments tested the hypothesis that biochar is inert in soil. However, we measured an increase in CO₂ production from soils after biochar amendment which increased with increasing rates of biochar. The ∂¹³C signature of the CO₂ evolved in the first several days of the incubation was the same as the ∂¹³C signature of the biochar, confirming that biochar contributed to the CO₂ flux. This effect diminished by day 6 of the incubation suggesting that most of the biochar C is slowly decomposing. Thus, aside from this short-term mineralization increasing soil C with young biochar may indeed be a long-term C storage mechanism.     

Microbial use of maize cellulose and sugarcane sucrose monitored by changes in the C/C ratio

An arable soil with organic matter formed from C₃-vegetation was amended initially with maize cellulose (C₄-cellulose) and sugarcane sucrose (C₄-sucrose) in a 67-day laboratory incubation experiment with microcosms at 25 °C. The amount and isotopic composition (¹³C/¹²C) of soil organic C, CO₂ evolved, microbial biomass C, and microbial residue C were determined to prove whether the formation of microbial residues depends on the quality of the added C source adjusted with NH₄NO₃ to the same C/N ratio of 15. In a subsequent step, C₃-cellulose (3 mg C g⁻¹ soil) was added without N to soil to determine whether the microbial residues formed initially from C₄-substrate are preferentially decomposed to maintain the N-demand of the soil microbial community. At the end of the experiment, 23% of the two C₄-substrates added was left in the soil, while 3% and 4% of the added C₄-cellulose and C₄-sucrose, respectively, were found in the microbial biomass. The addition of the two C₄-substrates caused a significant 100% increase in C₃-derived CO₂ evolution during the 5-33 day incubation period. The addition of C₃-cellulose caused a significant 50% increase in C₄-derived CO₂ evolution during the 38-67 day incubation period. The decrease in microbial biomass C₄-C accounted for roughly 60% of this increase. Cellulose addition promoted microorganisms strongly able to recycle N immediately from their own tissue by “cryptic growth” instead of incorporating NO₃⁻ from the soil solution. The differences in quality of the microbial residues produced by C₄-cellulose and C₄-sucrose decomposing microorganisms are also reflected by the difference in the rates of CO₂ evolution, but not in the rates of net N mineralization.     

Interactive effects of elevated CO2 and nitrogen fertilization levels on photosynthesized carbon allocation in a temperate spring wheat and soil system

Increasing atmospheric CO2 concentration impacts the terrestrial carbon(C) cycle by affecting plant photosynthesis, the flow of photosynthetically fixed C belowground, and soil C pool turnover. For managed agroecosystems, how and to what extent the interactions between elevated CO2 and N fertilization levels influence the accumulation of photosynthesized C in crops and the incorporation of photosynthesized C into arable soil are in urgent need of exploration.We conducted an experiment simulating elevated CO2 with spring wheat(Triticum aestivum L.) planted in growth chambers.13C-enriched CO2 with an identical 13C abundance was continuously supplied at ambient and elevated CO2 concentrations(350 and 600 μmol mol-1, respectively) until wheat harvest.Three levels of N fertilizer application(equivalent to 80, 120, and 180 kg N ha-1 soil) were supplied for wheat growth at both CO2 concentrations. During the continuous 62-d 13CO2 labeling period, elevated CO2 and increased N fertilizer application increased photosynthesized C accumulation in wheat by 14%–24% and 11%–20%, respectively, as indicated by increased biomass production, whereas the C/N ratio in the roots increased under elevated CO2 but declined with increasing N fertilizer application levels. Wheat root deposition induced 1%–2.5% renewal of soil C after 62 d of 13CO2 labeling. Compared to ambient CO2, elevated CO2 increased the amount of photosynthesized C incorporated into soil by 20%–44%. However, higher application rates of N fertilizer reduced the net input of root-derived C in soil by approximately 8% under elevated CO2. For the wheat-soil system, elevated CO2 and increased N fertilizer application levels synergistically increased the amount of photosynthesized C. The pivotal role of plants in photosynthesized C accumulation under elevated CO2 was thereby enhanced in the short term by the increased N application. Therefore, robust N management could mediate C cycling and sequestration by influencing the interactions between plants and soil in agroecosystems under elevated CO2.