Differentiating sources of CO2 from organic soil under bioenergy crop cultivation: A field-based approach using 14C

We tested here a plant-soil system to separate recent, plant-derived and native, soil-derived carbon in soil respiration. The approach uses a perennial crop cultivated on an organic soil where upper soil layers have been removed as a result of peat extraction. There, the ¹⁴C signal from native organic matter is highly depleted compared to that in vegetation established at the site after peat extraction ceased. Radiocarbon was analyzed in carbon dioxide respired from soil over one growing season, and a two-pool isotope mixing model was applied to calculate the relative contribution of old vs. new carbon sources. The analysis showed that the approach is reliable for source partitioning with isotopes. After six years of cultivation, old peat decomposition contributed less to total soil respiration than respiration of recent plant material (30% vs. 70% on average, respectively), but the relative proportions were highly variable over the growing season. The approach offers a new possibility to follow the fate of old, native soil organic matter in highly organic soils.     

Carbon dynamics in Appalachian peatlands of West Virginia and Western Maryland

Abundant production of organic matter that decomposes slowly under anaerobic conditions can result in substantial accumulation of soil organic matter in wetlands. Tedious means for estimating production and decomposition of plant material, especially roots, hampers our understanding of organic matter dynamics in such systems. In this paper, I describe a study that amended typical estimates for both production and decomposition of organic matter by measuring net flux of carbon dioxide (CO₂) over the peat surface within a conifer swamp, a sedge-dominated marsh, and a bog in the Appalachian Mountain region of West Virginia and western Maryland, USA. The sites are relatively productive, with net primary production (NPP) of 30 to 82.5 mol C m^?2 yr^?1, but peat deposits are shallow with an average depth of about 1 m. In summer, all three sites showed net CO₂ flux from the atmosphere to the peat during the daytime (?20.0 to ?30.5 mmol m^?2 d^?1), supported by net photosynthesis, which was less than net CO₂ flux from the peat into the atmosphere at nighttime (39.2 to 84.5 mmol m^?2 d^?1), supported by ecosystem respiration. The imbalance between these estimates suggests a net loss of carbon (C) from these ecosystems. The positive net CO₂ flux seems to be so high because organic matter decomposition occurs throughout the peat deposit — and as a result concentrations of dissolved inorganic carbon (DIC) in peat pore waters reached 4,000 Μmol L^?1 by late November, and concentrations of dissolved organic carbon (DOC) in peat pore waters reached 12,000 Μmol L^?1. Comparing different approaches revealed several features of organic matter dynamics: (i) peat accretion in the top 30 cm of the peat deposit results in a C accumulation rate of about 15 mmol m^?2 d^?1; however, (ii) the entire peat deposit has a negative C balance losing about 20 mmol m^?2 d^?1.     

The role of mineralization of the organic matter of soddy-podzolic and peat bog soils in the accumulation of <Superscript>137</Superscript>Cs by plants

The role of mineralization of soil organic matter (SOM) in the mobilization of ¹³⁷Cs was estimated on the basis of data on the biokinetic fractionation of the organic matter of soddy-podzolic sandy-loam and peat bog soils and on the coefficients of the soil-to-plant transfer of radiocesium under field conditions. The peat bog soils were richer than the soddy-podzolic soils in the total organic carbon (by 7.9–23.8 times), the potentially mineralizable carbon (by 2.4–6.5 times), and the carbon of the microbial biomass (by 2.9–4.6 times). The agricultural use of the soddy-podzolic and peat bog soils led to a decrease in the SOM mineralization capacity by 1.1–1.8 and 1.4–2.0 times, respectively. Simultaneously, the portions of the easily, moderately, and difficultly mineralizable fraction of the SOM active pool changed. The coefficients of the ¹³⁷Cs transfer from the peat bog soils to plants were 3.3–17.6 times higher than those for the soddy-podzolic soils. The content of ¹³⁷Cs in plants grown on the peat bog soils was 2–65 times higher than that in the mobile (salt-extractable) soil pool by the beginning of the growing season. Strong positive linear correlations were found between the coefficients of the soil-to-plant transfer of ¹³⁷Cs and the total content of the SOM, the content of the microbial biomass, the content of the potentially mineralizable carbon, and the intensity of its mineralization. It was concluded that the decisive factors controlling the intensity of the ¹³⁷Cs transfer from mineral and organic soils into plants are the SOM content and its mineralization potential. The mineralization of the SOM is accompanied by the release of both ¹³⁷Cs and mineral nitrogen; the latter facilitates the transfer of radiocesium into plants.     

长期施肥对冬小麦/夏玉米轮作下土壤呼吸及其组分的影响

为阐明施肥对农田土壤呼吸的影响,于2002年6月至2003年6月在河南封丘潮土上进行的长期试验地上测定了玉米/冬小麦轮作系统下的土壤呼吸,分析了土壤呼吸与土壤水分和温度的关系,并利用统计分析方法研究了土壤呼吸各组分的贡献。土壤呼吸变化与作物生长发育规律一致,施肥通过影响作物的生长发育而对土壤呼吸产生影响。不同作物生长期,根际呼吸、土壤原有机质以及前作根茬和有机肥中碳对土壤呼吸的贡献不同。玉米期土壤有机质、根际呼吸、前作根茬和有机肥中的碳对土壤呼吸的平均贡献率分别为70.19%、19.43%和10.37%;而小麦生长期则分别为23.75%、62.26%和14.11%。由于不同施肥处理的作物生长量、土壤有机质含量以及前作根茬和有机肥施入而进入的有机碳量不同,造成土壤呼吸个体上存在着较大差异。土壤有机质的消耗主要发生在玉米生长阶段。     

不同耕作方式下旱作玉米田土壤呼吸及其影响因素

为揭示不同耕作方式对旱作玉米田土壤呼吸的影响,对比研究深松耕、免耕、旋耕和翻耕4种耕作方式下土壤呼吸速率的动态变化及其与土壤水分、温度、有机质、全氮、pH值等的关系。结果表明,夏玉米生长季,4种耕作方式下土壤呼吸速率随生育时期均呈先增加后降低的趋势,平均土壤呼吸速率为深松耕>翻耕>旋耕>免耕;播种前至拔节期土壤温度为翻耕>深松耕>旋耕>免耕,抽雄期至成熟收获期为免耕>旋耕>深松耕>翻耕;各耕作方式下0～20cm层土壤有机质、全氮均逐渐增加,与免耕比较,翻耕有机质和全氮均降低;生育前期土壤pH值波动明显,抽雄期后趋于平缓,土壤pH值平均值为翻耕>旋耕>免耕>深松耕。各影响因素与土壤呼吸速率相关分析表明,深松耕和翻耕土壤水分、温度与土壤呼吸速率呈显著或极显著正相关;有机质与土壤呼吸速率呈负相关,且与深松耕措施下土壤呼吸速率呈显著负相关;除免耕外,其他耕作方式下土壤全氮、pH值与土壤呼吸呈负相关。该研究可为补充完善土壤呼吸排放机理、评估区域碳收支平衡及制定科学有效的土壤碳调控管理措施提供依据。     

Predicting soil organic matter stability in agricultural fields through carbon and nitrogen stable isotopes

In order to evaluate the sustainability and efficiency of soil carbon sequestration measures and the impact of different management and environmental factors, information on soil organic matter (SOM) stability and mean residence time (MRT) is required. However, this information on SOM stability and MRT is expensive to determine via radiocarbon dating, precluding a wide spread use of stability measurements in soil science. In this paper, we test an alternative method, first developed by Conen et al. (2008) for undisturbed Alpine grassland systems, using C and N stable isotope ratios in more frequently disturbed agricultural soils. Since only information on carbon and nitrogen concentrations and their stable isotope ratios is required, it is possible to estimate the SOM stability at greatly reduced costs compared to radiocarbon dating. Using four different experimental sites located in various climates and soil types, this research proved the effectiveness of using the C/N ratio and δ¹⁵N signature to determine the stability of mOM (mineral associated organic matter) relative to POM (particulate organic matter) in an intensively managed agro-ecological setting. Combining this approach with δ¹³C measurements allowed discriminating between different management (grassland vs cropland) and land use (till vs no till) systems. With increasing depth the stability of mOM relative to POM increases, but less so under tillage compared to no-till practises. Applying this approach to investigate SOM stability in different soil aggregate fractions, it corroborates the aggregate hierarchy theory as proposed by Six et al. (2004) and Segoli et al. (2013). The organic matter in the occluded micro-aggregate and silt & clay fractions is less degraded than the SOM in the free micro-aggregate and silt & clay fractions. The stable isotope approach can be particularly useful for soils with a history of burning and thus containing old charcoal particles, preventing the use of ¹⁴C to determine the SOM stability.     

Investigating drivers of microbial activity and respiration in a forested bog

Northern peatlands store nearly one-third of terrestrial carbon(C)stocks while covering only 3%of the global landmass;nevertheless,the drivers of C cycling in these often-waterlogged ecosystems are different from those that control C dynamics in upland forested soils.To explore how multiple abiotic and biotic characteristics of bogs interact to shape microbial activity in a northern,forested bog,we added a labile C tracer(13C-labeled starch)to in situ peat mesocosms and correlated heterotrophic respiration with natural variation in several microbial predictor variables,such as enzyme activity and microbial biomass,as well as with a suite of abiotic variables and proximity to vascular plants aboveground.We found that peat moisture content was positively correlated with respiration and microbial activity,even when moisture levels exceeded total saturation,suggesting that access to organic matter substrates in drier environments may be limiting for microbial activity.Proximity to black spruce trees decreased total and labile heterotrophic respiration.This negative relationship may reflect the influence of tree evapotranspiration and peat shading effects;i.e.,microbial activity may decline as peat dries and cools near trees.Here,we isolated the response of heterotrophic respiration to explore the variation in,and interactions among,multiple abiotic and biotic drivers that influence microbial activity.This approach allowed us to reveal the relative influence of individual drivers on C respiration in these globally important C sinks.     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

The response of soil organic matter to manure amendments in a long-term experiment at Ultuna, Sweden

In a long-term field experiment started in 1956 on a clay loam soil at Uppsala, Sweden, changes of organic carbon in the topsoils receiving various organic amendments at the rate of 200 kg C ha^'1 year^'1 were studied to determine soil organic matter characteristics, variations of δ¹³C in the soil and to estimate a carbon balance. Fallow and mineral fertilizer without N led to a significant decrease of soil organic matter (SOM) in the soil, green manure maintained the SOM content, and animal manure and peat increased the SOM content significantly. The stable portion of the added organic materials after 37 years of continuous input was 12·8, 27·3, and 56·7%, for green manure, animal manure and peat, respectively. This was reflected by half-lives of organic carbon originating from the amendments between 3·0 (green manure) and 14·6 years (peat). The isotopic composition of SOM changed both due to mineralization (continuous fallow) and the addition of amendments is topically different from soil humus (green manure, animal manure). The isotopic effect was used to calculate the percentage of carbon derived from animal manure present for the year 1993. This value (55·4%) was larger than that derived from the carbon balance, which indicated a priming effect of the animal manure on the initial soil humus. Mineralization of microbially available organic substances led to an increase in the degree of humification on plots not receiving organic amendments. Adding peat and animal manure resulted in a decrease of the humification index due to the continuous input of poorly humified material. The extinction ratio (E₄/E₆) and ratio of fulvic acid to humic acid changed considerably in the peat treated plots. Fourier transform infrared (FTIR)-measurements of the extracts showed that peat characteristics can be detected in peat treated soils. The other amendments did not alter the characteristics of the extractable humic substances.     

The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C

Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (<2 mm). The organic C content and its distribution among humic, humin and non‐humic fractions, as well as the isotopic signatures (Δ¹⁴C and δ¹³C) of organic carbon and of CO₂ produced during incubation of samples, all show that altered rock fragments contain a dynamic component of the carbon cycle. Rock fragments, especially the highly altered ones, contributed 4.5% to the total organic C content in the soil. The bulk organic matter in both fine earth and highly altered rock fragments in the A1 horizon contained significant amounts of recent C (bomb ¹⁴C), indicating that most of this C is cycled quickly in both fractions. In the A horizons, the mean residence times of humic substances from highly altered rock fragments were shorter than those of the humic substances isolated in the fine earth. Values of Δ¹⁴C of the CO₂ produced during basal respiration confirmed the heterogeneity, complexity and dynamic nature of the organic matter of these rock fragments. The weak ¹⁴C signatures of humic substances from the slightly altered rock fragments confirmed the importance of weathering in establishing and improving the interactions between rock fragments and surrounding soil. The progressive enrichment in ¹³C from components with high‐¹⁴C (more recent) to low‐¹⁴C (older) indicated that biological activity occurred in both the fine and the coarse fractions. Hence the microflora utilizes energy sources contained in all the soil compartments, and rock fragments are chemically and biologically active in soil, where they form a continuum with the fine earth.     

Abiotic drivers and their interactive effect on the flux and carbon isotope (C and δC) composition of peat-respired CO2

Feedbacks to global warming may cause terrestrial ecosystems to add to anthropogenic CO₂ emissions, thus exacerbating climate change. The contribution that soil respiration makes to these terrestrial emissions, particularly from carbon-rich soils such as peatlands, is of significant importance and its response to changing climatic conditions is of considerable debate. We collected intact soil cores from an upland blanket bog situated within the northern Pennines, England, UK and investigated the individual and interactive effects of three primary controls on soil organic matter decomposition: (i) temperature (5, 10 and 15 °C); (ii) moisture (50 and 100% field capacity – FC); and (iii) substrate quality, using increasing depth from the surface (0–10, 10–20 and 20–30 cm) as an analogue for increased recalcitrance of soil organic material. Statistical analysis of the results showed that temperature, moisture and substrate quality all significantly affected rates of peat decomposition. Q₁₀ values indicated that the temperature sensitivity of older/more recalcitrant soil organic matter significantly increased (relative to more labile peat) under reduced soil moisture (50% FC) conditions, but not under 100% FC, suggesting that soil microorganisms decomposing the more recalcitrant soil material preferred more aerated conditions. Radiocarbon analyses revealed that soil decomposers were able to respire older, more recalcitrant soil organic matter and that the source of the material (deduced from the δ¹³C analyses) subject to decomposition, changed depending on depth in the peat profile.     

Factors causing temporal and spatial variation in heterotrophic and rhizospheric components of soil respiration in afforested organic soil croplands in Finland

Partitioning soil respiration (SR) into its components, heterotrophic and rhizospheric respiration, is an important step for understanding and modelling carbon (C) cycling in organic soils. However, no partitioning studies on afforested organic soil croplands exist. We separated soil respiration originating from the decomposition of peat (SR_P), and aboveground litter (SR_L) and root respiration (SR_R) in six afforested organic soil croplands in Finland with varying tree species and stand ages using the trenching method. Across the sites temporal variation in SR was primarily related to changes in soil surface temperature (?5 cm), which explained 71–96% of variation in SR rates. Decomposition of peat and litter was not related to changes in water table level, whereas a minor increase in root respiration was observed with the increase in water table depth. Temperature sensitivity of SR varied between the different respiration components: SR_P was less sensitive to changes in soil surface temperature than SR_L or SR_R. Factors explaining spatial variation in SR differed between different respiration components. Annual SR_P correlated positively with peat ash content while that of SR_L was found to correlate positively with the amount of litter on the forest floor, separately for each tree species. Root respiration correlated positively with the biomass of ground vegetation. From the total soil respiration peat decomposition comprised a major share of 42%; the proportion of autotrophic respiration being 41% and aboveground litter 17%. Afforestation lowered peat decomposition rates. Nevertheless the effect of agricultural history can be seen in peat properties for decades and due to high peat decomposition rates these soils still loose carbon to the atmosphere.     

Abiotic drivers and their interactive effect on the flux and carbon isotope (14C and δ13C) composition of peat-respired CO2

Feedbacks to global warming may cause terrestrial ecosystems to add to anthropogenic CO₂ emissions, thus exacerbating climate change. The contribution that soil respiration makes to these terrestrial emissions, particularly from carbon-rich soils such as peatlands, is of significant importance and its response to changing climatic conditions is of considerable debate. We collected intact soil cores from an upland blanket bog situated within the northern Pennines, England, UK and investigated the individual and interactive effects of three primary controls on soil organic matter decomposition: (i) temperature (5, 10 and 15 °C); (ii) moisture (50 and 100% field capacity – FC); and (iii) substrate quality, using increasing depth from the surface (0–10, 10–20 and 20–30 cm) as an analogue for increased recalcitrance of soil organic material. Statistical analysis of the results showed that temperature, moisture and substrate quality all significantly affected rates of peat decomposition. Q₁₀ values indicated that the temperature sensitivity of older/more recalcitrant soil organic matter significantly increased (relative to more labile peat) under reduced soil moisture (50% FC) conditions, but not under 100% FC, suggesting that soil microorganisms decomposing the more recalcitrant soil material preferred more aerated conditions. Radiocarbon analyses revealed that soil decomposers were able to respire older, more recalcitrant soil organic matter and that the source of the material (deduced from the δ¹³C analyses) subject to decomposition, changed depending on depth in the peat profile.     

Diurnal and seasonal carbon dioxide exchange and its components in temperate grasslands in the Netherlands — an outline of the methodology

A methodological outline is presented of a study into the diurnal and seasonal cycle of carbon fluxes within grassland ecosystems in the Netherlands in relation to their environment. At experimental sites Lelystad and Zegveld ?redominantlyLolium perenne L. at a clay and peat soil, respectively — measurements will be made on (1) net CO₂ assimilation of the grassland vegetation using infrared gas analysis; (2) carbon distribution within the plant using¹⁴C pulse labeling; and (3) carbon and CO₂ fluxes associated with root respiration and soil organic matter decomposition using¹⁴C pulse labeling. At both sites and at experimental site Cabauw additional measurements will be made on total CO₂ fluxes between the grassland vegetation and the lower part of the atmospheric boundary layer. For the analysis of the experimental results and generalisation of the relationships between carbon fluxes and environmental and plant factors use will be made of dynamic simulation models of grass growth and soil organic matter dynamics.     

Approaching a Standardized Method for the Hot-Water Extraction of Peat Material to Determine Labile SOM in Organic Soils

Peatland soils are the most effective and important long-term terrestrial carbon (C) storages. To estimate potential C loss, a valid characterization of soil decomposability, in particular the labile fraction, is of great interest. One of the most labile fractions is hot-water-extractable organic matter (HWOM), often measured as hot-water-extractable carbon (C_hwe) and nitrogen (N_hwe). Various studies describe different extraction procedures for mineral soils. Because of methodical differences, it is difficult to compare extracted HWOM amounts directly to each other. For peatland soils, few studies exist. The aim of the present study is the development of a standardized method for the hot-water extraction of peat materials. Therefore, we extracted HWOM in various replicates from different peats on the basis of a standardized extraction method for mineral soils (1 h extraction at 100 °C). We tested how differences in soil/water ratios, extract treatment (filtering vs. not filtering), and sample pretreatment (freeze drying vs. air drying) influence HWOM amounts. The results clearly illustrated the influence of changing soil/water ratios on HWOM amounts. Mean C_hwe concentrations ranged between 8 and 34 g kg^?1 whereas N_hwe ranged between 0.2 and 2.6 g kg^?1. We recommend the extraction under soil/water ratios of 1/800 to provide sufficient volume of solvent for C_hwe. If relative differences for N_hwe amounts are greater than 15 percent, samples should be extracted again under soil/water ratios greater than 1/300 to avoid analytical errors due to unintended dilution effects. Filtering of centrifuged and decanted extracts before analysis is not necessary. Peat material should be either air dried or freeze dried before extraction.     

Sludge-derived organic carbon in an agricultural soil estimated by 13C abundance measurements

The objective of this study is to develop a method to follow the dynamics of sludge‐derived organic carbon, which will allow us to understand the behaviour of trace metals in the sludge‐treated soils. We studied, in a sandy agricultural soil of southwest France, cultivated with maize and amended with sewage‐sludge over 20 years, the dynamics of different sources of organic matter and compared this with a control, which had never received any treatment. For the first time, a method is proposed that will distinguish and quantify sludge‐derived organic carbon, maize‐derived organic carbon, and native organic carbon. This method is based on the mean differences in δ¹³C abundances between native (−26.5‰), maize (−12.5‰) and sludge (−25.4‰) organic carbon. Three hypotheses on the dynamics of soil organic matter sources are proposed: (i) isotopic differences observed between control and sludge‐treated soils are due only to the incorporation of sludge C, whereas in the others, the control was used to model the incorporation of (ii) maize C or (iii) native C in the sludge‐treated soils. The comparison of the stocks of each source (native C, maize C and sludge C) found in the bulk soil with the sum of corresponding stocks found in particle‐size fractions allowed us to reject the two first hypotheses and to validate the last one. Repeated applications of sewage‐sludge induced accumulation of sludge‐derived organic carbon in the topsoil, and simultaneously contributed to the preservation of maize‐derived organic carbon. When sludge applications ceased, the rapid decrease in soil organic matter stocks was mostly caused by the degradation of the sludge‐derived organic carbon sources. At the same time, the maize‐derived organic carbon shifted from the coarsest fraction (200–2000 μm) to the finest fraction (0–50 μm). Therefore, this study has shown that repeated applications of sewage‐sludge induced changes in soil organic matter dynamics over time.     

Verwertung organischer Substanzen durch mikrobielle Bodenbiomasse als eine Funktion chemisch-thermodynamischer Parameter

Carbon use efficiency of organic substances by soil microbial biomass as a function of chemical and thermodynamical parameters A simple calculation determining carbon use efficiency by soil microorganisms based on chemical and thermodynamical parameters of organic matter is proposed. The use efficiency characterizes carbon fraction of organic matter which is incorporated into the cells of the soil microbial biomass. The proposed approach is based on the transition of the organic matter enthalpy into the microbial biomass enthalpy considering the formation enthalpy of the end products of respiration like CO₂, NH₄⁺ and H₂O. The combustion energy content of organic matter was used for the calculations. This combustion energy content can be determined by simple analytical means. It can be derived from data given in the literature and for various agricultural products. The comparison of organic matter use efficiency data calculated as shown above with literature data produced by diverse methods showed a satisfactory correlation. The calculated enthalpy of formation and maintenance of a C-unit of soil microbial biomass under in situ conditions amounted to —153,3 kJ · gC^—1. This value is compared with the maintenance coefficient used in microbiology. The use of the suggested approach for the calculation of carbon use efficiency based on the substance composition of the organic matter allows a uniform, standardized procedure which is not dependent on specific experimental conditions.     

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.     

Differences in the activity and bacterial community structure of drained grassland and forest peat soils

The microbial activity and bacterial community structure were investigated in two types of peat soil in a temperate marsh. The first, a drained grassland fen soil, has a neutral pH with partially degraded peat in the upper oxic soil horizons (16% soil organic carbon). The second, a bog soil, was sampled in a swampy forest and has a very high soil organic carbon content (45%), a low pH (4.5), and has occasional anoxic conditions in the upper soil horizons due to the high water table level. The microbial activity in the two soils was measured as the basal and substrate-induced respiration (SIR). Unexpectedly, the SIR (μl CO₂ g⁻¹ dry soil) was higher in the bog than in the fen soil, but lower when CO₂ production was expressed per volume of soil. This may be explained by the notable difference in the bulk densities of the two soils. The bacterial communities were assessed by terminal restriction fragment length polymorphism (T-RFLP) profiling of 16S rRNA genes and indicated differences between the two soils. The differences were determined by the soil characteristics rather than the season in which the soil was sampled. The 16S rRNA gene libraries, constructed from the two soils, revealed high proportions of sequences assigned to the Acidobacteria phylum. Each library contained a distinct set of phylogenetic subgroups of this important group of bacteria.