Separation of soil respiration into CO2 emission sources using 13C natural abundance in a deciduous broad-leaved forest in Japan

Abstract

In determining the soil and ecosystem carbon balance, it is necessary to distinguish between autotrophic respiration and heterotrophic respiration. We attempted to measure the contribution of CO₂ emissions from plant roots (R_RHI), from soil organic matter (R_SOM), and from litter (R_L) to CO₂ emissions from the forest floor (soil respiration; R_S) in a deciduous forest of oak (Quercus serrata Thunb.) and hornbeams (Carpinus laxiflora Sieb. et Zucc. Bl., Carpinus tschonoskii Maxim. and Carpinus japonica Bl.) on Andosols in Japan, using a ¹³C natural abundance technique. The ¹³C natural abundances of roots (δ_RHI), litter (δ_L) and SOM (δ_SOM) in the surface soil were ?28.9, ?30.1 and ?24.3‰, respectively. This means that the differences between δ_SOM and δ_RHI are large enough to calculate the contributions of R_RHI, R_SOM and R_L to R_S based on the mass balance of the CO₂ isotope ratios. R_RHI and R_SOM had close relationships with soil temperature, and R_L was influenced by soil temperature and moisture. In summer, under high soil temperatures, R_RHI and R_SOM were the predominant sources of R_S and the proportion of R_RHI to R_SOM to R_L was 51:44:5. In winter, R_L was predominant and the proportion of R_RHI to R_SOM to R_L was 20:11:69. The estimated annual emissions of R_RHI, R_SOM and R_L were 1.45, 2.10 and 1.30 Mg C ha^?1, respectively; thus, the proportion of R_RHI to R_SOM to R_L was 30:43:27 on a whole-year basis.     

Long-term (42 years) effect of thinning on soil CO2 emission in a mixed broadleaved-Korean pine (Pinus koraiensis) forest in Northeast China

Thinning is an important forest management practice that has great potential to influence regional soil carbon storage and dynamics.The present study measured soil respiration(RS,the efflux of CO2 emitted)and its two components(heterotrophic(RH)and autotrophic(RA)respiration)from soil 42 years after thinning in comparison to un-thinning(control).Autotrophic respiration was significantly greater in the thinning plot,approximately 44%higher compared to the control,while both RSand RHwere slightly,but not significantly,higher in the thinning plot.Higher fine root biomass might have contributed to the higher RAin the thinning plot.Both RSand RHshowed clear soil temperature-dependent seasonal patterns,whereas RAwas less responsive to changes in temperature,especially within one specific season.The annual and season-specific temperature sensitivities of RSand RHwere lower in the thinning plot,specifically during the mid-growing season.Furthermore,variations in the season-specific temperature sensitivity of RSand RHwere less intense in the thinning plot.We conclude that forest thinning can reduce the temperature sensitivity of RSand RHduring the mid-growing season and increase soil CO2 emission in the long term.     

Dynamics of soil organic matter turnover and soil respired CO2 in a temperate grassland labelled with 13C

The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C‐release from soils is a major component of the global C balance. The use of ¹³C‐depleted CO₂ in a 10‐year free‐air carbon dioxide enrichment (FACE) experiment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for δ¹³C and C contents in the last year of the FACE experiment (2002) and in the two following growing seasons. After 10 years of exposure to CO₂ enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non‐fumigated plots. A ¹³C depletion of 3.4‰ was found in SOM (0–12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0–12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respiratory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant‐soil system it was shown that rhizosphere respiration is the dominant factor in soil respiration. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled.     

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.     

稻麦轮作系统中大气CO2浓度升高对冬小麦生长季土壤呼吸的影响

Studies on the effect of elevated CO2 on C dynamics in cultivated croplands are critical to a better understanding of the C cycling in response to climate change in agroecosystems. To evaluate the effects of elevated CO2 and different N fertilizer application levels on soil respiration, winter wheat （Triticum aestivum L. cv. Yangmai 14） plants were exposed to either ambient CO2 or elevated CO2 （ambient [CO2] ＋ 200 μmol mol-1）, under N fertilizer application levels of 112.5 and 225 kg N ha-1 （as low N and normal N subtreatments, respectively）, for two growing seasons （2006-2007 and 2007-2008） in a rice-winter wheat rotation system typical in China. A split-plot design was adopted. A root exclusion method was used to partition soil respiration （RS） into heterotrophic respiration （RH） and autotrophic respiration （RA）. Atmospheric CO2 enrichment increased seasonal cumulative RS by 11.8% at low N and 5.6% at normal N when averaged over two growing seasons. Elevated CO2 significantly enhanced （P 〈 0.05） RS （12.7%）, mainly due to the increase in RH （caused by decomposition of larger amounts of rice residue under elevated CO2） during a relative dry season in 2007-2008. Higher N supply also enhanced RS under ambient and elevated CO2. In the 2007-2008 season, normal N treatment had a significant positive effect （P 〈 0.01） on seasonal cumulative RS relative to low N treatment when averaged across CO2 levels （16.3%）. A significant increase in RA was mainly responsible for the enhanced RS under higher N supply. The correlation （r2） between RH and soil temperature was stronger （P 〈 0.001） than that between RS and soil temperature when averaged across all treatments in both seasons. Seasonal patterns of RA may be more closely related to the plant phenology than soil temperature. The Q10 （the multiplier to the respiration rate for a 10 ℃ increase in soil temperature） values of RS and RH were not affected by elevated CO2 or higher N supply. These results mainly suggested that the increase in RS at elevated CO2 depended on the input of rice residue, and the increase in RS at higher N supply was due to stimulated root growth and concomitant increase in RA during the wheat growing portion of a rice-winter wheat rotation system.     

稳定13C同位素示踪技术在农田土壤碳循环和团聚体 固碳研究中的应用进展

农田土壤有机碳库是全球碳循环的重要组成部分,其积累和分解直接影响陆地生态系统碳贮藏与全球碳平衡。土壤团聚体是土壤结构的物质基础和土壤肥力的重要载体,也是土壤有机碳的固定场所。稳定~(13)C同位素示踪技术是研究土壤碳动态变化的有效手段,能够揭示新输入碳在土壤及团聚体中赋存状态、周转过程以及微生物的调节机制。本文主要归纳与阐述了稳定~(13)C同位素示踪技术在农田土壤有机碳循环及土壤团聚体固碳机理方面的研究进展,提出~(13)C同位素示踪技术在未来土壤碳循环和固碳机制方面的主要研究方向。     

The turnover of carbohydrate carbon in a cultivated soil estimated by 13C natural abundances

Understanding the chemical composition of soil organic matter (SOM) requires the determination of the dynamics of each class of compounds. We measured the dynamics of carbon in neutral carbohydrates by use of natural ¹³C labelling in an experimental wheat and maize sequence extending over 23 years. The isotopic composition of individual neutral monosaccharides was determined in hydrolysed particle‐size fractions by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) of trimethylsilyl (TMS) derivatives. The sensitivity in terms of ¹³C/¹²C ratios ranged between 1 and 2‰ depending on the monosaccharide. The age distribution of neutral sugar carbon was very similar to that of total soil carbon. Particulate organic matter (POM) was characterized by the predominance of glucose and xylose of vegetal origin. In POM > 200 µm, the mean age of sugar‐C (5 years) was slightly less than that of total carbon (7 years). Xylose was younger than glucose. The fine fraction 0–50 µm contained mainly glucose, arabinose, galactose, xylose, fucose and mannose, which had predominantly microbial origins. The mean age of carbohydrate carbon in the fraction 0–50 µm was between 60 and 100 years and was similar to that of total organic carbon (OC). No difference in the age of carbon between the individual monosaccharides was found. The POM fraction 50–200 µm had an intermediate signature and turnover. Considering the typical lability of carbohydrates, the relatively great age of carbohydrate carbon may be explained by physical or chemical protection from degradation, as well as by recycling of soil organic matter carbon by soil microbes.     

南亚热带果园土壤二氧化碳释放变异性研究

Temporal variability in soil CO2 emission from an orchard was measured using a dynamic open-chamber system for measuring soil CO2 effiux in Heshan Guangdong Province, in the lower subtropical area of China. Intensive measurements were conducted for a period of 12 months. Soil CO2 emissions were also modeled by multiple regression analysis from daily air temperature, dry-bulb saturated vapor pressure, relative humidity, atmospheric pressure, soil moisture, and soil temperature. Data was analyzed based on soil moisture levels and air temperature with annual data being grouped into either hot-humid season or relatively cool season based on the precipitation patterns. This was essential in order to acquire simplified exponential models for parameter estimation. Minimum and maximum daily mean soil CO2 effiux rates were observed in November and July, with respective rates of 1.98 ± 0.66 and 11.04 ± 0.96 μmol m^-2 s^-1 being recorded. Annual average soil CO2 emission （FCO2） was 5.92 μmol m^-2 s^-1. Including all the weather variables into the model helped to explain 73.9% of temporal variability in soil CO2 emission during the measurement period. Soil CO2 effiux increased with increasing soil temperature and soil moisture. Preliminary results showed that Q10, which is defined as the difference in respiration rates over a 10 ℃ interval, was partly explained by fine root biomass. Soil temperature and soil moisture were the dominant factors controlling soil CO2 effiux and were regarded as the driving variables for CO2 production in the soil. Including these two variables in regression models could provide a useful tool for predicting the variation of CO2 emission in the commercial forest Soils of South China .     

Three-source partitioning of CO2 efflux from soil planted with maize by C natural abundance fails due to inactive microbial biomass

A theoretical approach to the partitioning of carbon dioxide (CO₂) efflux from soil with a C₃ vegetation history planted with maize (Zea mays), a C₄ plant, into three sources, root respiration (RR), rhizomicrobial respiration (RMR), and microbial soil organic matter (SOM) decomposition (SOMD), was examined. The δ¹³C values of SOM, roots, microbial biomass, and total CO₂ efflux were measured during a 40-day growing period. A three-source isotopic mass balance based on the measured δ¹³C values and on assumptions made in other studies showed that RR, RMR, and SOMD amounted to 91%, 4%, and 5%, respectively. Two assumptions were thoroughly examined in a sensitivity analysis: the absence of ¹³C fractionation and the conformity of δ¹³C of microbial CO₂ and that of microbial biomass. This approach strongly overestimated RR and underestimated RMR and microbial SOMD. CO₂ efflux from unplanted soil was enriched in ¹³C by 2.0‰ compared to microbial biomass. The consideration of this ¹³C fractionation in the mass balance equation changed the proportions of RR and RMR by only 4% and did not affect SOMD. A calculated δ¹³C value of microbial CO₂ by a mass balance equation including active and inactive parts of microbial biomass was used to adjust a hypothetical below-ground CO₂ partitioning to the measured and literature data. The active microbial biomass in the rhizosphere amounted to 37% to achieve an appropriate ratio between RR and RMR compared to measured data. Therefore, the three-source partitioning approach failed due to a low active portion of microbial biomass, which is the main microbial CO₂ source controlling the δ¹³C value of total microbial biomass. Since fumigation-extraction reflects total microbial biomass, its δ¹³C value was unsuitable to predict δ¹³C of released microbial CO₂ after a C₃-C₄ vegetation change. The second adjustment to the CO₂ partitioning results in the literature showed that at least 71% of the active microbial biomass utilizing maize rhizodeposits would be necessary to achieve that proportion between RR and RMR observed by other approaches based on ¹⁴C labelling. The method for partitioning total below-ground CO₂ efflux into three sources using a natural ¹³C labelling technique failed due to the small proportion of active microbial biomass in the rhizosphere. This small active fraction led to a discrepancy between δ¹³C values of microbial biomass and of microbially respired CO₂.     

Using natural 13C abundances to differentiate between three CO2 sources during incubation of a grassland soil amended with slurry and sugar

This study describes a novel approach to separate three soil carbon (C) sources by one tracer method (here ¹³C natural abundance). The approach uses the temporal dynamics of the CO₂ efflux from a C₃ grassland soil amended with added C₃ or C₄ slurry and/or C₃ or C₄ sugar to estimate contributions of three separate C sources (native soil, slurry, and sugar) to CO₂ efflux. Soil with slurry and/or sugar was incubated under controlled conditions, and concentration and δ¹³C values of evolved CO₂ were measured over a 2‐week period. The main assumption needed for separation of three C sources in CO₂ efflux, i.e., identical decomposition of applied C₃ and C₄ sugars in soil, was investigated and proven. The relative contribution to the CO₂ efflux was higher, but shorter with an increased (microbial) availability of the C source, i.e., sugar > slurry > SOM. The shortcomings and limitations as well as possible future applications of the suggested method are discussed.     

长期施肥下红壤旱地土壤CO2排放及碳平衡特征

在国家肥力网红壤旱地长期定位试验地上,采用静态箱/气相色谱法测定土壤CO2排放速率,同时利用根去除法区分根系对土壤呼吸的贡献,通过计算净生态系统生产力（NEP）,判断长期不同施肥下红壤旱地农田碳汇强度。结果表明,小麦、玉米生长季各处理的土壤和土体呼吸速率随着作物生长、温度升高均呈现明显的季节变化规律;玉米生长季土壤和土体累积呼吸量大于小麦生长季,小麦、玉米生长季均以NPKM处理土壤和土体呼吸累积呼吸量最大,且显著高于其它处理（P0.05）,NP和NPK处理次之,CK和NK处理最小（P0.05）;小麦、玉米生长季各处理根际呼吸占土壤呼吸的比例分别为7.6 %～17.4 %、4.7%～16.6 %,均以NPKM处理根际呼吸贡献率最大;小麦季NPKM处理、玉米季CK和NPKM处理的NEP值为负,是大气CO2的汇,且NPKM处理的净初级生产力与土壤呼吸的比值（NPP/Rs）最大,其它处理NEP值均为正,是大气CO2的源。有机无机肥配施（NPKM）相比其它处理具有较强的碳汇功能,是红壤旱地比较合理的施肥措施。     

森林生态系统中土壤有机质组分的固态13C核磁共振波谱表征

Forest management practices such as prescribed burning and thinning in forest ecosystems may alter the properties of soil organic matter (SOM).In this study,surface soils from field plots in the Bankhead National Forest,Alabama,USA,were used to investigate possible SOM transformations induced by thinning and burning.Elemental analysis and solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy were used to characterize SOM fractions in whole soils,humic substances,and density fractions.Our data revealed that the changes in SOM fractions due to the repeated burning carried out in the forest ecosystem studied were involved mainly with alkyl C,O-alkyl C,and carbohydrate functional groups,implying that most prominent reactions that occurred involved dehydrogenation,de-oxygenation,and decarboxylation.In addition,burning and thinning might have also affected the distribution and composition of free and occluded particulate SOM fractions.The limited structural changes in SOM fractions suggested that low-intensity prescribed fire in the forest ecosystem studied will not create major structural changes in SOM fractions.