Computing the mean residence time of soil carbon fractions using stable isotopes: impacts of the model framework

Soils contain the largest carbon (C) reservoir on Earth, but the mean residence time (MRT) of soil C is often poorly estimated, despite its importance for assessing the efficiency with which soils may serve as a sink for atmospheric C. The objective of this study was to evaluate how the structure of simple models of soil C dynamics affects the MRT determined from isotope‐mixing experiments, using data from field studies with either artificial labelling (FACE) or C3/C4 vegetation change. We first highlighted theoretically how non‐steady‐state conditions and the model structure (one single, two successive, or two parallel C pools) can have an impact on the MRT assessment. We then tested these different model structures against published data on the dynamics of different soil organic matter separates and their constituents. Our findings indicated that many of the reviewed studies assumed wrongly that the system was at steady state or could be described by a single‐pool approach. To select the correct model, exact knowledge of C input rates and several data points are needed from the beginning of the experiment. For steady‐state conditions an apparent temporal change of MRT computed from a single‐pool model is thus a clear indicator that a two‐pool approach must be chosen. The errors made by the wrong choice of model varied with the length of the experiment and usually resulted in an over‐estimate of MRT by a factor of 1.15 for some data published on physical size separates, but by a factor of up to 11 for individual microbial biomarkers such as muramic acid.     

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.     

土壤水稳定同位素研究进展

本文从影响土壤水稳定同位素变化的因素、时空变化规律以及"土壤-植物-大气"界面水分转化和循环过程等方面综述了国内外土壤水同位素研究的主要成果,认为同位素方法在研究土壤水运移、降水入渗及土壤蒸发问题上优势比较明显,并且有助于从宏观和微观上阐明土壤水的特征及其运动规律;指出了当前土壤水分同位素研究存在的问题和今后研究的重点,并对同位素技术与方法在土壤水中应用前景进行了展望。     

Comparison of chemical fractionation methods for isolating stable soil organic carbon pools

Stable soil organic matter (SOM) is important for long‐term sequestration of soil organic carbon (SOC), but the usefulness of different fractionation methods to isolate stable SOM is open to question. We assessed the suitability of five chemical fractionation methods (stepwise hydrolysis, treatment with H₂O₂, Na₂S₂O₈, NaOCl, and demineralization of the NaOCl‐resistant fraction (NaOCl + HF)) to isolate stable SOM from soil samples of a loamy sand and a silty loam under different land use regimes (grassland, forest and arable crops). The apparent C turnover time and mean age of SOC before and after fractionation was determined by ¹³C and ¹⁴C analysis. Particulate organic matter was removed by density fractionation before soils were exposed to chemical fractionation. All chemical treatments induced large SOC losses of 62–95% of the mineral‐associated SOC fraction. The amounts of H₂O₂‐ and Na₂S₂O₈‐resistant SOC were independent from land use, while those of NaOCl‐ (NaOCl + HF)‐ and hydrolysis‐resistant SOC were not. All chemical treatments caused a preferential removal of young, maize‐derived SOC, with Na₂S₂O₈ and H₂O₂ being most efficient. The mean ¹⁴C age of SOC was 1000–10000 years greater after chemical fractionation than that of the initial, mineral‐associated SOC and mean ¹⁴C ages increased in the order: NaOCl < NaOCl + HF ≤ stepwise hydrolysis ≪ H₂O₂≈ Na₂S₂O₈. None of the methods appeared generally suitable for the determination of the inert organic matter pool of the Rothamsted Carbon Model. Nonetheless, our results indicate that all methods are able to isolate an older, more stable SOC fraction, but treatments with H₂O₂ and Na₂S₂O₈ were the most efficient ones in isolating stable SOM.     

Soil carbon and nitrogen dynamics using stable isotopes in 19- and 10-year-old tropical agroforestry systems

Conversion of forests to agricultural land in the American tropics, through traditional agricultural practices such as shifting cultivation, has not been able to maintain stocks of soil organic carbon (SOC), and increasing population pressure has led to shortened fallow periods, causing further losses of soil fertility. However, land management practices such as agroforestry can provide a sustainable alternative to single cropping because of its ability to maintain or increase the SOC pool. This study quantified SOC and nitrogen (N) pools, gross SOC turnover, residue stabilization efficiency (RSE_AC) in the alley crop, soil δ¹³C partitioning, C₃-C abundance and δ¹⁵N dynamics in 19- and 10-year Gliricidia sepium and Erythrina poeppigiana alley cropping system. Each system was studied at two fertilizer levels (tree prunings only [−N or −A], and tree prunings plus chicken manure [+N], or Arachis pintoi as a groundcover [+A]), and was compared to a sole crop system. The SOC and N pools were significantly higher (p < 0.05) in the 19-year-old alley crop compared to the sole crop, but not significantly different (p < 0.05) in the 10-year-old system. Soil C and N (%) showed a similar trend as that of the SOC and N pools in both 19- and 10-year-old systems. Gross SOC turnover, to a 20 cm depth, ranged from 12 to 21 years in the 19-year-old alley crop compared to 50 years in the sole crop, and from 20 to 32 years in the 10-year-old alley crop compared to 106 years in the sole crop. The RSE_AC ranged from 10% to 58% in the 19-year-old system, and from 3% to 43% in the 10-year-old system. The δ¹³C signature of the soil shifted significantly (p < 0.05) towards that of C₃ vegetation in the alley crop due to the greater input of organic residues from tree prunings compared to the sole crop. The proportion of input from tree prunings only in the 19-year-old alley crop ranged from 14% to 20%, and from 9% to 11% in the 10-year-old system to a soil depth of 20 cm. The δ¹⁵N signature of the soil showed two patterns: that of the 19-year-old system being enriched in δ¹⁵N, and that of the 10-year-old system being depleted in δ¹⁵N compared to the sole crop. The addition of manure in the 19-year-old system has enriched the soil δ¹⁵N and in the 10-year-old system the soil was depleted due to the N₂-fixing groundcover A. pintoi.     

碳氮稳定同位素检测能力的验证——2013年实验室间比对分析结果的汇总

稳定性同位素示踪技术,特别是碳、氮稳定性同位素已广泛地应用于农业化学、地球化学和环境化学。碳、氮同位素比值的质谱检测结果直接关系到示踪试验的可靠性。在目前缺乏富集碳、氮同位素的标准物质的条件下,通过实验室间的比对可以验证各实验室对碳、氮同位素比值检测能力、检测结果的准确性和可比性。我所在2013年组织和实施了一次由全国14个单位的检测实验室共17台仪器参加的农业、生态和环境样品中碳氮百分含量及其稳定性同位素比值的实验室间比对。我所制备了多种被检测的样品,有含碳氮的化学肥料、土壤和植物样品;在稳定性同位素富集度上,有自然丰度的和不同富集程度的同位素样品。除固体样品外,还有3种不同丰度的N2O和CO2气体样品。在进行检测结果的统计和评价时,采用Z比分衡量各实验室检测结果的可信度。本文汇总了2013年实验室间比对的结果。     

The influence of precipitation pulses on soil respiration - Assessing the “Birch effect” by stable carbon isotopes

Sudden pulse-like events of rapidly increasing CO₂-efflux occur in soils under seasonally dry climates in response to rewetting after drought. These occurrences, termed “Birch effect”, can have a marked influence on the ecosystem carbon balance. Current hypotheses indicate that the “Birch” pulse is caused by rapidly increased respiration and mineralization rates in response to changing moisture conditions but the underlying mechanisms are still unclear. Here, we present data from an experimental field study using straight-forward stable isotope methodology to gather new insights into the processes induced by rewetting of dried soils and evaluate current hypotheses for the “Birch“-CO₂-pulse. Two irrigation experiments were conducted on bare soil, root-free soil and intact vegetation during May and August 2005 in a semi-arid Mediterranean holm oak forest in southern Portugal. We continuously monitored CO₂-fluxes along with their isotopic compositions before, during and after the irrigation. δ¹³C signatures of the first CO₂-efflux burst, occurring immediately after rewetting, fit the hypothesis that the “Birch” pulse is caused by the rapid mineralization of either dead microbial biomass or osmoregulatory substances released by soil microorganisms in response to hypo-osmotic stress in order to avoid cell lyses. The response of soil CO₂-efflux to rewetting was smaller under mild (May) than under severe drought (August) and isotopic compositions indicated a larger contribution of anaplerotic carbon uptake with increasing soil desiccation. Both length and severity of drought periods probably play a key role for the microbial response to the rewetting of soils and thus for ecosystem carbon sequestration.     

Aggregate and organic matter dynamics in reclaimed soils as indicated by stable carbon isotopes

Recovery of belowground ecosystem processes, such as soil aggregation and organic matter (OM) accumulation, in reconstructed soils is crucial to successful reclamation of disturbed lands. Objectives of this study were to track soil aggregate recovery in combination with aggregate associated OM on a chronosequence of reclaimed surface mine sites and a native, undisturbed reference site. Macroaggregate and micro-within-macroaggregate proportions increased with reclamation age, while microaggregate proportions decreased. Organic carbon (C) and total nitrogen (N) concentrations increased with reclamation age for each aggregate fraction and were higher in the OM fraction observed within soil aggregates than in the free OM fraction found between soil aggregates. Naturally occurring isotopic signatures of ¹³C decreased rapidly with reclamation age, indicating over 50% of total aggregate C to be new C from predominately C₃ plant community inputs after 26 years of reclamation. Soil aggregate size distribution trends of increasing macroaggregation and micro-within-macroaggregates along with rapid rates of OM accumulation with time indicated that reclaimed soils had recovered structurally towards a native soil condition after a period of 10-15 years.     

Sorption of radioactive and stable Co and Zn isotopes by a soddy-podzolic soil and chernozem

The effect of the metal concentration on the interphase distribution and sorption kinetics of ⁶⁰Co/Co and ⁶⁵Zn/Zn was studied in experiments with soil suspensions (at the phase ratio of 1: 10) and wetted soil (60% of the total water capacity). It was shown that the sorption of ⁶⁰Co (⁶⁵Zn) by the soil solid phase decreases with an increase in the concentration of the heavy isotope in the system owing to the exchange between the radioactive and stable isotopes of the chemical elements; this increases the mobility and potential availability of the radionuclides to plants by many times. No fixation of ⁶⁵Zn/Zn by the solid phase was revealed in the experiment with the contamination level exceeding the background content of Zn in the soddy-podzolic soil by two-three times. Upon the application of 100–600 mg of Zn/kg of soil, the concentration of Zn and the volumetric activity of ⁶⁵Zn in the soil solution decreased by two times in 12–9 and 15–8 days, respectively.     

Application of the methods of allied sciences in soil studies

The advances in allied natural sciences can be successfully used in soil science. In particular, the theory of chemical catalysis; the notion of active centers of catalyzers; the theory of crystal growth; the theory of physicochemical strength of solid bodies; and the theory of matrix processes developed in chemistry, biology, and mineralogy are being applied in soil research.     

酸性土壤有机碳两种测定方法的比较

土壤有机质虽然占矿质土壤总量的比例很小,但对土壤质量及功能的调节起着关键作用。因此,寻求合适的土壤有机碳测定方法十分重要。本研究以4个长期定位试验(A、B、C、D)为基础,在灵敏度、相关性等方面对外加热法和碳氮仪法作了比较。结果表明:对于酸性土而言无论测定整土还是团聚体有机碳,两种方法的准确度和灵敏度都很高,但外加热法能更灵敏地反应处理间的差异性;两种方法测定结果有很好相关性,但碳氮仪法测定结果较外加热法偏高;由于测定结果存在差异,在科研工作中两种方法不能混用,以免得出错误结论。     

土壤有机碳储量及其变化评估的研究方法

土壤作为陆地生态系统中最大的碳库和碳循环的关键要素,其碳储量和变化直接影响全球碳平衡和气候变化过程。分析和总结土壤有机碳(SOC)储量影响因素及其大小、变化的研究方法,半定量的相关性分析和荟萃分析等可以用来确定SOC 储量的主要影响因子;定量化的实测实验方法如容量法、燃烧法和稳定同位素法等可较为准确地测算SOC 含量并用于区域储量估算,但存在耗时费力等成本缺陷;基于经验模型、机器学习、遥感反演等技术实现大尺度、快速测量SOC 及其变化的方法获得较快的发展,但需要解决精度较低和结果不确定性等突出问题;基于过程的模型方法需要根据具体的研究目标进行抉择和改进,重点是模型的验证和校准;SOC-生态系统模型需要重点开发涡流协方差(EC)系统与生命周期评估(LCA)的耦合方法和标准程序;SOC-景观模型需要在景观三维表征、SOC 转移流在单元间相互作用过程模拟等方面多学科攻关。     

中国三类典型稻田CH4排放的稳定性碳同位素变化

Little is known about the stable carbon isotopes of methane(CH4) emitted(δ~(13)CH_(4emitted)) from permanently flooded rice fields and double rice-cropping fields.The CH4 emission and corresponding δ~(13)CH_(4emitted) under various field managements(mulching,water regime,tillage,and nitrogen(N) fertilization) were simultaneously measured in three typical Chinese rice fields,a permanently flooded rice field in Ziyang City,Sichuan Province,Southwest China,a double-rice cropping field in Yingtan City,Jiangxi Province,Southeast China,and a rice-wheat rotation field in Jurong City,Jiangsu Province,East China,from 2010 to 2012.Results showed different seasonal variations of δ~(13)CH_(4emitted) among the three fields during the rice-growing season.The values of δ~(13)CH_(4emitted) were negatively correlated with corresponding CH4 emissions in seasonal variation and mean,indicating the importance of CH_4 production,oxidation,and transport associated with isotopic fractionation effects to the δ~(13)CH_(4emitted).Seasonal variations of δ~(13)CH_(4emitted) were slightly impacted by mulching cultivation,tillage,and N application,but highly controlled by drainage.Meanwhile,tillage,N application,and especially mulching cultivation had important effects on seasonal mean CH4 emissions and corresponding δ~(13)CH_(4emitted) with low emissions accompanied by high values of δ~(13)CH_(4emitted).Seasonal mean values of δ~(13)CH_(4emitted) from the three fields were similar,mostly ranging from —60‰ to — 50‰,which are well in agreement with previously published data.These demonstrated that seasonal variations of δ~(13)CH_(4emitted) mainly depended on the changes in CH4 emission from rice fields and further indicated the important effects of methanogenic pathways,CH4 oxidation,and CH4 transport associated with isotope fractionation effects influenced by field managements on δ~(13)CH_(4emitted).     

水分利用效率与用水有效性: 基于气孔视角的稳定同位素应用研究

澳大利亚多年降水较少并且分布不均,同华北平原相似,水资源短缺是制约粮食生产和作物产量的限制因子,水资源消耗中,农业用水所占比例最大,提高植物(作物)的水分利用效率和用水有效性对于实现水资源和农业生产的可持续利用和发展具有重要意义。本文针对澳大利亚和华北平原的农业生产和水资源现状,基于植物气孔角度在以下3个方面进行论述:1水分利用效率与用水有效性的概念;2基于气孔视角的水分利用效率和用水有效性的分析;3不同作物品种水分利用状况的同位素判别与应用。除此之外,也对华北平原目前农业水分利用情况提出相应建议。     

Microbial phospholipid biomarkers and stable isotope methods help reveal soil functions

This review targets microbial phospholipid biomarkers, their isotope analysis and their ability to reveal soil functions. The amount and composition of phospholipid fatty acids (PLFAs) measured in environmental samples strongly depend on the methodology. To achieve comparable results the extraction, separation and methylation method must be kept constant. PLFAs patterns are sensitive to microbial community shifts even though the taxonomic resolution of PLFAs is low. The possibility to easily link lipid biomarkers with stable isotope techniques is identified as a major advantage when addressing soil functions. Measurement of PLFA isotopic ratios is sensitive and enables detecting isotopic fractionation. The difference between the carbon isotopic ratio of single PLFAs and their substrate (Δ¹³C) can vary between −6 and +11‰. This difference derives from the fractionation during biosynthesis and from substrate inhomogeneity. Consequently, natural abundance studies are restricted to quantifying substrate uptake of the total microbial biomass. In contrast, artificial labelling enables quantifying carbon uptake into single PLFAs, but labelling success depends on homogeneous and undisturbed label application. Current developments in microbial ecology (e.g. ¹³C and ¹⁵N proteomics) and isotope techniques (online monitoring of CO₂ isotope ratios) will likely improve soil functional interpretations in the future. ¹³C PLFA analysis will continue to contribute because it is affordable, sensitive and allows frequent sampling combined with the use of small amounts of ¹³C label.     

Carbon isotopes of water‐extractable organic carbon in a depth profile of forest soil imply a dynamic relationship with soil carbon

Although considerable research has been conducted on the importance of recent litter compared with older soil organic matter as sources of dissolved organic carbon (DOC) in forest soils, a more thorough evaluation of this mechanism is necessary. We studied water‐extractable organic carbon (WEOC) in a soil profile under a cool‐temperate beech forest by analysing the isotopic composition (¹³C and ¹⁴C) of WEOC and its fractions after separation on a DAX‐8 resin. With depth, WEOC became more enriched in ¹³C, which reflects the increasing proportion of the hydrophilic, isotopically heavier fraction. The ¹⁴C content in WEOC and its fractions decreased with depth, paralleling the ¹⁴C trend in soil organic matter (SOM). These results indicate a dynamic equilibrium of WEOC and soil organic carbon. The dominant process maintaining the WEOC pool in the mineral soil appears to be the microbial release of water‐soluble compounds from the SOM, which alters in time‐scales of decades to centuries.     

Incorporation of S into soil organic matter in the field as determined by the natural abundance of stable S isotopes

In agricultural systems with low S inputs, soil organic matter is a major source of S and the transformations between organic and inorganic S pools are important for the supply of S to plants. This study was conducted to determine the effect of S fertilizer on the size and activity of organic S pools. For 5 years S fertilizer with a known composition of stable S isotopes was applied to a rotation on a loamy soil and a coarse sandy soil at rates higher than the plant demand. Total organic S in soil organic matter was not affected by sulphur application, but a small increase occurred in the sulphate ester fractions (P<0.05). Inorganic sulphate concentrations in the soil reflected the S application in the year of sampling, whereas S applied in earlier years was not recognized. Organic matter below the plough layer in both soils was enriched with S, possibly as a result or organic matter leaching or an increased clay content in the subsoils. At 0–20 cm, the C:S ratio in organic matter was ca. 100 for both soils, decreasing to 73 and 46 at 60–80 cm for the coarse sandy soil and the loamy soils, respectively. In both soils, isotope data showed that ca. 30% of organic-bonded S at 0–20 cm originated from fertilizer S applied during the last 5 years, irrespective of the S application rate. At 20–40 cm the rate of incorporations was lower and at 40–60 cm no incorporation of fertilizer S into organic matter was recognized. The fertilizer application did not induce net changes in the total organic S fraction, but isotope data indicated that a considerable part of the organic S pool was involved in S cycling in the field.