Evaluation of a rapid microwave digestion method for determination of total organic carbon in soil

Precise measurement of soil organic carbon (SOC) is essential for constructing regional inventories, developing best agricultural management practices, and modeling purposes. Currently, the automated dry combustion method is considered standard, but the method is both costly and time-consuming. There is a need for a simple, easy to use and cost-effective method of organic C determination in soil. A simple method of total organic carbon (TOC) determination in soil that involved wet digestion of K₂Cr₂O₇-H₂SO₄-soil mixture in a commercial microwave oven followed by spectrophotomteric measurement of Cr (III) was evaluated. The method was compared with automated dry combustion and two other wet digestion methods. The method showed close agreement with dry combustion method (R² = 0.90; root mean square error = 0.70) and the TOC measured with the two methods did not differ for a range of soils drawn from lowland and upland land-uses and varying in pH (6.2–9.3), TOC (2.8–14 g kg^?1), and calcium carbonate content (0–6.7%). The recovery of the added organic C by the microwave method was 98.6 ± 4.2%. The results suggested that microwave-spectrophotometric method could be easily adopted in routine soil analysis as it is not only precise, rapid, and cost-effective but also produced small volume of reagent waste.     

A simple procedure for colorimetric determination of organic carbon in soil

Abstract

A simple procedure is described for the rapid determination of organic carbon content in a large number of soil samples using an automated segmented flow analyser. The method is a modification of the wet oxidation/ digestion methods based on potassium dichromate combined with a colorimetric method for determination of chromic oxide (Stevenson and Clare, 1962). Using a segmented flow analyser for the colorimetry, the procedure is capable of automated routine analyses of large numbers of samples and was shown to give high and repeatable recoveries of organic carbon from a range of compounds. Organic carbon values for a range of soils analysed by the reported method and by the dry combustion technique showed a high correlation.     

A rapid and precise method for routine determination of organic carbon in soil

Abstract

A simple method for routine determination of organic carbon in soil by a modified Mebius procedure is described. It involves (a) digestion of the soil sample with an acidified dichromate (K₂Cr₂O₇‐H₂SO₄) solution for 30 minutes in a Pyrex digestion tube in a 40‐tube block digester preheated to 170°C and (b) estimation of the unreacted dichromate by titration of the cooled digest with an acidified solution of ferrous ammonium sulfate with use ofN‐phenylanthranilic acid as an indicator. The method is more rapid and precise than the Mebius procedure commonly used for routine analysis of soils for organic carbon, and the only equipment required for its use is equipment now commonly used for routine Kjeldahl analysis of soils for total nitrogen.     

A microscale method for colorimetric determination of urea in soil extracts

Abstract

The diacetyl monoxime colorimetric method of determining urea in soil extracts was modified for microplate format. A 100‐μL aliquot of extract was treated with color reagent in a disposable plastic microtiter plate (96 wells/plate), and color was developed by heating the plate in a low‐temperature oven at 87°C for 55 min. After cooling for 20 min at ambient temperature, absorbance measurements were simultaneously performed on all 96 wells using a microplate reader. The microscale method was faster and more convenient than the conventional method; moreover, the volume of waste was markedly reduced. Studies to compare the two methods showed very little difference in accuracy, precision, or sensitivity.     

A high sample volume procedure for the colorimetric determination of soil organic matter

Abstract

A modified procedure for routine determination of soil organic matter is described. The procedure employs a wet combustion method of reducing the carbon in soil organic matter with dichromate in an acid media. The modification will permit 700–800 organic matter determinations per day with samples containing less than 8% 0M without undue difficulty.     

A rapid steam distillation method of assessing potentially available organic nitrogen in soil

Abstract

A rapid steam distillation of assessing potentially available organic nitrogen in soil is described. It involves determination of the ammonia‐N produced by steam distillation of the soil sample with pH 11.2 phosphate‐borate buffer solution for 8 min. The method is simple and precise, and its results are not significantly affected by air‐drying or air‐dry storage of the soil sample before analysis. It is well suited for use in soil testing laboratories because it does not require extraction, filtration or transfer steps. Studies using 33 Brazilian soils showed that the results obtained by this method were highly correlated with those obtained by aerobic and anaerobic incubation methods of assessing potentially available organic nitrogen in soil.     

A rapid colorimetric method for analysis of nitrate nitrogen by reduction to nitrite

Abstract

A new method was developed for the determination of nitrate based on a soluble powder containing zinc and a bluish diazotizing solution. In this procedure about 10% of the nitrate content of the sample is converted to nitrite, but the intensity of the color developed is proportional to the nitrate content. The color changes from blue through green to yellow. The nitrate concentration can be estimated visually or determined spectrophotometerically at λ_max 400 nm in the range of 4–40 mg/L N. When the color change is very small, a coupling solution is added, and the violet color that develops can be measured colorimetrically at λ _max 545 nm in the range of 0.5–5 mg/L N. This simple method employs harmless chemicals and is especially suitable for use in agriculture. Reliable results are obtained in less than 10 minutes.     

土壤碱溶有机质的测定研究与应用

本文研究了一种土壤供氮能力快速测定方法-土壤碱溶有机质的测定,以及碱溶有机质与常规土壤有机质的相关性、碱溶有机质与植株氮素吸收的相关性及其在测土推荐施肥中的初步应用。选择我国有代表性的潮土、黑土、红壤土样各50个,研究土壤碱溶有机质与常规方法-丘林法测定的土壤有机质的相关性,结果显示二者呈极显著相关（p>0.01）,相关系数R = 0.9345,相关直线回归方程为：土壤有机质（gkg-1） = 3.3265碱溶有机质（gkg-1）+ 6.9389。选择代表我国主要土壤类型的28个土样进行盆栽试验,研究土壤供氮能力测定方法与植株氮素吸收的相关性,结果表明,土壤碱溶有机质与不施氮处理植株吸氮量相关性极显著（相关系数为0.628,p>0.01）,且其相关系数高于土壤有机质和土壤全氮与植株吸氮量的相关系数。虽然土壤碱溶有机质与植株氮素吸收的相关性低于生物培养方法即好气培养2周后土壤矿质氮（硝态氮+铵态氮）与植株氮素吸收的相关性,但土壤碱溶有机质更适合测土推荐施肥实验室高效快速测定的需要,可见碱溶有机质是可浸提的土壤腐殖质,可用比色法快速测定并能较好估测土壤的供氮能力,值得推荐在测土配方施肥实验室应用。     

土壤碱溶有机质的测定研究与应用

研究了一种土壤供氮能力快速测定方法-土壤碱溶有机质的测定,以及碱溶有机质与常规土壤有机质的相关性、碱溶有机质与植株氮素吸收的相关性及其在测土推荐施肥中的初步应用。选择我国有代表性的潮土、黑土、红壤土样各50个,研究土壤碱溶有机质与常规方法-丘林法测定的土壤有机质的相关性,结果显示二者呈极显著相关(p>0.01),相关系数r=0.9345,相关直线回归方程为:土壤有机质(g kg-1)=3.3265μ碱溶有机质(g kg-1)+6.9389。选择代表我国主要土壤类型的28个土样进行盆栽试验,研究土壤供氮能力测定方法与植株氮素吸收的相关性,结果表明,土壤碱溶有机质与不施氮处理植株吸氮量相关性极显著(相关系数为0.628,p>0.01),且其相关系数高于土壤有机质和土壤全氮与植株吸氮量的相关系数。虽然土壤碱溶有机质与植株氮素吸收的相关性低于生物培养方法即好气培养2周后土壤矿质氮(硝态氮+铵态氮)与植株氮素吸收的相关性,但土壤碱溶有机质更适合测土推荐施肥实验室高效快速测定的需要,可见碱溶有机质是可浸提的土壤腐殖质,可用比色法快速测定并能较好估测土壤的供氮能力,值得推荐在测土配方施肥实验室应用。     

A comparison of some methods for soil organic carbon determination

Abstract

This study compared three dichromate‐oxidation methods adapted for use with 100‐mL digestion tubes and 40‐tube block digester (for controlled heating), the Walkley‐Black method, a loss‐on‐ignition procedure and an automated dry combustion method for the determination of organic carbon in soils of the northwestern Canadian prairie. The Walkley‐Black method required a correction factor of 1.40. The modified Tinsley method and the Mebius procedure, adapted for use with 100‐mL digestion tubes, recovered 95% and 98%, respectively, of soil carbon against the dry combustion procedure. The presence of elemental carbon in some soils probably caused, at least partially, the slightly incomplete recovery; thermal decomposition of dichromate may not have been accurately corrected for. A dichromate‐oxidation procedure with controlled digestion at 135°C gave 100% recovery, but somewhat more variable results. The loss‐on‐ignition procedure, even when allowance was made for clay content of the soils, was the least satisfactory of the methods tested. All procedures produced correlation coefficients of 0.980 or better against the dry combustion method.     

A rapid accurate wet oxidation diffusion procedure for determining organic and inorganic carbon in plant and soil samples

Abstract

A method is presented for determining organic carbon content of soils, extracts and plant tissue utilizing an acidic wet oxidation in modified culture tubes; samples containing carbonates are pretreated with dilute acid. Evolved CO₂ is trapped in a NaOH solution and quantified titrimetrically. The method described has a range of 0.4–11 mg. carbon and the range can be extended in either direction by modifying the CO₂ trapping/titration procedure. This method is as accurate and precise as more time consuming dry oxidation procedures.     

Reproducibility of a soil organic carbon fractionation method to derive RothC carbon pools

Fractionation of soil is undertaken to isolate organic carbon with distinct functional properties, such as stability and turnover times. Soil organic carbon (SOC) fractionation helps us to understand better the response of SOC to changes in land use, management or climate. However, fractionation procedures are often poorly defined and there is little information available on their reproducibility in different laboratories. In a ring trial, we assessed the reproducibility of a SOC fractionation method introduced by Zimmermann et al. (2007). The isolated fractions were linked to the model pool sizes of the Rothamsted carbon model (RothC). We found significant differences between six laboratories for all five defined fractions in three different soils with coefficients of variation ranging from 14 to 138%. During ultrasonic dispersion, the output power (energy per unit time) was identified as an important factor controlling the distribution of SOC within these five fractions, while commonly only the output energy is standardized. The amount of water used to wet‐sieve dispersed soil slurry significantly influenced the amount of extracted dissolved organic carbon (DOC). We therefore suggest using a fixed amount of power for ultrasonic dispersion (20 W) and a minimum amount of water for wet sieving (2000 ml). RothC pool sizes were predicted from the measured fractions and compared with RothC equilibrium pool size distributions. This model initialization using measured SOC fractions, however, led to an over‐estimation of stable RothC SOC pools when compared with pool size distributions derived from RothC equilibrium runs under a bare fallow soil model simulation. To improve the isolation of particulate organic matter from stable mineral‐bound organic matter, we suggest that the density should be increased from 1.8 to 2.0 g cm^?3 in the density fractionation step. We formulated a modified fractionation procedure, which aims specifically to enhance reproducibility across laboratories and to improve the match of the isolated SOC fractions with RothC's SOC pools.     

A rapid chloroform-fumigation extraction method for measuring soil microbial biomass carbon and nitrogen in flooded rice soils

 A chloroform-fumigation extraction method with fumigation at atmospheric pressure (CFAP, without vacuum) was developed for measuring microbial biomass C (C_BIO) and N (N_BIO) in water-saturated rice soils. The method was tested in a series of laboratory experiments and compared with the standard chloroform-fumigation extraction (CFE, with vacuum). For both methods, there was little interference from living rice roots or changing soil water content (0.44–0.55 kg kg^–1 wet soil). A comparison of the two techniques showed a highly significant correlation for both C_BIO and N_BIO (P<0.001) suggesting that the simple and rapid CFAP is a reliable alternative to the CFE. It appeared, however, that a small and relatively constant fraction of well-protected microbial biomass may only be lysed during fumigation under vacuum. Determinations of microbial C and N were highly reproducible for both methods, but neither fumigation technique generated N_BIO values which were positively correlated with C_BIO. The range of observed microbial C:N ratios of 4–15 was unexpectedly wide for anaerobic soil conditions. Evidence that this was related to inconsistencies in the release, degradation, and extractability of N_BIO rather than C_BIO came from the observation that increasing the fumigation time from 4 h to 48 h significantly increased N_BIO but not C_BIO. The release pattern of C_BIO indicated that the standard fumigation time of 24 h is applicable to water-saturated rice soils. To correct for the incomplete recovery of C_BIO, we suggest applying the k _C factor of 2.64, commonly used for aerobic soils (Vance et al. 1987), but caution is required when correcting N_BIO data. Until differences in fumigation efficiencies among CFE and CFAP are confirmed for a wider range of rice soils, we suggest applying the same correction factor for both methods. Received: 1 June 1999     

蒸汽浴- 流动分析仪法测定土壤有机碳含量的方法

为了准确高效批量测定土壤有机碳含量,采用比色管代替消煮管,蒸汽加热消煮土壤,利用流动分析仪法测定有机碳。并将此方法与重铬酸钾氧化-外加热法(常规通用)进行了比较,探讨方法的可行性。结果表明,蒸汽浴-流动分析仪法测定土壤有机碳含量重复性试验的相对标准偏差为3.19%～9.41%,回收率为92.21%～97.63%。方法精密度及准确性良好。所测结果与重铬酸钾氧化-外加热法无显著性差异(P>0.05),两种方法拟合的回归方程：y(蒸汽浴-流动分析仪法)=0.947x(重铬酸钾氧化-外加热法)+1.621(r²=0.933),两者呈良好的相关性。用两种方法测定200个土壤有机碳,蒸汽浴-流动分析仪法的检测用时是重铬酸钾氧化-外加热法用时的68%,人工操作用时是重铬酸钾氧化-外加热法用时的44%,蒸汽浴-流动分析仪法无需使用硫酸亚铁,重铬酸钾、硫酸和水用量为重铬酸钾氧化-外加热法的40%。蒸汽浴-流动分析仪法省工省时,人员工作强度低,试剂用量少,环境危害降低,检测准确高效,经济安全,可推荐为大批量土壤有机碳含量测定的方法。     

A rapid method for the determination of fixed ammonium in soil

Abstract

Five different procedures for the oxidation of organic carbon were compared in order to determine the most suitable method for use as a pretreatment before the determination of fixed NH₄‐N in soils. Three of the procedures involved a method based on the use of a solution containing 0.1M Na₄P₂O₇, 0.1M NaCl, and 6% H₂O₂, applied for three different periods of time, 10 days, 20 days, and whatever number of days are required until frothing no longer occurs. This third period of time is designated as NF (no frothing). The remaining two procedures involved a method based on the use of a 3M NaOCl solution applied for two different periods of time, 3‐cycles and 5‐cycles.

Two of the procedures, the NF treatment with H₂O₂+NaCl+Na₄P₂O₇ and the 5‐cycle treatment with NaOCl, were found to give the must efficient removal of organic C. The amount of residual N in the soil after the NF treatment was highly correlated with the amount of residual N in the soil after the 5‐cycle treatment. It is concluded that the 5‐cycle treatment is as effective as the NF treatment for the oxidation of organic C in the soil before carrying out the determination of fixed NH₄‐N. The 5‐cycle treatment has the advantage of being a faster and simpler analytical procedure than the NF treatment.     

克里格插值在区域土壤有机碳空间预测中的应用

基于河北省第二次全国土壤普查数据,对比了常用土壤有机碳相关因子土地利用和土壤类型与普通克里格插值结合前后对土壤有机碳密度空间预测精度的差异,探讨了普通克里格插值在区域土壤有机碳空间预测中的应用。研究结果表明,土地利用能够独立解释土壤有机碳密度总方差的19.0%,与普通克里格插值结合以后能够将对土壤有机碳密度总方差的解释程度显著提高到30.2%。低级土壤分类土属能够独立解释土壤有机碳密度总方差的45.0%,但与普通克里格插值结合以后对土壤有机碳密度总方差的解释程度为44.8%,两者相差不大。因此区域空间上能否进一步应用普通克里格插值优化土壤有机碳的空间预测与所选用的土壤有机碳相关因子有关。     

A simple soil organic carbon level metric beyond the organic carbon-to-clay ratio

Soil is a precious and non-renewable resource that is under increasing pressure and the development of indicators to monitor its state is pivotal. Soil organic carbon (SOC) is important for key physical, chemical and biological soil properties and thus a central indicator of soil quality and soil health. The content of SOC is driven by many abiotic factors, such as texture and climate, and is therefore strongly site-specific, which complicates, for example, the search for appropriate threshold values to differentiate healthy from less healthy soils. The SOC:clay ratio has been introduced as a normalized SOC level metric to indicate soils' structural condition, with classes ranging from degraded (<1:13) to very good (>1:8). This study applied the ratio to 2958 topsoils (0–30 cm) in the German Agricultural Soil Inventory and showed that it is not a suitable SOC level metric since strongly biased, misleading and partly insensitive to SOC changes. The proportion of soils with SOC levels classified as degraded increased exponentially with clay content, indicating the indicator's overly strong clay dependence. Thus, 94% of all Chernozems, which are known to have elevated SOC contents and a favourable soil structure, were found to have either degraded (61%) or moderate (33%) normalized SOC levels. The ratio between actual and expected SOC (SOC:SOC_exp) is proposed as an easy-to-use alternative where expected SOC is derived from a regression between SOC and clay content. This ratio allows a simple but unbiased estimate of the clay-normalized SOC level. The quartiles of this ratio were used to derive threshold values to divide the dataset into the classes degraded, moderate, good and very good. These classes were clearly linked to bulk volume (inverse of bulk density) as an important structural parameter, which was not the case for classes based on the SOC:clay ratio. Therefore, SOC:SOC_exp and its temporal dynamic are proposed for limited areas such as regions, states or pedoclimatic zones, for example, in a soil health monitoring context; further testing is, however, recommended.     

长期施肥对土壤有机碳和无机碳的影响

利用18年长期定位试验,研究了在不同施肥条件下,土壤有机碳和无机碳在0~50 cm土层分布特征。结果表明,施肥对土壤有机碳的影响随着土层深度的增加而下降,0~7.5 cm土层的土壤有机碳比7.5~15 cm、15~30 cm、30~50 cm分别增加了4.6%、22.0%、63.1%,而无机碳含量随着土层深度的增加而增加,与有机碳的变化规律正好相反。不同种类的肥料对土壤有机碳的影响也不相同,化肥、有机肥长期配合施用和长期施用有机肥可以在0~30 cm土层增加土壤有机碳含量,降低土壤中的无机碳含量,而长期单施化肥对土壤的有机碳和无机碳含量无明显差异。     

土壤有机碳稳定性影响因素的研究进展

增加土壤碳汇是应对全球气候变化的有效措施,作为土壤碳汇来源之一的有机碳在其中发挥重要作用。过去几十年,土壤有机碳的分子结构性质被认为是预测有机碳在土壤中循环的主要标准。然而最近的研究结果表明有机碳的分子结构并非绝对地控制着土壤有机碳的稳定,而土壤环境因子与有机碳的相互作用显著降低了土壤有机碳被降解的可能性。土壤微生物不仅参与有机碳的降解,其产物本身也是土壤有机碳的重要组成成分。非生物因子直接或间接地控制着土壤有机碳的稳定,包括土壤中的无机颗粒、无机环境以及养分状况等。其中,有机碳与土壤矿物的吸附作用和土壤团聚体的闭蓄作用被普遍认为高效地保护了有机碳。土壤矿物的吸附作用取决于其自身的矿物学性质和有机碳的化学性质。土壤团聚体在保护有机碳的同时也促进了有机碳与矿物的吸附,而有机-矿物络合物同样可以参与形成团聚体。此外,土壤无机环境也影响着有机碳循环。总之,土壤有机碳的稳定取决于有机碳与周围环境的相互作用。同时,有机碳的结构性质也受控于环境因素。然而,无论有机碳的结构性质,还是其所处的生物与非生物环境,都是生态系统的基本属性,且各属性间相互影响、相互作用。因此,土壤有机碳的稳定是生态系统的一种特有性质。