Soil Carbon Inventory for National Greenhouse Gas Inventory

Estimates of emissions and removals of SOC or stock changes in mineral and organic soils in land-use categories and subcategories are required for national greenhouse gas inventories. Stocks of SOC in different land-use categories need to be estimated for a selected inventory year using carbon stocks estimated at two points in time separated by several years because measurement may not be feasible over a period of 1 year. The methods described in this chapter could be adopted for generating emission and/or sequestration factors relevant to SOC in estimating national greenhouse gas inventories (Chapter 16).

13.2 Methods for Inventory of Soil Organic Carbon

Several methods are available and in use for estimating SOC, ranging from simple laboratory estimations to diffuse reflectance spectroscopy.

• Wet digestion or titrimetric determination (Walkley and Black method)

• Colorimetry

• Direct estimation of organic matter by loss-on-ignition

• Diffuse reflectance spectroscopy

The most common method used in the field is the wet digestion or titrimetric determination method, which is also a cost-effective method. The carbon, hydrogen and nitrogen (CHN) analyser, although very accurate, is rarely used in field studies because the instrument is expensive. Diffused reflectance spectroscopy is also expensive and yet to be widely used in field. Modelling is limited by the availability of models and data to represent local conditions. Remote sensing method can be used only for large projects, but still needs modelling and validation by data obtained from other methods.

(i) Wet digestion or titrimetric determination Wet digestion involves a rapid titration procedure to estimate the organic carbon content of soil (Kalara and Maynard 1991). Principle Organic matter is oxidized with a mixture of K2Cr2O7 and H2SO4. Unused K2Cr2O7 is back-titrated with ferrous ammonium sulphate (FAS). Organic carbon in the soil is oxidized to CO2.

Material A burette, pipette, 500 ml conical flask, measuring cylinder and analytical balance.

Reagents

• 1N K2Cr2O7solution: dissolve 49.04 g of K2Cr2O7 in minimum amount of distilled water and make up the final volume to 1 l

• 0.5 N ferrous ammonium sulphate (FAS) or Mohr's salt: dissolve 392 g FAS in distilled water. Add 15 ml of concentrated H2SO4 and make up the volume to 2 l with distilled water

• Diphenylamine indicator: dissolve 0.5 g of diphenylamine in a mixture of 100 ml of concentrated H2SO4 and 20 ml of distilled water

• Concentrated H2SO4 containing 1.25% Ag2SO4 (silver sulphate): if the soil is free of chlorides, use of Ag2SO4 can be avoided

• Sodium fluoride (NaF) or orthophosphoric acid 85%

Procedure

1. Weigh 0.5 g of powdered and sieved (2 mm) soil into a 500 ml conical flask

2. Add 10 ml of 1 N K2Cr2O7 solution and shake to mix

3. Add 20 ml of concentrated H2SO4 from the sides of the flask

4. Keep the contents of the flask undisturbed for 30 min

5. Add 3 g NaF or 10 ml of H3PO4 and 100 ml of distilled water and shake vigorously

6. Add 10 drops of diphenylamine indicator, which turns the solution violet

7. Titrate against 0.5 N FAS solution until the colour changes from violet to bright green and note the volume of solution used

8. Carry out a blank titration in a similar manner without the soil.

Calculations

- Weight of the sample = Sg

- Volume of FAS used in blank = Xg

- Volume of FAS used to oxidize SOC = Yg

- Volume of 1 N K2Cr2O7 used for the oxidation of carbon = (X-Y)/2

- Percentage of organic carbon in the soil = [(X-Y)/2 x 0.003 x 100]/S

Conclusion This is the most commonly adopted method in most laboratories with minimal facilities, as it does not require sophisticated equipment. This is also the least expensive method and the results are reasonably accurate. If CHN analyser is available, it is desirable to compare the results from the wet digestion method with those from the CHN analyser for validation and adopt a correction factor if necessary.

(ii) Direct estimation of organic matter by loss-on-ignition

Principle Organic matter is oxidized by heating at 375 °C and estimated by weight loss.

Material A muffle furnace, porcelain crucibles, a desiccator.

Procedure

1. Heat porcelain crucibles for 1 h at 375 °C.

2. Cool in open to about 150 °C. Place in a desiccator, cool for 30 min and weigh.

3. Weigh about 5 g oven-dried (to the nearest milligram) sample, passed through a 2 mm sieve, into each crucible.

4. Place the crucibles containing the samples in a muffle furnace at room temperature. Heat slowly (increase temperature by about 5 °C every minute) to 375 °C ± 5 °C.

6. Turn furnace off and let temperature drop to about 150 °C.

7. Remove crucibles and place in desiccator for 30 min. Weigh to the nearest milligram.

Calculation

Loss on ignition (%)

[weight of oven-dried sample (g) - weight of sample after ignition (g)] x 100

Organic matter % =

weight of oven-dried sample (g)

Merits and demerits The loss-on-ignition method of estimating organic matter is sufficiently accurate for most descriptive purposes. The method is most suitable for well-aerated samples (e.g. sandy and peat soils) with low clay mineral and inert carbon (charcoal) content.

However, the method is not suitable for calcareous soils. The procedure is prone to error as the weight loss may include carbon from carbonates and water and from hydroxyl groups from clay. Error is also caused by combustion of inert carbon compounds and volatilization of substances other than organic material. There is incomplete oxidation of carbonaceous materials in some soils at 375 °C.

(iii) Carbon, hydrogen, nitrogen (CHN) analyser Total organic carbon is a measure of the total amount of non-volatile, volatile, partially volatile and particulate organic compounds in a sample. Total organic carbon is independent of the oxidation state of the organic compounds and is not a measure of the organically bound and inorganic elements that can contribute to tests of biochemical and chemical oxygen demand.

Principle A CHN analyser analyses the carbon in solids based on the Dumas concept (Macko 1981) using helium and oxygen gases. The carbon in the sample is heated and carbon is oxidized (CO2) by oxygen in the presence of helium. The CO2 evolved is directly proportional to the content of carbon in the sample. The carbon evolved is detected by a CO2 detector. Results are represented as percentage.

Materials

• Induction furnace

° Leco WR-12, Dohrmann DC-50, Coleman CHN analyser, Perkin Elmer 240 elemental analyser, Carlo-Erba 1106

• Analytical balance: 0.1 mg accuracy

• Desiccator

• Combustion boats

• 10% hydrochloric acid

• Cupric oxide fines (or equivalent material)

• Benzoic acid or other carbon source as a standard.

Equipment preparation

• Clean combustion boats by placing them in the induction furnace at 950 °C. After being cleaned, combustion boats should not be touched with bare hands

• Cool boats to room temperature in a desiccator

Sample preparation

• If samples are frozen, allow samples to warm to room temperature

• Homogenize each sample mechanically

• Transfer a representative aliquot (5-10 g) to a clean container.

Collection and storage Samples can be collected in glass or plastic containers. The recommended total weight of the soil sample, consisting of multiple aliquots, is 25 g. If unrepresentative material is to be removed from the sample, it should be removed in the field and a note to that effect made on the field data sheet. Samples should be stored frozen and can be held for up to 6 months under such storage. Excessive temperatures should not be used to thaw the samples.

Laboratory procedures

Step 1: Dry each sample to constant weight at 70 °C. The drying temperature is relatively low to minimize loss of volatile organic compounds. Step 2: Cool the dried sample to room temperature in a desiccator. Step 3: Grind the sample using a mortar and pestle to break up aggregates. Step 4: Transfer a representative aliquot (0.2-0.5 g) to a clean, pre-weighed combustion boat. Step 5: Determine sample weight to the nearest 0.1 mg Step 6: Add several drops of hydrochloric acid to the dried sample to remove carbonates. Wait until the effervescing is complete and add more acid. Continue this process until the incremental addition of acid causes no further effervescence. Do not add too much acid at one time as this may cause loss of sample due to frothing. Step 7: Dry the acid-treated sample to constant weight at 70 °C. Step 8: Cool to room temperature in a desiccator.

Step 9 Add previously ashed cupric oxide fines or equivalent material (e.g.

alumina oxide) to the sample in the combustion boat. Step 10. Weigh the ascarite tube (A) before combustion. Step 11: Combust the sample in an induction furnace at a minimum temperature of 950 ± 10 °C and weigh the ascarite tube.

Calculations If an ascarite-filled tube is used to capture CO2, the carbon content of the sample can be calculated as follows:

where:

A = the weight (g) of CO2 determined by weighing the ascarite tube before and after combustion

B = dry weight (g) of the unacidified sample in the combustion boat 0.2729 = the ratio of the molecular weight of carbon to the molecular weight of carbon dioxide.

A silica-gel trap should be placed near the inlet end of the ascarite tube to trap any moisture driven off during combustion. Additional silica gel should be placed at the exit end of the ascarite tube to trap any water that may be formed by the reaction between the trapped CO2 and the NaOH in the ascarite.

If an elemental analyser is used, the amount of CO2 will be measured by a thermal conductivity detector. The instrument should be calibrated daily using an empty boat blank as the zero point and at least two standards. The standards should bracket the expected range of carbon concentrations in the samples. Conclusion A CHN analyser is a very reliable method, but the instrument is expensive to acquire and maintain. It can analyse a large number of samples at one time efficiently and effectively. This method is largely used for experimental studies and yet to find application in field projects.

(iv) Diffuse reflectance spectroscopy Diffuse reflectance spectroscopy (DRS) is a technology for non-destructive characterization of the composition of materials based on the interaction of visible-infrared light (electromagnetic energy) with matter. The method has the potential to increase efficiencies and reduce costs in both large-area applications (e.g. soil survey, watershed management, soil quality analysis) and site-specific management requirements. In particular, the ability to rapidly characterize a large numbers of samples using DRS opens up new opportunities in predicting and interpreting soil properties.

Principle Samples are illuminated with an artificial light source and the diffuse light reflected by the sample is collected and channelled through fibre optic cables to arrays of light detectors. The relative reflectance in each waveband comprises the reflectance spectrum for a sample, which is displayed and stored on a computer.

Procedure All soil spectral reflectance measurements are obtained using a Field spec ProFR spectroradiometer. The raw data must be processed before it can be used to predict soil properties. Multiple scans per sample allow the derivatives for a sample to be averaged and used in predicting soil properties.

Field sampling procedure

• Collect 20 soil cores (3 cm in diameter) to a depth of 1 m

• Mark each core location using a GPS so that the sample locations can be relocated in the future

• Dry and crush samples and pass through a 2 mm sieve

Laboratory procedure

• Pack air-dried soil samples, ground fine enough to pass through a 2 mm sieve, in polystyrene Petri dishes 55 mm in diameter and 12 mm deep

• Heap the Petri dishes with soil and scrape off excess soil using a blade to ensure a flat surface flush with the top of the dish

• Illuminate samples from above with two tungsten quartz halogen filament lamps in housings with aluminium reflectors

• Record the diffuse reflectance spectra of the samples using a FieldSpec FR spectroradiometer at wavelengths from 0.35 to 2.5 nm with a spectral sampling interval of 1 nm

• Record the average of ten spectra (the manufacturer's default value) at each position to minimize instrument noise

• Before reading each sample, record ten white reference spectra using calibrated spectralon placed at the same distance from the fibre optic as the soil sample

• Express reflectance readings for each wavelength band relative to the average white reference readings

Merits and demerits The main merits of DRS are higher precision and accuracy, repeatability and speed compared to conventional methods of soil analysis. With this method, a single operator can comfortably scan several hundred samples a day.

The main limitations of DRS include the need to build calibration libraries for a given population of soils for all the soil properties of interest and the complexity of the data analysis that this involves. Development of global calibration libraries in centralized laboratory facilities and software development for automated data analysis could help to reduce these limitations. Also, the method needs expensive instrumentation and trained staff.

Conclusion Further research and commercial development may lead to cheaper and more portable spectrometers, coupled with more flexible software and easier calibration methods. The technology should be increasingly used in a wide range of soil studies and surveys, and spectrometers are likely to become standard equipment in soil laboratories.

13.3 Broad Procedure for Soil Carbon Inventory

Making an inventory of soil carbon involves estimating the quantity of organic carbon present in the soil of a given land-use category or project activity at a given depth. Soil organic carbon is less frequently estimated compared to above-ground biomass, since the annual rate of change is low. A soil carbon inventory involves

• Estimation of bulk density of the soil at the specified depth

• Estimation of the concentration of organic carbon content in the soil sample

• Conversion of organic carbon content to tonnes of carbon per unit area (tC/ha) for a given depth of soil, using the bulk density.

The broad steps involved in inventory of soil carbon are as follows:

Step 1: Select the land-use category and project activities, stratify the area and demarcate project boundary according to the strata defined Step 2: Determine the frequency of measurement Step 3: Select the method for estimating

° Bulk density ° Soil organic carbon content

Step 4: Select the sampling technique

Step 5: Prepare for fieldwork

Step 6: Locate sampling points in the field

Step 7: Collect soil samples for laboratory analysis

Step 8: Measure bulk density parameters in the field

Step 9 Analyse the soil samples in the laboratory

Step 10. Enter field data and laboratory results into the database

Step 11: Calculate the quantity of soil organic carbon (tC/ha)

Determination of soil organic carbon requires access to laboratory facilities. It is important to decide whether soil organic carbon is a key pool and if this pool is likely to be impacted by the project activities and choose the frequency of estimation.

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Responses

  • ZULA
    How to make 1.25% ag2so4 involing h2so4?
    3 years ago

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