The verification method described here is adaptable for monitoring pedogenic, inorganic carbon sequestration in various agricultural soils amended with rocks containing alkaline earth metal silicates such as Wollastonite, basalt, and olivine. This method can be readily exploited by private or governmental entities for verifying soil inorganic carbon content in view of qualifying farmers for negative emission carbon credit. Enhanced weathering of minerals spread onto lands can also lead to carbon sequestration in scenarios beyond agriculture, such as pasture, forestry, or rehabilitated lands, and adrian soils.
The heterogeneity of agriculture soils, both adrial and depth-wise, makes it challenging to determine inorganic carbon content accurately, and subsampling of samples also contributes to reduced precision. The use of standard additions of calcium carbonate sample dividers, extensive analysis replication, and statistical analysis can help first-time researchers gain confidence with the proposed methodology. Demonstrating part of the field procedures will be Stephen Vanderburgt, a master's student from our laboratory.
Begin by determining leveling of each plot using a GPS receiver, then place flags at the boundaries of each plot to ease subsequent sampling. Collect core samples from random points within each subplot, one per subplot. Use a soil probe or a soil core sampler to collect the soil core down to three depth zones of zero to 15 centimeters, 15 to 30 centimeters, and 30 to 60 centimeters.
Use an extendible auger to collect deep soil samples from additional locations down to three depth zones of 60 to 100 centimeters, 100 to 175 centimeters, and 175 to 250 centimeters. Transfer the soil samples into buckets, one per each sample depth at each plot. Hand blend the soils in each bucket thoroughly, then place the portable moisture tester into the mixed soil sample and wait until the moisture content fixes at a stable point on the gauge of the device.
Press the holder button and record the value as the realtime moisture content of blended soils. Label sample bags properly with information about the plots, the soil depth, and date of sampling, then store the composite samples in the bags. Air dry the soil samples as soon as possible after sampling to minimize the oxidation of soil carbon.
Place the soil samples in cardboard boxes and place the boxes in a drying cabinet at 50 degrees Celsius for 24 to 48 hours, until the soil is dry. Store the air dried samples in sample bags until further analysis. Prior to soil fractionation, run the soil samples through a two millimeter sieve to remove large fragments of rocks and plant remains.
Oven dry the sieved soils by placing them in a muffle furnace maintained at 105 degrees Celsius, for at least 15 hours. For soil fractionation, place one kilogram of the oven dried sample onto the top mesh of the sieve shaker consisting of different mesh sizes. Shake the sieves at 60 RPM for 15 minutes.
Use pan fractions of less than 50 micrometers for analyses, as this is the pedogenic carbonate enriched soil fraction. To determine the inorganic carbon content of soil samples using calcimetry analysis, place five grams of a sieved soil sample in an appropriate Erlenmeyer flask. Suspend the sample in 20 milliliters of ultra pure water.
Add seven milliliters of four molar hydrochloric acid to a small flat bottomed glass test tube, then place this tube upright inside the flask using a pair of tweezers. Carefully attach the flask to the calcimeter by fixing the rubber stopper. Adjust and read the initial water level in the burette and seal its head space by turning the top valve to the measuring position.
Shake the flask, thereby knocking over the acid tube, until the water level in the burette reaches a constant value and no bubbling is observed in the solution. A typical set of data for a Wollastonite amended soil compared to a control untreated soil is shown here. The pH of the amended soil is higher by 1.15 units compared to the control, and the calcium carbonate content is nearly five times greater.
In the zero to 15 centimeter depth zone, the content was four times higher in the amended soil versus the control, and its pan fraction was enriched in calcium carbonate. The two deep profile samples had the highest contents, as these are naturally present carbonates in the C horizon. The various oxides present in the soil were determined by WDXRF.
The main oxides present are those that make up the main soil minerals, plant nutrients, and the alkaline earth metals. The XRD pattern of a Wollastonite amended soil is shown here. The main peaks present are quartz and albite, which are predominant minerals in sandy, loamy soils.
Peaks of the amended residual Wollastonite and of pedogenic calcite are also visible. Wollastonite amended soil was imaged using SEM after several weeks of weathering. A closer look at the Wollastonite particles shows the morphological changes occurring at the surface.
Microanalysis of the Wollastonite surface was performed by obtaining an elemental mapping of the sample. The EDS spectrum of the mapped area revealed its semi-quantitative chemical profile. The earlier elemental maps clearly show that the detected silicon and calcium are largely found in the acicular Wollastonite particles.
Spot EDS analysis was performed on the smaller fragments scattered in the soil sample. The fragments were rich in carbon and oxygen, suggesting that they are made up primarily of organic matter. When attempting this protocol, keep in mind that the sampling depth may vary in different areas, depending on the ease of sampling over the vertical profile, the thickness of the surface soil horizon, the depth of the water table, and soil structure.
Measuring stable isotopic and radiogenic carbon signatures over the profile of soil and subsoil could be incorporated in this procedure to further verify the sequestration of atmospheric CO2 in mineral amended fields.
