Monitoring Pedogenic Inorganic Carbon Accumulation Due to Weathering of Amended Silicate Minerals in Agricultural Soils.

Summary

The verification method described here is adaptable for monitoring pedogenic inorganic carbon sequestration in various agricultural soils amended with alkaline earth metal silicate-containing rocks, such as wollastonite, basalt, and olivine. This type of validation is essential for carbon credit programs, which can benefit farmers that sequester carbon in their fields.

Abstract

The present study aims to demonstrate a systematic procedure for monitoring inorganic carbon induced by enhanced weathering of comminuted rocks in agricultural soils. To this end, the core soil samples taken at different depth (including 0-15 cm, 15-30 cm, and 30-60 cm profiles) are collected from an agriculture field, the topsoil of which has already been enriched with an alkaline earth metal silicate containing mineral (such as wollastonite). After transporting to the laboratory, the soil samples are air-dried and sieved. Then, the inorganic carbon content of the samples is determined by a volumetric method called calcimetry. The representative results presented herein showed five folded increments of inorganic carbon content in the soils amended with the Ca-silicate compared to control soils. This compositional change was accompanied by more than 1 unit of pH increase in the amended soils, implying high dissolution of the silicate. Mineralogical and morphological analyses, as well as elemental composition, further corroborate the increase in the inorganic carbon content of silicate-amended soils. The sampling and analysis methods presented in this study can be adopted by researchers and professionals looking to trace pedogenic inorganic carbon changes in soils and subsoils, including those amended with other suitable silicate rocks such as basalt and olivine. These methods can also be exploited as tools for verifying soil inorganic carbon sequestration by private and governmental entities to certify and award carbon credits.

Introduction

CO₂ is a major greenhouse gas (GHG), and its concentration in the atmosphere is increasing continuously. Preindustrial global average CO₂ was about 315 parts per million (ppm), and as of April 2020, the atmospheric CO₂ concentration increased to over 416 ppm, hence causing global warming¹. Therefore, it is critical to reduce the concentration of this heat-trapping GHG in the atmosphere. Socolow² has suggested that to stabilize the concentration of atmospheric CO₂ to 500 ppm by 2070, nine 'stabilization wedges' will be required, where each stabilization wedge is an indiv....

Protocol

1. Soil sampling method and core collection

1. Divide the mapped and a demarcated agricultural land area of interest into different plots based on the land elevation, historical crop yield, and/or land management strategy. Determine the leveling of each plot using a GPS receiver, classify crop yield based on historical farm records (below-average, average, above-average), and the land management strategy used for each plot (types of soil amendments used, if any). Place the flags at the boundaries of each plot to.......

Discussion

Given that collecting samples from fertilized agricultural fields is usually difficult, it is suggested that samples should be collected before nutrient application. It is also advisable to avoid collecting samples from frozen fields. The sampling depth may vary in different areas depending on the ease of sampling over the vertical profile, and the depth of the water table. The selected soil sampling device is dependent on the soil structure and depth of interest³³. While it is more convenient to .......

Materials

Name	Company	Catalog Number	Comments
Analytical scale	Sartorius	Quintix 224-S1	Four decimals.
Calcimeter	Eijkelkamp	Model 08.53	To determine the wt% CaCO3-equivalent in the sample.
Drying cabinet/muffle furnace	Thermo Scientific	F48055-60	50°C or 103 ± 2°C.
HCl	Fisher Scientific	A144S-500	Reagent grade (36.5%-38.0%).
HNO3	Fisher Scientific	T003090500	Trace metal analysis grade (69.0%-70.0%)
Inductively Coupled Plasma Mass Spectrometer (ICP-MS)	PerkinElmer	NexION	To determine the concentration of Ca in the microwave-digested soil.
Microwave digester	PerkinElmer	Titan	To digest soils in concentrated HNO3.
pH meter	Oakton	700	Calibrated with standard solutions before each set of measurements; temperature corrected to 25 °C.
Scanning Electron Microscope -Energy Dispersive Spectroscope (SEM-EDS)	Oxford	X-Max20 SSD	To determine the morphology of soil particulates.
Sieve shaker	Retsch	AS-200	For soil fractionation.
Soil auger sampler	Eijkelkamp	01-16	Depths down to 700 cm.
Soil Dakota probe sampler	JMC	PN139	Depths down to 100 cm.
Soil probe sampler	JMC	PN031	Depths down to 30 cm.
Soil moisture meter	Extech	MO750	Measure moisture content up to 50%
Wavelength Dispersive X-ray Fluorescence spectroscope (WDXRF)	Malvern Panalytical	Zetium	To characterize elemental composition of soil.
X-ray Diffraction analyzer (XRD)	Panalytical	Empyrean	To characterize mineralogicalbproperties of soil.