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ABSTRACT

Soil carbon (C) plays a key role in mitigating and adapting to global climate change. In-situ

soil C measurement has faced many challenges including those related to aerial coverage,

economics, accuracy, and availability. The concept of paying for C credits to farmers and

ranchers who sequester C has necessitated availability of improved methods for in-situ

measurement of soil C at large scale. The objective of this review is to i) synthesize the existing

knowledge on methods of soil C measurement, (ii) discuss their pros and cons (iii)

review key factors affecting soil C measurement, and (iv) propose integrated data driven

method of soil C measurement using Machine Learning (ML)/Artificial Intelligence (AI)

approach. Lab and in-situ techniques of soil C determination are expensive, time consuming

and lack scale. Although, remote sensing (RS) technique is used to predict soil C maps at

large scale, it also lacks accuracy and requires high technical knowledge of image processing.

Soil C measurements are affected by key soil physical properties such as color, texture,

moisture content, bulk density etc. Thus, these factors must be considered while developing

innovative methods for soil C determination. A prototype handheld device is proposed to

measure these four properties along with Near Infrared (NIR) reflectance of soil that store

data in cloud using Wi-Fi signals. A data driven model is proposed that can use the data

from handheld devices and integrate with drone imagery to create soil C map of the entire

field and satellite imagery for the entire region. This model uses data from in-situ soil C

measurement technique in integrated form and soil C map can be updated every time the

handheld device is used at different locations of the field.

 

Introduction

Globally, soil holds total carbon (C) stock of

 2500 Pg (1Pg . 1015g) to 1-m depth, which is

approximately three times of that in the atmosphere

(800 Pg) [1–4]. Of the 2500 Pg of total C

stock; 1550 Pg is soil organic carbon (SOC) and

950 Pg is soil inorganic carbon (SIC) [1, 5]. In 2019,

about 4.8 billion hectares (B ha), 38% of global

land area, is agricultural land, of which one-third

(1.6 B ha) is cropland and two-thirds (3.2 B ha) is

meadows and pasture for grazing livestock [6,7].

Since 1990, the cropland area has increased by 5%

but permanent meadows and pastures decreased

by 4%, with an overall decrease of agricultural

land by 1% [7]. The human population was   250

million (M) in circa 1000 AD, increased to 6.1 B by

2000 and is projected to reach 9.8 B by the year

2050 [8,9]. This growing trend of population will

create greater demand for food and put pressure

on finite land and water resources [7]. Agriculture,

forestry, and other land uses (AFOLU) contribute

 23% of total anthropogenic greenhouse gas

(GHG) emission (2007-2016), of which 12-14% is

contributed by the agriculture land use [10]. The

conversion of natural vegetation to agricultural

land increases atmospheric CO2 concentration due

to increase in mineralization of SOC, accelerated

loss of erosion and reduction of input of biomass

-C [11–13]. Several studies [14–16] have documented

the loss of SOC stock upon conversion of

natural to agricultural ecosystems. However, the

depleted SOC stock can be re-sequestered through

adoption of improved rotations with deeper rooting

cultivars and species, use of organic amendments,

agroforestry [17–19]; along with conversion

to reduced and no-till (NT) practices [20,21] and

incorporation of soil organic matter (SOM) into

subsoil [22]. In contrast, high inputs of fertilizers

and use of conventional tillage can release soil C

stock into the atmosphere [23]. Strategies of SOC

protection and sequestration contribute 47% of

total of total potential mitigation (2.3 PgCO2e yr 1)

from grassland and agriculture, while 20% involves

other GHGs related with improved soil management

practices [24]. UN-Sustainable Development

Goals (SDGs) can also be advanced by increasing

or protecting soil C stock by long-term increase of

soil fertility, maintaining resilience to climate

change, reducing soil erosion, and improving habitat

conversion [25]. The United Nations Framework

Convention on Climate Change (UNFCCC) at

Congress of Parties (COP) 21 Lima-Paris Plan of

Action adopted new program of the Global

Climate Action Agenda (GCAA), known as “4 per

1000” initiative (https://www.4p1000.org/). It

aspires to increase SOM and C sequestration

through implementation of agriculture practices

such as agro-ecology, regenerative agriculture,

agroforestry, conservation agriculture (CA) or landscape

management at the annual rate of 0.4% to

40 cm depth [26,27]. Presently, the attention is also

directed to land-based efforts to reduce C emission,

remove CO2 from the atmosphere and provide

monetary credits to landowners [28,29].

The concept of C credit was introduced during

the UNFCCC in Kyoto, Japan in 1997 [30]. A C

credit is a tradable certificate or permit representing

the right to emit one metric ton (megagram or

Mg) of CO2 and has been used by many companies

to sell C credits to commercial and individual

customers [31,32]. This mechanism also provides

monetary value to the farmers or landowners who

are encouraged to protect or improve soil C stock

following sustainable agriculture methods [33].

The U.S. 117th congress passed the Growing

Climate Solutions Act of 2021 that authorizes

USDA to establish a voluntary GHG technical assistance

provider and third-party verifier certification

program to help reduce entry barriers into voluntary

environmental credit markets for farmers,

ranchers, and private forest landowners [34]. This

plan is proposed to convert diverse regions across

the U. S. into C sinks, aimed at offsetting the

nation’s 7Pg CO2e of GHG emission each year [35].

Private companies (i.e. Bayer, CIBO, ESMC,

Gradable, INDIGO, NORI, Soil and Water Outcomes

Fund, TRUTERRA) are also connecting with farmers

to follow sustainable agricultural practices (i.e. NT,

cover cropping) for 5-10 years and paying them a

US$7.5 to 24 ha 1 yr 1 [36,37]. These payments are

based on baseline field information, history of agriculture

farming and the future plan. Farmers have

to sign a contract for 5-10 years and follow the

specific guidelines to get paid for the C credit [36,

38]. The major problems for the farmers lie in getting

full benefits of C credit by using an easy and

affordable soil C measurement method [37].

Smallholder farmers are defined as households

who farm less than 2 hectares (ha) of land size and

obtain annual economic revenue from the same

farm [39]. Smallholder farming dominates the agricultural

farming in sub-Saharan Africa, South Asia

(i.e. India, China) where food security depends on

how small holders use their limited resources and

traditional knowledge to feed 80% of those populations

[40,41]. The UN Food System Summit

through its Action Track 3: Boosting Nature

Positive Production suggested an opportunity to

encourage small holder farmers in Africa and elsewhere

to improve soil health and fertility by using

the C credit program [42]. The farm size in Asia

and Africa is decreasing and the number of smallholder

farmers are in an increasing trend since

1950 and projected to increase through 2050 [43].

A study conducted by Shames [44] in Africa

showed that out of many challenges (project management,

cost, monitoring, unstable international

policy), inaccessible and expensive methods of soil

C measurement was the main challenge faced by

smallholder farmers. An economical and accessible

handheld device is instrumental in measuring soil

C and that will boost the C credit for all farm sizes around the globe. 

Researcher from the academic and industry sector

are working to develop data-driven model that

includes designing a handheld device, using cloud

to store data and artificial intelligence to predict

soil organic carbon or SOC [45,46]. Ewing et al. [45]

proposed affordable, accessible handheld device

(Our Sci Reflectometer; www.our-sci.net) an opensource

hardware tool licensed under the GNU

General Public License v3.0. This reflectometer

using the co-variates field estimable textural class

and slope class provides unbiased (r2.0.57;

n.1155; p.0.06) and actionable [area under

curve (AUC).0.88] data at field scale when compared

with African Soil Information Services (AFSIS;

www.soilgrids.org). Yard Stick (www.useyardstick.

com) is also developing a handheld device that

integrate Near Infrared (NIR) reflection, resistance

sensor to measure bulk density and GPS locator to

predict SOC using artificial intelligence. These initiatives

show the future scope of data-driven soil

carbon prediction at academic and industry sector.

Soil C sequestration is a potential climate solution

[24], and its credible and economic measurement

is key to addressing global climate change

[17]. Since the 1990s, several advanced analytical

methods have been developed to estimate soil C

stock [47–50]. Methods of soil C measurement can

be categorized into three groups: laboratory methods,

remote sensing techniques and in-situ procedures

[51]. These measurement methods have

their own merits, limitations, and challenges in

terms of cost involved, laboratory equipment

required, and accessibility to famers [52].

Therefore, the objective of this article is to (1) synthesize

the existing knowledge about the current

methods of soil C measurement, (2) discuss pros

and cons of those methods, (3) review key factors

affecting soil C measurement, and (4) propose

integrated methods to measure soil C using

Machine Learning (ML)/Artificial Intelligence (AI)

approach under in-situ conditions.

 

Methodology

The systematic review of literature was done thorough

Google Scholar, Web of Science, Scopus

using key search terms: (soil carbon measurement)

and (texture OR color OR moisture OR remote

sensing OR UAVs OR SOC OR in-situ OR machine

learning OR Artificial Intelligence OR laboratory OR

bulk density OR carbon credit). Scopus identified

6111 search results, Web of Science 9136, and

Google Scholar 36600 results of literature for string

search. These results were refined by using soil C

measurement in agricultural practices and fields

resulting in 352 articles in Scopus, 223 in Web of

science and 300 in Google Scholar. The bibliographic

details were imported into endnote and

duplicates were eliminated by applying exclusioninclusion

criteria and examining the title and keywords.

Finally, 161 potentially usable articles were

selected for this review.

These papers were categorized based on the

objectives of this review. Literature related to soil

C measurement (laboratory, remote sensing, and

in-situ) were reviewed and critically analysed to

prepare the pros and cons of each method under

different field studies. Four key soil properties

were identified, and tables developed on their

effect on soil C measurement. Literature was also

collated on use of machine learning and Artificial

Intelligence (AI) in predicting soil C at different

scales (field, farm and regional). Finally, with the

understanding of the different aspects of soil C

management a data driven model was 

suggested to predict soil C at different scales. 

Current methods of soil carbon

measurement

Direct measurement of soil C being difficult, some

methods are more direct, and some others are far

from direct with assumptions and sources of error

[51, 53]. Most available methods calculate soil C at

point source (lab methods) [47] or at large scale

(remote sensing or high-tech methods) [49, 54,55].

Laboratory analysis of soil C by the dry combustion

method is considered as the Gold Standard [56],

but it is expensive and time consuming to map

large areas [52]. Remote sensing techniques can

be useful for large scale measurement and mapping

of soil C [57] but have numerous errors [58].

The following section discusses commonly used

laboratory, remote sensing, and in-situ methods of

soil C measurement. 

Laboratory methods

Dry combustion or elemental analysis and wet oxidation

are commonly used laboratory methods for

soil C measurement [47, 59,60]. Dry combustion

method is regarded as the standard method to

conduct soil C analysis of field collected samples in

a laboratory [60]. This method is considered to

measure soil C with high precision and accuracy

[61,62]. It is quick and reliable with the use of an

automatic elemental analyzer. In the dry combustion

method, SOC is oxidized, and carbonate (CO3

-)

minerals are thermally decomposed in a medium

temperature resistance furnace. The CO2 produced

is then trapped in a suitable reagent and determined

titrimetrically or gravimetrically. The elemental

analyzers to measure soil C by the dry

combustion method are produced by the different

companies [60]. However, the working principle is

same for all, and uses a subsample of oven dried

soil (40  C) sieved through 250 microns

(lm.1 10 6 m) mesh. Some of the commonly

used elemental analyzer for C analysis are LECO

TruSpec CN, St. Joseph, MI [63]; Carlo-Erba Model

NA 1500 analyzer, Milan, Italy [64]; Perkin-Elmer

CHN2400, Northwalk, CT [65]; Thermo Scientific

Flash 2000, Massachusetts, USA [66]; VELP

Scientifica CN 802 CN elemental analyzer, Italy [67].

The oldest method of soil C measurement, the

wet oxidation method [68], is still used in many

countries [5, 69]. This method is simple, rapid and

needs minimal equipment [69,70]. Some major

problems with the wet oxidation method include

disposal of the waste produced during the procedure

and being less accurate in soils with 

carbonates [71]. Wet oxidation method is based on

oxidation of the C using potassium dichromate

(K2Cr2O7) in sulfuric acid (H2SO4). In the wet oxidation

method, only the most active organic C is oxidized,

leading to incomplete oxidation of organic

compounds [60].

The detailed process of Walkley Black (wet oxidation)

method for soil C measurement is

described by Nelson and Sommers [60]. Oven

dried (40  C) and finely grind [250 mm sieved] 1 g

soil is placed in an Erlenmeyer flask (125 ml), and

10 ml of 0.2M potassium dichromate solution is

added. Then 10 ml concentrated sulfuric acids

slowly added to this solution. The soil sample with

higher amount of SOC (>4%) requires a higher

amount of potassium dichromate and sulfuric acid

solution. After 30 min of oxidation under room

temperature, 50 ml (cm3) of distilled water, 3ml

(cm3) of concentrated orthophosphoric acid

(H3PO4) and four drops of the diphenylamine indicator

are added. SOC content is determined by

titration of the excess potassium dichromate using

Mohr’s salt solution (0.1 M). Each set of soil samples

require three blank reagents of Mohr’s salt

solution to record its exact molarity. SOC content

in the soil is determined using the following equation

(i): 

 

 

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