ISO/FDIS 23400

Guidelines for the determination of organic carbon and nitrogen stocks and their variations in mineral soils at field scale

Lignes directrices pour la détermination des stocks de carbone organique et d’azote et de leurs variations dans les sols minéraux à l’échelle d’une parcelle

General Information

• Status: Published
• ICS: 13.080.01 - Soil quality and pedology in general
• Technical Committee: ISO/TC 190 - Soil quality
• Drafting Committee: ISO/TC 190/WG 1 - Soil and climate change
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 15-Jun-2021
• Completion Date: 15-Jun-2021

Standards Content (sample)

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23400
ISO/TC 190
Guidelines for the determination of
Secretariat: DIN
organic carbon and nitrogen stocks
Voting begins on:
2021­06­15 and their variations in mineral soils at
field scale
Voting terminates on:
2021­08­10
Lignes directrices pour la détermination des stocks de carbone
organique et d’azote et de leurs variations dans les sols minéraux à
l’échelle d’une parcelle
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 23400:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2021
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ISO/FDIS 23400:2021(E)
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© ISO 2021

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ISO/FDIS 23400:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 3

5 Procedure..................................................................................................................................................................................................................... 3

5.1 Site investigation strategy ............................................................................................................................................................. 3

5.2 Sampling ....................................................................................................................................................................................................... 4

5.2.1 Sampling objectives ....................................................................................................................................................... 4

5.2.2 Sampling plan ..................................................................................................................................................................... 4

5.2.3 Sampling strategy............................................................................................................................................................ 5

5.2.4 Sample handling, storage and transport in the field ......................................................................10

5.2.5 Sample handling and storage in the laboratory .................................................................................10

5.2.6 Safety .......................................................................................................................................................................................11

5.2.7 Environmental Protection ....................................................................................................................................11

5.2.8 Quality assurance during sampling ..............................................................................................................11

5.2.9 Sampling report .............................................................................................................................................................11

5.3 Determination of the dry mass and the volume of the soil sampled .....................................................11

5.4 Chemical analysis ...............................................................................................................................................................................12

5.4.1 Sample processing for chemical analysis.................................................................................................12

5.4.2 Chemical analysis .........................................................................................................................................................12

6 Calculations of the organic C and N stocks .............................................................................................................................13

7 Measuring the temporal variations of soil organic carbon and nitrogen ..............................................14

7.1 General ........................................................................................................................................................................................................14

7.2 Calculation of SOC stock changes and uncertainties ...........................................................................................14

7.3 Possible source of errors .............................................................................................................................................................15

7.4 Information needed .........................................................................................................................................................................15

8 Reporting ...................................................................................................................................................................................................................15

8.1 Reporting for soil organic carbon and nitrogen stocks .....................................................................................15

8.2 Additional reporting for variation of soil organic carbon and nitrogen stocks............................16

Annex A (informative) Using minimum detectable difference to determine sample size .........................17

Annex B (informative) Equivalent soil mass procedure ................................................................................................................18

Bibliography .............................................................................................................................................................................................................................19

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ISO/FDIS 23400:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

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electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 190 Soil quality.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved
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ISO/FDIS 23400:2021(E)
Introduction

Soil comprise important pools in the biogeochemical cycles of carbon (C) and nitrogen (N), and thus

[1]

are critical for climate regulation either by emitting greenhouse gases (GHGs) or by sequestering C. .

Soils are the largest terrestrial reservoir of organic carbon, accounting for more carbon than contained

in atmosphere or biota. Consequently, relatively small changes in soil carbon stocks can equate to

considerable exchanges with other actively cycling carbon pools, such as the atmosphere. Estimation

of soil organic carbon stock changes is one of the main methods applied to determine long­term carbon

fluxes and to design carbon sequestration strategies. Soil organic carbon (SOC) is the balance between

inputs (e.g. plant residues, manure, etc.) and biologically mediated losses. Information on soil total

N stocks is valuable, because adequate N is critical for plant production while excessive N can be an

environmental hazard. Leakage of nitrous oxide (N O) from terrestrial systems to the atmosphere

(where it enhances radiative forcing and may catalyse stratospheric ozone (O ) destruction) is one

hazard associated with excessive soil N inputs. The ratio of organic C to total N stock can also provide

insight into SOC stability and potential for element retention in the soil. Climate policies promote actions

regarding the protection and increase of SOC stocks. Such measures require standardized methods to

assess the current SOC stocks at the relevant scale (e.g. plot, farm, region) and to verify the efficiency of

soil carbon sequestration actions. This document provides guidance on the measurement of carbon and

nitrogen stocks in soils and to the detection of their temporal variations.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 23400:2021(E)
Guidelines for the determination of organic carbon and
nitrogen stocks and their variations in mineral soils at
field scale
1 Scope

This document presents a method to quantify the soil organic carbon and nitrogen stocks in mineral

soils at plot scale. It also provides guidance on how to detect and quantify simultaneously the variations

of carbon and nitrogen stocks over time in mineral soils at field scale. It is based on several documents

[2] [3] [4] [5] [6] [7] [8]
already published , , , , , , .

This document does not apply to organic soils, soils with permafrost, wetland soils, or to soil layers

prone to submergence below the groundwater table.

NOTE 1 The possibility of increasing soil C storage is viewed as a means to sequester atmospheric carbon

dioxide (CO ) and mitigate greenhouse gas (GHG) emissions. Information on soil nitrogen (N) stocks is crucial

because it interacts with carbon cycling through plant nutrition and organic matter decomposition, and leakage

of N is of environmental concern (e.g. N O emissions, NO - leaching). Therefore, it is becoming increasingly

2 3

important to measure accurately the impact of changes of land uses and practices on organic carbon and nitrogen

stocks.

NOTE 2 While understanding changes in soil inorganic carbon it is important also to understand the land­

atmosphere exchange of CO , measuring stocks of soil inorganic carbon is outside the scope of this document.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments) applies.

ISO 16133, Soil quality — Guidance on the establishment and maintenance of monitoring programmes

ISO 18400­101, Soil quality — Sampling — Part 101: Framework for the preparation and application of a

sampling plan

ISO 18400­105, Soil quality — Sampling — Part 105: Packaging, transport, storage and preservation of

samples

ISO 18400­206, Soil quality — Sampling — Part 206: Collection, handling and storage of soil under aerobic

conditions for the assessment of microbiological processes, biomass and diversity in the laboratory

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
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ISO/FDIS 23400:2021(E)
3.1
spatial composite sample

two or more individual soil samples (e.g. cores) separated laterally in spacet having the same volume,

and coming from the same soil layer or depth increment
Note 1 to entry: Also called average sample or aggregated sample.

Note 2 to entry: Composite samples sometimes are collected to encompass more lateral variability and better

represent the mean of the measurement (e.g water content, or C concentration) than provided by a single soil

core.
3.2
land cover
observed (bio)physical cover of the Earth's surface
3.3
land use
socio­economic purpose of the land
3.4
land management practices

approach taken to achieve a land use outcome - the 'how' of land use (eg cultivation practices, such as

minimum tillage and direct drilling)
3.5
mineral soil
soil composed largely or entirely of mineral (inorganic) constituents
[SOURCE: ISO 14688­1:2017]
3.6
organic soil

soil in which the organic component is dominant with respect to the mineral component

Note 1 to entry: For the purpose of this standard, organic soils are taken to contain more than 50 % organic

matter by volume or more than 30 % organic matter by weight, i.e. 17 % of organic carbon. Please note that the

definition of ‘organic soils” varies between different soil classification systems.

3.7
organic soil layer

horizon dominated by organic material, consisting of undecomposed or partially decomposed litter,

such as leaves, needles, twigs, moss, and lichens, which has accumulated on the surface; they may be on

top of either mineral or organic soils
3.8
permafrost

ground consisting of mineral soil and sediment, rock, ice, peat ant other organic materials that remain

below 0 °C for at least two consecutives years
3.9
sampling point

precise position within a sampling site or within each soil constituting horizon from which samples are

collected

Note 1 to entry: The coordinates must include x and y dimensions to indicate lateral locations and may also

indicate the elevation of the soil surface in m relative to sea level.
3.10
undisturbed sample

sample obtained from the soil using a method designed to preserve the soil structure

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ISO/FDIS 23400:2021(E)
3.11
soil layer

layer of soil defined by its upper and deeper dimension (e.g. 0-30; 30-50 cm etc.) and/or by the sampling

procedure and it may comprise, or intersect, one or more soil horizons

Note 1 to entry: An horizon is a layer in soil that is roughly parallel to the ground surface and which is

[9] [10]

distinguished from layers above or below it on the basis of physical, chemical or biological differences ( , ).

Note 2 to entry: Horizon related sample: sample collected from and representing a defined soil horizon.

4 Principle

Organic carbon and nitrogen stocks in mineral soils reflect the balance between inputs and outputs of

C and N to the soil over decadal spans of time. Soil is heterogeneous due to variations in climate, parent

material, topography, organisms (including human activity) and time. Consequently, soil C stocks vary

with depth, location in space and sampling time. A proper sampling strategy should be implemented to

take this into account in order to get a representative estimate of C and N stocks. This generally entails

collecting several soil samples at different depths and locations.

To estimate soil organic carbon (SOC) and total nitrogen (TN) stocks, samples of a known volume shall

be collected, and the following determinations made:
— dry mass of the entire sample;
— dry mass of coarse (> 2 mm) mineral fragments or stones;
— fine soil (≤ 2mm) mass per volume sampled (“bulk density”);
— carbon and nitrogen concentrations in the fine soil fraction.

In general, significant field variations in organic carbon and nitrogen stocks occur very slowly, often

over a period of 5 to 10 years a minima, depending on climate and soil management practices. Careful

consideration of the complex factors governing the distribution of carbon and nitrogen stocks is

important for the sampling design over space and time to be able to differentiate spatial and temporal

variations.

Each step of the procedure (e.g. sampling, analysis) is associated with uncertainties, which can be

quantified in order to calculate the total uncertainties regarding stocks and stocks variations values.

However, it could suffice to collect replicate cores, recognizing that they will encompass variability in

space as well errors associated with all the steps. Separately quantifying analytical uncertainties can

verify that properly implemented methods using modern elemental analysers have small errors.

5 Procedure
5.1 Site investigation strategy

Site and soil description are necessary to interpret soil carbon stocks measurements and provide a

basis for extrapolation.

A site investigation strategy shall be prepared for the overall investigation. In addition to the sampling

strategy prepared in accordance with 5.2. This might include:
— description of the area of interest;

— current and past uses (e.g. crops, livestock, natural vegetation, restoration works) and management

(e.g. soil tillage, organic fertilization and amendment, cover crops, crop yields, crop residue removal);

— characterization of soils and profiles as deemed necessary, including for example soil type, layer

thicknesses, and basic physical and chemical characteristics;
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ISO/FDIS 23400:2021(E)

— use methods to record sampling locations that will permit precise positioning of subsequent (5 to

10 years in the future) sampling, including GPS coordinates with sub-metre resolution, distances to

other permanent features, installation of an electromagnetic marker.

Particular care shall be taken when developing the overall strategy and the sampling strategy so that

samples can be collected from the same sampling locations in future years to monitor changes in

soil organic carbon stock (in accordance with ISO 16133 relating to monitoring sites). Sufficient and

appropriate information on the site/area and soils should be collected to enable comparisons with the

results for other areas, when this is required.

NOTE 1 ISO 18400-202 gives detailed guidance on desk studies and site inspections (preliminary

investigations) and ISO 18400­205 gives further guidance relating to natural, near natural and cultivated sites.

ISO 18400-205 gives specific guidance on sampling in orchards etc. and wooded areas.

NOTE 2 ISO 25177 provides guidance on site and soil descriptions.
5.2 Sampling
5.2.1 Sampling objectives

The goal of soil sampling is to collect volumetric samples that represent the area of interest, and that

estimate mean soil OC and TN stocks (element masses per unit area to a specified soil depth and mass),

including estimates of variability (i.e scatter or dispersion of the data) about the means.

Determination of the soil organic carbon and total nitrogen stocks for a defined area (e.g., plot, field)

thus requires the boundaries of the area of interest to be delineated and the depth (range) of interest to

be decided.

The mass of soil also shall be stated. Assuming negligible geomorphological processes, comparisons

among soil OC or TN stocks should preferably be based on an equivalent soil mass, rather than on a

fixed volume.

It is also necessary to know the moisture content so that the results can be expressed on a dry weight

basis. The analytical measurements for C and N are made on the less than 2 mm fraction. It is therefore

necessary to know the mass of material (e.g. rocks, organic fragments) in the soil that is > 2 mm.

All organic matter in representative soil samples must be quantified, including the coarse (> 2mm)

organic fraction. Such materials can be ground or chopped to < 2 mm and included for analysis with the

entire < 2 mm mineral soil sample, or they can be isolated (e.g. as particulate or light fraction OM) and

analyzed independently, but they must not be discarded.
NOTE 1 Scale is discussed in ISO 18400-104, 5.6 and Annex E.

NOTE 2 Since sample processing and chemical analyses account for relatively small cost increments on sample

collection, it usually is preferable to perform independent analyses on separate sampling points and soil layers.

This provides important information on variability in three dimensions. In addition, when samples were taken

on at least two different collection dates, it makes it possible to distinguish temporal variability from spatial

variability.

NOTE 3 Depending on the specific program objectives, the coarse organic fraction could be determined (and

its C-N content measured) separately from the mineral soil to assess the time dynamics for specific purposes

(short term change of stocks, effect of a specific practice of OM management etc.). Particulate or light fraction

OM is often sensitive to management changes, and measuring > 2 mm fractions could provide early and valuable

indications of forthcoming changes in SOC stocks.
5.2.2 Sampling plan
A sampling plan shall be prepared in accordance with ISO 18400­101.

This should describe what is to be done to obtain the required samples and the practical requirements

for carrying out the work (i.e. how to implement the sampling strategy, see 5.2.3).

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ISO/FDIS 23400:2021(E)

Whatever the methods used to collect or form samples, their form and how they are to be taken should

be prescribed in the sampling plan.
5.2.3 Sampling strategy
5.2.3.1 General

A sampling strategy should be prepared in accordance with the guidance in ISO 18400-104 having

regard also to the guidance in ISO 18400-205. Usually based on the site investigation (5.1), the site may

[11]
be stratified in different zones using as a minimum the following variables :

— Land location (nearest settlement or roadway), legal land description, GPS coordinates;

— Typical soil texture, parent material, solum thickness, soil classification;

— Topography and landscape morphology (e.g. slope position, surface shape (concavity/convexity),

erosion forms, drainage and water regime);

— Biome, ecodistrict (if known), remote sensing images, vegetative cover, land use and management.

The sampling strategy should also:
— include all sampling activities that are to be undertaken;

— determine how to collect volumetric samples that represent the area of interest and that estimate

mean soil OC and TN stocks including estimates of variability about the means;
— provide information on spatial variation at the desired scale if required.

Mean elemental stocks of the area can be determined using composite sampling (see 5.2.2.2 – Figure 1)

or by averaging the stocks from independent sampling points (see 5.2.2.3 – Figure 1). The latter is

preferred because it will yield information on variability at the scale of the sampling pattern and it

allows pairing of sampling points from different sampling times to improve assessment of temporal

changes (see Clause 7.). However, depending on the budget, compositing can be required. Figure 1 gives

an overview of the different steps needed from sampling to calculating to obtain a mean elemental

value of the stock of an area.

Bulk density measurements and carbon and nitrogen concentrations should all derive from the same

core sample to determine the soil OC and TN stocks for that sample.
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ISO/FDIS 23400:2021(E)
Key
action
soil sample

Averaging the stocks from independent point samples is preferred because it will yield information on

variability at the scale of the sampling pattern and it allows pairing of sampling points among sampling

times to improve assessment of temporal changes. However, depending on the budget, compositing

can be required.

Depending on the specific program objectives, the coarse organic fragments could be determined (and

its C-N content measured) to assess the time dynamics for specific purposes (short term changes of

stocks, effect of specific practice of organic matter management, etc.). Coarse fragments of plant roots

and shoots and other organic materials may respond to management changes. Consequently, these

organic materials could be crushed or chopped to <2 mm and included with the < 2 mm mineral soil.

Figure 1 — Steps of soil samples collection, preparation and analysis for soil organic carbon and

total nitrogen stocks determination
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ISO/FDIS 23400:2021(E)

NOTE 1 A pre-sampling (e.g. with augers and/or soil pit description) can be performed to produce a first

qualitative assessment of soil carbon and nitrogen distribution and organize the sampling campaign.

NOTE 2 Composite sampling can also provide some information on variability depending on how it is applied.

The simplest way is to collect several (at least 2) composite samples.

NOTE 3 Initially it colud be useful to collect samples from more points than actually required so that

the minimum number of samples required to attain the minimum detectable difference can be calculated.

Furthermore, some samples might be lost.

NOTE 4 Mechanization of soil sample collection (e.g. using hydraulically driven soil core tubes) increases the

likelihood of obtaining the samples required for statistically significanct determination of soil OC and TN stocks,

Furthermore, such mechanization typically minimizes site disturbance, allowing subsequent samples to be

collected near (in space) to the initial ones. This might decrease the influence of spatial variability and increase

the detectability of temporal changes. However, the practicality of using such equipment, which is commonly in

the form of portable but heavy hand-operasted gear or a self-propelled tracked rig, will depend on the location

where it is to be used (e.g. topograsphy, vegetation).
5.2.3.2 Composite sampling

When desired, to reduce analytical costs, composite sampling may be carried out in accordance with

ISO 18400­104 which, among other things, provides guidance on:
— how to form composite samples;

— how many composite samples to take from an area of a given size (see Table 1 and ISO 18400­104:2018,

7.3.2).

Table 1 — Number of zones for composite sampling in relation to the total area of the site (after

ISO 18400-104:2018, Table 4)
Area Minimum number of Zones
A n
0 to 2 1
> 2 to 5 2
> 5 to 10 3
> 10 to 15 4
> 15 to 20 5
> 20 to 30 6

NOTE 1 For areas larger than those given, the following equation should be used to specify the number of zones to be

sampled: nA=+1 .

NOTE 2 The underlying assumption is that properties are generally uniform within the area to be investigated – if this

...