Cotrufo et al. (2023). The robust concept of mineral‐associated organic matter saturation: A letter to Begill et al., 2023

), its controls, and effects on SOC

dynamics (e.g., Cotrufo et al.,

2019

; Six et al.,

2002

; Sokol et al.,

Using soils from the German Agricultural Soil Inventory, Begill

et al. (

) have questioned the concept of soil C saturation. They

posit that the available mineral surface area is not a physical limitation
on MAOC accumulation because organic matter (OM) can bind to other
OM bonded to minerals, in unlimited “skyscraper- like” structures. We
challenge their conclusion here based on the following points.

1. Physical fractionation method: The fine fraction most likely

contains particulate organic matter (POM)

The conceptual definitions of fractions do not fully correspond to
their procedural definitions (Leuthold et al.,

). Begill et al. dis-

persed aggregates using sonication, which is known to disintegrate
POM and disperse OM in solution even at energies below 100 kJ
(Leuthold et al.,

). They also used flocculant to precipitate

the OM in solution (

m) as mineral- associated OM (MAOM).

To discount POM contamination, they noted the consistent C- to-
nitrogen (N) ratio (10.7) of their fine fractions (

Figure

), assuming

POM has a higher C:N than that. However, the latter has been con-
tradicted by observations of C:N ratios of 10– 12 for fine POM (Six
et al.,

1998

). Furthermore, since they selected soils with C:N

13,

several may have POM C:N ≤ 10.7 and, for soils with C:N ratios
of 10.7– 13, a minor shift from the linear relation would indicate
POM contamination. That is what their data show; all fine fractions
with high C contents have C:N

10.7 (

Figure

), suggesting POM

contamination.

Potential overestimation of MAOC by Begill et al. (

) is further

suggested by slightly higher contributions of MAOC to SOC than other
studies, with average values of 86% for cropland and 72% for grassland,
versus global and continental- scale averages of 72%– 79% for croplands
and 70%– 72% for grasslands (Lugato et al.,

; Sokol et al.,

https://doi.org/10.1111/gcb.16804

2. Dominance of Gleysols among high organic carbon soils

Gleysols are fundamentally different from mineral soils for which
the C saturation concept was defined. They accumulate C because
of prevalent anoxic conditions and not necessarily through physi-
cochemical stabilization (Six et al.,

2002

). For the POM- rich Gleys-

ols, POM contamination to the fine fraction would certainly be
expected. Gleysols should therefore be avoided when assessing
MAOC saturation.

This article is a Letter to the Editor by Begill et al.,

. See also response to this letter by Poeplau et al.,

https://doi.org/10.1111/gcb.16920

F I G U R E 1

(a) Reproduction of carbon (C) versus nitrogen (N) in the fine fraction samples from Begill et al. (

https://plotd igiti zer.com/app

, figure 5). Ratios of C:N

for the high C samples were extracted using

plotdigitizer

(

) and are reported in red. (b) Second- order polynomial

versus linear fit of the relationship between soil organic carbon (SOC) and mineral- associated organic carbon (MAOC) for European
croplands (CR), forest (FR), and grassland (GR) soils. Data are from Cotrufo et al. (

2019

) and Lugato et al. (

https://github.com/eluga to/

). Regression models were

tested using the

anova

function of R software version 4.0.5 (MAOM_vs_SOC_model_regression.r), available at

SOC_satur ation

. [Colour figure can be viewed at

wileyonlinelibrary.com

]

5987

LETTER TO THE EDITOR

3. Support for previous report of C saturation

Begill et al. (

) questioned our observation of MAOC saturation

in European grasslands and forests (Cotrufo et al.,

2019

) because

there were no soils with

47 g MAOC kg

−1

soil. The samples were

selected to represent the distribution of total SOC and soil texture

observed across over

∼

20,000 European sampling locations (Lugato

et al.,

). Thus, that all the soils had

47 g MAOC kg

−1

soil was a

finding rather than the result of a biased selection.

While we recognize that our data were sparser for higher SOC

soils, there is adequate evidence for saturation behavior (

Figure

) for

all land uses: polynomial fits of the MAOC- to- SOC relationship were
statistically better and different from linear fits at

.001 (

Figure

Observing the MAOC- to- SOC relationships presented by Begill

et al. (figure 3), it appears that a second- order polynomial might have
fit better than the linear relationships for the low and medium fine
fraction soils (figure 3a,b), suggesting a saturation behavior, while
the linear relationship for the high fine fraction soils (figure 3c), may
be driven by a few samples with very high SOC (possibly Gleysols).

4. Is organic matter associated with organic matter bonded to

minerals equally stable?

Rather than challenging the C saturation concept, the growing evi-
dence for a patchy “skyscraper” model for what we define as MAOC
and the range in

C ages for MAOC (Six et al.,

2002

), should serve as

a reminder that separating a MAOC fraction by size and/or density
is not necessarily sufficient to identify the persistent SOC pool. It
is well known that MAOM can exchange with dissolved OM (Kle-
ber et al.,