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\begin{document}

\title{Organic carbon content in the particulate matter emitted by rural soils.
A laboratory assessment.}



\author[1]{Mariano Mendez}%
\author[1]{Nancy Belén Ramirez Haberkon}%
\affil[1]{Consejo Nacional de Investigaciones Cientificas y Tecnicas}%


\vspace{-1em}



  
  \date{June 09, 2024}


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\selectlanguage{english}
\begin{abstract}
The aim of this study was to assess the organic carbon (OC) content in
the PM10 emitted by agricultural soils and rural roads under controlled
conditions. Samples were collected from agricultural soils and rural
roads. The PM10 was generated and collected using an electrostatic
precipitator coupled with the Easy Dust Generator (EDG). This procedure
ensures that the PM10 collected come specifically from soil. OC contents
were measured in both the soil and PM10. The enrichment ratio (ER) was
calculated as the ratio between OC content in the PM10 and OC content in
the soil. The results showed that OC content in the PM10 ranged from 2.7
to 3.5 \% in agricultural soils and from 1.4 to 2.9 \% in rural roads.
These values were comparable to the OC contents observed in fine
particles transported by the wind, but lower than OC contents observed
in PM10 samples collected in rural areas using active samplers and
filters. A quadratic function described the association between OC in
PM10 and OC in the soil. A negative potential function described the
association between ERs and OC in the soil. Both associations suggested
a saturation of OC in PM10 when the OC content in the soil was high.
This information is crucial for a better comprehending of the dust
emission role in the redistribution of OC within terrestrial ecosystems
and to the atmosphere and oceans.%
\end{abstract}%



\sloppy


\textbf{Organic carbon content in the particulate matter emitted by
rural soils. A laboratory assessment.}

Nancy B. Ramirez Haberkon\textsuperscript{1}, Mariano J.
Mendez\textsuperscript{1,2*}

\emph{1 Institute for Earth and Environmental Sciences of La Pampa
(INCITAP, CONICET-UNLPam), Argentina, cc 300, 6300 Santa Rosa,
Argentina}

\emph{2 National University of La Pampa, Faculty of Agronomy (UNLPam),
Argentina, cc 300, 6300 Santa Rosa, Argentina}

\emph{* Corresponding author. E-mail address: marianomendezz@hotmail.com
(Dr. Mendez Mariano Javier.).}

\textbf{Abstract}

The aim of this study was to assess the organic carbon (OC) content in
the PM10 emitted by agricultural soils and rural roads under controlled
conditions. Samples were collected from agricultural soils and rural
roads. The PM10 was generated and collected using an electrostatic
precipitator coupled with the Easy Dust Generator (EDG). This procedure
ensures that the PM10 collected come specifically from soil. OC contents
were measured in both the soil and PM10. The enrichment ratio (ER) was
calculated as the ratio between OC content in the PM10 and OC content in
the soil. The results showed that OC content in the PM10 ranged from 2.7
to 3.5 \% in agricultural soils and from 1.4 to 2.9 \% in rural roads.
These values were comparable to the OC contents observed in fine
particles transported by the wind, but lower than OC contents observed
in PM10 samples collected in rural areas using active samplers and
filters. A quadratic function described the association between OC in
PM10 and OC in the soil. A negative potential function described the
association between ERs and OC in the soil. Both associations suggested
a saturation of OC in PM10 when the OC content in the soil was high.
This information is crucial for a better comprehending of the dust
emission role in the redistribution of OC within terrestrial ecosystems
and to the atmosphere and oceans.

\textbf{Keywords}

Soil organic carbon; PM10; Dust; Unpaved rural roads; Agricultural soil

\textbf{Introduction}

Dust-borne carbon plays a significant role in the carbon cycle and is
closely linked to global warming and climate change (Wang and Jia,
2013). Soils are the main terrestrial reservoir of organic carbon (OC),
which accumulates in the first layer of the soil. Lands used for
agriculture and livestock are a large source of suspended dust
containing OC. Particles with an aerodynamic diameter less than 10 \selectlanguage{greek}µ\selectlanguage{english}m
(PM10, respirable dust) are emitted from topsoil layer by wind erosion,
tillage and traffic. Quantifying the OC content in PM10 emitted by rural
soils provides valuable insights for dust models to estimate the
redistribution of OC within terrestrial ecosystems and to the atmosphere
and oceans.

The precise determination of the OC content in PM10 emitted by rural
soils remains limited. Field studies on wind erosion have reported OC
content for particles larger than 10 microns (Li et al., 2009).
Environmental monitoring in rural areas has provided information on OC
in PM10 (Borlaza et al., 2022). However, PM10 collected with active
samplers represents a mixture of PM10 from primary biogenic aerosols,
dispersal units, plant residues and biomass burning among other (Huffman
et al., 2020). Only a limited number of studies have investigated the OC
content in PM10 specifically emitted by rural soil (Mendez et al., 2017;
Iturri et al., 2017). However, these studies have focused on a narrow
range of soils, and there is currently a lack of research examining
different rural soils.

In the central semiarid Pampas of Argentina, agricultural soils and
rural roads occupy the rural areas. Recent estimates suggest that the
actual PM10 emission from rural roads is 409 times higher than that from
agricultural soils (Ramirez Haberkon et al., 2021). Despite the
significant contribution of rural roads to PM10 discharge into the
atmosphere, the OC content in the PM10 specifically emitted by them
remain unknown. The aim of this study was to assess the OC content in
the PM10 emitted by agricultural soils and rural roads under controlled
conditions.

\textbf{Materials and methods}

Three agricultural soils for grain production (AG), three agricultural
soils for forage and grain production (AFG), three unpaved rural roads
inside farm field (RRI) and three unpaved rural roads outside farm field
(RRO) were sampled in the eastern region of La Pampa province, Argentina
(see supplementary material Table 1). From each soil, three subsamples
of 2.5 cm depth were collected, and the following determinations were
performed: texture, OC content and erodible fraction (see supplementary
material Table 2).

For each subsample, PM10 was generated using the Easy Dust Generation
(EDG) and the PM10 was collected using an electrostatic precipitator as
described by Mendez et al. (2017). The collection procedure consisted in
introducing 40 g of soil in the rotating chamber of the EDG for 30
minutes. In the rotation chamber aggregates broke up for collision
generating dust, which was collected, by electrostatic differences, on
an aluminum plate located inside an electrostatic precipitator. The
collected dust was carefully separated from the aluminum plate using a
brush, weighed, and stored in a plastic tube (Polistor 45 PST). This
procedure was repeated until approximately 0.5 g of PM10 was obtained.
Additional technical specifications, photographs, and detailed
procedural information can be found in Mendez et al. (2017).

The OC enrichment ratio (ER) was calculated as the quotient between OC
content in the PM10 and OC content in the soil. An ER higher than 1
meaning that the OC content in PM10 is greater than OC content in the
soil. An ER lower than 1 meaning that the OC content in PM10 is smaller
than OC content in the soil.

To assess the differences among means, ANOVA and LSD Fisher test
comparisons were performed using the Infostat program (See supplementary
material 1*) at a significance level of 0.05. Furthermore, for
describing the relationships between OC in the PM10, OC content in the
soil, and OC enrichment ratio, regression analysis in Microsoft Excel
was employed.

\textbf{Results and discussion}

The OC content in the PM10 ranged from 1.9 to 3.3 \% and the OC content
in the soil ranges from 0.3 to 1.6 \% (Fig. 1). In all rural soils
analyzed the OC content in the PM10 was higher than the OC content in
the soil. The same results were found in two agricultural soils of the
same region (Mendez et al., 2017; Iturri et al., 2017). It is well known
that OC tends to accumulate in clays and fine silts, which are two
particle fractions within the size range of PM10 (Jagadamma and Lal,
2010).

Environmental monitoring in rural areas reported OC content in the PM10
from 4 to 8 times higher than OC content in the PM10 found in this study
(Borlaza et al., 2022). Environmental monitoring used active samplers
which probably collected PM10 from multiple sources. Primary biogenic
aerosols, dispersal units, plant residues, vehicle emissions (tillage
operations) and biomass burning are sources of PM10 with high OC content
in rural areas (Huffman et al., 2020). This could explain the high OC
content in PM10 found in environmental monitoring of rural areas.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

The OC in AG soils was higher than in AFG, but the OC in the PM10 was
similar (between 3 and 3.3 \%) (Fig. 1). The OC in RRI was higher than
in RRO, but the OC in the PM10 was similar (between 1.9 and 2.2 \%)
(Fig. 1). The OC content in PM10 emitted by agricultural soils (AG and
AFG) was between 1.3 and 1.7 higher than that emitted by rural roads
(RRI and RRO). To our knowledge, this is the first report of the OC
content in the PM10 emitted by rural roads. The OC contents in the PM10
emitted by rural roads is valuable information to quantify the OC losses
from the soil and the OC discharges into the atmosphere by dust
emission. In semiarid pampas, actual PM10 emission per unit of area of
rural roads was 400 times higher than PM10 emission of agricultural
soils (Ramirez Haberkon et al., 2021).

Figure 1. Contents of organic carbon (OC) in soil and in PM10. AG:
agricultural soils for grain production (n=9); AFG: agricultural soils
for forage and grain production (n=9); RRI: rural roads inside farm
fields (n=9); RRO: rural roads outside farm fields (n=9). Different
lowercase letters indicate statistical differences between land
management systems (p\textless{}0.05). Bars represent standard
deviations.

The OC in the PM10 was associated to the OC in the soil. This
relationship was described by a quadratic function (Fig. 2), suggesting
that OC was accumulated in PM10 until reaches a maximum. The maximum OC
in the PM10 was 3.5 \% and it was reached in soils with more than 1.3 \%
OC. This relationship has not been explored in previous studies for
PM10, but it was explored for other soil fractions. In the clay fraction
(particles \textless{}2 \selectlanguage{greek}µ\selectlanguage{english}m) a maximum OC content was reached when the OC
in the soil was high than 3 \% (Jagadamma and Lal, 2010). A limit of
accumulation of OC also was found for particles fraction \textless{}20
\selectlanguage{greek}µ\selectlanguage{english}m (Hassink, 1997). Our results and those of previous studies suggest
that the OC can be accumulated in the PM10 until a limit (saturation).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2. Organic carbon (OC) content (\%) in PM10 and OC enrichment
ratio (ER) as a function of OC content in the soil. AG: agricultural
soils for grain production; AFG: agricultural soils for forage and grain
production; RRI: rural roads inside farm fields; RRO: rural roads
outside farm fields. **Statistically significant (p\textless{}0.01)

The OC ER was positive in all rural soils studied (Fig. 2). Other
studies also found positive OC ER in the PM10 and in the fine fractions
transported by the wind (Iturri et al., 2017; Mendez et al., 2017; Webb
et al., 2012). The OC ER was associated to the OC in the soil, and it
was described by a negative potential function (Fig. 2). Soils with low
OC contents (rural roads) showed high ERs and soils with high OC
contents (agricultural soils) showed low ERs. OC content in soils of
rural roads was low (Fig. 1) and most of the OC was associated to the
PM10 (Fig. 2). The low OC content in soils of rural roads can be
attributed to the absence of a continuous supply of labile OC (due to
lack of vegetation) and high rates of wind erosion (Ramirez Haberkon et
al., 2021). In rural roads, OC tends to accumulate in PM10, which
represents the stable fraction of OC that is protected chemically and
physically (Hassink, 1997). The stable fraction of OC is less sensitive
to soil management and wind erosion (Webb et al., 2012). Vegetal
coverage in agricultural soils played a crucial role in preventing the
loss of labile OC through wind erosion, while also facilitating the
accumulation of labile OC in the soil.

\textbf{Conclusions}

Our study revealed important findings regarding the OC content in PM10
emitted by agricultural soils and rural roads. This study provides the
first report on the OC content in the PM10 emitted by rural roads in the
region. The OC content in the PM10 was consistently higher than that in
the soil across all soils analyzed, indicating an enrichment of OC in
the PM10. This finding aligns with previous studies demonstrating the
accumulation of OC in fine particles.

This study reveals an association between organic carbon (OC) content in
PM10 and OC content in the soil. An association between OC ER and OC
content in the soil was also found. Both associations indicate that OC
accumulates in PM10 until it reaches a maximum (Saturation), which was
found to be 3.5\% for the analyzed soils. These findings enhance our
understanding of the dynamics of OC in PM10 and its association with the
OC content in the soil. Our study highlights the importance of
considering the OC content in the PM10 emitted by rural soils for
quantifying OC losses from the soil and OC discharges into the
atmosphere through dust emissions. Further investigations should examine
the OC content in PM10 emitted by rural soils across diverse soil
textures and geographical regions.

\textbf{Acknowledgements}

This study was financed by Consejo Nacional de Investigaciones
Cient\selectlanguage{ngerman}íficas y Técnicas, Argentina (CONICET), Universidad Nacional de La
Pampa, Facultad de Agronomía, Argentina (UNLPam) and Agencia Nacional de
Promoción de la Investigación, el Desarrollo Tecnológico y la Innovación
(PRESTAMO BID PICT 2019-N° 03297).

\textbf{References}

Borlaza, L.J., Weber, S., Marsal, A., Uzu, G., Jacob, V., Besombes, J.,
Chatain, M., Conil, S., Jaffrezo, J., 2022. Nine-year trends of PM10
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https://doi.org/10.5194/acp-22-8701-2022.

Hassink, J., 1997.~The capacity of soils to preserve organic C and N by
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Huffman, J.A., Perring, A.E., Savage, N.J., Clot, B., Crouzy, B.,
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Jagadamma, S., Lal, R., 2010. Distribution of organic carbon in physical
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Li, J., Okin, G.S., Epstein H.E., 2009. Effects of enhanced wind erosion
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Mendez, M.J., Aimar, S.B., Aparicio, V., Ramirez Haberkon, N.B.,
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Ramirez Haberkon, N.B., Aparicio, V.C., Mendez, M.J., 2021. First
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respirable dust (PM10) emitted from unpaved rural roads of Argentina.
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https://doi.org/10.1016/j.scitotenv.2021.145055.

Wang, H., and Jia, X., 2013, Field observations of windblown sand and
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Webb, N.P., Chappell, A., Strong, C.L., Marx, S.K., Mctainsh, G.H.,
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http://dx.doi.org/10.1111/j.1365-2486.2012.02780.x\selectlanguage{english}
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