Keywords
Soil organic carbon; PM10; Dust; Unpaved rural roads; Agricultural soil
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 µm
(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.
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.
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.