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.