Microplastics are emerging as a steadily increasing environmental threat. Wastewater treatment plants efficiently remove microplastics from sewage, trapping the particles in the sludge and preventing their entrance into aquatic environments. Treatment plants are essentially taking the microplastics out of the waste water and concentrating them in the sludge, however. It has become common practice to use this sludge on agricultural soils as a fertilizer. The aim of the current research was to evaluate the microplastic contamination of soils by this practice, assessing the implications of successive sludge applications by looking at the total count of microplastic particles in soil samples. Thirty-one agricultural fields with different sludge application records and similar edaphoclimatic conditions were evaluated. Field records of sludge application covered a ten year period. For all fields, historical disposal events used the same amount of sludge (40 ton ha-1 dry weight). Extraction of microplastics was done by flotation and particles were then counted and classified with the help of a microscope. Seven sludge samples were collected in the fields that underwent sludge applications during the study period. Soils where 1, 2, 3, 4, and 5 applications of sludge had been performed had a median of 1.1, 1.6, 1.7, 2.3, and 3.5 particles g-1 dry soil, respectively. There were statistical differences in the microplastic contents related to the number of applications that a field had undergone (1, 2, 3 < 4, 5). Microplastic content in sludge ranged from 18 to 41 particles g-1, with a median of 34 particles g-1. The majority of the observed microplastics were fibers (90% in sludge, and 97% in soil). Our results indicate that microplastic counts increase over time where successive sludge applications are performed. Microplastics observed in soil samples stress the relevance of sludge as a driver of soil microplastic contamination.

Microplastic pollution is becoming a major challenge with the growing use of plastic. In recent years, research about microplastic pollution in the environment has become a field of study with increased interest, with ever expanding findings on sources, sinks and pathways of microplastics. Wastewater treatment plants effectively remove microplastics from wastewater and concentrate them in sewage sludge which is often used to fertilise agricultural fields. Despite this, quantification of microplastic pollution in agricultural fields through the application of sewage sludge is largely unknown. In light of this issue, four wastewater treatment plants and 16 agricultural fields (0-8 sewage sludge applications of 20-22 tons ha^-1 per application), located in the east of Spain, were sampled. Microplastics were extracted using a floatation and filtration method, making a distinction between light density microplastics (ρ < 1 g cm^-3) and heavy density microplastics (ρ > 1 g cm^-3). Sewage sludge, on average, had a light density plastic load of 18,000 ± 15,940 microplastics kg^-1 and a heavy density plastic load of 32,070 ± 19,080 microplastics kg^-1. Soils without addition of sewage sludge had an average light density plastic load of 930 ± 740 microplastics kg^-1 and a heavy density plastic load of 1100 ± 570 microplastics kg^-1. Soils with addition of sewage sludge had an average light density plastic load of 2130 ± 950 microplastics kg^-1 and a heavy density plastic load of 3060 ± 1680 microplastics kg^-1. On average, soils' plastic loads increased by 280 light density microplastics kg^-1 and 430 heavy density microplastics kg^-1 with each successive application of sewage sludge, indicating that sewage sludge application results in accumulation of microplastics in agricultural soils.

The growing evidence of microplastic pollution in terrestrial ecosystems reveals adverse effects of microplastics on soil biota and plant growth. However, since large scale assessments are lacking, it is possible that the laboratory based experiments conducted have assumed unrealistic microplastic concentrations in soils. In this paper we present regional scale data on the presence of microplastics in soils under different land uses in the central valley of Chile, which is characterized by urbanization, agricultural, and mining operations. We identified microplastics in soils under four different land use systems having different management intensities (crop lands, pastures, rangelands, and natural grasslands), and all somewhat prone to accumulate microplastics from different sources. We analyzed 240 soil samples from Chile's central valley, trying to identify the most probable sources of the microplastics. Our hypothesis was that microplastics were ubiquitous in the environment and that their concentration peaks follow the intensity of fertilizer use (phosphorus), soil heavy metals concentrations derived from nearby mining operations (Zn and Cu), and distance to roads and urban areas. We did find evidence of microplastic pollution in crop lands and pastures (306 ± 360 and 184 ± 266 particles kg^-1, respectively), but we did not observe pollution of rangelands and natural grasslands. Distance to mining operations, roads, or urban areas did not increase the microplastic particles count. Our observations contradict the common belief that microplastics are ubiquitous in the environment and relate the pollution problem more to agricultural activities. However, our data do not provide sufficient evidence to identify the pollution source. This is the first study that reports on microplastic occurrence in soils at a broad geographical scale. For greater insight on this topic more studies that contribute monitoring data about microplastics in soils are urgently needed.

Microplastic accumulation in soil may have a detrimental impact on soil biota. The lack of standardized methods to identify and quantify microplastics in soils is an obstacle to research. Existing techniques are time-consuming and field data are seldom collected. To tackle the problem, we explored the possibilities of using a portable spectroradiometer working in the near infrared range (350-2500 nm) to rapidly assess microplastic concentrations in soils without extraction. Four sets of artificially polluted soil samples were prepared. Three sets had only one polymer polluting the soil (low-density polyethylene (LDPE), polyethylene terephthalate (PET), or polyvinyl chloride (PVC)). The fourth set contained random amounts of the three polymers (Mix). The concentrations of microplastics were regressed on the reflectance observed for each of the 2150 wavelengths registered by the instrument, using a Bayesian approach. For a measurement range between 1 and 100 g kg-1, results showed a root-mean-squared-deviation (RMSD) of 8, 18, and 10 g kg-1 for LDPE, PET, and PVC. The Mix treatment presented an RMSD of 8, 10, and 5 g kg-1 for LDPE, PET, and PVC. The repeatability of the proposed method was 0.2-8.4, 0.1-5.1, and 0.1-9.0 g kg-1 for LDPE, PET, and PVC, respectively. Overall, our results suggest that vis-NIR techniques are suitable to identify and quantify LDPE, PET, and PVC microplastics in soil samples, with a 10 g kg-1 accuracy and a detection limit ≈ 15 g kg-1. The method proposed is different than other approaches since it is faster because it avoids extraction steps and can directly quantify the amount of plastic in a sample. Nevertheless, it seems to be useful only for pollution hotspots.

A critical aspect of predicting soil organic carbon (SOC) concentrations is the lack of available soil information; where information on soil characteristics is available, it is usually focused on regions of high agricultural interest. To date, in Chile, a large proportion of the SOC data have been collected in areas of intensive agricultural or forestry use; however, vast areas beyond these forms of land use have few or no soil data available. Here we present a new SOC database for the country, which is the result of an unprecedented national effort under the framework of the Global Soil Partnership. This partnership has helped build the largest database of SOC to date in Chile, named the Chilean Soil Organic Carbon database (CHLSOC), comprising 13 612 data points compiled from numerous sources, including unpublished and difficult-to-access data. The database will allow users to fill spatial gaps where no SOC estimates were publicly available previously. Presented values of SOC range from $\mathrm{6}\times {\mathrm{10}}^{-\mathrm{5}}$ % to 83.3 %, reflecting the variety of ecosystems that exist in Chile. The database has the potential to inform and test current models that predict SOC stocks and dynamics at larger spatial scales, thus enabling benefits from the richness of geochemical, topographic and climatic variability in Chile. The database is freely available to registered users at https://doi.org/10.17605/OSF.IO/NMYS3 (Pfeiffer et al., 2019b) under the Creative Commons Attribution 4.0 International Public License.