Accumulation of microplastics in soil after long-term application of biosolids and atmospheric deposition.

Land-applied biosolids can be a considerable source of microplastics in soils. Previous studies reported microplastics accumulation in soils from biosolid application, however, little is known about the contribution of atmospherically deposited microplastics to agricultural soils. In this study, we quantified and characterized microplastics in soils that have been amended with biosolids over the past 23 years. We also collected atmospheric deposition samples to determine the amount and type of plastics added to soils through atmospheric input over a period of about 2 years. Soil samples were taken from a replicated field trial where biosolids have been applied at rates of 0, 4.8, 6.9, and 9.0 t/ha every second crop. The biosolids were anaerobically digested and dewatered, and were applied by spreading onto the soil surface. Soil and atmospheric samples were extracted for microplastics by Fenton's reaction to remove organic matter followed by flotation in a zinc chloride solution to separate plastic from soil particles. Samples were analyzed for microplastics by optical microscopy and Laser Direct Infrared Imaging Analysis (LDIR). The mean number of microplastics identified from biosolids samples was 12,000 particles/kg dry biosolids. The long-term applications of biosolids to the soil led to mean plastics concentrations of 383, 500, and 361 particles/kg dry soil in the 0-10 cm depth for low, medium, and high biosolids application rates, respectively. These plastic concentrations were not significantly different from each other, but significantly higher than those found in non biosolids-amended soil (117 particles/kg dry soil). The dominant plastic types by number found in biosolids were polyurethane, followed by polyethylene, and polyamide. The most abundant plastics in soil samples were polyurethane, polyethylene terephthalate, polyamide, and polyethylene. Atmospheric deposition contributed to 15 particles/kg dry soil per year and was mainly composed of polyamide fibers. This study shows that long-term application of biosolids led to an accumulation of microplastics in soil, but that atmospheric deposition also contributes a considerable input of microplastics.

Long-term accumulation, depth distribution, and speciation of silver nanoparticles in biosolids-amended soils.

Biosolids can be a source of metals and metal nanoparticles. The objective of this study was to quantify and characterize the accumulation and transport of silver (Ag) in a natural soil that has received agronomically recommended rates of biosolids as fertilizer from 1994 to 2017. Total Ag concentrations were measured in biosolids and soil samples collected from 0 to 10 cm between 1996 and 2017. The depth distribution of Ag in soil to 60-cm depth was measured in 2017. Electron microscopy, in combination with X-ray spectroscopy, and X-ray absorption spectroscopy were used to characterize the Ag. The Ag concentrations in the biosolids-amended soil increased steadily from 1996 until 2007, after which the concentrations leveled off at about 1.25 mg Ag kg-1 soil. This corresponded with a decrease of Ag concentrations in biosolids over time. The majority of the Ag (82%) was confined to the top 10 cm of the soil, small amounts (14%) were detected at 10-to-20-cm depth, and trace amounts (4%) were detected at 30-to-40-cm depth. The Ag in the biosolids was identified as S-containing nanoparticles (Ag2 S) with a diameter of 10-12 nm; however, in soil, the Ag concentrations were too low to allow identification of Ag speciation. This study shows that in a real-world field scenario, biosolids applied at agronomic rates represent a long-term, economically viable source of crop nutrients without increasing the concentration of total Ag in soil above a maximum of 1.5 mg Ag kg-1 . This concentration is below estimated ecotoxicity limits for Ag2 S in soil.

In situ degradation of biodegradable plastic mulch films in compost and agricultural soils.

The global use of agricultural plastic films, which provide multiple benefits for food production, is expected to grow by 59% from 2018 to 2026. Disposal options for agricultural plastics are limited and a major global concern, as plastic fragments from all sources ultimately accumulate in the sea. Biodegradable plastic mulches could potentially alleviate the disposal problem, but little is known about how well they degrade under different environmental conditions. We quantified the degradation of biodegradable plastic mulches in compost and in soil at warm and cool climates (Tennessee and Washington). Mulch degradation was assessed by Fourier-transformed infrared (FTIR) spectroscopy, molecular weight analysis, thermogravimetric analysis (TGA), nuclear-magnetic resonance (NMR), and mulch surface-area quantification. Biodegradable plastic mulches degraded faster in compost than in soil: degradation, as assessed by surface-area reduction, in compost ranged from 85 to 99% after 18 weeks, and in soil from 61 to 83% in Knoxville and 26 to 63% in Mount Vernon after 36 months. FTIR analyses indicate that hydrolytic degradation of ester bonds occurred, and a significant reduction of molecular weight was observed. TGA and NMR confirmed degradation of biodegradable polymers. Our results indicate that biodegradable plastic mulches degrade in soil, but at different rates in different climates and that degradation occurs over several years. Faster degradation occurred in compost, making composting a viable disposal method, especially in cool climates, where mulch fragments in soil may persist for many years.

Release of micro- and nanoparticles from biodegradable plastic during in situ composting.

Plastic is ubiquitous in modern life, but most conventional plastic is non-biodegradable and accumulates as waste after use. Biodegradable plastic is a promising alternative to conventional plastic. However, biodegradable plastics must be thoroughly evaluated to ensure that they undergo complete degradation and have no adverse impact on the environment. We evaluated the degradation of biodegradable plastics during 18-week full-scale composting, and determined whether additives from the plastics are released upon degradation. Two biodegradable plastic films-one containing polybutylene co-adipate co-terephthalate (PBAT) and the other containing polylactic acid/poly-hydroxy-alkanoate (PLA/PHA)-were placed into meshbags and buried in the compost. Degradation was assessed by image analysis, scanning electron microscopy, Fourier-transformed infrared spectroscopy, electrophoretic mobility, δ13C isotope analyses, and single particle mass spectrometry of mulch fragments. The results showed >99% macroscopic degradation of PLA/PHA and 97% for PBAT film. Polymers in the biodegradable films degraded; however, micro- and nanoparticles, most likely carbon black, were observed on the meshbags. Overall, biodegradable plastics hold promise, but the release of micro- and nanoparticles from biodegradable plastic upon degradation warrants additional investigation and calls for longer field testing to ensure that either complete biodegradation occurs or that no long-term harm to the environment is caused.

Biosolids and Tillage Practices Influence Soil Bacterial Communities in Dryland Wheat.

Class B biosolids are used in dryland wheat (Triticum aestivum L.) production in eastern Washington as a source of nutrients and to increase soil organic matter, but little is known about their effects on bacterial communities and potential for harboring human pathogens. Moreover, conservation tillage is promoted to reduce erosion and soil degradation. We explored the impacts of biosolids or synthetic fertilizer in combination with traditional (conventional) or conservation tillage on soil bacterial communities. Bacterial communities were characterized from fresh biosolids, biosolid aggregates embedded in soil, and soil after a second application of biosolids using high-throughput amplicon sequencing. Biosolid application significantly affected bacterial communities, even 4 years after their application. Bacteria in the families Clostridiaceae, Norcardiaceae, Anaerolinaceae, Dietziaceae, and Planococcaceae were more abundant in fresh biosolids, biosolid aggregates, and soils treated with biosolids than in synthetically fertilized soils. Taxa identified as Turcibacter, Dietzia, Clostridiaceae, and Anaerolineaceae were highly abundant in biosolid aggregates in the soil and likely originated from the biosolids. In contrast, Oxalobacteriaceae, Streptomyceteaceae, Janthinobacterium, Pseudomonas, Kribbella, and Bacillus were rare in the fresh biosolids, but relatively abundant in biosolid aggregates in the soil, and probably originated from the soil to colonize the substrate. However, tillage had relatively minor effects on bacterial communities, with only a small number of taxa differing in relative abundance between traditional and conventional tillage. Although biosolid-associated bacteria persisted in soil, potentially pathogenic taxa were extremely rare and no toxin genes for key groups (Salmonella, Clostridium) were detectable, suggesting that although fecal contamination was apparent via indicator taxa, pathogen populations had declined to low levels. Thus, biosolid amendments had profound effects on soil bacterial communities both by introducing gut- or digester-derived bacteria and by enriching potentially beneficial indigenous soil populations.

Interaction of Lumbricus terrestris with macroscopic polyethylene and biodegradable plastic mulch.

Polyethylene mulch films used in agriculture are a major source of plastic pollution in soils. Biodegradable plastics have been introduced as alternative to commonly-used polyethylene. Here we studied the interaction of earthworms (Lumbricus terrestris) with polyethylene and biodegradable plastic mulches. The objective was to assess whether earthworms would select between different types of mulches when foraging for food, and whether they drag macroscopic plastic mulch into the soil. Laboratory experiments were carried out with earthworms in Petri dishes and mesocosms. The treatments were standard polyethylene mulch, four biodegradable plastic mulches (PLA/PHA [polylactic acid/polyhydroxy alkanoate], Organix, BioAgri, Naturecycle), a biodegradable paper mulch (WeedGuardPlus), and poplar litter, which served as control. Four and three replicates for the Petri dish and mesocosm experiments were used, respectively. Macroscopic plastic and paper mulch pieces (1.5 cm × 1.5 cm and 2 cm × 2 cm) were collected from an agricultural field after a growing season, after being buried in the soil for 6 and 12 months, and after being composted for 2 weeks. We found that earthworms did not ingest polyethylene. Field-weathered biodegradable plastic mulches were not ingested either, however, after soil burial and composting, some biodegradable plastics were eaten and could not be recovered from soil any longer. Earthworms, when foraging for food, dragged plastic mulch, including polyethylene and biodegradable plastic, and poplar leaves into their burrows. The burial of macroscopic plastic mulch underground led to a redistribution of plastics in the soil profile, and likely enhances the degradation of biodegradable mulches in soil, but also can lead to leaching of plastic fragments by macropore flow.

Biosolids applications to tall fescue have long-term influence on soil nitrogen, carbon, and phosphorus.

Repeated applications of biosolids provide long-term benefits by increasing soil organic matter and N supply but can cause excess accumulation of P. Our objective was to determine the residual effects of repeated surface applications of biosolids on N availability and fate, tall fescue ( Schreb.) response, soil C, and P. A field experiment was started in 1993 to compare two biosolids products, each applied at three rates (6.7, 13.4, and 20.1 Mg ha yr), with synthetic N fertilizer (0 and 403 kg N ha yr as ammonium nitrate). Treatments were surface applied for 10 yr, followed by a 9-yr residual period where all plots received a reduced rate of inorganic N (202 kg N ha yr). Annual measurements included forage yield, N uptake, and soil nitrate N. Soil samples collected in 2002 and 2011 were analyzed for total C and N and Bray-1 P. Cumulative apparent N recoveries in harvested grass (1993-2010) were 51% for biosolids N and 72% for ammonium nitrate. Net fall soil nitrate N summed for the period 1993-2002 ranged from <1 to 3% of N applied. The N applied that was accounted for in forage and soil averaged 74% for biosolids and 73% for ammonium nitrate. Soil C increased in the biosolids treatments, and the increase was equivalent to 27% of biosolids C. Bray-1 P remained at excessive levels (338-629 mg P kg soil) 9 yr after the last biosolids application.

Long-term crop and soil response to biosolids applications in dryland wheat.

Biosolids have the potential to improve degraded soils in grain-fallow rotations. Our objectives were to determine if repeated biosolids applications in wheat ( L.)-fallow could supply adequate but not excessive N for grain production and increase soil C without creating a high risk of P loss. A replicated on-farm experiment was established in 1994 in central Washington, comparing anaerobically digested biosolids with anhydrous NH and a zero-N control. Biosolids were applied at 5, 7, and 9 Mg ha every fourth year through 2010 and incorporated 10 cm deep, while anhydrous NH plots received 56 kg ha N every second year. Grain yield and protein were determined. Soil chemical, biological, and bulk density analyses were made in 2012. Medium and high biosolids rates significantly increased grain yield (3.63 vs. 3.13 Mg ha) and protein (103 vs. 85 g kg) compared with anhydrous NH averaged across all crops. The medium biosolids rate had significantly lower bulk density (1.05 vs. 1.22 g kg) and greater total C (0-10-cm depth) (16.9 vs. 9.4 g kg), mineralizable N (156 vs. 52 mg kg), and extractable P (114 vs. 16 mg kg) than anhydrous NH. The P index site vulnerability increased from low for anhydrous NH to medium for the biosolids treatments. Soil NO-N was nearly always <10 mg N kg soil (0-30-cm depth). Medium and high biosolids rates significantly increased bacteria/fungi ratios, Gram-negative bacteria, and anaerobic bacteria markers compared with anhydrous NH. Biosolids can be an agronomically and environmentally sound management practice in wheat-fallow systems.

Validation of a 2 percent lactic acid antimicrobial rinse for mobile poultry slaughter operations.

Poultry processing antimicrobial interventions are critical for pathogen control, and organic, mobile operations in Washington seek alternatives to chlorine. Laboratory and field studies (three replications each) evaluated lactic acid efficacy as a chlorine alternative. For the laboratory study, retail-purchased, conventionally processed chicken wings inoculated with Salmonella were randomly assigned to the following treatments: Salmonella inoculation followed by no treatment (10 wings) or by 3-min rinses of water, 50 to 100 ppm of chlorine, or 2% lactic acid (20 wings for each rinse treatment). Wings were sampled for Salmonella enumeration on xylose lysine desoxycholate agar. During pastured poultry processing at mobile slaughter units for each field study replication, 20 chicken carcasses were randomly assigned to each treatment: untreated control or 3-min immersion in lactic acid or chlorine. Whole-carcass rinses were examined for aerobic plate count (APC) on tryptic soy agar and coliforms on violet red bile agar. Untreated controls were also examined for Salmonella. In the laboratory study, lactic acid produced a significant (P < 0.01) Salmonella reduction compared with the inoculated no-rinse, water, and chlorine treatments, which were statistically similar to each other. In the field study, no Salmonella was detected on untreated controls. Lactic acid produced significant >2-log (P < 0.01) reductions in APC and coliforms, whereas chlorine resulted in slight, but significant 0.4-log reductions (P < 0.01) and 0.21-log reductions (P < 0.05) in APC and coliforms compared with untreated controls. Considering laboratory and field studies, lactic acid produced greater reductions in Salmonella, APC, and coliforms, validating its effectiveness as a chlorine alternative in mobile poultry slaughter operations.

Estimating plant-available nitrogen release from manures, composts, and specialty products.

Recent adoption of national rules for organic crop production have stimulated greater interest in meeting crop N needs using manures, composts, and other organic materials. This study was designed to provide data to support Extension recommendations for organic amendments. Specifically, our objectives were to (i) measure decomposition and N released from fresh and composted amendments and (ii) evaluate the performance of the model DECOMPOSITION, a relatively simple N mineralization/immobilization model, as a predictor of N availability. Amendment samples were aerobically incubated in moist soil in the laboratory at 22 degrees C for 70 d to determine decomposition and plant-available nitrogen (PAN) (n = 44), and they were applied preplant to a sweet corn crop to determine PAN via fertilizer N equivalency (n = 37). Well-composted materials (n = 14) had a single decomposition rate, averaging 0.003 d(-1). For uncomposted materials, decomposition was rapid (>0.01 d(-1)) for the first 10 to 30 d. The laboratory incubation and the full-season PAN determination in the field gave similar estimates of PAN across amendments. The linear regression equation for lab PAN vs. field PAN had a slope not different from one and a y-intercept not different than zero. Much of the PAN released from amendments was recovered in the first 30 d. Field and laboratory measurements of PAN were strongly related to PAN estimated by DECOMPOSITION (r(2) > 0.7). Modeled PAN values were typically higher than observed PAN, particularly for amendments exhibiting high initial NH(4)-N concentrations or rapid decomposition. Based on our findings, we recommend that guidance publications for manure and compost utilization include short-term (28-d) decomposition and PAN estimates that can be useful to both modelers and growers.