Sean M. Schaeffer

Polylactic acid (PLA) is among the most widely used biodegradable, biobased polymers, yet it can persist in soils and waters, where accumulation may affect plants and agroecosystems. This narrative, mechanistic review asks why PLA behaves differently under industrial composting and ambient soil conditions, drawing on original studies and using soils as the interpretive lens, with aquatic and waste management systems for contrast. We examine hydrolysis, Norrish photochemistry, biofilm formation, enzymatic cleavage, environmental controls, polymer crystallinity, physical accessibility, and microbial community interactions. Elevated temperature and moisture promote rapid degradation during industrial composting, whereas ambient conditions slow hydrolysis and restrict microbial access. We propose contextual recalcitrance as an organizing framework, in which persistence arises from interactions among polymer structure, environmental conditions, physical accessibility, and the local availability of depolymerizing traits rather than intrinsic chemical resistance. Within it, we advance trait gating by primary depolymerizers and cross-feeding of soluble lactate as a testable conceptual model rather than a demonstrated soil succession because community-resolved field evidence is lacking. We conclude that environmental biodegradability should be assessed using endpoints beyond CO₂ evolution, including dissolved organic carbon, microbial assimilation, and organic matter partitioning, to distinguish fragmentation, depolymerization, assimilation, mineralization, and stabilization of PLA-derived carbon in soils.

A method to recover and quantify micro- and nano-plastics (MPs and NPs) formed in the soil during biodegradation is needed to accurately assess the degradation and environmental impact of biodegradable plastic products. The presence of MPs and NPs in soil may alter soil properties like aggregation behavior or have toxic effects on soil biota. Existing MP recovery methods are not always suitable for measuring biodegradable polymers like polybutylene adipate terephthalate (PBAT); some common digestion procedures with acids or oxidizers can destroy PBAT-based biodegradable MPs. Identification methods like micro-FTIR and micro-Raman spectroscopy are also limited by the minimum size of particles that can be recovered and analyzed. Therefore, this method was developed to extract and quantify PBAT from soil to assess the mass fraction of MPs and NPs in the soil without chemically transforming PBAT. In the protocol, a chloroform-methanol solution is used to selectively extract PBAT from the soil. The solvent is evaporated from the extract, and then the extract is redissolved in deuterated chloroform. The extract is analyzed by proton nuclear magnetic resonance spectroscopy (1H-QNMR) under quantitative parameters to quantify the amount of PBAT in each sample. Solvent extraction efficiencies for PBAT range from 76% in a shady loam soil to 45% in an Elkhorn sandy loam soil. PBAT recovery may be reduced for photo-oxidized materials compared to pristine ones and may be reduced in soils with high clay content. Extraction efficiencies do not depend on PBAT concentration within the test range, but lower extraction efficiencies were observed for NPs than for MPs. PBAT quantification results were comparable to the quantification of plastic degradation by measuring cumulative soil respiration in a laboratory incubation study.

A method to recover and quantify micro- and nano-plastics (MPs and NPs) formed in the soil during biodegradation is needed to accurately assess the degradation and environmental impact of biodegradable plastic products. The presence of MPs and NPs in soil may alter soil properties like aggregation behavior or have toxic effects on soil biota. Existing MP recovery methods are not always suitable for measuring biodegradable polymers like polybutylene adipate terephthalate (PBAT); some common digestion procedures with acids or oxidizers can destroy PBAT-based biodegradable MPs. Identification methods like micro-FTIR and micro-Raman spectroscopy are also limited by the minimum size of particles that can be recovered and analyzed. Therefore, this method was developed to extract and quantify PBAT from soil to assess the mass fraction of MPs and NPs in the soil without chemically transforming PBAT. In the protocol, a chloroform-methanol solution is used to selectively extract PBAT from the soil. The solvent is evaporated from the extract, and then the extract is redissolved in deuterated chloroform. The extract is analyzed by proton nuclear magnetic resonance spectroscopy (1H-QNMR) under quantitative parameters to quantify the amount of PBAT in each sample. Solvent extraction efficiencies for PBAT range from 76% in a shady loam soil to 45% in an Elkhorn sandy loam soil. PBAT recovery may be reduced for photo-oxidized materials compared to pristine ones and may be reduced in soils with high clay content. Extraction efficiencies do not depend on PBAT concentration within the test range, but lower extraction efficiencies were observed for NPs than for MPs. PBAT quantification results were comparable to the quantification of plastic degradation by measuring cumulative soil respiration in a laboratory incubation study.

Bacterial viruses are extremely small and abundant particles that can control the microbial abundance and community composition through infection, which gradually showed their vital roles in the ecological process to influence the nutrient flow. Compared to the substrate control, less is known about the influence of soil viruses on microbial community function, and even less is known about microbial and viral diversity in the soil system.

Agricultural practices alter the structure and functions of soil microbial community. However, few studies have documented the alterations of bacterial communities in soils under long-term conservation management practices for continuous crop production. In this study, we evaluated soil bacterial diversity using 16S rRNA gene sequencing and soil physical and chemical properties within 12 combinations of inorganic N fertilization, cover cropping, and tillage throughout a cotton production cycle. Soil was collected from field plots of the West Tennessee Agriculture Research and Education Center in Jackson, TN, United States. The site has been under continuous cotton production for 38 years. A total of 38,038 OTUs were detected across 171 soil samples. The dominant bacterial phyla wereProteobacteria,Acidobacteria,Actinobacteria,Verrucomicrobia, andChloroflexi, accounting for ∼70% of the total bacterial community membership. Conventional tillage increased alpha diversity in soil samples collected in different stages of cotton production. The effects of inorganic N fertilization and conventional tillage on the structure of bacterial communities were significant at all four sampling dates (p< 0.01). However, cover cropping (p< 0.05) and soil moisture content (p< 0.05) only showed significant influence on the bacterial community structure after burn-down of the cover crops and before planting of cotton (May). Nitrate-N appeared to have a significant effect on the structure of bacterial communities after inorganic fertilization and at the peak of cotton growth (p< 0.01). Structural equation modeling revealed that the relative abundances of denitrifying and nitrifying bacteria were higher when conventional tillage and vetch cover crop practices were applied, respectively. Our results indicate that long-term tillage and fertilization are key factors increasing the diversity and restructuring the composition of bacterial communities, whereas cover cropping may have shorter-term effects on soil bacteria community structure. In this study, management practices might positively influence relative abundances of bacterial functional groups associated with N cycling. The bacteria functional groups may build a network for providing N and meet microbial N needs in the long term.

Soil bacterial communities are a critical component for stability and function of terrestrial ecosystems. Viruses are ubiquitous in soils and have significant impacts on the structure and functions of bacterial host communities. However, little is known about the variation of bacterial and viral communities and their connection under various land use management practices. In this study, we evaluated bacterial and viral diversity using 16s rRNA sequencing and RAPD-PCR, respectively, and estimated viral and bacterial abundances by enumeration method, and used structural equation model to reveal relationship between viruses, bacteria, and soil properties, in soils from a long-term conservation management experimental site in western Tennessee USA. Inorganic N fertilization, cover cropping, and tillage treatments showed no significant differences in bacterial alpha-diversity but significantly influenced the structure, as suggested by differences in beta-diversity of bacterial communities. Higher soil pH and water content were favorable to bacterial abundances. Cover cropping, soil water content, and bacterial abundances may explain the variation of viral abundances and community structure in soil. Structural equation modeling showed that bacterial abundances positively influenced viral abundances, and in turn virus abundances and bacterial alpha diversity affected the level of extractable dissolved organic C, which exerted a feedback effect on the structure of bacterial communities. This feedback loop suggests that viral lysates might significantly contribute to reshaping bacterial community structure and indirect influence bacterial alpha diversity. This supports the theory of the “viral shunt” in soil ecosystems. This study indicated that long-term conservation management can reshape soil bacterial and viral communities and influence their interactions in biogeochemical processes.