General information regarding microbial communities associated with PBSA degradation
We analysed the archaeal, bacterial, and fungal communities that were adhered to PBSA films in soils with and without (NH4)2SO4 addition (PSN and PS treatments, respectively), those present in the soil surrounding the PBSA in the aforementioned treatments, and those present in soil not treated with PBSA (control soil and control SN treatments) after 90 days of incubation; for comparison, we also analysed the communities present in the initial soil without incubation (initial soil treatment). Taxonomic resolution was conducted at the ASV level (Additional file 2: Tables S1, S2, and S3). In total, we detected 27, 6,487, and 788 archaeal, bacterial, and fungal ASVs, respectively, across all PBSA and soil microbial communities (Additional file 2: Tables S1, S2, and S3). The archaeal, bacterial, and fungal ASVs corresponded to 3, 418, and 164 genera, respectively. Among these, we detected 7 (26% of total archaeal ASVs), 2,113 (33% of total bacterial ASVs), and 170 ASVs (22% of total fungal ASVs) for archaea, bacteria, and fungi in PBSA, respectively (Additional file 2: Tables S1, S2, and S3). In soil samples from PS and PSN treatments, we detected 20 (74% of total archaeal ASVs), 5,429 (84% of total bacterial ASVs), and 313 ASVs (40% of total fungal ASVs) for archaea, bacteria, and fungi, respectively (Additional file 2: Tables S1, S2, and S3). The relative archaeal (ASV level), bacterial (phylum level), and fungal (class level) abundances are shown in Fig. 1. Briefly, microbial community composition (at the coarse taxonomic level) of initial soil and control soils were comparable for archaeal (dominated by Nitrososphaeraceae), bacteria (dominated by Proteobacteria), and fungi (dominated by Sordariomycetes) (Fig. 1). While pure PBSA addition did not significantly change archaeal community composition, it increased relative abundance of Proteobacteria and strongly reduced abundance of Sordariomycetes (Fig. 1). Adding (NH4)2SO4 into PBSA–soil system increased Proteobacteria and Sordariomycetes (Fig. 1). The microbial community composition at coarse taxonomic level of PBSA was significantly different from those of the surrounding soil (Fig. 1). PSN treatment significantly reduced microbial diversity on PBSA.
Effects of PBSA on microbial ASV richness
Both prokaryotic (archaeal and bacterial) and fungal ASV richness in the control soil without PBSA (control S) and control soil with (NH4)2SO4 addition (control SN) were similar to those in the initial soil (Fig. 2). Bacterial ASV richness in soil under PS treatment was not significantly affected by PBSA, while archaeal ASV richness was slightly decreased (by 13%) and fungal ASV richness was strongly reduced by approximately 45% (Fig. 2a, c, e). (NH4)2SO4 as a single factor (control SN treatment) did not significantly affect the microbial richness. Contrarily, adding (NH4)2SO4 into PBSA–soil system caused a significant reduction in archaeal, bacterial and fungal ASV richness in soil under PSN treatment by 31%–79%. The richness of bacteria and fungi colonising PBSA in the PS treatment accounted for 60% and 43%, respectively, of the richness of the surrounding soil (Fig. 2). Moreover, bacterial and fungal ASV richness in the communities adhered to PBSA following PSN treatment accounted for only 14% and 38%, respectively, of the corresponding richness of the soil microbiome (Fig. 2).
Effects of PBSA on soil microbial community composition and activity
Both prokaryotic and fungal community compositions in control soil were almost identical to those in the initial soil (Fig. 3a, c, e). We found small but significant changes in bacterial and fungal community compositions in soils under control SN and PS treatment but not in archaeal community composition. After adding PBSA, the relative abundance of Proteobacteria (30.6%, 2.7% contributed by Bradyrhizobium ASV 15) in PS-treated soil slightly increased compared with its abundance in control soils, whereas the relative abundance of Actinobacteriota and Acidobacteriota slightly decreased (Fig. 1b). For fungi, there were large changes in the dominant fungal classes in PS-treated soil compared with its abundance in initial and control soils. Eurotiomycetes (24.0%, 20.6% contributed by Exophiala equina ASV 02) and Leotiomycetes (28.6%, 23.1% contributed by Tetracladium furcatum ASV 03) were co-dominant with Sordariomycetes (31.4%, 9.5% contributed by Ilyonectria macrodidyma ASV 07 and 4.7% contributed by Chaetomium globosum ASV 09), whereas Dothideomycetes almost disappeared from the PS-treated soil (Fig. 1c).
Adding (NH4)2SO4 together with PBSA strongly shifted the microbial community composition of soil under PSN treatment from that of the initial, control soils and control soils with (NH4)2SO4 addition. Archaeal, bacterial and fungal community compositions in soil were more sensitive to the combination of the two amendment additions (PBSA and (NH4)2SO4 addition) (Figs. 1, 3). After adding (NH4)2SO4 and PBSA, the abundance of Candidatus Nitrocosmicus ASV 59 in the soil of the PSN treatment increased compared with its abundance in initial and control soils, whereas the abundance of Nitrososphaeraceae ASV 13 slightly decreased, and some other archaeal ASVs disappeared. Strong hyper-dominant patterns were observed for both bacterial and fungal communities in PSN-treated soil. We found that only Proteobacteria (66.6%, 20% contributed by Achromobacter insolitus ASV 02) was dominant in the bacterial community and that Sordariomycetes (80.3%, 43.0% contributed by Fusarium solani ASV 01 and 10.2% contributed by Clonostachys rosea ASV 11) were dominant in the fungal community in PSN-treated soil (Fig. 1b, c). Interestingly, in two of five soil samples under PSN treatment, PBSA decomposition rates were lower than those of the other three replicates, whereas Bacteroidota (contributed by Chitinophaga ASV 41) and Eurotiomycetes (contributed by E. equina ASV 02) almost completely disappeared (Fig. 1b, c).
Different colonisers of PBSA under PS and PSN treatments
The compositions of archaeal, bacterial, and fungal communities in PBSA receiving PS and PSN treatments were significantly different from those of their soil counterparts (Additional file 1: Table S4). Among the total 27 archaeal ASVs, only seven were present on PBSA under PS treatment. Among these, Nitrososphaeraceae ASV 13, Candidatus Nitrocosmicus ASV 59, and Nitrososphaeraceae ASV 72 were dominant (Fig. 1a and Additional file 2: Table S1). The bacterial community in the PBSA samples under PS treatment was dominated by Proteobacteria (56.1%, 7.5% contributed by Caulobacter rhizosphaerae ASV 08, 3.5% contributed by Azotobacter chroococcum ASV 18) with a small contribution from Actinobacteriota (10.3%, 1.1% contributed by Agromyces ramosus ASV 30) and Bacteroidota (8.7%, 1.1% contributed by Ohtaekwangia ASV 97) (Figs. 1b, 4 and Additional file 2: Table S2). In the fungal community, Leotiomycetes (28.3%, 22.8% contributed by Tetracladium furcatum ASV 03), Eurotiomycetes (27.2%, 22.6% contributed by E. equina ASV 02) and Sordariomycetes (27.6%, 18.1% contributed by I. macrodidyma ASV 07) were co-dominant in the PBSA samples under PS treatment (Figs. 1c, 5 and Additional file 2: Table S3). For PBSA under PSN treatment, only Nitrososphaeraceae ASV 13 was detected in two of five independent PBSA replicates and Candidatus Nitrocosmicus ASV 59 in one replicate, whereas in the other three replicates, archaea were completely absent (Fig. 1a). In the PSN treatment, only a few bacterial phyla and fungal classes could colonise the PBSA (Fig. 1b, c). Proteobacteria (94.6%, 19.4% contributed by Stenotrophomonas maltophilia ASV 03, 18.9% contributed by A. insolitus ASV 02 and 15.3% contributed by Achromobacter denitrificans ASV 04) and Sordariomycetes (78.9%, 49.1% contributed by F. solani ASV 01 and 14% contributed by C. rosea ASV 11) dominated the bacterial and fungal communities, respectively, of PBSA samples under PSN treatment (Figs. 1b, c, 4, 5, Additional file 2: Tables S2 and S3).
Soil pH response to PBSA addition and links to microbial community composition and richness
The initial soil pH of 7.5 slightly decreased when (NH4)2SO4 was added (pH 7.3) (Additional file 1: Fig. S2a). After 90 days of incubation, pH values in the control soil and PS-treated soil were similar to the initial soil pH. Contrarily, soil pH after 90 days of incubation significantly decreased under control SN and PSN treatment (pH value 6.7 and 6.3, respectively). pH values were significantly positively correlated with archaeal, and bacterial richness (R = 0.54–0.55, P = 0.011–0.014, Additional file 1: Fig. S2b, c). We found no correlation between pH values and fungal richness (Additional file 1: Fig. S2d). Overall, treatments explained 13%, 23%, and 28% of variations in archaeal, bacterial, and fungal community composition, respectively (Additional file 1: Fig. S3a–c). PBSA explained 54%, 35%, and 64% of the total explainable variation in archaeal, bacterial, and fungal community composition, respectively. The addition of (NH4)2SO4 did not explain archaeal community composition, but explain 13%, and 11% of the total explainable variation in bacterial, and fungal community composition. pH alone did not explain the microbial community compositions. However, the combinations of pH and (NH4)2SO4 addition explained 31%, 43%, and 21% of the total explainable variation in archaeal, bacterial, and fungal community composition, respectively.
Potential lipase-producing and PBS/PBSA-degrading microbes
Considering the 40 most abundant bacteria, potential lipase-producing bacteria were almost absent or were detected in low relative abundance in the initial, control soils and control soils with (NH4)2SO4 addition, whereas potential PBS-degrading bacteria were not detected in the initial soil (Fig. 4). In control soils and control soils with (NH4)2SO4 addition, only Paenibacillus amylolyticus ASV 54 (a potential lipase producer and potential PBS degrader) [1, 46] was detected with 1.8% and 0.006% relative abundance, respectively. In PS- and PSN-treated soils, P. amylolyticus was not detected or detected at very low relative abundance (≤ 0.01%). Potential lipase-producing bacteria were found to be associated with both soil and PBSA samples across all other treatments (PS, and PSN treatments). Most of them were more enriched on PBSA than in soil from the same treatment. However, the community of these potential lipase-producing and PBS-degrading bacteria differed among treatments. After incorporating PBSA into soil, five out of 10 potential lipase-producing bacteria were detected in PS-treated soil at very low relative abundances (≤ 0.3%). In PBSA samples under PS treatment, eight out of 10 potential lipase-producing bacteria were detected, and Cupriavidus pauculus ASV 48, Variovorax ginsengisoli ASV 39 were enriched. In the PSN treatment, A. insolitus ASV 02 was highly enriched in both soil and PBSA samples. Pantoea agglomerans ASV 12, and Stenotrophomonas maltophilia ASV 03 were detected in soil and highly enriched in PBSA samples under PSN treatment.
Among the 40 most abundant fungi, all potential PBS-degrading fungi were F. solani ASV 01, ASV 05, ASV 27, and ASV 47  (Fig. 5). F. solani was also reported to potentially produce lipase . In the initial, control soils and control soils with (NH4)2SO4 addition, three out of four potential PBS-degrading F. solani ASVs were detected with 0.04–1.4% relative abundance. Under PS treatment, two out of four F. solani ASVs were found to be associated with both soil and PBSA samples (0.08–1.7% relative abundance). Under PSN treatment, F. solani ASV 01 was highly enriched in both soil and PBSA samples with relative abundances of 43–49%. The community of other potential lipase-producing fungi also differed among the different treatments. Chaetomium globosum were the most abundant in the initial (11.9%), control soils (5.7%) and control soils with (NH4)2SO4 addition (10.5%). Under PS treatment, E. equina ASV 02 was enriched in both soil and PBSA samples (21–23%). Under the PSN treatment, E. equina ASV 02 and Pseudogymnoascus roseus ASV 12 were enriched in both soil and PBSA samples (7–14% and 4–12%, respectively).
Beneficial and pathogenic microorganisms associated with PBSA: top 40 bacteria and fungi
We found that both beneficial and pathogenic microbes colonised PBSA samples, and they were listed among the top-40 most abundant bacteria and fungi (Figs. 4, 5). Three main groups of potentially beneficial bacteria associated with PBSA were as follows: (i) biological control agents (A. ramosus, and P. agglomerans), (ii) plant growth-promoting microorganisms (A. chroococcum and C. rhizosphaerae), and (iii) N fixers (A. chroococcum, Bradyrhizobium, Devosia insulae, Rhizobium cellulosilyticum, and R. leguminosarum). Among the beneficial bacteria, C. rhizosphaerae (7.5%), A. ramosus, A. chroococcum, Bradyrhizobium, and R. leguminosarum were enriched (1.1–7.5%) on PBSA under PS treatment. Most beneficial bacteria, especially N-fixing bacteria (three of five ASVs), were suppressed in PBSA under PSN treatment (Fig. 4). Only a biological control agent (P. agglomerans) was enriched in PBSA under PSN treatment. In fungi, we detected only two potential beneficial groups associated with PBSA: (i) biological control agents (Chaetomium globosum, C. rosea, Dactylellina haptotyla, Gibberella fujikuroi, and Minimedusa polyspora) and (ii) plant growth promoters (G. fujikuroi, Mortierella elongata and Solicoccozyma terrea). Among them, D. haptotyla was suppressed in PBSA under PSN treatment (Fig. 5). Only one biological control agent (C. rosea) was enriched in PBSA under PSN treatment.
Opportunistic human pathogenic bacteria and plant pathogenic fungi were the frequently detected pathogenic microorganisms in PS-treated PBSA samples. Opportunistic human pathogenic bacteria dominated the 40 most abundant bacterial ASVs, and these included A. denitrificans, A. insolitus, C. pauculus, Lelliottia amnigena and P. agglomerans. Among them, only C. pauculus was enriched in PS-treated PBSA samples, although it was not detected in either the initial or both control soils (control S and SN). Furthermore, A. denitrificans and A. insolitus (not detected in initial and control soils and almost absent in control soil with (NH4)2SO4 addition) was highly enriched in PSN-treated PBSA samples (15% and 19%, respectively). Apart from opportunistic pathogenic bacteria, there was an animal and human pathogen (Brucella melitensis) that was only detected in PSN-treated PBSA sample, where it was abundant. The 40 most abundant fungi were dominated by plant pathogens (Fig. 5). Among them, I. macrodidyma was enriched after PBSA was incorporated into the soil (PS treatment). PSN treatment markedly enriched the relative abundances of C. rosea and F. solani, whereas it significantly suppressed those of I. macrodidyma. Some plant pathogens, such as C. globosum and F. solani were also classified as opportunistic human pathogens. In this study, we found that individual bacterial and fungal ASVs may represent more than one function, even the opposite one. For example, P. agglomerans is an opportunistic pathogen that also helps in biological control, and C. rosea is classified as either a biological control or a plant pathogen depending on the plant species being considered.
Effects of PBSA on beneficial and pathogenic microbes in soil
PBSA not only offers a habitat for beneficial and pathogenic microbes, but also alters the relative abundances of such microbes in soils. The relative abundances of these microbes detected on PBSA and in soil did not always correspond with each other. We detected three patterns: (i) enriched both in soil and in PBSA [bacteria: Bradyrhizobium (N fixation), fungi: Dactylellina haptotyla (biological control, and saprotroph), E. equina (opportunistic human pathogen and saprotroph), and I. macrodidyma (plant pathogen)]; (ii) enriched only on plastic but not in soil [bacteria: A. ramosus (biological control), C. rhizosphaerae (plant growth-promoting bacteria), C. pauculus (opportunistic human pathogen), R. leguminosarum (N fixation), fungi: C. rosea (biological control and plant pathogen)] and (iii) enriched only in soil but not on plastic [fungi: A. alternatum (biological control), G. fujikuroi (biological control, plant growth promoter, and plant pathogen), Gibellulopsis nigrescens (plant pathogen)]. Addition of (NH4)2SO4 together with PBSA (PSN treatment) highly enriched both bacteria [A. denitrificans and A. insolitus (denitrification and opportunistic human pathogens)] and fungi [C. rosea (biological control and plant pathogen) and F. solani (plant pathogen and opportunistic human pathogen)] both in soil and on PBSA. E. equina (opportunistic human pathogen and saprotroph) was relatively resistant to the combined effect of (NH4)2SO4 and PBSA addition as its relative abundance was not significantly different between PS- and PSN-treated soils.