From plastic mulching to microplastic pollution : An effect assessment of microplastics in the soil-plant system

Resúmen

Plastic debris originating from mulching films has made a significant contribution to agricultural microplastic pollution.  However, the accumulation of plastic residues and the occurrence of microplastics in agricultural fields are missing. Biodegradable were introduced to agricultural systems due to its biodegradability. However, fully biodegradation under in-situ conditions have rarely been observed. Instead, biodegradable materials tend to break down into small bio-plastic fragments and microplastics. Worryingly, the ecological effects of LDPE and biodegradable microplastics in soil-plant systems remains largely unknown. For this reason, this PhD thesis aims to fill some knowledge gaps concerning the effects of plastic mulching and microplastic pollution on terrestrial ecosystems. To achieve this, a field observation, and a net house mesocosm experiment were carried out.

In Chapter 2, a field observation was conducted in Northwestern China. We selected two regions with a long history of plastic mulching use but with two different farming systems. The first study region (S1) was characterized by small scale farmlands and low intensity machinery use. The second study region (S2) was characterized by large scale farmlands and high intensity machinery use. Plastic residues were collected from the top 0-30 cm of soil. Our results showed that in S1, fields with 6-8 years continuous plastic mulching accumulated significantly higher amounts of macroplastics than fields with 30 years intermittent plastic mulching. In S1, macroplastics in the 10–50 cm2 size category accounted for over 40% of the collected macroplastics, followed by macroplastics from 2–10 cm2 which accounted for over 30%. In S2, the collected macroplastics were mainly from the size category 0.25–2 cm2 (40.6%) and 2–10 cm2 (41.1%). Comparing the two study regions, macroplastics were more fragmented in S2 than in S1. This result was in line with the occurrence of microplastics in the two study regions. In S1, microplastics were only detected in fields that were exposed to more than 30 years of mulching use, while in S2, microplastics were detected in all the selected fields. The study emphasized that farming systems play important roles in the accumulation of macroplastics and the formation of macroplastics to microplastics.

To investigate the effects of microplastics in the soil-plant system. In 2019, a 105-day outside net-house pot experiment was conducted. Common bean (Phaseolus vulgaris L.; P. vulgaris) was selected as the model plant. Two types of microplastics were used: LDPE microplastics (LDPE-MPs) and biodegradable microplastics (Bio-MPs) of polybutylene adipate terephthalate mixed with polylactic acid (PBAT+PLA). The MPs used in this experiment measured 250-500μm and 500-1000 μm (weight ratio 3:2). Microplastics were mixed into soil at doses of 0.5%, 1.0%, 1.5%, 2.0% and 2.5% w/w (weight of microplastic to dry soil).

In Chapter 3, a suite of proxies on crop growth were measured on the 46th day of growth (at the end of the vegetative stage) and the 105th day of growth (after harvest). The results showed that LDPE-MPs exerted no significant effects on shoot and root biomass as compared to the control treatment (no microplastics) and Bio-MPs at higher doses of 1.5%, 2.0% and 2.5% w/w significantly inhibited the root and shoot biomass. All Bio-MPs treatments and ≥1.0% LDPE-MPs showed significantly higher numbers of specific root nodules. Overall, Bio-MPs showed stronger effects on the growth of P. vulgaris than LDPE-MPs. The results indicated that the existence of microplastics in agricultural soils affect P. vulgaris growth. We speculate that the different responses of the shoots and roots of P. vulgaris to LDPE-MPs and Bio-MPs might be attributed to the different compositions of the two types of microplastics, thus affecting the different responses of soil nutrients and rhizosphere bacterial communities.

In Chapter 4, we examined the dynamics of total organic matter, soil labile organic matter fractions of soil dissolved organic carbon (DOC), permanganate oxidizable carbon (POXC), available nitrogen (AN) of N-NH+3 and N-NO-3, and dissolved organic nitrogen (DON) on the 46th day and the 105th day of plant growth. Overall, Bio-MPs exerted stronger effects on soil carbon and nitrogen cycling than LDPE-MPs. The SOM results indicated that Bio-MPs experienced a rapid biodegradation from the start of the experiment to the 46th day, while LDPE-MPs did not degrade during the whole growth period. Compared to the control treatment, LDPE-MPs exerted no significant effects on DOC, AN and DON. 2.0% and 2.5% treatments of Bio-MPs showed significantly higher DOC and DON (46th and 105th day), while ≥1.5% Bio-MPs showed significantly lower AN availability (46th day). In order to gain more insights into microplastic pollution in soil-plant systems, we further explore the effects of microplastics on the rhizosphere microbial community.

Chapter 5 examined the changes in rhizosphere microbial communities exposed to three selected microplastic doses, 0.5%, 1.0% and 2.5% w/w. according to our results, for both types, 0.5% and 1.0% w/w showed higher diversity in rhizosphere microbial communities, while 2.5% w/w showed similar or lower diversity, as compared to the control. This implied that a higher dose (2.5% w/w) of microplastics might have exerted selective pressure on the rhizosphere microbial communities. The rhizosphere microbial communities in 1.0% and 2.5% w/w Bio-MPs were clearly separated from other treatments. The relative abundance of specific bacterial taxa (i.e. Comamonadaceae Rhizobiaceae and Micrococcaceae) that related to soil organic matter degradation and nutrient cycling were affected by the occurrence of both types of microplastics, indicated that both LDPE-MPs and Bio-MPs could exert profound effects on the rhizosphere microbial community and has the potential to affect soil ecological function and nutrient cycling.

To conclude, in Chapter 6, we discussed the implications and outlook for future research based on our results. This thesis provided a “field to pot experiment” perspective of the escalating plastic pollution problem. The accumulation of macroplastics and the transformation from macro to micro size under different farming systems were studied. The pot experiment results not only showed that LDPE and biodegradable microplastics could substantially affect plant growth, soil carbon and nitrogen cycling, and soil rhizosphere microbial communities but also showed that these effects may have far reaching impacts for future crop production, soil health and food safety.