Rebuttal of the arguments put forward in the Letter to the Editor by Nizzetto et al.

In their Letter to the Editor, Nizzetto et al. challange a recent article in which I show that there has been unwarranted alarmism about biodegradable mulch films due to the publication of numerous articles based on preliminary data that are irrelevant for drawing conclusions on environmental risk. The tendency to over-emphasise results in order to attract attention is a growing problem in the scientific world and has been studied by many scholars. Nizzetto et al. accuse me of not using scientific methodology and of not disclosing that I am a scientist working for a company that produces biodegradable plastics. In this rebuttal I show that Nizzetto et al.'s accusations suffer from a number of logical fallacies, in particular the "straw man" fallacy and the "ad hominem" fallacy.

The pathology of hype, hyperbole and publication bias is creating an unwarranted concern towards biodegradable mulch films.

The idea that it is a risk to promote biodegradable mulch films on a large scale is becoming established at academic level based on a series of articles similar in approach and conclusions. However, a critical analysis shows that the results do not justify the alarmist tones. The negative effects of hand-cut pieces of virgin material added in pots at concentrations up to 714 times the application doses are ascribed to the "accumulation" and "contamination" of "residues" and "debris" of biodegradable plastics. Yet, no accumulation and no contamination of biodegradable microplastics has actually been shown. No Predicted Environmental Concentration was established, thus the use of the term risk is inappropriate. The hypothesis of transient phytotoxicity of organic matter under decomposition i.e., an artificial outcome of the experimental scheme used, was not considered. A scrupulous approach to terminology is very important for the quality of communication and for the development of innovations. Scientific communication is a delicate process in which and to avoid hyperbole, there must be strict logical and lexical consistency between results and conclusions. Guidelines on the communication of the results of studies on biodegradable mulch must be developed to avoid the spread of unjustified concerns.

Microorganisms that produce enzymes active on biodegradable polyesters are ubiquitous.

Biodegradability standards measure ultimate biodegradation of polymers by exposing the material under test to a natural microbial inoculum. Available tests developed by the International Organization for Standardization (ISO) use inoculums sampled from different environments e.g. soil, marine sediments, seawater. Understanding whether each inoculum is to be considered as microbially unique or not can be relevant for the interpretation of tests results. In this review, we address this question by consideration of the following: (i) the chemical nature of biodegradable plastics (virtually all biodegradable plastics are polyesters) (ii) the diffusion of ester bonds in nature both in simple molecules and in polymers (ubiquitous); (iii) the diffusion of decomposers capable of producing enzymes, called esterases, which accelerate the hydrolysis of esters, including polyesters (ubiquitous); (iv) the evidence showing that synthetic polyesters can be depolymerized by esterases (large and growing); (v) the evidence showing that these esterases are ubiquitous (growing and confirmed by bioinformatics studies). By combining the relevant available facts it can be concluded that if a certain polyester shows ultimate biodegradation when exposed to a natural inoculum, it can be considered biodegradable and need not be retested using other inoculums. Obviously, if the polymer does not show ultimate biodegradation it must be considered recalcitrant, until proven otherwise.

Is composting of packaging real recycling?

In 1994, the European directive on packaging and packaging waste introduced the principle that biodegradable packaging can be recovered together with bio-waste by organic recycling (e.g. composting). Recently, critical voices have been raised against this principle on the basis that packaging does not add nutrients to the compost and is also "lost", i.e. mostly mineralized in CO2 and water. These opinions do not take into account the specificity of composting and are unfounded. The term compost comes from composite. In fact, it is necessary to mix together feedstocks with different biodegradation behaviour and different C/N ratios to start a composting process and obtain quality compost. For example, cellulose is a feedstock at medium biodegradation rate that brings energy and biomass. Energy is needed to heat the compost pile and ensure that the composting process, including pasteurization, takes place without any external energy source. On the other hand, lignin is quite recalcitrant, brings no energy to the process and forms the basic structure of compost. Cellulose does not contain nitrogen, but it is the most relevant feedstock in composting. Likewise, packaging is nitrogen-free and can be equated with cellulose in terms of biodegradation behaviour and role in the composting process. In fact, biodegradability of packaging is assessed by using cellulose as the reference material. A compostable packaging, whether based on cellulose-fibres (paper, cardboard) or based on biodegradable plastics behaves similarly to other composting feedstock and contributes to the composting process and to the production of good quality compost.

Biodegradation of plastics in soil and effects on nitrification activity. A laboratory approach.

The progressive application of new biodegradable plastics in agriculture calls for improved testing approaches to assure their environmental safety. Full biodegradation (≥90%) prevents accumulation in soil, which is the first tier of testing. The application of specific ecotoxicity tests is the second tier of testing needed to show safety for the soil ecosystem. Soil microbial nitrification is widely used as a bioindicator for evaluating the impact of chemicals on soil but it is not applied for evaluating the impact of biodegradable plastics. In this work the International Standard test for biodegradation of plastics in soil (ISO 17556, 2012) was applied both to measure biodegradation and to prepare soil samples needed for a subsequent nitrification test based on another International Standard (ISO 14238, 2012). The plastic mulch film tested in this work showed full biodegradability and no inhibition of the nitrification potential of the soil in comparison with the controls. The laboratory approach suggested in this Technology Report enables (i) to follow the course of biodegradation, (ii) a strict control of variables and environmental conditions, (iii) the application of very high concentrations of test material (to maximize the possible effects). This testing approach could be taken into consideration in improved testing schemes aimed at defining the biodegradability of plastics in soil.

Monitoring biodegradation of poly(butylene sebacate) by Gel Permeation Chromatography, (1)H-NMR and (31)P-NMR techniques.

The increasing use of new generation plastics has been accompanied by the development of standard methods for studying their biodegradability. Generally, test methods are based on the measurement of CO(2) production, i.e. the mineralization degree of the tested materials. However, in order to describe the biodegradation process, the determination of the residual amount of tested material which remains in the environment and its chemical characterization can be very important. In this study, the biodegradation in soil of a model polyester (poly(butylene sebacate)) was monitored. Gel Permeation Chromatography and Nuclear Magnetic Resonance ((31)P-NMR and (1)H-NMR) were used in order to obtain information about the polyester structure and the possible by-products that can be found in soil during and at the end of the incubation. The polyester mineralization (i.e. the CO(2) production) was tested according to ASTM 5988 standard method for 245 days. When the polyester mineralization was about 21% and 37% (after 78 and 140 days of incubation) and at the end of the process (63% of mineralization, 100% if compared to the cellulose used as reference material), the soil was extracted with chloroform (solvent of the tested substance) and the extracts were analyzed using GPC and NMR acquisitions. The analytical acquisitions showed high molecular weight polyester in soil during the incubation (78 and 140 days): the polyester concentration decreased but its structure remained almost the same with a slow decreasing in molecular weight. At the end of the test (245 days) no film of the polyester could be extracted from the soil: NMR acquisitions and GPC analyses of the extracts suggested a strong degraded structure of the residual polyester. Even if at the end of the process only 63% of carbon had been lost by mineralization, the whole of the added polyester seems to have disappeared after about eight months of incubation, suggesting substantial biomass formation.

Laboratory test methods to determine the degradation of plastics in marine environmental conditions.

In this technology report, three test methods were developed to characterize the degradation of plastic in marine environment. The aim was to outline a test methodology to measure the physical and biological degradation in different habitats where plastic waste can deposit when littered in the sea. Previously, research has focused mainly on the conditions encountered by plastic items when floating in the sea water (pelagic domain). However, this is just one of the possible habitats that plastic waste can be exposed to. Waves and tides tend to wash up plastic waste on the shoreline, which is also a relevant habitat to be studied. Therefore, the degradation of plastic items buried under sand kept wet with sea water has been followed by verifying the disintegration (visual disappearing) as a simulation of the tidal zone. Most biodegradable plastics have higher densities than water and also as a consequence of fouling, they tend to sink and lay on the sea floor. Therefore, the fate of plastic items lying on the sediment has been followed by monitoring the oxygen consumption (biodegradation). Also the effect of a prolonged exposure to the sea water, to simulate the pelagic domain, has been tested by measuring the decay of mechanical properties. The test material (Mater-Bi) was shown to degrade (total disintegration achieved in less than 9 months) when buried in wet sand (simulation test of the tidal zone), to lose mechanical properties but still maintain integrity (tensile strength at break = -66% in 2 years) when exposed to sea water in an aquarium (simulation of pelagic domain), and substantially biodegrade (69% in 236 days; biodegradation relative to paper: 88%) when located at the sediment/sea water interface (simulation of benthic domain). This study is not conclusive as the methodological approach must be completed by also determining degradation occurring in the supralittoral zone, on the deep sea floor, and in the anoxic sediment.

A screening model for fate and transport of biodegradable polyesters in soil.

A numerical model for predicting the fate and transport of biodegradable polyester residues in soil, following successive applications of mulch film, was developed and applied. The polymer, applied on surface soil, was assumed to be converted into by-products (monomers), according to a first order kinetics with constant K(1deg). The monomers released were assumed to sorb on soil organic matter (according to a first-order kinetics with constant K(s)), to be leached with the seepage water, through vertical advection and hydrodynamic dispersion, and biodegraded (according to a first-order kinetics with constant K(b)). Results suggested that, to assess a possible build-up of mulch film (as a polymer) on the surface soil, the degradation constant K(1deg) relating the polymer conversion to by-products should be known, whereas the biodegradation constant K(b) indicates there is no danger of groundwater pollution. Likewise, on the basis of by-product concentration in deep soil, soil pollution should not occur.