**4. Discussion**

## *4.1. Economic Evaluation*

The results shown in Table 5 indicate that all materials had similar yields to PE film, but the trend is that some of the biodegradable materials obtain higher yields, confirming previous evidences such as that of [32] who reported higher pepper yields with similar biodegradable materials compared to PE.

Total costs and net margins (Table 6) in the PE situations are quite similar, with an increase of 0.11% in the costs when considering landfill and 0.18% when plastic is recycled. These results sugges<sup>t</sup> that the cost of waste treatment and recycling do not significantly a ffect final profitability. This contrasts strongly with the widespread perception among farmers that waste managemen<sup>t</sup> is costly in terms of time and money. Our estimations support the authorities' e fforts to hold farmers responsible for the wastes they generate in their activities until the end of their cycle.

However, given that there are no significant yield di fferences between materials, it is important to note that subsidies would be insu fficient to compensate for the extra cost of the material if identical yields were obtained, with the only exception of Ecovio ® and Sphere ®. This result is maintained even taking into account the total recycling cost. Therefore, the current level of subsidies (35%) does not seem to be a strong enough incentive for all the biodegradable materials to be adopted by farmers. An alternative to the current system should provide for compensation to cover the di fference in cost with regard to PE. Calculations show that the rate of subsidy should be 50.1% for Mater-Bi ® and 37.6% for Bioflex ® to assure these options to be as profitable as PE. When the total cost of recycling is considered, then the necessary subsidy would reach 48.7% for Mater-Bi ® and 35.9% for Bioflex ®.

With regard to paper mulches, although their costs are between 5.5% and 9.3% higher than PE, they obtain higher net margins due to the influence of savings on field conditioning operations and higher yields. Arrosi ®240 is the best option among paper mulches, with increases in net margin by 22.8%. Once again, this result is highly dependent on the higher yields. When yields are considered the same as obtained by PE, then the over-cost of paper materials is not compensated by savings in waste managemen<sup>t</sup> costs. In this case, the percentage of subsidies needed to make them as profitable as the PE option would be 48.2% for Mimgreen ®, 45.1% for Arrosi ® 69 and Arrosi ® 240, and 58.6% for Arrosi ® G1a.

In summary, although six of the eight materials evaluated as alternatives to PE have proved to be more profitable, only two of them (Ecovio ® and Sphere ®) are good potential alternatives from an economic point of view under the current subsidies received despite their higher market price. Two main reasons explain this result: first, because they achieve crop yields similar to PE, and secondly, because they save waste treatment costs that compensate their higher market prices. Biodegradable plastics benefit from public support to compensate for part of the rise in market prices but the results show that the current subsidies system does not guarantee the profitability of all the materials analyzed. In fact, the most expensive materials (Mater-Bi ® and Bioflex ®) are not good economic alternatives when the yields are the same as PE. Similarly, [1] showed that the use of biodegradable mulches with tomato crop in di fferent localities was only profitable in certain specific locations and with some materials.

Interestingly, two of the evaluated biodegradable films (Ecovio ® and Sphere ®) are good economic alternatives to PE under the current public paymen<sup>t</sup> system. This contrasts with the widespread use of the PE, which probably comes from its low cost in comparison with biodegradable materials. By contrast, our calculations show that biodegradable films can be better alternatives in the short-term even in the case of no waste management. The net margins when using these biodegradable materials are even better when recycling is considered mandatory. Of course, there may be other non-economic reasons that may inhibit broader adoption of bio materials and papers. Breakdown during the growing season and fragments of mulches after tillage may be aesthetically displeasing to farmers and consumers thus inhibiting their adoption. In the case of papers, it may also exist a negative perception linked to the greater discomfort for their installation beyond the cost of time that has been included in our calculations.

#### *4.2. Environmental Implications of the Use of Plastic Films and Papers*

In addition to the short-term economic considerations, other environmental aspects related to the use of mulching materials should be taken into account. It is necessary to emphasize the increasing problems caused in the environment by the plastics. For example, [33] indicated that the presence of PE in horticultural soils in Argentina can represent around 10% of the soil and [34] a ffirmed that the amount of plastic waste in an average vegetable field of China could reach 317.4 kg ha−1. Although no similar data have been found for Europe, there is strong evidence that the presence of plastic

residues also a ffect the soil quality. For example, [13] reported that amounts of residual mulch films of 320 kg ha−<sup>1</sup> could interfere in tomato crop yields, causing decreases by 5.9% in yields. It has been demonstrated that this e ffect on the soil's productive capacity increases with the concentration of plastic particles in the soil. This evidence is a further argumen<sup>t</sup> in favor of making the complete managemen<sup>t</sup> of waste mandatory for farmers, and therefore a strong support for the use of other biodegradable materials.

However, it should be remembered that there is a growing number of studies warning of the consequences of the use of many of the so-called "biodegradable" materials, as they do not degrade completely in soil. A recent study of [23] hypothesized the case where a farmer tills all the biodegradable mulch at the end of the crop cycle into the soil. The standard method tests applied to plastics (ASTM D5988 and ISO 17556) consider a degradation rate of 90% biodegradation rate within to 2 years; considering this, 45% of this plastic will remain in the field during the first year. After the second year, a 10% of the first year plastic will probably remain in soil and the plastic from the second application with its 10% remaining to the third year. If this 10% is assumed never to degrade, then it will accumulate every year. The authors hypothesize that 350 kg ha−<sup>1</sup> of non-degradable plastic will represent 6.45% decreased yield on the fifth year of using biodegradable films and tilling them at the end of the crop season. Unfortunately, there is no standard method to measure the rate of degradation after incorporation in the soil and the percentages could be very variable.

In the case of some of our tested materials, some evidences are reported in literature. [35] established that Bioflex ® material lost 73% of their initial weight after 145 days after soil incorporation (DASI), while Sphere lost only 42% in the same period. On the other hand, Mater-Bi ® generated fragments of a wide range of sizes (up to 2664 mm2) which maybe will interfere with tillage, another aspect to take into consideration. By contrast, the paper Mimgreen ® presented the smallest fragments and surface after 200 DASI.

With regard to paper mulches, no waste managemen<sup>t</sup> has to be implemented and no accumulation of waste in the soil is expected to interfere with the crop, so, in principle, their e ffects are likely to be less harmful than plastics. However, papers are insu fficiently explored until now and their environmental effects in the long-term and these advantages have to be proven. If these advantages are verified, then the papers should be eligible for public support.
