Organic Broadcaster

Research shows biodegradable mulch film slow to degrade in field

By Jennifer Moore-Kucera

Biodegradable biobased mulch film was added to the National List of Allowed and Prohibited Substances for organic production in October 2014. Several recent studies have shown wide variability in degradation, raising concerns about film fragments that may persist in the field. As organic farmers take advantage of the new allowance for using biobased film in their fields this season, they should be aware of the research—there are many unanswered questions that remain about biodegradable mulch films.

One of the major goals of biobased mulch films is to replace non-renewable petroleum-based polyethylene plastic mulch films with a product that degrades in the field with minimal or no negative impacts—an extremely important and challenging task. With plastic mulch used on more than 400,000 ha in the U.S. alone (2006 estimate), we are definitely moving in the right direction. It is very exciting to be playing a role in this valuable research.

Research Results
In 2010 and 2013, I participated in a multi-disciplinary, multi-state research project funded by USDA-NIFA-SCRI (SCRI-SREP Grant Award No. 2009-02484) that evaluated soil quality impacts following burial of four types of biodegradable mulch (plus a no-mulch control) in tomato production in high tunnels and the open field. We estimated the degradation of these mulches over a 24-month period following till-down by measuring the percent mulch area remaining (PMAR) in the soil at three geographically distinct U.S. locations (Knoxville, Tenn.; Lubbock, Texas; and Mount Vernon, Wash.).

To simulate the fate of mulches incorporated into soil by plowing following an annual crop production cycle, we placed weathered mulch pieces (103 cm2) that had been used during the 2010 growing season in nylon mesh bags and buried them within the cropping area. We tested four mulches, including two commercially available films (BioAgri® Ag-Film and BioTelo Agri), one experimental mulch (Spunbond-PLA-10), and one commercially available cellulosic-based mulch (WeedGuardPlus) that served as a positive control.

These photographs show the amount of degradation experienced by three types of biobased mulches at the Mount Vernon, Wash. test plot. Photo scan by Jennifer Moore-Kucera and Chenhui Li

After 6 months (top panels):
A) Spunbond-PLA-10 = 104.4 cm2
B) BioAgri Ag-Film = 101.8 cm2
C) BioTelo Agri = 109.6 cm2

After 24 months (bottom panels):
a) Spunbond-PLA-10 = 101.0 cm2
b) BioAgri Ag-Film = 107.2 cm2
c) BioTelo Agri = 108.0cm2

PMAR of WeedGuardPlus was 0% within 12 months at all three locations, whereas PMAR of experimental Spunbond poly-lactic acid mulch was >90% at 24 months at the three locations. Our results also showed wide variability in degradation levels across the three locations for the BioAgri and BioTelo products. For example, after 24 months of burial, only 2% of mulch remained at Lubbock compared to 49% remaining on average at Knoxville and 89% at Mount Vernon.

This wide variability suggests that certain factors stimulate or inhibit the degradability of these products. We evaluated both biological and chemical soil properties to identify differences between the Lubbock site (with the most degradation in the shortest period of time) and the other two sites (Knoxville and Mount Vernon). We found that the Lubbock site had the greatest soil diurnal temperature (the difference between the maximum and minimum soil temperature during the day), the greatest maximum daily soil temperature, an alkaline pH, and a soil microbial community characterized by a relatively high abundance of fungi. While these factors may play important roles in how quickly mulch films degrade, further testing is needed to better understand the factors that affect biodegradability.

Issues with Degradation
Nitrogen might be one factor that affects degradation. In general, when a material is high in carbon but low in nitrogen, breakdown by microorganisms is very slow. When nitrogen is applied to system, breakdown proceeds more quickly. Although nitrogen was not significantly different across the three locations at 6, 12, 18, and 24 months post burial, nitrogen is a very dynamic compound—it is quickly flushed out of the soil or removed through crop uptake) that likely plays an important role in biodegradable mulch degradation and should be explored in subsequent research.

In other research, we isolated native soil microorganisms that colonized biodegradable mulch films. Fungal colonizers were related to others previously found to colonize and sometimes degrade conventional and biodegradable films. Although they colonized the biodegradable plastic mulch materials, our fungal isolates did not significantly degrade any of them. Given that less than 10% of microorganisms in soil have been cultured, it remains possible that a specific organism or group of organisms may be isolated that could break down these materials. Such an organism could then perhaps be impregnated within the mulch, or sprayed onto the mulch in the field prior to soil incorporation to help aid in effective breakdown. As the majority of our fungal colonizers were potentially pathogenic and/or produced mycotoxins, such an isolate would need to be tested for safe agricultural use.

Another issue is the rate of breakdown. Assuming organisms that could break down biodegradable mulch films do exist in the soil, there is no evidence from our study and scant evidence from the literature that they break down these materials in a timeframe that might prevent accumulation in the soil when biodegradable mulch is applied year after year. Brodhagen et al. (2014) discuss the biology of biodegradable film breakdown by soil microorganisms, and the potential for ecotoxicity of accumulated plastic materials in the soil. (See link in the box of resources.)

Users of biodegradable mulch films may opt to test these materials in small plots before whole farm adoption. Important variables to take note of include:

• Soil type and key soil properties (e.g., texture, organic matter, pH, inorganic nitrogen status)
• Weather conditions (e.g., air temperature, soil temperature if possible, humidity, wind, and UV index)
• Management practices (e.g., how long mulch is used before incorporation, amount and timing of inputs including fertilizers and agrochemicals, irrigation/precipitation quantity and timing, etc.)

Recommendations
When and how the materials are incorporated may play a role in degradability. Incomplete degradation can become a nuisance for field implements (plastic becomes wrapped around blades and moving parts) or create unknown challenges in subsequent cropping cycles.
It is important to plow under the biodegradable mulch using implements that will create the smallest possible fragments. In general, smaller fragments will increase the surface area available for microbial attack and enhance degradability.

Timing of incorporation also may impact degradation. For example, it may be advantageous to time incorporation prior to precipitation or apply water.

And, going back to the idea that nitrogen can improve degradation, consider adding nitrogen if soils are low in inorganic or organic sources. This will reduce the carbon:nitrogen ratio and enhance degradability.

Future of Biodegradable Mulch Films
Of course, each year, manufacturers are modifying and improving their products. Therefore, the formulations of the biodegradable mulches we used in 2010 may be different today. Integrating manufacturers’ knowledge with the research from agronomists, soil chemists and microbiologists will help advance this area of research, and hopefully identify reliable biodegradable mulch films.

Further studies under field conditions also are needed to assess the ecosystem-wide impacts of biodegradable mulches. Concerns are long-term effects of small, film fragments that may persist in the environment, or potential interactions between various agrochemicals and mulch fragments.

For further information, please see our team’s website at http://mtvernon.wsu.edu/hightunnels/.

Jennifer Moore-Kucera is an Associate Professor of Soil & Environmental Microbiology in the Department of Plant and Soil Science at Texas Tech University.

Dr. Carol Miles, Professor in the Department of Horticulture at Washington State University, contributed to this article.

Additional Papers
1. Li, C., Moore-Kucera, J., Lee, J., Corbin, A., Brodhagen, M., Miles, C., Inglis, D., 2014. Effects of biodegradable mulch on soil quality. Applied Soil Ecology, 79, 59-69. http://www.sciencedirect.com/science/article/pii/S0929139314000651

  1. Li, C.H., Moore-Kucera, J., Miles, C., Leonas, K., Lee, J., Corbin, A., Inglis, D., 2014. Degradation of potentially biodegradable plastic mulch films at three diverse U.S. locations. Journal of Agroecology and Sustainable Food Systems, 38(8), 861-889. http://www.tandfonline.com/doi/abs/10.1080/21683565.2014.884515#.VICmLsnZeSo

  2. Moore-Kucera, J., Cox, S.B., Peryron, M., Bailes, G., Kinloch, K., Karich, K., Miles, C., Inglis, D.A., Brodhagen, M., 2014. Native soil fungi associated with compostable plastics in three contrasting agricultural settings. Applied microbiology and biotechnology, 98, 6467-6485. http://link.springer.com/article/10.1007/s00253-014-5711-x

  3. Bailes, G., Lind, M., Ely, A., Powell, M., Moore-Kucera, J., Miles, C., Inglis, D., Brodhagen, M., 2013. Isolation of native soil microorganisms with potential for breaking down biodegradable plastic mulch films used in agriculture. Journal of Visualized Experiments (75) e50373, doi:10.3791/50373. http://www.jove.com/video/50373/isolation-native-soil-microorganisms-with-potential-for-breaking-down.

  4. Corbin, A.T., Miles, C.A., Cowan, J., Hayes, D.G., Moore-Kucera, J., Inglis, D.A., 2013. Current and future prospects for biodegradable plastic mulch in certified organic production systems. eXtension http://www.extension.org/pages/67951/current-and-future-prospects-for-biodegradable-plastic-mulch-in-certified-organic-production-systems.

  5. Brodhagen, M., Peryon, M., Miles, C., Inglis, D.A., 2014. Biodegradable plastic agricultural mulches and key features of microbial degradation. Applied Microbiology and Biotechnology. DOI 10.1007/s00253-014-6267-5. http://link.springer.com/article/10.1007/s00253-014-6267-5.

USDA Organic Regulations for Biobased Mulch Film

On Oct. 30, 2014, the USDA organic regulations were amended to allow the use of biodegradable biobased much film in organic crop production. Here’s the wording:

Biodegradable biobased mulch film.
A synthetic mulch film that meets the following criteria:
(1) Meets the compostability specifications of one of the following standards: ASTM D6400, ASTM D6868, EN 13432, EN 14995, or ISO 17088 (all incorporated by reference; see §205.3);
(2) Demonstrates at least 90% biodegradation absolute or relative to microcrystalline cellulose in less than two years, in soil, according to one of the following test methods: ISO 17556 or ASTM D5988 (both incorporated by reference; see §205.3); and
(3) Must be biobased with content determined using ASTM D6866 (incorporated by reference; see §205.3).

NOP Clarification
Miles McEvoy, Deputy Administrator of the National Organic Program, issued a Policy Memorandum Jan. 22, 2015 to certifying agents to clarify how to review the biobased requirement for these products. The memo specifies that the polymer feedstocks must be biobased, meaning an “organic material in which carbon is derived from a renewable resource via biological processes.” Pigments and processing aids are not considered feedstocks. Biodegradable mulch film that contains non-biobased synthetic polymer feed-
stocks, such as petrochemical resins, does not comply with the USDA organic regulations.

OMRI Weighs In
The Organic Materials Review Institute (OMRI) provided this comment on its website, www.omri.org.
“Based on OMRI’s research and understanding of these products, most and possibly all, of the currently marketed biobased mulch films contain some petrochemical feedstocks, and the feedstocks are typically less than 50% biobased. It is unlikely that OMRI will be able to add any biodegradable, biobased mulch films to the OMRI Products List© this year.
“We do not know the feasibility or timeframe in which mulch film manufacturers may develop new formulas that comply with the NOP policy for biobased content.”

From the March | April 2015 Issue

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