Abstract
Introduction
There are quite a few reasons why biobased (originating from living organisms) fertilizers are urgently needed. The prime mineral that makes up fertilizers is phosphate. Phosphate is a mined resource and, like oil reserves, phosphate supplies are in depletion. Rock phosphate imports into the European Union mostly come from China, the United States or African countries. A way to counteract any future disruptions to the supply of phosphate is to recover the mineral from waste streams, particularly from waste water, for example from agricultural processes like the washing of potatoes. In this process, struvite which is a phosphate rich salt is produced. This can be used for fertilizer formulations.
The second major point is that society needs to reduce the impact of mineral-based fertilizers on the environment. At the very start of modern fertilization, nutrients were supplied without refining, and often were dosed in large amounts. This led to the pollution of ground water due to the leaching of nutrients into the ecosystem. A way to counteract this was to refine the fertilizers by coating them.
What was at first a major step forward for agriculture would soon emerge as a problem that was not foreseen. The coating materials applied were mostly fossil-based and, in the end, they are very similar to plastic. The use of these materials leads to the accumulation of microplastics in soils and furthers their entry into the food chain.
A third reason to pursue the use of biobased substances is for the reduction of carbon dioxide (CO2) emissions. The utilization of local waste streams saves a huge amount of transport and mining costs while at the same time recovers valuable raw materials that are usually disposed of.
Are There Sustainable, Biobased Fertilizers on the Market Today?
This is a difficult topic to talk about. On one hand, one can argue that manure is a biobased, and, yes, it is even an organic fertilizer. But modern ways of animal husbandry have led to a very different composition of manure that does not pass the tests and certifications necessary for organic products. The application of such manure for fertilization can be realized on a small scale on organically managed farms, as is the case on many farms in Austria. But, for the large agricultural areas of Europe, the amounts of organic manure that would be necessary for large-scale fertilization would never be sufficient. This was why mineral fertilization was introduced in the first place.
Today, there are quite a few technologies that allow for the manufacturing of biobased fertilizers through the application of biotech methods. One of the main reasons why these fertilizers are not exploited yet is due to their cost when compared with the low import prices of mineral fertilizers. The implementation of new technologies is of course linked to high investments which companies are not necessarily willing to make due to the possible risk of failure. This is one of the major reasons why the EU is determined to encourage good practices that support the biobased circular economy through the implementation of the European Green Deal. This initiative promotes the use of waste materials and the implementation of sustainable processes.
Fertilizers Need Nutrient Efficiency Improvements
The negative effects caused by the release of fertilizers into the environment are putting pressure on agrochemical producers to improve the efficiency of their products. Ideal fertilizers should have a higher nutrient use efficiency to reduce their detrimental effects on soil, water and atmospheric environments. At the same time, only one single fertilizer application should be applied during a growing season.
However, currently, the nutrient use efficiency (NUE), defined as the percentage of a nutrient that contributes to biomass production for nitrogen-based fertilizers, ranges from 30% to 50%, which clearly indicates that efficiency improvements are needed. To compensate for these losses, agrochemicals are applied in excessive amounts, meaning at least 70% of the fertilizer is lost either through leaching, thereby causing eutrophication that can lead to algae blooms. In addition, this excessive application also increases the release of nitrous oxide,6 a powerful greenhouse gas that is emitted into the atmosphere that contributes to climate change.
The Challenge of Developing Sustainable Fertilizers
The process of developing a sustainable fertilizer is quite challenging. Many aspects need to be taken into consideration such as selecting the appropriate materials for the new products. The necessary technologies and methods to develop the fertilizer need to be determined. Sometimes, you may even think that you have a viable product, but this is not in itself a guarantee that the product will work in the end.
All the steps and processes along the value chain to develop a sustainable fertilizer must be considered, and this is the challenging part. You might have a side stream that has the material you need for the fertilizer development, for example phosphate, but there might also be components like heavy metals that should not end up in your product. It might even not be possible to remove those impurities.
A major point is also that the techniques that work at a small scale in a laboratory might not be feasible at an industrial scale. The reaction you perform in a beaker of 200 mL might not work in a reactor with 2,000 L. How different coatings for a fertilizer can perform at different scales can come as a big surprise to fertilizer developers and scientists (it was for me). So, you should remember always that the forces at work in a huge coating drum cannot easily be extrapolated from a small lab scale coater.
Using wood side streams for fertilizer coatings
Pulping and paper-making processes are very old. Considerable effort has been invested by the industries concerned to make them more sustainable. In the past, milling effluents were released into rivers with major impacts on the environment. The implementation of waste water treatment has had a lot of positive effects, but for some time the lignin fraction of the process, that comes from a wood side stream, mainly has been burned for energy production.
Many scientists have thought of creative ways to utilize this fraction by extracting chemicals and small molecules. Also, applications in construction and agriculture have been developed. Our group at the Institute of Environmental Biotechnology at the University of Natural Resources and Life Sciences Vienna (BOKU) has long worked on developing new enzymatic processes, especially to make lignosulfonates usable for new applications. A process developed by Priv. Doz. Dr. Gibson Nyanhongo and his co-workers finally gave way to the better utilization of lignosulfonates.
The enzyme-based process we use to produce the fertilizer coatings has many positive effects, with one showing a big potential to reduce CO2 emissions. We could show in our recent paper A biobased, bioactive, low CO2 impact coating for soil improvers that compared to polyurethane coatings, a large amount of CO2 can be saved by evading the incineration process and by using enzymatic synthesis in comparison to the chemical synthesis of fossil-based coatings.
Exciting Developments in SUSFERT
We could show environmental non-hazardous application of polymerized lignosulfonates in plant trials. Essentially, we did not experience any negative impacts on the model plants in corn, wheat, tomato and salad when the coating material was applied. The CO2 emission analysis performed by the University of Antwerp on the coatings produced with xylitol and glycerol compared to polyurethane and polyethylene coatings showed potential CO2 savings of up to 3.4 kg CO2-eq/kg product.
In addition, plant growth-promoting bacteria spores were introduced into the coating. Not only could we show the viability of the bacilli on the fertilizer, but also their positive effects as plant growth promoters could be shown by an increased biomass production of the test plant used. The authors of the SUSFERT paper were all extremely excited about the results which were published in the Royal Society of Chemistry's Green Chemistry journal.
This project has received funding from the Bio Based Industries Joint Undertaking (BBI-JU) under the European Union's Horizon 2020 research and innovation program under grant agreement No. 792021.