The focus on biomass recycling has increased markedly in recent years, fuelled by the societal goal of a more circular economy. This includes the recycling of nutrient-rich organic wastes and residues for biobased fertiliser production. Many of these new biobased fertilisers have been introduced to the European market, and a few years ago, a new EU fertiliser regulation entered into force. This aims to ensure that biobased fertilisers can enter the market with mineral fertilisers on equal terms, contributing to the circular economy of Europe. It also ensures quality standards, making biobased fertilisers reliable and safe for the user in terms of nutrient efficiency, handling and storage. In addition, it enables certification through the CE-system, certifying these biobased fertilisers contain no harmful substances or have no adverse effects on soil, environmental, or human health.
Agronomic fertiliser value
For farmers, one of the most important drivers for crop productivity is the application of fertilisers containing essential plant nutrients (nitrogen, phosphorus, potassium etc.). To ensure profitability for the farmer, efficacy of the nutrients applied for enhancing crop yield is crucial, as is the cost of fertiliser. To convince farmers of shifting from conventional mineral towards more biobased fertilisers, their fertiliser value needs to be rigorously documented.
In the EU Horizon-2020 project LEX4BIO (https://lex4bio.eu), we have tested the nitrogen fertiliser value of a range of biobased fertiliser products compared to mineral nitrogen fertiliser in field trials over two years (with maize, winter- and spring cereals) at four locations in Europe. Seven commercial pelletized solid biobased fertilisers were tested at all locations, while a number of local biobased fertiliser products (e.g. biogas digestate, food-industry by-products or compost) were tested at single sites only; in total 18 products.
The biobased fertilisers tested showed a relatively high average nitrogen fertiliser replacement value of 70% (relative to mineral fertiliser) in the crop which they were applied to. However, fertiliser value varied widely between the biobased fertilisers, with eight of them above 75%, six between 60 – 75%, while only four below 60% (11% for a compost). However, most biobased fertilisers contain some organically bound nitrogen, which is not immediately available to the first crop, but with some potential residual effect in subsequent crops. When the first and second year effects were added, the accumulated fertiliser value reached close to 100% for most of the biobased fertilisers we studied (except for compost). From a multi-annual perspective, most of them therefore showed high agronomic performance, similar to mineral fertilisers.
Risk of ammonia losses to the atmosphere
Some biobased fertilisers can be prone to nitrogen loss by ammonia volatilisation to the atmosphere, which is detrimental to the environment, but also lower their fertiliser value. We found potential ammonia loss to vary greatly across the wide range of biobased fertilisers, with highest and most rapid loss occurring from biogas digestates, while for other it was either low or occurred after a delay period. For biobased fertilisers at risk of ammonia loss, soil incorporation, as opposed to soil surface application, could be an effective mitigation strategy.
Effects on soil health
In addition to providing essential nutrients to crops, biobased fertilisers can also have positive effects on soil health. We found that biobased fertilisers generally improved soil health indicators (biological, chemical and physical) more than conventional mineral fertilisers across different European soils, but with highly variable effects between the biobased fertilisers.
Pros and cons of biobased fertilisers
Overall, we identified multiple trade-offs between the properties of the biobased fertilisers. Some were positive for soil health, but had a very low fertiliser value (e.g compost). Some were efficient nitrogen fertilisers, but did not improve soil health (e.g. potato cell water). Others had both positive effects on soil health and nitrogen efficiency, but a higher risk of ammonia loss.
We therefore cannot draw general conclusions about biobased fertilisers as a group. However, the most important determinant for farmer adoption will be price and cost of application. Many of the biobased fertilisers are not yet available in large quantities or at sufficiently low cost per amount of nutrient. Developments in the coming years will show whether supply can increase to a point where prices will fall to a level where they can compete with mineral fertilisers and in turn increase their use. Moreover, an appreciation of their other positive properties may also stimulate their use.
In any case, our results indicate the agronomic performance of most of the biobased fertilisers we studied is relatively good. This is good news for the farmers, the climate and the environment.
Lars Stoumann Jensen
Professor of Soil Fertility and Organic Waste Recycling, Department of Plant & Environmental Sciences, University of Copenhagen
Denmark
Lærke Wester-Larsen
Postdoc Researcher, Department of Plant & Environmental Sciences, University of Copenhagen
Denmark
