The potential to use circular nitrogen from residual streams and green alternatives, as opposed to conventionally manufactured fertilisers, is the subject of a new project led by researchers at Wageningen University.

The use of fertilisers containing nitrogen and phosphorus has been determined as critical to maintaining agricultural productivity, particularly in the context of increasing food demand from a growing worldwide population.

Although the air consists of 78% nitrogen, its natural form—N2—is not directly available to plants and must be applied in fertiliser form to enable plant consumption.

Nitrogen fertiliser is produced from atmospheric nitrogen via the Haber-Bosch reaction, an energy-taxing process that relies heavily on natural gas and therefore has substantial consequences for climate change.

According to researchers, there are various alternatives for nitrogen fertilisation, such as extracting nitrogen from the air without using natural gas or reusing nitrogen from waste, both of which are being explored in new research conducted by Wageningen University.

Lead researcher at Wageningen University & Research’s greenhouse horticulture business unit, Alexander van Tuyll explained: "For example, in the Haber-Bosch process, green hydrogen could be used instead of natural gas.

"When it comes to residual streams, nitrogen can be recovered from sources such as wastewater or animal manure. These alternatives are already being explored and applied by scientists, however, the available knowledge is still quite fragmented."

The lead researcher presented additional insight into the challenges facing nitrogen extraction from residual streams, concluding that not enough of the substance is available to cover demand of synthetic nitrogen fertiliser for both greenhouse horticulture and arable farming. 

"For crops grown in recirculating systems, like vegetables, the requirements for fertilisers differ from those in arable farming or ornamental plant production.

"One key requirement is solubility, so the fertiliser can be delivered efficiently via drip irrigation systems.

"In a recirculating greenhouse system where drainwater is recaptured, our research found that contamination levels must be much lower than European safety limits – doable, but important to know,” van Tuyll added.

The researchers also compared energy consumption for various methods as well as qualitative factors, concluding that even when using green hydrogen, the Haber-Bosch process remains far more energy-intensive than extracting nitrogen from residual streams.

However, extracting nitrogen from residual streams tends to have a lower concentration, which researchers indicated can present additional logistical challenges.

The study is the first in a three-part series developed by the university aimed at exploring the requirements and options of circular fertilisers for greenhouse horticulture.