Crop cultivation without nitrogen fertiliser using nitrogen-fixing cyanobacterial extracts for low environmental impact

In this study, we attempted to develop a biological fertiliser using the nitrogen-fixing cyanobacterium, Trichormus sp. PCC7120, with the aim of constructing a sustainable farming method for growing crops without chemical fertilisers. We attempted to hydroponically culture two types of rice (Oryza sativa L. 'Sasanishiki' and 'Koshihikari'), broccoli (Brassica oleracea var. italica), and melon (Cucumis melo L.) using intracellular extracts from Trichormus sp. PCC7120, which was prepared using heat treatment or acid hydrolysis. Early crop growth, including length and weight, was compared among three groups: (i) pure water, (ii) chemical fertiliser, and (iii) Trichormus extract groups. Sasanishiki grew most efficiently in the 80% heat-treated and 2.5% acid-hydrolysed extracts than in pure water, whereas Koshihikari grew most efficiently in the 40% heat-treated extract than in pure water. Sasanishiki and Koshihikari consumed ammonium, potassium, and various proteinogenic amino acids in the Trichormus extract. Broccoli exhibited more length and weight when cultivated with 10-20% acid-hydrolysed extracts than in pure water and consumed glucose, along with phosphorus and glycine, in the extract. Melon exhibited more length and weight when cultivated with 10% acid-hydrolysed extracts than in pure water and actively consumed glucose, serine, glycine, and alanine in the extract. Crop growth with Trichormus extracts was comparable with that of the chemical fertiliser group. This report shows that ammonium, phosphorus, potassium, glucose, and amino acids in nitrogen-fixing cyanobacterial extracts contribute to crop growth, and these extracts may thus be valuable as biological fertilisers in crop cultivation.