Recent Advances in Biological Nitrogen Fixation

Nitrogen is essential for the growth and functioning of all living organisms; however, only 2% of Earth’s nitrogen is available for them [1]. It can mostly be found in the atmosphere as N₂, which can be assimilated only by a small group of microorganisms called diazotrophs, which are able to reduce dinitrogen into NH₃ [2]. This process, referred to as biological nitrogen fixation, is most effective in symbiotic systems formed between soil saprophytic bacteria called rhizobia and legume plants, and can provide considerable amounts of reduced nitrogen for agricultural systems and uncultivated areas [3]. New discoveries and concepts related to biological nitrogen fixation often result in useful improvements in agronomy; therefore, new ideas focusing on enhancing or broadening its application still need to be developed.

The papers in this Special Issue include a review and original research articles covering different aspects of biological nitrogen fixation and organisms associated with this phenomenon. In their extensive review, Santos et al. [4] focused on the demand for environmentally friendly technologies based on microorganisms and their plant hosts that can be used instead of chemical fertilizers and pesticides. In this paper, (a) standards for inoculant production, (b) ways to deliver inoculants to the crop, (c) compatibility between inoculants and pesticides, and (d) detrimental effects of pesticides on inoculants were thoroughly discussed. Maitra et al. [5] proposed new nontoxic polymers that considerably enhance bacterial survival ability, which can be used in preparation of rhizobial liquid inoculants. This might be an interesting finding, since improvement of inoculant formulas are still needed to ensure longer storage with high viability of cells and better competitive properties of strains introduced as biofertilizers. Siczek et al. [6] and Helios [7] described solutions related to sustainable agriculture based on legume–rhizobia symbiotic systems. One of these studies focused on the dynamics of bacterial and fungal communities studied after enrichment of soil with different crop residues in a field trial, and showed that application of legumes as a forecrop not only increased the soil nutrient pool but also had a strong impact on fungal community, acting against phytopathogens, which may result in lower fungicide requirements in the following growing seasons [6]. The second paper presented the use of white clover as undersowing for basket willow (Salix viminalis L.), which resulted in reduced weed infestation of young plantations of willow, thus lowering the need for herbicide treatment [7]. As the growth of willow undersown with clover was comparable with plants fertilized with nitrogen, this practice can be assumed as a good alternative for mineral N fertilization of short-rotation woody crops. Radzka et al. [8] focused on maximization of profits of biological nitrogen fixation by plants and described how different sowing densities of soybean could affect the amount of nitrogen obtained from symbiotic reduction of atmospheric N₂. Smytkiewicz et al. [9] described the effect of rhizobial metabolites called chitolipooligosaccharides on the growth, development, and yielding of peas, and demonstrated that such a preparation could be an efficient growth stimulator for legumes. Finally, Marzec-Grządziel et al. [10] described a symbiotic system emerging between ruddy clover (Trifolium rubens L.), an endemic species which is considered endangered in European countries) and their microsymbionts, and showed the perspective for development of an inoculant formulation for this valuable nectariferous plant.

The set of papers presented in this Special Issue proves that studies on biological nitrogen fixation can be conducted in numerous and different directions. Each of them brings new knowledge of this phenomenon or describes a new invention that can be useful for the benefit of humans; therefore, I warmly encourage reading these publications.