The influence of phosphorus fertiliser addition on soybean nitrogen fixation and yield

Walling, Hannah (2025) The influence of phosphorus fertiliser addition on soybean nitrogen fixation and yield. PhD thesis, Lancaster University.

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Abstract

This thesis tests the hypothesis that phosphorus (P) fertiliser addition can optimise nitrogen (N2) fixation processes and increase soybean yield. Using a combination of controlled environment studies, field trial and global meta-analysis, this thesis aids in achieving sustainable soybean production through building improved understanding of the effects of P fertiliser addition to inform future fertiliser guidelines, crop models and management practices. Global meta-analysis showed an increase in soybean response to P fertiliser addition with seed yield increasing by 25%. This also highlighted the complexity of soybean yield response to P fertiliser, with a several key management and environmental conditions having a significant effect, including soil P concentration, pH, fertiliser type and rate of application and climatic conditions – indicating soybean yield cannot be increased by single P fertiliser applications alone. Controlled environment studies revealed P addition significantly increased nitrogen (N2) fixation. Key nodule traits significantly correlated with shoot N; however, further work should examine the mechanistic pathways driving the increase in nodule formation. Interestingly, controlled environment studies revealed nodule function was not influenced by P fertiliser addition. Instead, regulatory mechanisms such as maintenance of nodule P concentration and leghaemoglobin concentration under low P conditions maintained N2 fixation. Through combined analysis of multiple growth parameters and measures of plant physiology, seed yield was found to increase under P fertiliser addition. Seed P concentration also increased following P fertiliser addition. Results of this thesis contribute to our understanding of soybean response to P fertiliser addition, particularly the improvement to key nodule traits to improve N2 fixation and the partitioning and remobilisation of resources to improve yield. This now needs to be upscaled at differing environmental and management conditions and incorporated into crop models to ensure the sustainable use of P fertiliser in soybean production globally.