Effect of Fertigation with Struvite and Ammonium Nitrate on Substrate Microbiota and N₂O Emissions in a Tomato Crop on Soilless Culture System

Struvite and ammonium nitrate (AN), as wastewater-recovered products, are possible alternatives as raw materials for nutrient solutions. However, their impact on the rhizosphere microbiota and N2O emissions is scarcely known. Therefore, the present research studies the ecological changes in the bulk-substrate microbiome and its correlation with N2O emissions in a perlite-based system tomato crop under (i) conventional synthetic fertigation management; (ii) fertigation with struvite; and (iii) struvite and AN. A high bacterial diversity and the natural presence of plant-growth-promoting rhizobacteria in a soilless system are highlighted. However, the different N-NH4+:N-NO3− ratios influence the ecological niches of ammonia-oxidizing archaea (AOA) and bacteria (AOB), with a stronger response by AOB community, while AOA kept constant regarding the fertilization applied. Despite this, enrichment of N-transforming bacterial phylotypes was relatively enhanced (mainly Nitrosomonas, Nitrosospira, and Nitrospira) concomitant with the production of N2O emissions when ammonium fertilization was overapplied. In the absence of a plant, N2O emissions were positively correlated, respectively, with Nitrosospira and AOB:AOA ratio, suggesting potential indicators for ammonium availability in the substrate. Fertilizer blends using recovered nutrients are a feasible alternative for increasing circularity in horticulture. Nevertheless, optimum fertilizer management is needed due to its influence on rhizosphere microbiota and N2O emissions.