Mined phosphate rock is the largest source of phosphorus (P) for use in agriculture and agro-industry, but it also is a finite resource irregularly distributed around the world. Alternatively, waste water is a renewable source of P, available at the local scale. In waste water treatment, biological nitrogen (N) removal is applied according to a wide range of variants targeting the abatement of the ammonium content. Ammonium oxidation to nitrate can also be considered to mitigate ammonia emission, while enabling N recovery. This review focuses on the analysis of alternatives for coupling biological N treatment and phosphate precipitation when treating waste water in view of producing P-rich materials easily usable as fertilisers. Phosphate precipitation can be applied before (upstream configuration), together with (concomitant configuration), and after (downstream configuration) N treatment; i.e., chemically induced as a conditioning pre-treatment, biologically induced inside the reactor, and chemically induced as a refining post-treatment. Characteristics of the recovered products differ significantly depending on the case studied. Currently, precipitated phosphate salts are not typified in the European fertiliser regulation, and this fact limits marketability. Nonetheless, this topic is in progress. The potential requirements to be complied by these materials to be covered by the regulation are overviewed. The insights given will help in identifying enhanced integrated approaches for waste water treatment, pointing out significant needs for subsequent agronomic valorisation of the recovered phosphate salts, according to the paradigms of the circular economy, sustainability, and environmental protection.

Struvite and ammonium nitrate are products obtained from widely studied processes to remove phosphorus (P) and nitrogen (N) from waste streams. To boost circularity in horticulture, these recovered products should be applied to edible crops. Particularly, struvite has not been implemented in fertigation as the unique source of P fertilizer. Therefore, a soilless system greenhouse experiment was conducted for tomato crops during two growing seasons. This study aims to compare the agronomic and environmental effectiveness of recovered products used in a nutrient solution for fertigation (NS) to synthetic fertilizer treatment. Moreover, two different N concentrations of the NS were tested to evaluate the impact on the N-leaching. Additionally, struvite dissolution tests were performed to ensure its solubility. Satisfactory results of struvite solubilization were obtained. Results show that both nutrient-recovered products can be used as fertilizers in NS, due to their non-statistical significance in total yield production and fruit quality. However, ammonium nitrate treatment, depending on the crop variety, showed a lower marketable yield. Moreover, the variation on N concentration input exhibited leachate concentration differences, with N leached percentage values from 36 to 13%. These results give deeper insights into the future potential utilization of nutrient-recovered products and technical data to optimize fertigation strategies.

Non-continuous flooding irrigation practices, such as alternate wetting and drying (AWD) and mid-season drainage (MSD), have been implemented in rice agroecosystems to reduce water use and mitigate climate change. Draining fields reduces methane (CH₄) emissions, as soil aeration decreases the abundance and activity of soil methanogens. Mitigation effects during the growing season have been widely studied. However, there is a knowledge gap regarding potential effects these growing season practices might have on subsequent fallow season emissions. This is relevant when assessing overall annual CH₄ emissions, particularly in systems in which fallow seasons account for a significant part of these. A field experiment was implemented in the Ebro Delta region (Catalonia, Spain) with the objective of identifying potential effects of growing season AWD and MSD on CH₄ emitted during the following flooded fallow season, in comparison to continuously flooded fields. Both emissions and the structure of soil microbial communities were analyzed for rice field plots under the assessed irrigation strategies during the growing season and later for a continuously flooded mesocosm across the fallow season. Both practices achieved an average 86% decrease in CH₄ fluxes when compared to continuous flooding during the growing season. AWD resulted in the highest fallow season emissions, leading to increases in overall annual cumulative CH₄ emissions (+8%), global warming potential (+30%) and yield-scaled global warming potential (+70%) compared to continuous flooding. Growing season AWD decreased the relative abundance of both methanogens and methanotrophs in the fallow season. Reduced methanotroph communities might lead to lower CH₄ consumption, resulting in higher fallow season emissions and offsetting the mitigation effect achieved during the growing season. Under the studied conditions, MSD represented a more effective mitigation strategy. These results highlight the importance of considering both rice growing and fallow season when assessing climate change mitigation strategies.

Phosphorous and nitrogen recovery from wastewater as struvite and ammonium nitrate (AN) may be viable alternative fertilizers to boost circularity in horticulture. A 2-year fertigated crop rotation in soil under greenhouse conditions was evaluated to determine the efficiency of both recovered products as raw materials for a nutrient solution (NS) manufacture. The effects of these treatments versus synthetic fertilizers were compared in terms of crop performance, plant nutrient uptake, soil chemistry and microbiota. This is the first study to implement struvite through fertigation as the sole source of P in soil crops. Results showed that both recovered products can be used as fertilizers in NS, due to the similar response to the control for different parameters and crops (tomato, lettuce, and cauliflower). However, the AN treatment showed lower yield in the first tomato crop, which results may depend on the cultivar ammonium tolerance. Besides, the concentration of heavy metals in fruits/leaves was below the permissible limits. Total and Olsen phosphorous soil analysis revealed no differences among treatments, resulting in a similar performance of P-struvite to commercial phosphate. Bulk soil bacteria structure, richness and relative dominance were increased over time, while archaea only showed lower evenness, both despite the fertilization strategy. Shannon diversity was not significantly affected. A predominance of ammonia-oxidizing bacteria (AOB) versus archaea (AOA) was observed, while nitrite-oxidizing bacteria (NOB), dominated by Nitrospira, increased with fertigation. Our results demonstrate that fertilizer blends for NS containing recovered nutrients are a feasible alternative to synthetic fertilizers. GRAPHICAL ABSTRACT

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.

The impact of long-term organic (ECO) versus conventional (CON) agricultural management on subsurface soil microbiota diversity and soil physicochemical properties remains unclear in Mediterranean vineyards. This study evaluated long-term ECO and CON effects in the Alt Penedès terroir (Spain), focusing on subsurface soil microbial diversity and soil characteristics. ECO increased the fungal-to-bacterial ratio and ammonium-oxidizing bacteria but reduced total subsurface soil bacterial populations and soil organic carbon. While ECO did not enhance annual yield production in the vineyard, fungal abundance, and ammonium-oxidizing archaea, it slightly increased the overall alpha diversity (Shannon and Inverse Simpson indexes) and significantly altered taxa composition in subsurface soil with a more robust and modular community. Crop management, soil texture, training system, and rootstock, but not vine variety, significantly influenced beta diversity in subsurface soil. The Mantel test revealed subsurface soil texture, Ca²⁺/Mg²⁺ ratio, and salinity as the main key soil drivers shifting the microbial community (beta diversity), while C/N and topsoil organic matter significantly correlated with bacterial abundance; NH₄⁺ correlated with fungal abundance; and N-Kjeldahl, pH, and Mg²⁺/K⁺ correlated with alpha diversity. Integrating soil microbiota and physicochemical monitoring allowed us to confirm the positive effect of long-term agroecological practices on subsurface soil health and to identify the critical factors shaping their microbial communities in Mediterranean vineyards.