Design and agro-environmental evaluation of advanced fertilisers based on bio-residues, rhizobacteria and the circular economy principles

Laburpena

To reduce the negative environmental consequences of agriculture intensification and to achieve sustainability, the European Commission has fostered strategies based on the circular economy to reduce dependence on non-renewable sources. The use of bio-residues is a leading strategy to provide nutrients to crops and improve nutrient use efficiency. The objective of this work was to design and evaluate advanced fertilisers based on different combinations of bio-residues and rhizobacteria, as part of a global strategy based on a circular economy aimed at reducing the use of conventional mineral fertilisers in agriculture. Two families of advanced fertilisers were designed: the first based on compost and the second based on biochar. The first one consisted of two products: i) compost + biochar; and ii) compost inoculated with Bacillus siamensis (SCFB3-1 strain) formulated with biochar as a carrier (‘doped compost’). The second family consisted of one product, namely biochar + anaerobic digestate (AD). The aqueous extracts of the designed products were analysed for phytotoxicity and toxicity in three species of soil rhizobacteria. All products were phytostimulants at the dilution ratio of 1:25 (w:v), whereas for the ratio 1:10, they had either no phytotoxic or moderately phytotoxic effects. Conversely, more concentrated ratios were phytotoxic. No toxicity to rhizosphere bacteria was observed. The products compost + biochar and biochar + AD were preliminary tested in microcosm conditions; in such a trial, maize plants treated with either of the two products produced higher biomass than the non-fertilised control, although the N content in the biomass decreased. In commercial fields, the ‘doped compost’ worked better in terms of crop yield than the combination of compost and biochar. Moreover, a reduced dose (minus 20%) of mineral fertiliser combined with ‘doped compost’ (2 t ha−1) produced a higher yield in melon and pepper than the control that received a full mineral fertiliser dose (24% to 33% higher in melon and 2% to 4% in pepper). Furthermore, the same reduced dose of mineral fertiliser combined with biochar + AD (250 to 500 t ha−1) produced a higher melon yield (2% to 16% higher) and a similar pepper yield compared with the control that received the full mineral dose. In addition to the positive agronomic effects on crop yield from the environmental side, the first advantage is the reduction in the dose of mineral fertilisers. However, more relevant environmental benefits have been encountered in the soil microbiome, as revealed by the metataxonomic approach. Foremost, the composition of the bulk soil microbiome remained unaltered by the treatments. However, the treatments increased the soil microbiome activity, which consumed soil nitrogen (N); thus, the risk of N lixiviation was reduced, although it resulted in a lower N content in the crop biomass. We have demonstrated that the increased yield, even for less N available, is positively correlated with the enhanced microbiome activity but also with the presence of certain bacteria clusters considered plant growth promoting rhizobacteria (PGPR), whose relative abundance in the rhizosphere has been increased by the treatments. Moreover, microbiome diversity and species richness were also enhanced by the treatments, which theoretically has a positive effect on crop yield. We finally hypothesised that not only does the inoculated B. siamensis exert a direct effect on the crop, but it also causes changes in the rhizosphere. Further research is needed to understand the process involved in such a mechanism.