Salinity and N input drive prokaryotic diversity in soils irrigated with treated effluents from fish-processing industry

Abstract

Wastewater reuse for irrigation has become an important practice in many countries in the context of global water scarcity. However, knowledge about the potential soil impact of reusing treated fish-processing (TFP) effluents for irrigation is still limited. The aim of this study was to investigate the response of the soil prokaryotic community in general, and the nitrifying taxa in particular, to TFP-effluent irrigation. We analyzed the impacts of irrigation with two effluent dilutions (EF1 and EF2, with electrical conductivities of 2.7 and 6 mS cm−1, respectively) or water (W) as a control on soil chemical properties, dehydrogenase and nitrifying activities, amoA gene abundances of ammonia-oxidizing bacteria (AOB) and archaea (AOA), and soil prokaryotic community structure and diversity. At the end of the irrigation experiment, soil ammonium, nitrate plus nitrite, dehydrogenase and nitrifying activities, soil electrical conductivity (EC), and sodium adsorption ratio (SAR) were significantly higher in TFP-irrigated treatments than in water irrigated controls. Prokaryotic richness and diversity indices followed the pattern W > EF1 > EF2, and negatively correlated with soil EC, SAR, ammonium, nitrate plus nitrite, and total N concentrations. In particular, EF2-irrigation stimulated soil copiotrophic bacteria (e.g. Proteobacteria and Bacteroidetes) to the detriment of oligotrophic members such as Acidobacteria. TFP-effluent irrigation also influenced the relative abundance of the amoA gene of AOB but not that of AOA; and the composition of nitrifying taxa, by inducing a significant increase in OTUs whose closest cultured matches were ‘Candidatus Nitrosocosmicus franklandus’ and Nitrosospira briensis Nsp10. Overall, irrigation with the more diluted effluent (classified as slight to moderate degree of restriction by local regulations) induced a reduction of soil prokaryotic diversity, whereas the less diluted effluent (severe irrigation restriction) promoted the greatest changes in the prokaryotic community due to the increase in soil salinity and N content.