The high boron (B) content in desalinated seawater is a concern for crop development. However, in spite of the importance of the soil microbial community in soil fertility, the below-ground impacts of B are still unknown. Here, in a soil-ryegrass model system, the activity, biomass and diversity of the soil microbial community were evaluated in response to irrigation with: i) 0.3 mg B L-1; ii) 1 mg B L-1; and 50 mg B L-1. We assessed two different compounds of boron: boric add (H3BO3) and disodium tetraborate decahydrate (Na2B4O7 center dot 10H(2)O). Overall, the 1 mg B L-1 dose was identified as the threshold limit that did not irreversibly harm soil sustainability. In contrast, the highest B dose had a noticeable impact on the nitrogen (N) cycle of the soil, as demonstrated by an increase in the water-soluble N content and a decrease in urease activity. Analysis of the phospholipid fatty adds (PLFAs) revealed that the effect of B on the soil microbial biomass was dependent on the chemical form used. High B doses reduced soil microbial respiration and influenced the composition of the bacterial and fungal communities, with fungal diversity being diminished, as revealed by sequencing approaches. (C) 2019 Elsevier B.V. All rights reserved.