Effects of agricultural biostimulants on soil microbial activity and nitrogen dynamics

We investigated the effects of two commercially available soil biostimulants, designated Z93 and W91, on key microbial and nutrient cycling processes in the soil, by conducting short-term (1 week) and longer-term (8 weeks) soil incubations in the laboratory. In the short-term soil incubations, the two compounds differed in their effects on microbial activity: Z93 was effective over a wide range, stimulating substrate-induced respiration (SIR) and dehydrogenase activity (DHA) at remarkably low concentrations (0.5–500 nl/g soil); W91 stimulated SIR at these concentrations, but also inhibited DHA. In longer-term soil incubations, we amended batches of soil with either finely-ground alfalfa leaves, wheat straw, or added no amendments, to alter patterns of soil nitrogen mineralization and immobilization. We treated these soils with Z93 and W91 at two concentrations (0.005 and 0.5 μl/g soil), and incubated them for up to 8 weeks. These extremely low doses of both Z93 and W91 influenced soil SIR, DHA, and cellulase activity significantly (P<0.05). Both compounds also influenced soil nitrogen dynamics significantly; the extent depending upon the quality of the organic amendments. In the alfalfa-amended soil there was a steep increase in NO₃-N concentration during the incubation due to the rapid mineralization of nitrogen-rich alfalfa material. However, in this soil, both Z93 and W91 reduced NO₃-N concentrations greatly after 56 days. In the straw-amended soil, mineral nitrogen concentrations were very low, probably due to rapid immobilization of nitrogen by microbial biomass. In this soil, treatment with both compounds decreased microbial biomass nitrogen and increased dissolved organic nitrogen (DON), relative to that in the controls. Our results suggest that the two biostimulants can stimulate both the breakdown and mineralization of soil organic materials, perhaps by selectively inhibiting or stimulating particular components of the microbial community, leading to lasting (8 weeks or longer) increases in soil nitrogen availability.