Can biostimulants and tillage rotations improve dryland crop productivity and soil microbes?

Soil microbes are essential for soil nutrient cycling. However, frequent tillage and the use of synthetic agrochemicals can reduce soil microbial diversity and enzyme activity. In this study, the effects of four tillage treatments (mouldboard plough, shallow tine‐tillage, no‐tillage, and tillage rotation) and two rates of synthetic agrochemicals (standard and reduced, with biostimulants) on soil microbial diversity and enzyme activity were investigated between 2018 and 2020 in a Mediterranean climate zone in South Africa. It was hypothesized that a reduction in tillage frequency and quantity of synthetic agrochemical application would lead to greater microbial diversity and enzyme activity. Soil samples were collected from the 0‐ to 150‐mm layer of a field trial under a dryland crop rotation system. Soil microbial species richness and abundance were assessed using the Shannon–Wiener diversity and evenness indices. The activities of four microbial enzymes—β‐glucosidase, acid phosphatase, alkaline phosphatase, and urease—were used to evaluate ecosystem functioning. The combined effects of tillage rotation with a shallow tine implement and the application of biostimulants failed to significantly improve soil microbial diversity, enzyme activity, and crop productivity relative to other treatments. However, the combination did not reduce the wheat (Triticum aestivum) grain yield and quality, and soil biological parameters. Furthermore, the less intensive tillage treatments, ST, NT, and ST‐NT‐NT‐NT, resulted in higher enzyme activity than the mouldboard treatment. Therefore, we suggest that combining non‐intensive tillage with reduced synthetic agrochemical use can be a safer, more environmentally friendly alternative to intensive tillage and high agrochemical application in dryland cropping systems.