Small‐sized soil organisms drive the legacy of soil addition in a degraded grassland

Abstract Plants can alter the abiotic and biotic components of the soil ecosystem, and this can result in soil legacies that facilitate or inhibit the growth of succeeding plants. To what extent soil legacy effects are due to soil biotic or abiotic characteristics is still poorly understood. In a microcosm experiment, we grew one plant species ( Leymus Chinensis ) in soil containing legacy effects of a prior soil inoculation event where soil from donor sites was inoculated at a receiver site. We tested whether soil legacies originating from the field plots differentially shaped the assemblage of soil microbial communities and plant performance, and how these effects are changed by the removal of specific size groups of soil biota. Our microcosm experiment showed that soil legacies from a one‐time soil addition experiment in a degraded grassland can affect the soil microbial composition 4 years later. This legacy effect was positively influenced by the amount of soil originally added to the field plot. By testing four types of biotic fractions differing in size, we found that small‐sized organisms (<20 μm), in particular fungi, were the most important for legacy formation and plant growth. Our results highlight the importance of inoculum density and smaller‐sized microbes in the formation of soil legacies. We conclude that the interaction between plant and soil microbes is a driver in the restoration of degraded grassland after soil addition. Disentangling the contributions of different biotic groups in the soil to plant growth can provide a theoretical basis for the preparation of bio‐inoculants that can be used for the sustainable restoration of degraded ecosystems. Read the free Plain Language Summary for this article on the Journal blog.