The grape string theory is inspired by Mortierella and Trichocladium species that promote soil aggregation more than indigenous microorganisms

Abstract

Soil aggregates are the core physically distinct units that are embedded in the complex soil matrix. The roles of saprophytic fungi in soil aggregation and their relevant mechanism is not fully understood. Here, a microcosm was conducted to study the roles of two saprophytic fungi on soil aggregation. The plant growth promoting fungi Mortierella (K1) and Chaetomium (F1) were selected as representative species. After the original aggregates were destroyed using a 100 mesh sieve, the sterilized soil was placed in a petri dish, and the fungal strains were inoculated after sterile water was added to bring the soil to 60 % of the maximum field water capacity. The treatments contained a negative control that was not inoculated and single inoculated F1 and K1, as well as their combined inoculation. Non-sterilized soil as a positive control. All the soils were cultured in the dark for 45 days at 20 °C and followed by aggregate grading. Biological aggregates and abiotic soil fragments were clearly distinguished using scanning electron microscopy. The mysterious nature of the effective entanglements of fungal hyphae to soil particles and the special gossamer in the pores within aggregates have been clearly uncovered. Both fungi can effectively promote the formation of macroaggregates and the accumulation of organic carbon and nitrogen in the macroaggregates. The extracellular polymeric substance (EPS) uronic acid produced by both fungi has a significant positive effect on soil aggregation, but the impact of EPS-humic acid and EPS-protein is a nondeterministic process that occurs during the 45 days of incubation. The results of separating aggregates showed that the driving effect of a single fungus on soil aggregation was stronger than that of complex indigenous microbial communities. Therefore, the theory of grape cluster-like aggregates was proposed. Grape-like microaggregates (<0.25 mm) and abiotic soil fragments can further form a grape cluster composed of large aggregates (>1 mm) owing to the adherence and entanglement of filamentous hypha. Compared with diverse and competitive microbial communities, fungi of the same or similar species are conducive to mutual attraction and linkage between hyphae, which more strongly promote soil aggregation. Our research provides direct evidence that saprophytic fungi promote soil aggregation, and highlights a new theory of aggregate formation that will contribute to a better understanding of the mechanisms that regulate soil aggregation.