Effects of Different Substitution Rates of Organic Fertilizers on Soil Multifunctionality and Its Microbial Driving Mechanisms

The aim of this study was to investigate the effects of varying substitution proportions of organic fertilizers replacing chemical fertilizers on dryland wheat productivity, agronomic efficiency of nitrogen (N) and phosphorus (P) fertilizers, soil multifunctionality, and microbial community structure. It focused on elucidating the microbial mechanisms underlying soil multifunctionality-driven crop growth, to provide a scientific basis for optimizing fertilization practices, promoting soil health, and advancing sustainable agricultural development in the Loess Plateau.

The dryland wheat fertilization experiment was conducted from 2019 to 2022, with six treatments, including no fertilizer (CK), chemical fertilizer alone (CF), and substitution of chemical fertilizer N with 80%, 60%, 40%, and 20% organic fertilizer N(MN: M₂₀N₈₀, M₄₀N₆₀, M₆₀N₄₀, M₈₀N₂₀). Macrogenome sequencing technology was used to obtain 62 functional genes related to carbon, nitrogen, phosphorus, and sulfur cycling, and to analyze the differences in community composition and diversity, exploring how microbial communities in dryland farmland ecosystems affect wheat growth through soil multifunctionality.

The yield, N and P fertilizer agronomic efficiency, and soil multifunctionality under MN were significantly higher than those under CK and CF, with M₄₀N₆₀ showing the best performance. Compared with CK, M₄₀N₆₀ had the greatest improvement in average yield and soil multifunctionality, reaching 60.1% and 191.6%. The results of microbial community analysis showed that fertilization significantly changed the composition and diversity of bacterial and fungal communities. The dominant bacterial phyla in the bacterial community were Actinobacteria (32.4%), Proteobacteria (31.1%), and Acidobacteria (13.5%). The dominant fungal phyla in the fungal community were Ascomycota (0.6%), Mucoromycota (0.2%), and Basidiomycota (0.1%). The distribution patterns of species richness and the Shannon index were different. The Shannon index showed CK<CF<MN, and it increased and then decreased with the decrease of organic fertilizer substitution proportion; the species richness was CK>CF>MN. The results of modified stochasticity ratio indicated that community assembly of bacterial and fungal communities was dominated by deterministic and stochastic processes, respectively. The correlation results indicated that soil multifunctionality was significantly positively correlated with yield and bacterial Shannon index but had no significant relationship with species richness. After incorporating multiple soil variables into the structural equation, the bacterial Shannon index remained positively correlated with soil multifunctionality, and soil multifunctionality exerts a positive effect on wheat yield. Random forest analysis indicated that the predictive effect of rare bacterial taxa on soil multifunctionality was stronger than that of rich bacterial taxa.

Substitution of chemical fertilizer N with 40% organic fertilizer N (M₄₀N₆₀) could achieve a synergistic improvement in crop productivity and soil health. It was recommended to include it in the recommended fertilization program for dryland agriculture on the Loess Plateau.