Bacterial diversity and network modularity determine alfalfa yield in flood lands

Advancements in planting and fertilization strategies can improve forage productivity in animal husbandry ecosystems. However, the way in which soil microbes regulate forage yield during this process remains unclear and limits our understanding of the underlying mechanisms that determine the impact of agriculture on forage yield. Here, a 4-year field experiment was conducted using three levels of planting density (high at 20 kg·hm−2, medium at 15 kg·hm−2, and low at 10 kg·hm−2) and two levels of fertilization (high at 150 kg·hm−2 P2O5 with 150 kg·hm−2 of urea and low at 75·kg·hm−2 P2O5 with 75 kg·hm−2 of urea) in nutrient-deficient flood lands. This study aimed to survey the forage yield, soil variables, community features (including diversity, community composition, network patterns, and potential functions) of soil bacteria and fungi, and the linkages. The findings indicated that planting density and fertilization had remarkable impacts on bacterial and fungal diversity, co-occurrence network patterns, and the nitrogen-cycle-related potential of bacteria (including nitrogen fixation, nitrification, denitrification, aerobic ammonia oxidation, and nitrite respiration). The combination of medium planting density with low fertilization resulted in higher levels of forage production, bacterial Shannon diversity, N-fixation potential, and network modularity than did the other treatments. Furthermore, significant positive correlations were observed among bacterial diversity, network modularity, available soil nutrients, and forage production. The soil that supported a large number of bacterial taxa with co-occurring network modularity had high soil nutrient levels and forage production. Structural equation modeling and linear regression models demonstrated that the contribution of soil bacteria to forage yield was higher than that of fungi and that planting density and fertilization indirectly associated microbial function by altering bacterial diversity and network modularization, which also positive associated forage production. Therefore, there is a need to emphasize bacterial alpha-diversity and potential interaction patterns to sustain forage yields, which leads to improvements in future planting and fertilization strategies in animal husbandry ecosystems.