Abstract
Introducing beneficial microbial inoculants to degraded lands represents a promising strategy, but their effects on plant-soil system performance are highly context-dependent. In my thesis, We combined the use of microbial inoculants with organic fertilizer to investigate the effects of inoculant type, inoculation period, and organic fertilizer level on plant-soil performance
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and the ecological role of the resident soil microbial community.
In Chapter 2, we examined the individual and combined effects of four commercial strains on soil nutrient properties and their survival dynamics in degraded soils. Results showed that mixed microbial consortia performed bettered than single strains in terms of inoculum population dynamics in the soil, although introduced populations still decreased after 45 days due to resource limitation. Mixed microbial cultures may allow their components to interact with each other synergistically, but the effects are limited by resource availability over time.
To subsequently test their synergistic effects, Chapters 3 and 4 assessed the practical applications of the selected inoculants using a pot experiment. The results of Chapters 3 and 4 confirmed that mixed inoculants performed better than single inoculants in stimulating plant growth and soil nutrient properties. However, in Chapter 4, the effects were only observed during the first 90 days and did not increase with repeated inoculations. In addition, the single and mixed inoculants transiently modulated the structure of the resident microbial community, which showed resilience to subsequent inoculations. These results suggest that, although the effects are different between mixed and single inoculants, their practical effects on plant-soil performance are time-limited. The initial inoculation plays a more important role in influencing the whole system.
Chapters 5 and 6 examined the practical effects of mixed microbial inoculants under field conditions after one year and three years, respectively. A different pattern was found between field and control experiments: inoculations increased the soil nutrient availability with increasing fertilizer level in the soil incubation test, but the medium fertilizer level provided the most optimal plant growth in the field, which was associated with plant nutrient acquisition and root morphology. This suggests that the effects of microbial inoculants can be influenced by other factors under field conditions, such as the activities of resident microbes. Chapter 6 further used a modeling approach to identify specific soil microbial taxa that can act as potential indicators relating to the relative success of microbial inoculant application. These indicators represent taxa that may assist or impede the ability of inoculants to improve plant metabolite levels.
In summary, as an alternative option to conventional strategies, microbial inoculant applications show great potential for improving degraded land productivity, but the effects of beneficial microbial inoculants are highly context-dependent. The inoculant type, concentration, inoculation frequency, and soil-resident microbes all can play important roles in affecting the ultimate effects of microbial inoculants on both soil nutrient properties and plant productivity. Integrated analysis of those key factors can help determine the most appropriate management strategies for improving such degraded ecosystems.
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