Effects of the Lower Irrigation Limit and Drip Irrigation Flow Rate on Root Traits and Bacterial Community Structure of Tomatoes Grown in Coconut Coir

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Water management is a key factor affecting crop physiology and rhizosphere microbial ecology in protected agriculture. However, the responses of root development and bacterial community structure to different lower irrigation limits and drip irrigation flow rates in coir substrates remain poorly understood. This study investigated greenhouse-grown tomatoes in the Gobi region under three lower irrigation limits (30%, 50%, and 70% of substrate water-holding capacity) and two drip irrigation flow rates (1 and 2 L·h−1). Substrate moisture, root traits, bacterial community structure, co-occurrence networks, and functional potential were evaluated. Increasing the lower irrigation limit maintained higher and more stable substrate moisture and generally enhanced root length, root surface area, root tip number, and root activity. W3Q1 promoted greater spatial expansion of the root system, whereas W2Q2 produced the largest root volume and maintained relatively high root activity. Irrigation combination significantly altered bacterial community structure but had limited effects on bacterial richness. W2Q1 enriched Bacillota-related taxa, whereas W2Q2 enriched Pseudomonadota and exhibited greater bacterial network connectivity and modularity. FAPROTAX-based functional prediction indicated that the bacterial communities were mainly associated with chemoheterotrophy, organic matter decomposition, and nitrogen transformation. Overall, the lower irrigation limit primarily regulated substrate moisture and root traits, whereas the effects of drip irrigation flow rate were trait- and irrigation-limit-dependent, with significant main and interaction effects on root volume. These findings provide a basis for optimizing precision irrigation in coir-substrate tomato cultivation in arid greenhouse systems.

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