Competition and co-association, but not phosphorous availability, shape the benefits of phosphate solubilising root bacteria for maize (Zea mays).

Predicting the conditions under which rhizobacteria benefit plant growth remains challenging. Here we tested the hypothesis that benefits from inoculation with phosphate-solubilising rhizobacteria will depend upon two environmental conditions: phosphate availability and competition between bacteria. Maize-associated rhizobacteria with varying phosphate solubilisation ability were used in experiments in soil, sterilised soil and gnotobiotic microcosms under conditions of varying orthophosphate availability, while we manipulated intensity of competition by varying the number of isolates in plant inocula. Growth promotion by microbes did not depend on phosphate availability but was affected by interactions between inoculants: beneficial effects of one <em>Serratia </em>isolate were detectable only when plants were inoculated with a single strain and beneficial effects of a competition sensitive <em>Rhizobium </em>was only detectable in sterilised soil or in microcosms inoculated with single strains. Moreover, microcosm experiments suggested that facilitation of a parasitic isolate, not competitive interactions between bacteria, prevented plants from gaining benefits from a potential mutualist. Competition and facilitation affected colonization of plants in microcosms but growth promotion by <em>Serratia</em> was more affected by inoculation treatment than culturable densities on roots<em>.</em> Experimental manipulation of seed inocula can reveal whether plant growth stimulation is robust with respect to competition, as well as the ecological strategies of different rhizobacteria. From an applied perspective, phosphate solubilisation may not provide the mechanism for bacterial growth promotion but may indicate mutualistic potential due to phylogenetic associations. Importantly, benefits to plants are vulnerable to interactions between rhizobacteria and may not persist in mixed inoculations.预测根际细菌（rhizobacteria）促进植物生长的具体条件，仍是一项颇具难度的研究课题。本研究验证了一项假说：溶磷根际细菌（phosphate-solubilising rhizobacteria）的接种促生效应，取决于两类环境条件：磷素有效性与细菌间的竞争作用。 本研究选取溶磷能力各异的玉米关联根际细菌，分别在自然土壤、灭菌土壤（sterilised soil）以及限菌培养微宇宙（gnotobiotic microcosms）体系中开展实验，设置不同浓度的正磷酸盐（orthophosphate）有效性梯度；同时通过调整植物接种物中分离菌株（isolates）的数量，调控细菌间的竞争强度。 微生物的促生效应并未随磷素有效性变化而改变，但会受到接种菌剂间互作的调控：某一株沙雷氏菌属（*Serratia*）分离菌株的促生效果仅在植物接种单一菌株时可被检测到；而一株竞争敏感型根瘤菌属（*Rhizobium*）分离菌株的促生效应，仅在灭菌土壤或接种单一菌株的微宇宙体系中可被观测到。 此外，微宇宙实验结果表明，阻碍植物从潜在互利共生菌株中获取促生益处的因素并非细菌间的竞争互作，而是对某一寄生分离菌株的协同促进作用。竞争与协同互作均会影响细菌在植物体内的定殖，但沙雷氏菌属（*Serratia*）的促生效应受接种处理（inoculation treatment）的影响程度，远高于其在植物根系的可培养菌落密度（culturable densities）。 通过实验调控种子接种菌剂（seed inocula），可以揭示植物生长刺激效应在竞争条件下的稳定性，以及不同根际细菌的生态策略。从应用视角来看，溶磷能力或许并非细菌促生作用的核心机制，但基于系统发育关联（phylogenetic associations），该特性可作为指示细菌具备互利共生潜力的标志。值得注意的是，植物获得的促生益处极易受根际细菌间互作的影响，在混合接种（mixed inoculations）的条件下，此类促生效应可能无法持续存在。