Plant fine root exudation carbon (C) and mycorrhizal colonization are two critical “outsourcing” strategies for water and nutrient exploitation. Quantifying the plasticity of multiple fine root traits under water stress is important for understanding plant adaptations to drought. Here, we carried out a short-term rainfall exclusion experiment in a common garden with five temperate tree species in northeast China, and quantified the co-variation of root exudation C, mycorrhizal colonization, and four morphological and four chemical traits in response to drought. Root exudation C was positively correlated with “fast” root morphological traits (specific root length and area), but negatively correlated with “slow” ones (root diameter and tissue density). Percentage of mycorrhizal colonization decreased as increasing root non-structural carbon concentration, but was weakly correlated with the “fast-slow” morphological traits. Drought increased root diameter, tissue density, nitrogen concentration, exudation C, and mycorrhizal colonization, but decreased non-structural carbon concentration, and specific root length and area. Our findings suggest that fine roots investment in rhizosphere microorganisms through exudation C, together with fungal collaboration and “fast-slow” gradient would encompass a whole root economics space. Drought promotes “slow” root traits for hydraulic safety, but increases root resource acquisition by “outsourcing” processes, i.e., increased C investments in rhizosphere microbes (through root exudation C) and mycorrhizal collaboration.