Abstract Plant roots release organic compounds that enhance soil nutrient availability, often in coordination with root morphology and chemistry. Interspecific variation in resource-use strategies, particularly those associated with successional status, can shape adaptive responses involving root exudation. However, the role of root exudation under severe nutrient deficiency in tropical rainforests remains unclear. We examined the effects of 11 years of nitrogen (N) and phosphorus (P) fertilization on root exudation in seven common tree species, including dominant dipterocarps, in a Bornean tropical rainforest. Root exudation rates remained largely unchanged in climax species following fertilization but increased in pioneer species following P fertilization. Pioneer species also showed a greater increase in root P concentrations than climax species. An interaction between specific root length (SRL) or root diameter and P fertilization indicated that roots with higher SRL or smaller diameters exhibited higher exudation rates under low-P conditions. Notably, one climax non-dipterocarp species with the highest exudation and SRL showed reduced exudation following fertilization. These results suggest that root exudation is not uniformly enhanced under P deficiency across species. Instead, pioneer species may increase exudation in response to P-induced N limitation and/or to shift toward greater reliance on exudate-mediated P mobilization, reflecting their high nutrient demand. Coordinated changes in exudation and root morphology likely contribute to species-specific adaptive mechanisms under P deficiency. Thus, in lowland tropical rainforests, the ecological role of root exudation should vary across species with contrasting resource-use strategies, highlighting diverse adaptive responses and belowground carbon allocation patterns under nutrient deficiency.
Hirano et al. (Thu,) studied this question.