End-of-wet-season responses of potential nitrogenase activity, soil nutrients and plant nutrient allocation to long-term precipitation manipulation in a tropical forest

Biological nitrogen fixation supplies bioavailable nitrogen to tropical forests, but whether precipitation-related variation in potential nitrogenase activity is coordinated with plant nutrient allocation and photosynthesis remains unclear. Following 12 years of precipitation manipulation, we conducted a single sampling campaign at the end of the 2023 wet season in plots assigned to ambient precipitation (Control), delayed wet season (DW), or wetter wet season (WW) treatments. We measured litter and soil acetylene reduction activity (ARA), used here as an operational indicator of potential nitrogenase activity under common laboratory incubation conditions, together with soil nutrients, foliar nitrogen and phosphorus fractions, and photosynthetic traits. Relative to the Control, WW increased ARA by 102% in litter and 300% in the 0–10 cm soil layer. WW also increased soil inorganic nitrogen and available phosphorus and was generally associated with greater foliar inorganic nitrogen, Rubisco, metabolic phosphorus, and net photosynthetic rate, although responses differed among species. DW altered cell-wall nitrogen in a species-dependent manner, increasing it in three species and decreasing it in two species. Across plots, soil ARA covaried with soil nitrate ( ρ = 0.79, FDR-adjusted p = 0.030), and nitrate covaried with mean net photosynthetic rate ( ρ = 0.74, FDR-adjusted p = 0.034). The plot-level relationships weakened, and the plant-level associations did not persist after treatment adjustment, indicating that both largely reflected concurrent differences among treatments. These end-of-wet-season observations indicate contrasting, species-dependent coordination of microbial activity and plant nutrient-use strategies following long-term manipulation of precipitation amount and seasonal distribution, but do not resolve causal pathways or seasonal and annual dynamics.