Response of foliar mineral nutrients to long-term nitrogen and phosphorus addition in a tropical forest

Mineral nutrients play a pivotal role in plant growth, development and reproduction. The uptake and distribution of nutrients are generally limited by soil nutrient availability, affected by global change. However, the responses of foliar mineral nutrient concentrations to changes in soil nutrient availability remain largely untested in tropical forests. We used a field-based experiment with nitrogen (N) and phosphorus (P) addition in a tropical forest in southern China to investigate the effect of N and P fertilization on exchangeable soil and foliar mineral nutrients of five understorey species. Our results show that N addition did not change soil pH, but reduced exchangeable soil Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺ and Mn²⁺ concentrations. In contrast, P addition increased soil pH, while it had no effects on exchangeable cations. Nitrogen addition significantly decreased foliar [Ca], [Mg] and [Mn], while P addition markedly reduced foliar [Cu] for most of the studied species. Foliar [K] and [Fe] were not affected by fertilization. The results indicate that long-term N addition reduced the concentrations of several mineral nutrients in leaves, and we did not observe any adaptive mechanisms (e.g. increased transpiration to take up mobile soil nutrients) in the studied species to maintain foliar nutrient concentrations at high N supply. Synthesis. The responses of other foliar nutrient concentrations to N and P co-addition were more complicated than those of N and/or P, and the specific decreases in mineral nutrient concentrations following long-term N addition may influence key physiological process in the studied tropical understorey species, and ultimately reduce their fitness and survival under high N deposition. A free Plain Language Summary can be found within the Supporting Information of this article.