Nitrogen deposition and phosphorus addition alter mobility of trace elements in subtropical forests in China

Abstract

Trace elements are important in sustaining the functioning of forest ecosystems. However, knowing how nitrogen (N) deposition and phosphorus (P) addition alter the mobility of trace elements, and how trees equilibrate root capture and leaf resorption of trace elements remains unclear. We measured concentrations of five elements in rhizospheric soils, fine roots, twigs, and leaves in Chinese fir plantations fertilized with P (5 g P m−2 a−1) and/or N (5 and 10 g N m−2 a−1) over 5 years in subtropical China and calculated these mobility factors of four trace elements and Mg from soils to roots and from roots to twigs and leaves. Results showed that N addition increased the capacity of roots to capture Mg and to twigs re-adsorb Fe. Phosphorus regulated the employment of a Mn acquisition strategy under N deposition. Nitrogen addition increased the capacity of roots to capture and the efficiency of leaves to re-adsorb Mn without P addition while only increased the Mn capturing capacity with P addition. The Mg, Fe, and Mn were more sensitive than Cu and Zn to N deposition and P addition. The capture-resorption tradeoff depended more on element types than N and P supplies. In addition, the tradeoff between capture and resorption could be reflected by the transport capacity since transport capacities of Mg, Fe, Mn, and Zn were positively correlated with their corresponding capture capacities or resorption efficiencies. Furthermore, structural equation modeling showed the element mobility tended to be dominated by P rather than N. These findings suggest subtropical forests would adapt to atmospheric N deposition and soil P limitation by adjusting the tradeoff between capture and resorption strategies. The establishment of trace element mobility and tradeoff patterns provides insights into trace element cycling under human-driven nutrient imbalance scenarios.