Abstract: Estuarine marsh is one of the most sensitive ecosystems to global change and human activities. Estuarine marsh generally acts as a sink of exogenous nitrogen (N), with significant effects on N-cycling. Under the scenario of enhanced N and sulfur (S) loads in the Minjiang River estuary, it is of great significance to explore the responses of N distribution in plant-soil system. We conducted a field experiment with four treatments (T₀, control; T_N, high N load treatment, 98.0 g N·m^-2·a^-1; T_S, high S load treatment, 216.0 g S·m^-2·a^-1; and T_NS, high N and S load treatment, 98.0 g N·m^-2·a^-1 + 216.0 g S·m^-2·a^-1) in a typical Phragmites australis marsh in the Minjiang River estuary. We investigated nitrogen distribution patterns in the plant-soil system under elevated N and S inputs. Results showed that high N and S load significantly altered the contents of total nitrogen (TN), ammonium (NH₄⁺-N) and nitrate (NO₃^--N) in marsh soils. Compared with the T₀ treatment, the TN, NH₄⁺-N and NO₃^--N contents in the T_N treatment increased by 9.1%, 1.3% and 13.4%, the TN content in the T_S treatment increased by 9.4%, while the NH₄⁺-N and NO₃^--N contents decreased by 2.7% and 33.6%, respectively. The TN and NH₄⁺-N contents in the T_NS treatment decreased by 13.3% and 6.7%, respectively, while NO₃^--N content increased by 37.9%. Under different treatments, the TN contents were consistently higher in leaves and lower in roots of P. aus-tralis. Compared with the T₀ treatment, root TN contents increased by 1.5% under the T_N treatment but decreased by 7.0% and 15.4% under the T_S and T_NS treatments, while those in leaves in the T_N, T_S and T_NS treatments increased by 10.4%, 1.9% and 7.9%, respectively. Among different treatments, the N stocks in the plant-soil system in the T_N and T_S treatments were much higher, while that in the T_NS treatment was the lowest. Under different N and S loading conditions, P. australis adapted to environmental variations by altering its N uptake and translocation strategies. It adopted a “limited uptake and limited translocation” strategy under high N loading condition, a “massive uptake and massive translocation” strategy under high S loading condition, and a “limited uptake but massive translocation” strategy under high N and S loading condition.

Key words: high N and S load, N distribution, plant-soil system, Phragmites australis marsh, Minjiang River estuary