Effects of enhanced nitrogen load on variation and turnover of phosphorus in the plant-soil system of Suaeda salsa marsh in the Yellow River estuary, China

Purpose To evaluate the effects of enhanced nitrogen (N) load on turnover of phosphorus (P) in Suaeda salsa marsh in the Yellow River estuary, the variation and allocation of P in the plant-soil system were investigated in the growing season. Methods This study was explored by field N addition experiment which included four load levels: NNL (no N load treatment, 0 gN m(-2) yr(-1)), LNL (low N load treatment, 3.0 gN m(-2) yr(-1)), MNL (medium N load treatment, 6 gN m(-2) yr(-1)), and HNL (high N load treatment, 12 gN m(-2) yr(-1)). Results In most sampling periods, the TP contents in soils in the LNL, MNL, and HNL treatments showed a decreasing trend compared to the NNL treatment. With increasing N load levels, the contents of non-available decomposed P in soils decreased by 3.08 similar to 5.22%, while those of easily available and moderately available P increased by 0.36 similar to 8.92% and 1.21 similar to 24.24%, respectively. The variations of NaOH-P-o, Resin-P-i,P- and NaHCO3-P-o contents primarily rested with the duration of N import, while those of NaOH-P-i, Residual-P, NaHCO3-P-i,P- and HCl-P-i contents mainly depended on the levels of N load. Dissimilar variations of TP contents in plant tissues in the growing season were observed among N load levels, which were dependent on the structure and function of organs and the nutrient function of P at distinct growth stages. The P absorption coefficients (PAC) of plants in N load treatments increased by 31.82 similar to 36.36%, while the P utilization coefficients (PUC) decreased by 4.96 similar to 13.33%. Conclusion This paper found that the S. salsa was very likely to adjust its growth rhythm and P absorption/utilization capacity to adapt N load environment. Particularly, as N load reached medium level, the P supply relationships between aboveground and belowground tissues and the P transferences in the plant-soil system were greatly altered, indicating that the S. salsa might have a special response to the medium N enrichment. Thus, the potential impacts of medium N load level on S. salsa marsh should be emphasized in the future.