Differential responses of Bolboschoenus planiculmis and Phragmites australis to hydroperiod and nitrogen supply

Bolboschoenus planiculmis and Phragmites australis are two wetland plants found in Momoge National Nature Reserve (MNNR) in northeastern China. Recently, the water depth in this region has substantially increased during peak plant growth phases due to return flow from nearby paddy fields. The total nitrogen (TN) concentration of the water in the wetland’s inlet is 1.153 ± 0.112 mg/L. During this study, artificial ponds were used to simulate these natural changes. Experiments were then conducted to examine the response of growth, the ramet number, the biomass accumulation, and the allocation of the two aforementioned species in different hydroperiods (a static water depth and an increasing water depth) and soil nitrogen conditions (0.5, 1.1, and 1.7 mg g⁻¹ TN). We found that P. australis demonstrated a significantly higher biomass accumulation, plant height, and ramet number under the greater water depth compared to the 30 cm static water depth. In contrast, B. planiculmis’ values were substantially lower under the increasing water depth than those under the 10 cm static water depth. P. australis exhibited an initial increase followed by a decline in response to nitrogen addition, independent of hydroperiod conditions. Conversely, under static water conditions, B. planiculmis’ total biomass and aboveground biomass increased significantly with the addition of nitrogen. This indicates that the interaction between the nitrogen supply and the hydroperiod of wetland plants is species-specific and dependent on nitrogen availability. Although an increased nitrogen supply resulted in higher belowground biomass, plant height, and ramet and tuber numbers for B. planiculmis under the increasing water depth, these values were still significantly lower than those under the static water conditions. This indicates that nitrogen enrichment did not counteract the negative effects of the increasing water depth on B. planiculmis’ growth. The findings of this experiment will help predict the vegetation dynamics of MNNR wetlands under future habitat changes driven by the return flow from paddy fields.