Soil C, N, and P contents and organic phosphorus mineralization in constructed wetlands with different litter input in northern China

Purpose

Constructed wetlands have profound influences on efficient wastewater purification and treatment. However, what extent and how different kinds of constructed wetland can effectively influence the distribution of nutrients content and mineralization? Specially, whether the response of the changes of soil nutrients content and mineralization to different amounts of litter input was consistent? It has not been resolved.

Methods

In this study, five constructed wetland systems (i.e., the Circulating Water Treatment Pond 1 (CW), Recirculating Water Treatment Pond 2 (RCW), Reclaimed Water Treatment Pond (RW), Plant Oxidation Pond (POP), and Mixed Oxidation Pond (MOP)) in the Beijing Olympic Forest Park were studied. CW, RCW, and RW belong to the composite vertical-flow systems, while POP and MOP belong to the free surface systems. Field litter input (5 and 20 g, respectively) with five replicates applied to the constructed wetland systems were conducted. The contents of soil total carbon (TC), soil total nitrogen (TN), soil total phosphorus (TP), and phosphorus mineralization rates were quantified. Ordinary kriging interpolation was used to characterize the spatial distribution of soil TC, TN, TP and phosphorus mineralization rates.

Results

The results showed that the contents of soil TC and TN in the composite vertical-flow systems (CW, RCW, and RW) were greater than those in the free surface systems (POP and MOP), while it was contrary for the content of soil TP. Soil organic phosphorus (accounting for 45.80 ± 8.12%) and inorganic phosphorus (accounting for 51.81 ± 7.46%) were the main components of soil TP. Phosphorus mineralization rates in the composite vertical-flow systems were greater than the free surface systems. The phosphorus mineralization rates were the smallest in MOP (-2.06 mg·kg⁻¹·d⁻¹) and the highest in RW (0.32 mg·kg⁻¹·d⁻¹). Litter input decreased the contents of soil TC and TN in the composite vertical-flow systems and MOP, while increased in POP. Soil TP content after the litter input increased in CW, RCW, and MOP, while decreased in RW and POP. The litter input was beneficial for improving the phosphorus mineralization rates. The effects of 5 g litter input on the changes of the contents of soil TC, TN, TP and phosphorus mineralization rates were stronger than that of 20 g litter input.

Conclusion

Our study has supplemented the inconclusive results of the influences of different constructed wetlands and amounts of litter input on soil nutrient content and mineralization. The findings of this study could provide data support for better constructed wetland management, which could help the managers understand the mechanisms of improving the efficiency of wastewater treatment in constructed wetlands.