The influence of revetment types on soil denitrification in the adjacent tidal urban riparian zones

Abstract

Riparian zones adjacent to revetments, as the last position of the entire riparian, are the last barrier to intercept runoff nitrogen pollution. They are also the areas where hyporheic exchange occurs when soil is submerged by river water. Revetments can be classified into impervious revetments and permeable revetments according to their permeability. It has been realized that impervious revetments are not conducive to exchange between rivers and riparian zones. Tidal riparian zones undergo water level changes more frequently than normal riparian zones. Thus, revetments in tidal riparian zones may have a greater impact on nitrogen cycling. However, the quantitative influence of revetment types on nitrogen removal in adjacent tidal urban riparian zones is not clear, resulting in the lack of scientific support for the revetments construction. Removal of nitrogen from riparian zones primarily occurs through plant uptake, microbial immobilization, and denitrification. However, nitrogen will be released into soil again after plants die or microorganisms decompose. While denitrification can completely remove nitrogen through transferring nitrate (NO3−) into gaseous nitrous oxide (N2O) and dinitrogen (N2). In this study, we used three revetment types (permeable concrete pile revetments (PR), impervious masonry stone revetments (IR), and no revetments (NR)) as examples to quantify the effects of revetment types on soil denitrification in adjacent tidal urban riparian zones at three distances from the revetments (0.3 m, 0.6 m, 1.0 m) by measuring the soil (0–20 cm) denitrification potential (DP) and N2O production rates (N2OR). Results indicated that compared with soil denitrification (0.32 ± 0.06 mg kg−1h−1) in natural riparian zones (NR), PR could significantly improve it (0.97 ± 0.07 mg kg−1h−1), while IR significantly inhibited it (0.23 ± 0.04 mg kg−1h−1) in the adjacent tidal urban riparian zones (P < 0.01). Changes in soil properties caused by revetment types accounted for the differences in soil denitrification. Soil organic carbon (SOC) was a main direct chemical factor, and soil texture was a main indirect physical factor. Based on these results, revetments construction did not necessarily inhibit soil denitrification in natural riparian zones, and priority should be given to building permeable revetments to improve the soil denitrification provided by the riparian ecosystem.