A small proportion of litter-derived nitrogen is assimilated by plant biomass or immobilized in sediments regardless of harvest management as detected by 15N-labeled Phragmites litter in a constructed wetland.

Emergent aquatic macrophytes play an important role in the removal of nutrients in constructed wetlands (CWs). However, plant biomass supplies litter after the onset of senescence. Although litter-derived nitrogen (N) has been considered a nutrient source for the internal loading that may reduce CW performance, little is known about the quantitative N dynamics associated with litter decomposition. Thus, a controversial question remains about whether plant harvest is needed to manage CWs. In this study, we evaluated the decomposition and the fate of N derived from 15N-labeled Phragmites litter in a CW for 1 year. To simulate respective natural conditions, two treatments, including (1) a single winter harvest and (2) no harvest where the latter supplies a greater stem litterfall, were compared. Although the dry weight of the added stem litter was approximately 4.7 times larger in the no harvest plot than in the harvest plot, the total N content of the initial 15N-labeled litter was only 1.2 times higher in the no harvest plot than in the harvest plots because of the low N concentration in the stem litter. The litter functioned as a minor N sink within the first 6 months of decomposition, and it then shifted to functioning as a minor N source after 1 year of decomposition. The recovery of litter-derived N in the sediment and plant biomass was low (less than 10% of the initial litter N), and much of the remaining N might have been released into ambient water or lost through denitrification. Furthermore, our results suggested a potentially low contribution of litter-derived N to internal N loading for at least 1 year regardless of the harvest management treatment.

Effect of Phragmites japonicus harvest frequency and timing on dry matter yield and nutritive value.

Phragmites is a cosmopolitan perennial emergent macrophyte that is distributed worldwide. In recent years, Phragmites has attracted attention for its potential use as roughage. Given the increasing demand for feed and the number of constructed wetlands (CWs) vegetated with Phragmites, Phragmites is expected to play an important role in roughage production. Thus, it is vital to understand the effects of harvest timing and frequency on dry matter yield, nutritive value, and nitrogen (N) removal to establish appropriate vegetation management. In two CWs in Southwest China, four treatments with different harvesting frequencies were evaluated in monospecific areas of P. japonicus. The four treatments included no harvest, single harvest at 6 months, two harvests at 2 and 4 months, and three harvests at 2, 4, and 6 months. A sharp decline in the total digestible nutrients (TDN) concentration and the rate of increase in dry matter (DM) yield was associated with the heading timings, and the seasonal variations in TDN were likely influenced by carbohydrate accumulation in the stems. The three harvest treatment contributed to substantially improve the N and DM yields without decreasing the nutritive value but negatively affected the growth in the following year. Therefore, not only the combinations of harvest timing and frequency but also other management practices, including partial harvesting, may be needed to optimize CW performance and roughage production.

Timing of harvest of Phragmites australis (CAV.) Trin. ex Steudel affects subsequent canopy structure and nutritive value of roughage in subtropical highland.

In recent decades, constructed wetlands dominated by common reeds [Phragmites australis (CAV.) Trin. ex Steudel] have been utilized for treating nitrogen-rich wastewaters. Although plant harvest is a vegetation management in constructed wetlands for the purpose of improving nutrient removal, harvested biomass has become a problem in many places. The reed has attracted increasing interest for its potential as high-quality roughage for ruminants. Therefore, it is crucial to understand the effect of reed harvest timing on subsequent regrowth, reconstruction of canopy structure, and nutritive value of regrown biomass for roughage when defining an appropriate vegetation management in constructed wetlands. The shoots of common reeds were harvested in January (winter), March (spring), and May (early summer) in a free-water surface constructed wetland in southwest China. Harvesting in winter enhanced the shoot regrowth and concentrations of total digestible nutrients (TDN), probably due to vigorous translocations of nonstructural carbohydrates from rhizomes. Harvesting in spring and early summer decreased aboveground biomass, nitrogen (N) standing stock, and concentrations of TDN. From fifty to 110 days after harvest, the TDN had sharply declined to values similar to non-harvested stands. Thus, to obtain high-quality roughage, it is recommended that regrown shoots be harvested again within a year in the early growing stage after the first harvest in winter.

Impact of plant harvest management on function and community structure of nitrifiers and denitrifiers in a constructed wetland.

Plant harvest is one of the most important management practices in constructed wetlands. In this study, we evaluated the impact of harvesting Phragmites australis (Cav.) Trin. ex Steudel on the activity and community structure of nitrifiers and denitrifiers in a free-water surface constructed wetland. The nitrifiers were targeted using bacterial and archaeal-amoA that encode ammonia monooxygenase, and the denitrifiers were targeted using nirK and nirS that encode the nitrite reductase. The community structures were evaluated using denaturing gradient gel electrophoresis. The potential nitrification and nitrate reduction rates were shown to be significantly higher in the harvested plant rhizosphere than in a non-harvested control plot. The potential nitrification rate positively correlated with the potential nitrate reduction rate and influenced the community structure of nirK. In addition, plant canopy developed differently after harvest and simultaneously changed the microclimate beneath the plant community. These results suggest that plant harvest management could change subsequent plant development and associated microenvironments, thereby affecting the function and community structure of nitrifiers and denitrifiers. Our study highlights the importance of plant harvest management within constructed wetlands to regulate the functions of nitrification and denitrification.