Assessing wetland nitrogen removal and reed (Phragmites australis) nutrient responses for the selection of optimal harvest time

Highlights

• Later harvest times had less impact on wetland TN and NH₄⁺–N removal.

• Mid- or late-wilting harvest should be selected according to reed nutrient response.

• Root stoichiometric characteristics were related to wetland nitrogen removal.

Abstract

Wetlands play an important role in reducing the impact of nitrogen pollution on natural aquatic environments. However, during the plant wilting period (winter) there will inevitably be a reduction in nitrogen removal from wetlands. Understanding optimum harvest time will allow the use of management practices to balance the trade-off between nitrogen removal and the sustainability of wetlands. In this study, we investigated wetland nitrogen removal and reed (Phragmites australis) nutrient responses for two years [first year: influent total nitrogen (TN) 17.6–34.7 mg L⁻¹; second year: influent TN 3.2–10.0 mg L⁻¹] to identify the optimal harvest time: before wilting, mid-wilting, or late wilting. Harvesting decreased wetland nitrogen removal in both years, with later harvest time producing a smaller decrease in TN and ammonium-nitrogen (NH₄⁺–N) removal. In addition to harvest before wilting, aboveground reed harvest at mid-wilting harvested more nutrients [carbon (C) 7.9%, nitrogen (N) 46.6% and phosphorus (P) 43.6%] in the first year, while harvest at late wilting harvested more nutrients (C 4.9%, N 7.8% and P 24.1%) in the second year, although this was not statistically significant. The late wilting harvest caused fewer disturbances to root stoichiometric homeostasis in the first year, while mid-wilting harvest promoted root nutrient availability in the second year. In addition, redundancy analysis (RDA) showed that root stoichiometry was interrelated with wetland nitrogen removal. Our results suggest that optimal harvest time was late wilting on the basis of wetland nitrogen removal, or either mid- or late wilting according to reed nutrient response to influent nitrogen concentration in some years. Our results provide crucial information for winter wetlands management.

Introduction

Nitrogen is essential for regulating primary productivity and biotic diversity in both aquatic and terrestrial systems, while excess nitrogen has many deleterious effects on ecological function and even on human health (Tartowski and Howarth, 2013). Human activity has greatly increased the rate of nitrogen creation. In the early 1990s, anthropogenic activities contributed 156 Tg N y⁻¹ to global nitrogen budgets, a factor of 10 increase over 1860, and projections are 267 Tg N y⁻¹ in 2050 (Galloway et al., 2004). Nitrogen release into aquatic ecosystems is responsible for algal blooms and eutrophication (Beman et al., 2005); over-application of nitrogen fertilizer has increased nitrogen losses to water bodies through leaching and surface runoff, which exacerbates water quality deterioration (Chen et al., 2015). Wetlands are ecotones between aquatic and terrestrial ecosystems that play an important role in intercepting nitrogen sources from land and reducing nitrogen levels in water bodies (Wang et al., 2016; Schwammberger et al., 2019).

Reed (Phragmites australis) is a species with cosmopolitan distribution, which adapts with high competitive ability and colonizes habitats such as riverbanks, estuaries, lake shores, and coasts (Landucci et al., 2013; Packer et al., 2017). Reedbeds have important values for biodiversity, representing habitats for plants, vertebrates, and invertebrates, including many rare and endangered species, and are typically managed by winter cutting (Deak et al., 2015). Reed cutting effectively rejuvenates reed stands by removing dead biomass and enhances the homogenous vegetation structure to achieve the desired conservation benefits (Deak et al., 2015); its expansion to nearby meadows supports species diversity, which associated with higher plant species richness (Heneberg et al., 2017). Cut reedbeds also attract more species to trap nests, which is likely the result of better food availability in cut reed compared to dense reed stands (Heneberg et al., 2017). However, reed cutting significantly alters habitat diversity, exhibiting an overall negative effect on the number of reed-associated vertebrates and invertebrates (Valkama et al., 2008). The most widespread and productive wetlands in the world are reed wetlands, and reed cutting in winter remains the most typical management activity in wetlands and one of the most effective methods of fighting against water eutrophication (Trnka et al., 2014; Deak et al., 2015).

Nitrogen is removed from wetlands via a complex variety of physical, chemical, and biological processes, including ammonia volatilization, ammonification, nitrification, denitrification, assimilation by plants and microorganisms, and adsorption by substrate (Hu et al., 2016). Plants and microorganisms play an important role (Wang et al., 2017). More than 50% of nitrogen removal occurs through the bacterial nitrification-denitrification process (Liu et al., 2019), with plants shaping the temporal and spatial patterns of bacterial community structures in wetlands (Oopkaup et al., 2016). Total nitrogen (TN) and ammonium-nitrogen (NH₄⁺–N) removal in winter is lowered by up to 30% and 40%, respectively, compared to that observed in summer (Ouellet-Plamondon et al., 2006; Sun et al., 2019). As nitrogen concentrations in many wetlands have been increasing owing to regular input by anthropogenic disturbances, the development of winter management strategies for wetlands has become more critical (Uddin and Robinson, 2018). The mineral demands of plants tend to be greatest for nitrogen, whose nitrogen uptake mainly occurs during the growing season (Allen et al., 2013). If not harvested, the nitrogen absorbed by plants may return to the wetland system as wilting or rotting litter, resulting in increased nitrogen concentration in the effluent (Huang et al., 2013). Harvesting reed annually in winter can enhance wetland nitrogen removal compared with no harvesting (Shuai et al., 2016). However, reed harvest is mostly limited to large wetland areas because it requires immense manpower, material, and financial resources (Poulin et al., 2009). Thus, optimal harvest time should be selected to rationalize resources and maintain the nitrogen removal effects from wetlands in the low-temperature season.

Nutrient resorption from senescing plant tissues is a critical strategy and ecological process in nutrient conservation (Lü et al., 2013). The subtropical monsoon climate zone, where there are plenty of reed wetlands, has a high accumulated temperature and long frost-free period. Reed usually sprouts in March and enters the wilting period in November with limited root dormancy (Shuai et al., 2016). As roots serve as the main storage of nutrients in winter, harvest time notably affects the proportion of biomass and nutrients (Hubner et al., 2011; Liu et al., 2013), which further affects subsequent plant growth and the nutrient cycling process. To determine whether harvest time affects nutrient absorption of reed roots in winter and nitrogen removal from wetlands, we harvested aboveground biomass at three different periods of reed wilting: before wilting, mid-wilting, and late wilting. Specifically, we aimed to test the following hypotheses: (1) different harvest time of reed leads to different levels of nitrogen removal from wetlands in winter; and (2) the nitrogen removal is related to plant nutrient allocation, especially in the remaining organ root. Our results provide crucial information for wetlands management, to promote wetland environmental protection and contribute to eutrophication treatment strategies.