Distribution of Triarrhena lutarioriparia and its reserve characteristics of nitrogen and phosphorus in Dongting Lake

. Triarrhena lutarioriparia , a typical and most abundant macrophyte in Dongting lake wetland, was in the state of abandonment following the papermaking industry revocation in the lake basin. In order to provide scientific basis for precise management of T. lutarioriparia , the T. lutarioriparia distribution charateristics in Dongting Lake and its storage characteristics of nutrients were investigated in this study. Remote sensing interpretation results showed that the total area of T. lutarioriparia in Dongting Lake wetland was 58,450 ha, 48.31% of which distributed in South Doting Lake wetlands. The nutrients contents were significantly different in T. lutarioriparia tissues, ranking in the descending order of spikes (TN 27.90 mg/g, TP 3.46 mg/g)>leaves (TN 16.38 mg/g, TP 2.11 mg/g)>stems (TN 5.38 mg/g, TP 0.85 mg/g). The total P quantities in each T. lutarioriparia tissue were ranked in the order: stems (560.26 t)>leaves (396.52 t)>spikes (284.67 t), while the total N quantities were within the range of 2170.02~2801.3 t. It was estimated that about 7712.99 t of TN and 1241.45 t of TP were annually removed from Dongting Lake by reaping T. lutarioriparia . The nutrients stored in the dead tissues of T. lutarioriparia might possess non-negligible impact on the water quality of Doting Lake.


Introduction
Dongting Lake is known as the "Kidney of the Yangtze River", and its runoff is about 1/3 of the total runoff of the Yangtze River [1]. Dongting Lake is a very important natural flood storage reservoir and internationally important wetlands, bearing major responsibilities for water ecology safety in the middle and lower reaches of the Yangtze River Basin [1]. Aquatic plants are indispensable parts both for both the structure and function of lake ecosystems [2]. Triarrhena lutarioriparia, a unique fibrous perennial herbaceous macrophyte in China, widely distributed in Dongting lake wetland [3][4][5]. The ecological functions of Triarrhena lutarioriparia such as purification of water bodies are extremely important to the ecosystem of Dongting Lake wetland. Meanwhile, compared with Phragmites australis, T. lutarioriparia has higher biomass per unit area and excellent cellulose quality [3,6]. Thus, T. lutarioriparia has been applied as the main raw material for pulp production in Dongting Lake area since the early 1960s [7][8][9][10]. In order to protect Dongting Lake from pulping and papermaking wastewater pollution, an implementation plan was issued by Hunan Provincial People's Government for guiding exit of papermaking enterprises in Dongting Lake Area (Government Office Letter No. 36, 2018), requesting paper-making enterprises around Dongting Lake to fully withdraw from pulp production capacity and to withdraw from paper-making production capacity in 2019. The overall withdrawal of papermaking makes it difficult to develop alternative resource utilization methods for T. lutarioriparia in the lake area within a short time. Therefore, a large amount of T. lutarioriparia will stay uncut in the lake wetland, causing problems in how to manage and protect them from exerting non-negligible impact on the water quality of Doting Lake [11][12][13].
Mastering the distribution status and nutrient characteristics of T. lutarioriparia is highly essential to formulate an effective management for it [14]. However, as the artificial planting development of T. lutarioriparia in recent years, the current distribution of T. lutarioriparia in Dongting Lake is still unclear. Moreover, the nutrient characteristics in T. lutarioriparia have been given less concern. The lack of research on T. lutarioriparia greatly increases the difficulty in the precise management of T. lutarioriparia. Therefore, remote sensing interpretation methods were used in this study to explore the distribution characteristics of T. lutarioriparia in Dongting Lake wetland. The dry-weight biomass and nutrient contents of each T. lutarioriparia tissue in different areas of Dongting Lake were investigated, and the total quantities of nutrients stored in T. lutarioriparia were further calculated. Results obtained in this study can provide scientific basis and strong supports for management of T. lutarioriparia after the withdrawal of the paper industry in Dongting Lake district.

Sample Collection and Pretreatment
After preliminary investigation and research in November 2018, a global positioning system was used to locate T. lutarioriparia in the whole lake area of Dongting Lake, and 6 sampling points were selected to collect T. lutarioriparia samples. The stems, leaves, and spikes of each plant of T. lutarioriparia are separately processed and marked on site. Then the collected T. lutarioriparia samples are washed with ultrapure water, dried and weighed. After green removing at 105 ℃ for 30 min, the plant samples were dried to constant weights at 80 ℃. The dried samples were then placed in a desiccator, cooled to room temperature and weighed, and water contents of the plant samples were measured.

Remote sensing interpretation of T. lutarioriparia resources in Dongting Lake
The geological conditions and plant species of the Dongting Lake area were investigated. Furthermore, the variations in remote sensing pictures of T. lutarioriparia due to seasonal changes were analyzed. Based on the cloud analysis of difference in water levels due to seasonal changes and the remote sensing pictures, the distribution of T. lutarioriparia in Dongting Lake wetland was interpreted by applying the Landsat 8 OLI-TIRS image data [15,16]. The remote sensing images of the Dongting Lake area in early spring of 2018 were selected for interpretation, and ENVI 5.3 and ArcMap 10.2 software were used to process the images. The maximum likelihood method is used to supervise and classify the images through the visual method. In order to improve the accuracy of the interpretation work, the unclear distribution areas of T. lutarioriparia in Dongting Lake wetlands were clarified by on-site survey and sampling.

Data analysis
The total nitrogen (TN) and phosphorus (TP) contents in the remaining litter (leaves, stems and spikes) were measured using Kjeldahl digestion followed by colorimetric analysis. Data shown in the figures herein are averages. Analysis of the variance was performed by oneway ANOVA and two-tailed paired t test using the PASW statistics software package version 18 (SPSS, Inc., Chicago, IL). Differences were considered significant when the significance level was <0.05 (e.g. α=0.05).

The distribution characteristic of T. lutarioriparia in Dongting Lake
Comprehensively considering the three factors including T. lutarioriparia distribution, natural conditions and geographic location, six sampling points (D1-D6 seen in Figure 1) were set up in the field investigation for the precise interpretation of the distribution of T. lutarioriparia in the Dongting Lake area. Figure 1 illustrated the interpretation results of T. lutarioriparia distribution in Doting Lake wetland. The specify areas of T. lutarioriparia in different districts were shown in Table  1. The total distribution area of T. lutarioriparia in early 2018 was nearly 58,467 ha ( Table 1). As shown in Table 1, the specify areas of T. lutarioriparia in different districts were ranked in the descending order: South Dongting Lake (50.40%) > East Dongting Lake (31.39%) > West Dongting Lake (18.21%). In the basin of South Dongting Lake, T. lutarioriparia with the area of 29,457.78 ha was mainly distributed in district of Yuanjiang city, accounting for 95.8% of the total T. lutarioriparia in South Dongting Lake. In the basin of East Dongting Lake, the distribution area of T. lutarioriparia in Yueyang City and Xiangyin city has an area of 9,707.68 ha and 7,948.50 ha, accounting for 96.40% of the total distribution area in this basin. In the basin of West Dongting Lake, Triarrhena lutarioriparia was mainly distributed in Hanshou County followed by Anxiang County. Among all the districts in Dongting Lake basin, Yuanjiang City has the widest distribution area with 28,235.89 ha of T. lutarioriparia, followed by Yueyang County (9,707.68 ha) and Xiangyin County (7,948.5 ha).

Nitrogen and Phosphorus distribution in T. lutarioriparia
Ten copies of each T. lutarioriparia tissue were collected from sampling sites (D1-D6). Basic properties including biomass and water content were statistically calculated and showed in Table 2. The proportion for dry-weight biomass of over-ground T. lutarioriparia tissues was ranked in the descending order of stems (70.73%) > leaves (20.98%) > spikes (8.28%). The TN and TP contents of T. lutarioriparia in D1-D6 sampling sites of Doting Lake wetlands were demonstrated in Figure 2 and Figure 3, respectively. The TN (5.76 ~11.07 mg/g) and TP contents (1.14 ~2.11 mg/g) varied siginificantly with different T. lutarioriparia tissues (P<0.05), both exhibiting the sequence of spikes > leaves > stems. The average contents of TP in spikes, leaves, stems and roots of T. lutarioriparia tissues were 3.76 mg/g, 2.07 mg/g, 0.85 mg/g and 2.08 mg/g, respectively. Meanwhile, the average contents of TN in spikes, leaves, stems and roots of T. lutarioriparia tissues were 27.58 mg/g, 15.37 mg/g, 4.42 mg/g and 6.23 mg/g, respectively. No significant differences were observed in the nutrient content of T. lutarioriparia from different area of Dongting Lake wetlands (P>0.05). The nutrient contents in rhizosphere soil where T. lutarioriparia grew were shown in Table 3. The TN and TP contents in rhizosphere soils were within the range of 11.8~13.4 mg/g and 6.13-11.1 mg/g. There is a positive correlation (R2> 0.99) between the nutrient contents in T. lutarioriparia and its rhizosphere soil.

Quantities of Nitrogen and Phosphorus stored in T. lutarioriparia
According to statistics, the total output of T. lutarioriparia in Dongting Lake is about 900,000 tons/year (dry weight). Based on the biomass proportion and the nutrient (N and P)content of each T. lutarioriparia tissue, the annual yield and nutrient reserves of each T. lutarioriparia tissue were calculated and shown in

Conclusion
The total growing area of T. lutarioriparia in Dongting Lake wetland was about 58,450 ha, and T. lutarioriparia mainly distributed in South Doting Lake wetlands, accounting for 50.40% of the total growing area. The annual yields (calculated in dry weight) of the over-ground parts of T. lutarioriparia tissues in Dongting Lake wetland were 74,500 tons, 188,800 tons, and 636,700 tons for spikes, leaves and stems, respectively. The nutrients showed similar distributing characteristics in over-ground tissues of T. lutarioriparia. Both the average contents of TN and TP exhibited the high-to-low sequence of spikes (TN 27.58 mg/g, TP 3.76 mg/g) > leaves (TN 15.37 mg/g, TP 2.07 mg/g) > stems (TN 4.42 mg/g, TP 0.85 mg/g). The quantities of nitrogen and phosphorus that can be annually removed from Doting Lake by cutting the T. lutarioriparia were 7712.99 tons and 1241.45 tons, respectively. Thus, the dead tissues of the uncut T. lutarioriparia with large amount of nutrient reserves may possess non-negligible impact on the water quality of Doting Lake.