## 2 Materials and Methods  
### Study Site and Sampling  
The field sampling was conducted in the Nansi Lake Basin (NLB), which is located in the Jining, Shandong Province of China. NLB belongs to warm temperate monsoon climate, has a high temperature, is rainy in summer, and sunny and cold weather in winter. The annual average temperature is 13.3-14.1 \({}^{\circ}\)C. The average frost-free period is 199 days and the annual precipitation is 597-820 mm.  
With a total area of 1266 km\({}^{2}\), the Nansi Lake (NL) is the largest inland lake in the Shandong Province. It has 53 tributaries and the Xinxue River (XR) is a large tributary that connects to NL from the southeast. The Xinxue River Constructed Wetland (XRCW) were designed and operated in 2008, downstream of XR, and were 5100-m-long and 270-m-wide [17]. The Nansi Lake estuary (NLE) is an extensive flat wetland mouth of XRCW that transfers water from the XRCW into NL. NLE has similar characteristic and habitat with XRCW, and also receives the rush from lake water. XRCW and NLE have significantly higher plant richness and diversity than NL and XR. Concretely, the main plants in XRCW and NLE are _P. australis_, _Acorus calamus_, and _A. philosoroides_, etc., while NL and XR have fewer plant cover. Sediments can be divided into sand, silt, and clay, based on the particle size. In NLB, the corresponding proportions of sand, silt, and clay in NL and XR were on average, 20.21%, 43.30%, and 36.48%, and those in XRCW and NLE were 13.16%, 76.54%, and 10.31%, respectively, based on the previous reports [18; 19]. In this study, within the geographic range of 34 deg42 50.02-34 deg52 18.51 N and 117 deg04 45.30-117 deg18 29.41 E, we collected sediment samples from the XR and NL for characterizing the natural wetlands and from the XRCW and NLE, as representative of constructed wetlands.  
The sediment sampling lasted for ten days from May to June of 2017. Sediments of the constructed wetlands were collected first and then the sediments of XR and NL were collected accordingly. Preliminary investigation found that NL and XR were rarely invaded by alligator weed with an invaded area of less than 5%, while XRCW and NLE were largely invaded, with the invaded area exceeding 30%. Thus, the invaded sites and non-invaded sites were selected as representative of invaded habitats and normal habitats in this study. Specifically, we selected eight invaded sites and seven non-invaded sites in XRCW, and three invaded sites and four non-invaded sites in NLE. Two and five non-invaded sites were selected in NL and XR, respectively. In total, 29 sites were selected and the details of the sampling sites are shown in Figure 1. Previous reports showed that OC and ON were sensitive to soil depths in terrestrial ecosystem [20; 21], thus sediments from two depths (0-15 cm and 15-25 cm) were collected to analyze the vertical distribution pattern of C and N fractions. At each site, sediment cores of 0-25 cm were excavated, the five-point sampling method was applied to collect sediment samples of 500-1000 g wet weight from two depths, using a GRASP sediment sampler (GRASP ZYQ-WN, Beijing, China). Meanwhile, surface sediment samples of 30-50 g were collected and stored in 4 degC ice box for bacterial analysis, and surface sediments were also in-situ collected using a cutting ring of 100 cm\({}^{3}\) to analyze soil bulk density and moisture contents.