2.3 | Field sampling and laboratory analysis
We randomly harvested 30 bunches of C. songorica in each plot in late August of 2018, corresponding to peak biomass time. Each bunch of plants with soil were dug out to a depth of 15 cm, with approximately 90% of the roots harvested. The non-rhizosphere soil was defined as those shaken off from the roots while the rhizosphere soil as those that adhere to the roots tightly and cannot be shaken off (Chaudhary et al., 2015). Plants of C. songorica were separated into shoots and roots, and oven-dried at 65°C for 72 h and weighed. Soil samples from the same plot were mixed to form a composite sample, thus, we obtained 12 rhizosphere and 12 non-rhizosphere composite soil samples (4 treatments × 3 replications).
Each composite soil sample was equally separated into three portions: one air-dried, one kept fresh at 4°C, and one stored at -80 °C. The fresh sample was used to determine the soil moisture and inorganic nitrogen concentration. Concentrations of ammonium nitrogen and nitrate nitrogen were measured using a continuous flow analyzer (Auto Analyzer III, Bran+Luebbe, Germany), and the inorganic nitrogen was their sum. The air-dried samples were used to determine the concentrations of total carbon, total nitrogen, total phosphorus, available phosphorus, and the pH values. Concentrations of total carbon and total nitrogen in soil/root samples were determined by an element analyzer (Elementar Vario Micro Cube, Germany), whereas soil total phosphorus and available phosphorus by a spectrophotometer. Soil pH was measured by a pH meter. The frozen samples were used to extract DNA for evaluating the richness and composition of microbial communities.