2.4 Soil sampling and lab analysis
A field with uniform soil salinity status was selected for the plot experiment, and the vegetation of this field was mostly Aeluropus littoralis and reeds with the coverage of about 30%. This field was leveled using a rototiller to ensure the homogeneous condition of surface soil. Plots were built after the field leveling and soil sampling at 0-15 cm layer was conducted from each plot on late May 2015 after the plots were established (before calcium superphosphate and urea application). Soil samples taken at this time was use as the initial soil conditions. During the experiment, soil sampling were repeatedly performed on early June 2016, early June 2017 and early June 2018, respectively, i.e., after the harvest of winter wheat and before the sowing of paddy rice. In each plot, soil samples were collected using corers (5.0 cm diameter) and three soil cores were taken and then mixed to form one unique representative sample. A total of sixteen composite samples were obtained for each soil sampling.
Soil sample from each plot were subdivided in three subsamples: the first one was air dried, crushed and passed through 1 mm and 0.15 mm sieves for soil physicochemical analysis, the second one was sieved with the mesh size of 2 mm and stored at 4 ℃ for soil microbial analysis, and the third one was passed through 2 mm sieves and stored at -80 ℃ for soil DNA extraction, amplification and pyrosequencing. The analyzed soil physicochemical and microbial attributes included soil salinity (ECe), pH, sand (SA) and clay (CL) particle content, soil organic carbon (SOC), bulk density (ρb), cation exchange capacity (CEC), total nitrogen (TN), available nitrogen (AN), available phosphorous (AP), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), carbon mineralization rate (CMR), net nitrogen mineralization rate (NMR) and potential nitrification rate (PNR). The soil physical attributes mentioned above were measured only for initial soil samples since these attributes were assumed to be static and not change over a short period of time (Yao et al., 2013). Table 2 gives the analyzed physicochemical and microbial properties as well as the analytical protocols selected.
Table 2
Table 3 presents some measured soil physicochemical properties of initial soil samples across the study location. Apparently, the soils in our experimental site are characterized by high salinity, high soil compaction, low organic matter, and low nutrient supply capacity. This was in line with previous reports for coastal mudflat soil (Yao et al., 2016; Luo et al., 2017).
Table 3