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