2.3. Sampling and chemical analyses
Soil samples were collected from each plot before the experiment and
after the rice had been harvested to analyze the basic soil properties
including pH, SOC, TN, NH4+-N,
NO3−-N, AP, AK, permanganate oxidizes
carbon (POXC), and other available medium and trace elements. Soil
samples were collected for determining
NH4+-N and
NO3−-N contents at the seeding,
tillering, elongation, earing, flowering, and maturation stages,
respectively. Quadruplicate soil
samples from a depth of 0−20 cm for each plot were collected and mixed
thoroughly. The soil samples were
air-dried at room temperature and then sieved to < 2 mm and/or
< 0.15 mm after removing the residues and plant roots. Soil pH
was measured with a pH meter (PH-21, Sartorius, Goettingen, Germany) in
a 1:2.5 mass: volume soil/water suspension (Thomas, 1996). SOC content
was determined using the potassium dichromatic oxidation titration
method (Walkley & Black, 1934). TN content was determined using the
Kjeldahl method (Pansu & Gautheyrou, 2006). AP was extracted with
sodium bicarbonate solution and then was measured by the molybdenum
antimony blue colorimetric method (Olsen, 1954). AK was extracted with 1
N ammonium acetate and then was determined by flame spectrometry
(Jackson, 1958). POXC was measured using the colorimetric method after
oxidating by 0.02 M KMnO4 (Blair, Lefroy, & Lisle,
1995). Available medium and trace elements were extracted with
DTPA-CaCl2-TEA solution and then were measured using an
ICP-OES (ICAP7000, ThermoFisher, USA)
(Jackson, 1958). Soil NH4+-N and
NO3−-N were extracted with 2 M KCl and
then were measured using a flow analyzer (San++System,
SAKLAR, Netherlands). The shoot and/or grain of rice and wheat were also
collected at seeding, tillering, elongation, earing, flowering, and
maturation stages, respectively. The oven-dried and ground shoot and
grain were digested with sulfuric acid-hydrogen peroxide, and the N
content was determined by the Kjeldahl method. The grain yield and the
grain moisture were measured during harvesting. Grain yield was adjusted
to standard moisture of 15% for rice and 12% for wheat. The N
agronomic efficiency (NAE, kg
kg−1 N) was calculated as follow:
\begin{equation}
\begin{matrix}\text{NAE}=\frac{Y_{N}-Y_{0}}{F_{N}}\#\left(\ AUTONUM\ \ \backslash*\ Arabic\ \right)\\
\end{matrix}\nonumber \\
\end{equation}where \(\mathrm{Y}_{\mathrm{0}}\) and \(\mathrm{Y}_{\mathrm{N}}\) (kg
ha−1) are the dry grain yields without N fertilizer
and with N treatments respectively, \(\mathrm{F}_{\mathrm{N}}\) (kg N
ha−1) is the N fertilization rate.