2.3. Soil sampling and measurements
Each year, soil samples were gathered in 0-20, 20-40, 40-60, and 60-80 cm layers between the two rows of sunflowers using a soil auger at sunflower harvest stage. The soil samples were naturally air-dried, sieved (0.149 mm) before SOC and TN determination. SOC content (g kg-1) was measured by K2Cr2O7-FeSO4oxidation, and the TN content (g kg-1) was analyzed by Kjeldahl method (Bao 2007). SOC and TN storages (Mg ha-1) were determined as below:
\(SOC\ storage=\ SOC\ content\ \times\ BDi\ \times\ Hi\ \times\ 10\)(1)
\(TN\ storage=\ TN\ content\ \times\ BDi\ \times\ Hi\ \times\ 10\)(2)
Where BDi, Hi, and 10 represent the soil bulk density (g cm-3), depth (m) and conversion factor, respectively.
The C:N ratio was determined by dividing the SOC content by the TN content. The stratification ratios of SOC and TN contents in 0-20 cm layer to that in the 20-60 cm and 60-80 cm soil layers were calculated as the method of Franzluebbers (2002). The methods of the measurements for soil moisture, soluble salt content, and crop yield were described in Zhang et al. (2020).