Table 1
2.2. Data analysis
To evaluate the effects of conservation tillage practices on crop yield, SOC concentration and other soil physical and chemical characteristics, we used the natural log of the response ratio (lnR) to calculate the effect size. The natural log of the response ratio is widely-used in meta-analysis of ecology studies (Hedges, Gurevitch, & Curtis, 1999), and calculated using Equation (1)
\(\ln R=ln\frac{x_{1}}{x_{2}}\) (1)
Where, x1 and x2 stand for crop yield / SOC concentration under conservation tillage and conventional tillage, respectively. The ln R is the effect value of conservation tillage. The effects of other soil physical and chemical indicators were also calculated using Equation (1).
SOC concentration at the depth of 0-5 and 0-20 cm were collected. Where studies reported soil organic matter (SOM), Equation (2) was used to convert SOM into SOC concentration.
\(\ SOC=0.58\ \times SOM\times 0.1\) (2)
Where, SOC is soil organic carbon concentration, g kg-1; SOM is the concentration of soil organic matter, %; 0.58 is the conversion factor (Nelson & Sommers, 1982).
Soil organic carbon storage (SCS) in the 0-20 cm soil depth was also collected because this is an important index to evaluate soil fertility and a more robust indicator of SOC sequestration than concentration data as it takes into account differences in bulk density among management practices. For most studies, only SOC concentration and soil bulk density were provided, and SCS was calculated using the equivalent mass method (Ellert & Bettany, 1995) with Equation (3):
\(SCS=[M\times SOC+(M_{\max}-M)\times\text{SOC}_{\text{extra}}]\times 0.001\)(3)
\(M=BD\times 0.2\times 10000\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \)(4)
where, SCS is soil organic carbon stocks, Mg/ha; M is the determined equivalent soil mass of 0-20 cm and Mmax means the maximum soil mass at depth of 0-20 cm under different tillage practices (NT, RT, ST, and CT); SOC is soil organic carbon concentration, g kg-1; SOCextrais the added SOC concentration, g kg-1; 0.001 is the coefficient of mass unit kg converted to Mg. For the calculation of soil mass, Equation (4) was used, where BD is the soil bulk density, Mg m-3; 0.2 is the thickness of the soil layer, m; 10000 is the coefficient of area unit m2 converted to ha.
Based on the differences in SCS between conservation tillage and conventional tillage, the annual SOC sequestration rate (SOCSR, Mg ha-1yr-1) of different conservation tillage measures in Northeast China were also calculated, using Equation (5):
\(\text{SOC}_{\text{SR}}=\frac{\text{SCS}_{t}-\text{SCS}_{c}}{d}\)(5)
where, SCSt and SCSc represent SOC stocks (0-20 cm) under conservation tillage and conventional tillage, respectively (Mg ha-1); d is the duration of experiments (yr).
2.3. Statistical analysis
Analysis of variance (ANOVA) was conducted to test the treatment effects on SOC and yield with the least significant difference (LSD) at P < 0.05 level. Pearson correlation analysis was conducted between SOC/crop yield and site conditions along with other soil physical and chemical indicators to determine their relationships. For all analyses, we used SPSS software 25.0 (SPSS Inc., Chicago, IL, USA).