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).