2.6 Data analyses
All statistical analyses were conducted using SYSTAT 14.0 (SYSTAT Software, Richmond, CA, USA). Pearson’s correlation coefficient was applied to evaluate the relationship between environmental factors and CO2 efflux rate, MBC and qCO2 in the C treatment. To evaluate the effect of treatment on soil moisture, CO2 efflux rate, MBC and qCO2 over the experimental period and also during each cultivation period, repeated-measures analysis of variance (RM-ANOVA) was conducted, in which treatment and sampling time were treated as fixed effects and permitted to interact. When ANOVA indicated a significant difference for treatments, mean comparisons were performed with the Tukey-Kramer multiple comparison test. In addition, to assess the interaction effect of biochar application and FYM application during each cultivation period on CO2 efflux rate, MBC and qCO2, two-way RM-ANOVA was conducted. Surface SOC stock was calculated by multiplying soil C content by soil bulk density in each treatment plot. Tukey-Kramer test was used to determine the differences between treatments, in SOC stock in Sep 2017, SOC stock in Dec 2019 and SOC increment. Student’s t-test was used to determine the differences between SOC stock in 2017 and Dec 2019 for each treatment. In all cases,P < 0.05 was considered significant.
To estimate the annual CO2 flux, we used an modified Arrhenius relationship between the measured CO2 efflux rate and environmental factors such as soil moisture and soil temperature by multiple regression analysis, as shown in Sugihara et al. (2012), as follows:
Cem = aMb exp(-E / RT)
where Cem is the hourly CO2 efflux rate (mol C ha−1 hr−1), M is the volumetric soil moisture content (m3m−3; 0.12 < M < 0.27), E is the activation energy (J mol−1), R is the gas constant (8.31 J mol−1 K−1),T is the absolute soil temperature (K), b is a coefficient related to the contribution of soil moisture, and a is a constant. Because of the considerable annual variation in rainfall and disturbance by plowing and cultivation, we separated the period from the start of the cultivation and performed the above analysis for each year, i.e., first-year (from Sep 2017 to Jul 2018; 11 months), second-year (from Aug 2018 to Jul 2019; 12 months), and third-year (from Aug 2019 to Dec 2019; 4 months).