3.1. Tillage effects on winter wheat
3.1.1 Tillage effect on wheat PSE and SWSp
The number of paired observations for PSE and SWSp were 120 and 334, respectively (Figures 2a and 2b). Data showed significant heterogeneities, as shown by high Qt values of 112 for PSE and 321 for SWSp. Compared to CT, conservation tillage methods overall increased PSE by 22.6% (P ≤ 0.05) (Figure 2a). The biggest increase in PSE was obtained with RT (25.1%) followed by ST (23.9%) and NT (15.9%) (P ≤ 0.05) across conservation tillage methods. The response ratio (RR) of PSE to conservation tillage techniques was significant in overall soil textures (Figure 3a). The medium and coarse-textured soils increased PSE by 22.1% and 16.4%, respectively, while fine-textured soils increased PSE by 25.7%. The enhancement of conservation tillage on PSE was greater when MAP was >600 mm than 400-600 and <400 mm. Conservation tillage also increased PSE in the regions where MAT was 8-15 ℃ than >15 ℃ and <8 ℃ (Figure 3a).
The conservation tillage methods raised SWSp by 17.8% as compared to CT (P ≤0.05, Figure 2b). This beneficial impact on SWSp varied with tillage methods. In comparison to CT, SWSp increased by 26.9%, 24.4%, and 13.1% from NT, RT, and ST, respectively (P ≤ 0.05). Under all soil textures, the RR of SWSp under conservation tillage methods was significant as compared to CT (Figure 3b). SWSp increased by 26.4% in fine-textured soils, while PSE increased by 22.1% and 17.7% in medium- and coarse-textured soils, respectively. The benefits of conservation tillage on SWSp were greater under MAP of 400-600 mm. The SWSp improved with conservation tillage methods in the regions where MAT was below 8 °C (Figure 3b).
3.1.2. Tillage effect on wheat grain yield, ET and WUE
Total 331 observations for grain yield, 34 for ET and 103 for WUE were measured. The data were heterogeneous for grain yield (Qt = 318), ET (Qt = 29), and WUE (Qt = 101) (Figures 2c, 2d, and 2e). Compared to CT, overall conservation tillage methods enhanced grain yield by 24.1% (P ≤ 0.05, Figure 2c). According to the categorical meta-analysis, the grain yield was increased by 31.4%, 21.1%, and 18.2% with NT, RT, and ST, respectively. The fine and coarse textured soils increased grain yield by 20.3% and 22.1%, respectively, and medium textured soils increased wheat yield by 24.1% (Figure 2c). The enhancement of conservation tillage methods on grain yield was greater in the regions where MAP was >600 mm. Conservation tillage methods also increased grain yield in the regions where MAT was 8-15 ℃ (Figure 3c).
There was a significant effect of overall conservation tillage methods on wheat ET. The NT and RT increased ET by 11.6% and 14.1%, respectively (P ≤ 0.05 , Figure 2d). However, ST had no significant effect on ET. Fine-textured soils increased ET by 18% while medium and coarse textured-soils enhanced ET by 2% each. The enhancement of conservation tillage methods on ET was greater when MAP was <400 mm. Conservation tillage methods increased ET in the regions where MAT was >15 ℃ (Figure 3d).
Winter wheat WUE increased by 12.1% under overall conservation tillage methods over the fallow period (P ≤ 0.05, Figure 2e). The NT, RT and ST increased the WUE by 18.4%, 11.3% and 5.4%, respectively, as compared to CT. Overall soil types exhibited an increase in wheat WUE when conservation tillage methods were applied, although medium-textured soils exhibited the greatest increase (22.3%) compared to fine and coarse-textured soils (Figure 3e). Conservation tillage methods had a stronger positive impact on WUE in regions having the MAP <400 mm. Conservation tillage methods increased WUE in areas where MAT was 8–15 °C (Figure 3e).