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Figure 1. 

Effects of different application rates of biochar on soil physicochemical properties in 2018 to 2019: (a) SOC, soil organic carbon; (b) AP, soil available phosphorus; (c) AK, soil available potassium; (d) CEC, cation exchange capacity; (e) BD, bulk density; (f) Fe_d, free iron oxide; (g) Fe_o, amorphous iron oxide; (h) Fe_p, complexed iron. Error bars indicate standard deviations (n = 3). B0, B10, B20, B30, and B40 are treatments with N fertilization and application of 0, 10, 20, 30, and 40 t biochar ha⁻¹, respectively. Different lowercase letters indicate statistically significant differences (p < 0.05) between treatments in the same year. B indicates biochar effect; Y indicates cropping year effect; B × Y indicates interaction effects between biochar and year; ns, not significant; * p < 0.05; ** p < 0.01; *** p < 0.001. The SOC and BD for 2019 and the CEC for 2018 were redrawn from Liu et al.^[58] and Li et al.^[60], respectively.

Figure 2. 

Soil aggregate stability under different application rates of biochar in 2018 to 2019: (a) WSA_>0.25, water-stable macroaggregates (%); (b) GMD, geometric mean diameter; (c) MWD, mean weight diameter; (d) FD, fractal dimension. Error bars indicate standard deviations (n = 3). B0, B10, B20, B30, and B40 are treatments with N fertilization and application of 0, 10, 20, 30, and 40 t biochar ha⁻¹, respectively. Different lowercase letters indicate statistically significant differences (p < 0.05) between treatments in the same year. B indicates biochar effect; Y indicates cropping year effect; B × Y indicates interaction effects between biochar and year; ns, not significant; * p < 0.05; ** p < 0.01; *** p < 0.001.

Figure 3. 

Effects of different addition rates of biochar on (a) total phospholipid fatty acid (PLFA) content; (b) arbuscular mycorrhizal fungi (AMF) PLFAs; (c) actinomycete PLFAs; (d) Gram-positive bacteria (G+) PLFAs; (e) Gram-negative bacteria (G−) PLFAs; (f) G+/G− PLFA ratios; (g) bacterial PLFAs. (h) fungal PLFAs; and (i) fungal/bacterial PLFA ratios. (i) Error bars indicate standard deviations (n = 3). B0, B10, B20, B30, and B40 are treatments with N fertilization and application of 0, 10, 20, 30, and 40 t biochar ha⁻¹, respectively. Different lowercase letters indicate statistically significant differences (p < 0.05) between treatments in the same year. B indicates biochar effect; Y indicates cropping year effect; B × Y indicates interaction effects between biochar and year; ns, not significant; * p < 0.05; ** p < 0.01; *** p < 0.001. The total PLFA content, fungal PLFA content, bacterial PLFA content, and their PLFA ratios from 2019 were redrawn from Liu et al.^[58].

Figure 4. 

Effects of different addition rates of biochar on (a) vegetable yield; (b) nitrogen use efficiency; (c) nitrogen uptake; and (d) nitrogen partial factor productivity, NPFP; and (e)–(h) the linear relationships between vegetable yield, NUE, nitrogen uptake, NPFP, and biochar addition rate in 2018 to 2019. Error bars indicate standard deviations (n = 3). B0, B10, B20, B30, and B40 are treatments with N fertilization and application of 0, 10, 20, 30, and 40 t biochar ha⁻¹, respectively. Different lowercase letters indicate statistically significant differences (p < 0.05) between treatments in the same year. B indicates biochar effect; Y indicates cropping year effect; B × Y indicates interaction effects between biochar and year; *** p < 0.001. Solid lines and shaded areas indicate linear regression lines and 95% confidence intervals. The vegetable yield in 2018 was redrawn from Li et al.^[60].

Figure 5. 

Effects of different addition rates of biochar on (a) soil quality indices; (b) their linear relationships with vegetable yield; (c) nitrogen use efficiency; and (d) nitrogen uptake from 2018 to 2019. B0, B10, B20, B30, and B40 are treatments with N fertilization and application of 0, 10, 20, 30, and 40 t biochar ha⁻¹, respectively. Different lowercase letters indicate statistically significant differences (p < 0.05) between treatments in the same year. B indicates biochar effect; Y indicates cropping year effect; B × Y indicates interaction effects between biochar and year; ns, not significant; ** p < 0.01; *** p < 0.001. Solid lines and shaded areas indicate linear regression lines and 95% confidence intervals.

Figure 6. 

(a) Mantel test of NUE, N uptake, vegetable yield, and NPFP with environmental factors; (b) Random forest model analysis to determine the primary factors influencing N uptake, NUE, and vegetable yield. Percentage increases in the MSE (mean squared error) of variables were used to estimate the importance of these predictors, where higher MSE% values imply more important predictors. * p < 0.05; ** p < 0.01; *** p < 0.001.

Figure 7. 

Partial least squares path model showing the relationships among biochar application rate, soil basic properties, microbiome, soil structure, nitrogen utilization and vegetable yield (a); as well as the standardized effects of these variables on nitrogen utilization (b), and vegetable yield (c). Soil basic properties include the SOC, AP, AK, CEC, Fe_d, Fe_o, and Fe_p. Microbiome includes the G+ bacterial, G− bacterial, Fungal and Bacterial. Soil structure includes the WSA_>0.25 and BD. Nitrogen utilization includes the NUE and N uptake. Red and blue arrows indicate significant positive and negative relationships, respectively. Dashed arrows denote nonsignificant relations. The number adjacent to each line represents the standardized path coefficient; *, p < 0.05; **, p < 0.01; ***, p < 0.001.