Effects of mineral N type, plant C type and ecosystems
Mineral N addition generally increased plant-derived C decomposition (RR++: 0.35, 95% CI from 0.25 to 0.46, Fig. 2a), while decreased soil PEs (RR++: -0.79, 95% CI from − 1.19 to -0.38, Fig. 2b) and net C sequestration (RR++: -0.14, 95% CI from − 0.23 to -0.06, Fig. 2c). Specifically, NH4+ input increased plant-derived C decomposition (RR++: 0.25, 95% CI from 0.09 to 0.47, Fig. 2a) and inhibited soil PEs (RR++: -1.51, 95% CI from − 2.12 to -0.91, Fig. 2b), thereby making no changes on soil net C sequestration (RR++: 0.00, 95% CI from − 0.12 to 0.12, Fig. 2c). The NH4+ and NO3− mixed addition decreased soil net C sequestration (RR++: -0.26, 95% CI from − 0.40 to -0.13, Fig. 2c) by increasing plant-derived C decomposition (RR++: 0.42, 95% CI from 0.25 to 0.58, Fig. 2a) rather than soil PEs (RR++: -0.89, 95% CI from − 1.52 to -0.26, Fig. 2b). However, urea input decreased soil net C sequestration (RR++: -1.45, 95% CI from − 1.73 to -1.18, Fig. 2c) by increasing plant-derived C decomposition (RR++: 0.71, 95% CI from 0.47 to 0.96, Fig. 2a) and soil PEs (RR++: 1.87, 95% CI from 0.95 to 2.80, Fig. 2b). Further, the RR++ of plant-derived C decomposition were 0.29 in exudate C addition soil (95% CI from 0.15 to 0.43), 0.31 in residue C addition soil (95% CI from 0.13 to 0.48) and 1.05 in pyrolytic C addition soil (95% CI from 0.66 to 1.45, Fig. 2a). The RR++ of soil PEs were − 1.60 in exudate C addition soil (95% CI from − 2.14 to -1.06), 0.02 in residue C addition soil (95% CI from − 0.65 to 0.69) and 1.56 in pyrolytic C addition soil (95% CI from 0.06 to 3.07, Fig. 2b). The RR++ of net C sequestration were − 0.06 in exudate C addition soil (95% CI from − 0.18 to 0.05), -0.21 in residue C addition soil (95% CI from − 0.34 to -0.07) and − 0.30 in pyrolytic C addition soil (95% CI from − 0.59 to -0.01, Fig. 2c).
The effects of mineral N addition on plant-derived C decomposition, soil PEs and net C sequestration varied with ecosystems. Mineral N addition increased plant-derived C decomposition (RR++: 0.71, 95% CI from 0.47 to 0.96, Fig. 2a) and soil PEs (RR++: 1.87, 95% CI from 0.95 to 2.80, Fig. 2b), and decreased soil net C sequestration (RR++: -1.45, 95% CI from − 1.73 to -1.18, Fig. 2c) in grassland soil. For forest soil, mineral N addition increased plant-derived C decomposition (RR++: 0.58, 95% CI from 0.42 to 0.75, Fig. 2a) and inhibited soil PEs (RR++: -2.50, 95% CI from − 3.12 to -1.87, Fig. 2b), thereby making no changes on soil net C sequestration (RR++: 0.04, 95% CI from − 0.12 to 0.19, Fig. 2c). However, the input of mineral N did not affect plant-derived C decomposition (RR++: 0.00, 95% CI from − 0.15 to 0.16, Fig. 2a), soil PEs (RR++: -0.33, 95% CI from − 0.91 to 0.26, Fig. 2b) and soil net C sequestration (RR++: 0.00, 95% CI from − 0.15 to 0.15, Fig. 2c) of cropland soil.
Factors influencing the effect size of mineral N addition
The RR of mineral N on plant-derived C decomposition was positively correlated with SOC, total nitrogen (TN), soil clay content, exogenous additives C/N, and incubation temperature, but was negatively correlated with incubation time (Fig. 3). The RR of mineral N on soil PEs was positively correlated with soil C/N, soil pH and incubation time, while was negatively correlated with SOC, TN, soil clay content, exogenous additives C/N, and incubation time (Fig. 3). The RR of mineral N on net C sequestration was positively correlated with SOC, TN, and incubation time, while was negatively correlated with soil C/N, soil clay content, and incubation temperature (Fig. 3). The weighted random-forest model further found that exogenous additives C/N was the most important factor mediating the effects of mineral N addition on plant-derived C decomposition and soil PEs, and soil C/N was the most important factor mediating the effect of mineral N addition on net C sequestration (Fig. 4).