SOC stock, AC and PC
Long-term fertilization significantly increased SOC stock by 56.8% and 76.3% under NPKM and 1.5NPKM treatments, respectively (Fig. 1). The AC was significantly increased by 53.0% under 1.5NPKM while there was no significance difference under NPK and NPKM treatments (Fig. 2). The PC was significantly increased by 76.7% and 108.5% under NPKM and 1.5NPKM treatments, but there was no significance difference under NPK (Fig. 3).
The fertilization treatments and soil depths as well as their interaction had significance effects on SOC stock, AC and PC (P < 0.001, Table S2). SOC stocks, AC and PC were generally decreased with soil depth at all treatments (0–20 cm > 20–40 cm > 40–60 cm) (Fig. 1, Fig. 2, Fig. 3). In the top layer (0–20 cm), the PC were significantly increased by 36.6%, 77.8%, 117.1% under NPK, NPKM and 1.5NPKM, respectively. While SOC and AC only significantly increased by 56.2%, 80.3% and 40.7%, 53.9% under NPKM and 1.5NPKM, respectively. In the sub layer (20–40 cm), the SOC and PC were significantly increased under 1.5NPKM and NPKM, 1.5NPKM, respectively, but there was no significance difference for AC among all fertilization. In the deep layer (40–60 cm), the SOC was significantly increased by 62.3%, 85.8% under NPKM and 1.5NPKM, respectively. The PC was significantly increased by 113.0% under 1.5NPKM compared with Control, while there were no significance differences for AC among all fertilization.
Fertilization increased the proportion of PC content in the total SOC pool from 44.4–49.5%, 49.8% and 52.2% across all sites under NPK, NPKM and 1.5NPKM, respectively (Fig. 4). At different sites, there were similar trends. At GZL, the proportion of PC in the total SOC pool were from 39.9–45.5%, 45.6 %, 48.4 %, and at ZZ from 46.1% to 51.9 %, 51.4 %, 53.7 %, and at CQ from 46.0–48.5%, 50.1%, 52.0%, and at QY from 45.7–52.1%, 52.4%, 54.7%, respectively. We also found that the 1.5NPKM treatment resulted in PC representing more than 50% of SOC across the sites. Across the three sites, PC was greater than AC under 1.5NPKM treatments, expect GZL site.
Response ratio of SOC, AC and PC
The results show that long-term fertilization significantly increased the RR of SOC, AC and PC across all depths and sites (Fig. S2, Fig. 5). The average RR of SOC were significantly increased by 23.7%, 60.4%, 75.6%, and 29.2%, 42.1%, 43.6% for AC while 66.5%, 85.7% and 92.2% for PC under NPK, NPKM and 1.5NPKM, respectively.
The RR of SOC, AC and PC to different fertilization at different depths and sites were differed. For total SOC, the response ratio at all depth were 1.5NPKM > NPKM > NPK. For AC, the response ratio in the top layer (0–20 cm) was 1.5NPKM > NPKM > NPK, and in the sub (20–40 cm) was 1.5NPKM > NPK > NPKM and deep layer (40–60 cm) was NPK > NPKM > 1.5NPKM. For PC, the response ratio in the top layer (0–20 cm) was NPKM > 1.5NPKM > NPK, and in the sub (20–40 cm) was 1.5NPKM > NPK > NPKM and deep layer (40–60 cm) was NPKM > NPK > 1.5NPKM.
In addition, for total SOC, the response ratio at the GZL is 1.5NPKM > NPKM > NPK, at the ZZ is 1.5NPKM = NPKM > NPK, at the CQ is NPKM > NPK = 1.5NPKM, at the QY is NPKM > 1.5NPKM > NPK. For AC, the response ratio at the CQ site was NPKM > NPK > 1.5NPKM, at the GZL and QY sites was NPKM > 1.5NPKM > NPK, and at the ZZ site was 1.5NPKM > NPK > NPKM. For PC, the RR at GZL, CQ and QY sites were 1.5NPKM > NPKM > NPK, whilst at the ZZ site it was 1.5NPKM > NPK > NPKM.
Modeling drivers of response ratio
The RR was correlated with soil chemical properties (SOC, TN, C/N and pH), environmental variables (MAT and MAP) and soil texture (Clay) (Fig. S2). It shows that RR was positively with SOC (r = 0.32), TN (r = 0.31) and C/N (r = 0.16). However, it decreased with MAT (r = -0.23) and MAP (r = -0.14). In addition, it also shows that RR was negative with the Clay (r = -0.26).
SEM analysis showed that SOC, Clay, MAP and fertilization input had direct effects on RR. Together, these variables predicted 52% of variance in RR (Fig. 6). Specifically, soil chemical properties, including SOC had direct positive effects on RR. Environmental variables, including MAT and MAP, had direct and indirect by mediating soil texture and SOC. Soil texture, including Clay, had direct effect on RR. Taking the indirect and direct effects together, Clay was the most important factors influence the RR in cropland, followed by MAP, MAT, fertilization input and SOC (Fig. 7).