### Total P and available P

Fertilizer application significantly (*P*â€‰<â€‰0.05) increased total P concentration within 0â€“20Â cm soil depth and available P concentration within 0â€“40Â cm soil depth (Fig.Â 2). Compared to CK, total P concentration in the 0â€“20Â cm soil depth was increased by 26.6%, 4.1%, and 68.2% in NPK, NPKâ€‰+â€‰S, and OM treatments, respectively, while available P concentration was increased by 315.9%, 108.3% and 667.5% in NPK, NPKS and OM treatments, respectively. Phosphorous was accumulated in the 0â€“40Â cm soil depth in both NPK and OM treatments, majority of which was in the cultivated 0â€“20Â cm soil depth (Fig.Â 2). The majority of P was accumulated in the 60â€“80Â cm soil depth under NPKâ€‰+â€‰S treatment, but no significant difference was observed among treatments (*P*â€‰>â€‰0.05). The highest increase in available P concentration in NPKâ€‰+â€‰S treatment observed in the 60â€“100Â cm soil depth, with the increase of 111% and 115% in 60â€“80Â cm and 80â€“100Â cm soil depths, respectively, over CK treatment.

### Phosphorous activity constant

The phosphorus activity coefficient (PAC, the ratio of available P to total P) is an important indicator of soil P availability and the transformation of P fractions^{37}. When the PAC is less than 2.0%, the total P is not easily converted to available P^{38}. The PAC ranged from 1.5% to 13.8% within 0â€“100Â cm soil depth and the largest value (>â€‰3.6%) was associated with OM treatment, especially at the 0â€“20 and 20â€“40Â cm soil depths (Fig.Â 3). The PAC values under NPK, NPKâ€‰+â€‰S and OM treatments increased by 7.6%, 4.5% and 11.5% in the 0â€“20Â cm soil depth and 4.2%, 1.3%, and 5.8% in 20â€“40Â cm soil depth, respectively as compared to the CK treatment. However, PAC value for soil depth below 40Â cm showed the trend, NPKâ€‰<â€‰CKâ€‰<â€‰OMâ€‰<â€‰NPKâ€‰+â€‰S treatments (*P*â€‰<â€‰0.05).

### The accumulation of total P and available P in soil profile

Total P accumulation under NPK, NPKâ€‰+â€‰S and OM treatments within the 0â€“100Â cm soil profile was 511, 1,068 and 1,353Â kg P ha^{âˆ’1} more than that under CK treatment (Table 2). Increase in total P in NPK and OM treatments within the top 0â€“20Â cm soil depth accounted for 95.9% and 79.5% of total P increase within 0â€“100Â cm soil profile, respectively. However, total P in NPKâ€‰+â€‰S treatment was mainly accumulated between 20â€“100Â cm soil depths, with increased values of 215, 201, 556 and 90Â kg P ha^{âˆ’1} for 20â€“40, 40â€“60, 60â€“80 and 80â€“100Â cm soil depths, respectively over the CK treatment (Table 2). The NPK and OM treatments showed increase in available P concentration at the 0â€“40Â cm soil depths (*P*â€‰<â€‰0.05). The available P concentrations under NPK and OM treatments increased by 181.7 and 356.1Â kg P ha^{âˆ’1} in the 0â€“20Â cm soil depths, and 69.7 and 127.0Â kg P ha^{âˆ’1} for 20â€“40Â cm soil depth compared to that under CK treatment. Available P under NPKâ€‰+â€‰S treatment increased throughout the entire 0â€“100Â cm soil profile compared to that associated with the CK treatment (*P*â€‰<â€‰0.05) (Table 2).

### P fractions

Organic P (Po) concentration decreased with the increase of soil depth regardless of treatment, while inorganic P (Pi) concentration increased under CK and NPKâ€‰+â€‰S treatments and decreased under NPK and OM treatments (Fig.Â 4). The P fertilizer application significantly increased Pi concentration in the 0â€“100Â cm soil depth (*P*â€‰<â€‰0.05) and Po concentration at 40â€“80Â cm soil depth (*P*â€‰<â€‰0.05) compared with that of the CK treatment. The Pi under NPK and OM treatments accumulated mainly in the 0â€“20Â cm soil depth resulting in higher Pi concentration of 12.4% and 62.1%, respectively, than that at deeper soil depths (Fig.Â 4). Applied P fertilizer mainly presented as Pi in 0â€“40Â cm soil depth for the NPK and OM treatments, and in 0â€“100Â cm soil depth for NPKâ€‰+â€‰S treatment over CK treatment.

In general, concentration of all Pi fraction under NPKâ€‰+â€‰S, NPK and OM treatments increased depending on soil depth except for Ca_{10}-P compared to that associated with the CK treatment (Fig.Â 5). Different fertilizer treatments have significantly impacted Ca_{2}-P and Ca_{8}-P concentrations as deep as 0â€“40Â cm depth (*P*â€‰<â€‰0.05), showing a decreasing trend as OMâ€‰>â€‰NPKâ€‰>â€‰NPKâ€‰+â€‰S. Compared with CK treatment, the Ca_{2}-P concentrations associated with OM, NPK and NPKâ€‰+â€‰S treatments showed significant increase of 5.2 times, 7.5 times and 1.4 times, respectively in the 0â€“20Â cm soil depth and 2.4 times, 2.8 times and 0.5 times, respectively in 20â€“40Â cm soil depth (*P*â€‰<â€‰0.05). The Ca_{8}-P concentration associated with NPK, OM and NPKâ€‰+â€‰S treatments showed significant increase of 4.5 times, 10.4 times and 1.1 times, respectively in the 0â€“20Â cm soil depth and 1.3 times, 2.9 times and 0.5 times, respectively in the 20â€“40Â cm soil depth (*P*â€‰<â€‰0.05). The NPK and NPKâ€‰+â€‰S treatments showed significant increase in Al-P concentrations of 1.30 times and 7.61 times, respectively within 0â€“100Â cm soil depth (*P*â€‰<â€‰0.05), while OM treatment showed significant increase of Fe-P concentration of 79.0â€“235.8% within 0â€“60Â cm soil depths (*P*â€‰<â€‰0.05), compared with the CK treatment. Fertilizer application increased O-P concentration with significant differences observed between different treatments (*P*â€‰<â€‰0.05).

The concentrations of different Po fractions decreased with the increase of soil depth (Fig.Â 6) except for HSOP. The added P fertilizer mainly transformed to MSOP and HSOP (Fig.Â 5). The LOP and MLOP forms under OM treatment increased by 77.4% and 14.45%, respectively in the 0â€“20Â cm soil depth as compared to that associated with the CK treatment. The LOP under NPKâ€‰+â€‰S increased by 17.7% and 3.5% for 0â€“20Â cm and 20â€“40Â cm soil depth, respectively, compared with CK treatment. MSOP concentrations under NPK and NPKâ€‰+â€‰S treatments decreased by 0.6% and 2.4% within 0â€“20Â cm soil depth, while increased by 6.0â€“38.3% and 4.0â€“12.4% within 20â€“80Â cm soil depths, respectively. OM treatment significantly increased MSOP concentration by 11.6â€“30.3% within 0â€“80Â cm soil depth with the peak at 60â€“80Â cm soil depth. Generally, P fertilizer application increased HSOP concentration at 0â€“100Â cm soil profile by 8.5â€“45.0%, 0.7â€“17.0% and 1.6â€“23.8% for NPK, NPKâ€‰+â€‰S and OM treatments, respectively, with significantly different at 20â€“60Â cm soil depth.

### Accumulation of different P fractions

Total P accumulation increased mainly in the 0â€“20Â cm soil depth as impacted by long-term fertilization in this study (Table 2). Compared with CK treatment, the accumulation of Fe-P, Ca_{8}-P, Ca_{2}-P, O-P and Al-P in NPK treatment increased by 144.9, 93.5, 68.0, 67.8, and 36.8Â kg P ha^{âˆ’1}, respectively; while the accumulation of Fe-P, Ca_{8}-P, Ca_{2}-P, MSOP, and Al-P associated with OM treatment increased by 347.3, 203.8, 91.8, 129.5 and 39.2Â kg P ha^{âˆ’1}, respectively. The accumulation of Fe-P and Al-P associated with NPKâ€‰+â€‰S treatment were increased as well (Table 3).

### Proportions of P fractions

The concentrations of different P fractions increased significantly in the 0â€“40Â cm soil depth (*P*â€‰<â€‰0.05) (Fig.Â 7). Compared with CK, the proportions of Po fractions of the total P associated with NPK and NPKâ€‰+â€‰S treatments decreased by 0.3â€“3.5%, while the proportions of Ca_{2}-P, Ca_{8}-P and Fe-P of total P associated under NPK and NPKâ€‰+â€‰S treatments increased by 1.0â€“4.0 percentage. The greatest variations among these fractions were observed at the 0â€“20Â cm soil depth. However, OM treatment did not impact the proportion of different Po fractions of the total P compared with CK treatment. Field treatments did not impact the proportion of different P fractions of the total P below 40Â cm soil depths.