# Effects of Long-Term Fertilization on Phosphorus Form and Availability in Black Soil
## 3 Results
### Effects of Long-Term Fertilization on the Phosphorus Content of Each Form of Soil  
The long-term use of different fertilizer application methods and dosages affected the content of each form of phosphorus in the soil. The content of each form of inorganic phosphorus in the soil could be ranked follows: HCl-Pi > NaOH-Pi > NaHCO\({}_{3}\)-Pi > H\({}_{2}\)O-Pi. Additionally, the content of each form of organic phosphorus could be ranked as follows: HCl-Po > NaOH-Po > NaHCO\({}_{3}\)-Po > H\({}_{2}\)O-Po (Figure 2).  
Figure 1: Soil total and available phosphorus content under long-term fertilizer application. Note: Different lowercase letters indicate significant differences among treatments at the \(p<0.05\) level.  
The soil active phosphorus, moderately active phosphorus, and residual phosphorus contents were significantly increased in the NPK treatment samples compared with the CK treatment samples, with increases ranging from 12.0% to 438.1%. Compared with the NPK treatment, the M1 NPK, M2 NPK, and 1.5M1 NPK treatments resulted in significant increases in soil inorganic phosphorus and organic phosphorus contents for all forms. Among them, the soil H\({}_{2}\)O-Pi and H\({}_{2}\)O-Po contents increased the most, ranging from 141.7% to 417.7% and from 236.7% to 711.5%, respectively. The percentage of significant increases in the content of Res-\(p\) ranged from 40.4% to 43.1%, with the highest increase found in the content of the soil that received the 1.5M1 NPK treatment. However, the M1 NPK-treated soil showed the highest levels. This meant that the long-term application of chemical fertilizers could increase the content of all forms of phosphorus, and the use of  
Figure 2: Phosphorus content in various forms of soil under long-term fertilizer application (mg-kg\({}^{-1}\)). Note: Different lower case letters in the figure indicate significant differences among soil inorganic phosphorus treatments at the \(p\) < 0.05 level, and different upper case letters indicate significant differences among soil organic phosphorus treatments at the \(p\) < 0.05 level.  
an organic fertilizer in combination with a chemical fertilizer could increase the content of all forms of phosphorus by more than that from the use of the chemical fertilizer alone, with 1.5M1 NPK treatment having a more significant effect.  
Effects of Long-Term Fertilizer Application on the Distribution Proportion of Various Forms of Soil Phosphorus  
As can be seen from Figure 3, NPK increased the proportion of active and moderately active phosphorus in the soil by 1.24% to 11.86%, while the proportion of residual phosphorus and mildly active phosphorus decreased by 7.9% and 11.61%, respectively, when compared with the CK treatment; compared with the NPK treatment, the proportion of active and moderately active phosphorus of the soil when organic manure was applied alongside the chemical fertilizer was significantly increased by 0.08% to 6.03%, with the 1.5M1 NPK treatment showing the highest increases, while the relative proportions of both mildly active phosphorus and residual phosphorus decreased, with the decreases ranging from 2.17% to 9.62%. The 1.5M1 NPK treatment had the lowest decreases. This indicated that the long-term application of chemical fertilizer could promote the conversion of phosphorus to high- and medium-activity phosphorus, and the effect of the combined use of organic and chemical fertilizers was better than that of the long-term application of only chemical fertilizers, in which the effect of the use of 1.5M1 NPK was better.