# Effects of Long-Term Fertilization on Phosphorus Form and Availability in Black Soil
## 4 Discussion  
The results showed that long-term fertilizer application increased TP and AP levels in the soil. Previous studies also found that the use of organic manure with chemical fertilizer significantly increased the soil TP and AP contents, with the increase significantly higher than that resulting from the use of chemical fertilizer treatments only [26,27]. In this study, we found that using 1.5M1 NPK was better than the other treatments in terms of the TP and AP contents. This is partly because the chemical fertilizer was spread as a base fertilizer and because the applied organic fertilizer itself has a high phosphorus content. The organic fertilizer can supplement the soil phosphorus and contains a high concentration of AP. Therefore, the higher the application rate, the greater the increase in the soil TP and AP contents. This reduces the adsorption of phosphorus by the soil, improves the effectiveness of the soil phosphorus, and enhances the intensity of the soil phosphorus supply [28,29]. In this study, the proportion of soil active phosphorus and moderately active phosphorus significantly increased with the combined use of organic and chemical fertilizers, with 1.5M1 NPK resulting in the largest increase. With equal amounts of N, the long-term application of a phosphorus fertilizer with an organic fertilizer can supplement the soil's phosphorus content. Organic fertilizers contain many nutrients, such as phosphorus at 1.5 times the concentration; therefore, with the application of both low and high amounts of organic fertilizer, a more obvious increase in the conversion of phosphorus to its active state can be noticed. This, in turn, reduces the proportion of mildly active phosphorus [30]. Long-term fertilizer use does not change the soil phosphorus form, but it does affect the content and transformation of the various forms of phosphorus in the soil. In general, HCI-P and Res-P account for most of the soil phosphorus, and H\({}_{2}\)O-P accounts for the lowest  
Figure 5: Correlation analysis among soil phosphorus forms under long-term fertilization. Note: * indicates significant correlation at the \(p\) < 0.05 level; ** indicates highly significant correlation at the \(p\) < 0.01 level. The hue of the ellipse corresponds to the positive or negative correlation, the colour corresponds to the size of the correlation coefficient, the redder the correlation coefficient, the larger the correlation coefficient, and the shape of the ellipse corresponds to the size of the correlation coefficient, by default the more flattened the correlation coefficient is, the larger it is. proportion when fertilizers are applied over time. However, NaOH-P may also account for a larger proportion when different fertilizers are applied. [PERSON] et al. [31] believe that NaOH-P mainly comes from humus compounds produced by the decomposition of plant residues. Dosing organic fertilizer can increase the NaOH-Pi and HCI-P content in the soil [32]. The long-term application of chemical and organic fertilizers mainly increases the proportion of available inorganic phosphorus in the soil and decreases the proportion of residual and organic phosphorus, thus increasing the soil phosphorus effectiveness and promoting the conversion of potential phosphorus. In the present study, we found that the effects of the long-term application of different measures of fertilizer at the molecular level mainly manifested in the changes in orthophosphate. Different long-term amounts of applied fertilizer have been suggested to affect the distribution and form of P. The long-term application of a chemical fertilizer significantly increases the orthophosphate content of the soil because chemical fertilizers tend to have a higher proportion of orthophosphate content [33; 34]. When the chemical fertilizer was paired with the organic fertilizer, the orthophosphate concentration in the soil's P increased [35].