# Effects of Long-Term Fertilization on Phosphorus Form and Availability in Black Soil
## 1 Introduction  
The northeast black soil region, a world-renowned region for the distribution of black soils, has a total arable land area of 35.84 million hectares. The quality of arable land not only affects crop yield but also impacts China's food security [1]. The overuse of agrochemicals accelerates the degradation process of black soil and threatens its sustainable use [2]. Therefore, it is necessary to take scientific and effective measures to fertilize the soil and increase grain yield. Studies have shown that a combination of organic and inorganic fertilizers ensures an adequate supply of total nitrogen. Not only does this strategy significantly improve the sustainability of crop yields, but it also effectively promotes the conservation of carbon and nitrogen [3; 4; 5; 6; 7]. The soil phosphorus concentration for optimal maize and soybean yield is at a threshold of 15 mg\(\cdot\)kg\({}^{-1}\)[8]. An increase in the soil organic carbon (SOC) level can significantly enhance the available phosphorus content in the soil and the phosphorus activation coefficient [9]. When SOC reaches a certain threshold, manure can completely replace chemical fertilizers to maintain high maize yields [10]. Straw returning significantly increases the phosphorus content in various soil components, showing better improvements in enhancing soybean yields. Further in-depth analysis of soil phosphorus forms and their effectiveness under different fertilization treatments would be meaningful for improving soil phosphorus use efficiency and promoting agricultural production [11]. Phosphorus is one of the main nutrient elements required for crop growth and development [8]; however, this increases environmental risks [12]. Excessive phosphorus application leads to the accumulation of a large amount of phosphorus in the soil, which increases many agricultural environmental risks [13]. At present, the most commonly used method of soil phosphorus assessment is the modified Hedley sequential fractionation method by [PERSON] et al. [14], which can comprehensively explore the forms of phosphorus in the soil. Among the six major categories and nine sub-categories considered [15; 16], water-extracted phosphorus (H\({}_{2}\)O-P) and sodium bicarbonate-extracted phosphorus (NaHCO\({}_{3}\)-P) are considered available forms of phosphorus that can be directly absorbed by plants or utilized by microorganisms in a short period of time. On the other hand, sodium hydroxide-extractable phosphorus (NaOH-P) is poorly soluble and cannot be directly utilized by plants in the short term. The sustained use of straw and green manure in conjunction with inorganic fertilizers has been shown to markedly enhance the levels of NaHCO\({}_{3}\) phosphorus and NaOH organic and inorganic phosphorus in paddy soil [17], and after the continuous application of a chemical phosphorus fertilizer for 6 years, the main form of soil phosphorus in black soil was NaOH-P [18]. To compensate for the limitations of conventional chemical sequential extraction methods, solution \({}^{31}\)P NMR spectroscopy was utilized to differentiate phosphorus forms based on their physiological effects on plants. According to the position, intensity, and fine structure of the resonance peaks in the \({}^{31}\)P NMR spectrum, different phosphorus compounds in the soil, such as orthophosphate, pyrophosphate, monoester phosphate, and diester phosphate, can be quantitatively distinguished [13; 19]. Generally speaking, the long-term application of chemical fertilizers will significantly increase the content of orthophosphate in the soil, and the increase in orthophosphate content in black soil under the long-term combined application of chemical and organic fertilizers is even more significant [20]. Most current studies on soil phosphorus fractions are centered on the effects of a single phosphorus fertilizer reduction or organic fertilizer addition, and there are fewer studies on the synergistic effects of nitrogen fertilizer reduction and organic fertilizer.