3.2 Soil total inorganic phosphorus and total organic phosphorus content Nitrogen and P single fertilization and N-P combined had highly significant ( p < 0.01) effects on soil total inorganic P and total organic P content ( Figures 2A, C ).
In general, under P 0 conditions, soil total N content tended to increase with increasing N application, while total P, available P content, pH, inorganic P content in all forms, and organic P content in all forms showed a decreasing trend.
Soil Ca 8 -P, Al-P, and Ca 10 -P in the N treatment was significantly ( p < 0.05) smaller than that of the unapplied N treatment [except for the third and Ca 8 -P in the 4th crop was significantly ( p < 0.05) larger than that in the unapplied N treatment; the Ca 10 -P in the 2nd crop under N 1 treatment and Al-P under N 2 treatment were not significant; the O-P showed a trend of increasing and then decreasing except for the 2nd crop, which showed a trend of decreasing and then increasing], and reached the maximum value under N 2 treatment. 3.4 Content of each organic phosphorus fraction of soil The effects of N application, P application, and N-P combined the content of organic P fractions in soils under alfalfa production were highly significant ( p < 0.01) ( Figure 4 ).
Under P 1 conditions, the percentages of Ca 2 -P, Fe-P, MROP, and HROP showed a decreasing trend with increasing N application, all of which reached the minimum value under N 3 conditions, the percentages of Ca 8 -P, O-P, and LOP showed an increasing trend, and the percentages of Al-P, Ca 10 -P, and MLOP showed insignificant changes. 3.6 Relationship between soil physicochemical properties and content of inorganic and organic phosphorus fractions Soil available P, total N, and pH were the main drivers affecting soil P fractions, explaining 67.3%, 25.3%, and 6.5%, of their variation, respectively ( Figure 6 ).