Abstract:Taking the seedlings of the dwarf rootstock M9T337 as the test material, 4 nitrogen (N) levels (N 0, 150, 300, 450 mg/kg, represented by N0, N1, N2, N3, respectively) and 2 phosphorus (P) levels (P2O5 100, 200 mg/kg, represented by P1 and P2, respectively) was set up to study the effects of combined application of N and P fertilizer on the growth of apple rootstock seedlings, the transformation of soil inorganic P forms and its absorption and utilization, in order to provide a reference for the efficient use of P fertilizer in orchards. The results showed that different N and P treatments significantly affected the biomass and root morphology of M9T337 seedlings, and N2P2 treatment performed best, followed by N1P2 treatment. P accumulation and P fertilizer utilization efficiency in seedings reached the maximum under N2P2 and N2P1 treatments, respectively. Under the same P level, appropriate N addition could promote the absorption of P by seedlings and effectively improve the P fertilizer utilization efficiency of plants. High N treatment (N3P1, N3P2) significantly inhibited the uptake of P in soil by seedlings, which was not conducive to the growth of rootstock seedlings. The soil available phosphorus (Olsen—P) content was mainly affected by the amount of P applied, and reached the maximum value of 27.86 mg/kg under N2P2 treatment. The maximum alkaline phosphatase activity reached at 2.12 mg/(g·d) under N2P1 treatment. Compared with P applied alone, combined application of N and P increased the proportion of Ca8—P and Al—P in the soil P pool that could be absorbed and utilized by plants, and reduced the proportion of Ca10—P that was difficult for plants to absorb and utilize, and the proportion of Ca2—P was also reduced. With the increase of N application rates, Ca8—P and Al—P showed a trend of firstly increasing and then decreasing, and both reached the maximum under N2P2 treatment. Ca10—P showed a gradually decreasing trend with the increase of N application rates. There was a stable trend of Fe—P and O—P content under different N application rates. Reasonable N and P combination could improve the availability of P in soil by changing the composition of soil inorganic P pool, and promote the growth of rootstock seedlings and the absorption of P in soil.