To provide a scientific basis for the optimal fertilization strategy of Cyclocarya paliurus plantations, the effects of different levels of nitrogen (N) and phosphorus (P) application on soil enzyme activities, their enzyme stoichiometric ratios and available nutrients in the soil were investigated.

A field experiment in the C. paliurus plantations was established by the two-factor complete block design, with three N levels (N₀, N₁, and N₂) and three P levels (P₀, P₁, and P₂), then the relationship between soil enzyme activity and available nutrients was elucidated.

1) Different application ratios of N and P fertilizers affect soil β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), acid phosphatase (AP) and their enzyme stoichiometric ratios in the soil of C. paliurus plantations, while in most cases there are interactions between N and P (P<0.05); 2) In general, a combined application of N and P fertilizers improved soil enzyme activities of BG, NAG, LAP and AP. However, the highest activity of BG, NAG and AP was all observed in the N₂P₂, whereas the greatest value of LAP activity was detected in the N₂P₁. The activity of BG and NAG in the non-rhizosphere soil decreased with soil depth, while the LAP activity in the rhizosphere soil showed up-tendency with the increase of soil depth; 3) The application of N and P fertilizers also influenced enzyme activity C∶N ratio (EA_C:N), enzyme activity C∶P ratio (EA_C:P) and enzyme activity N∶P ratio, (EA_N:P) in both rhizosphere and non-rhizosphere soils. Compared to the non-rhizosphere soil, the values of EA_C:N and EA_C:P in the rhizosphere are higher but the EA_N:P value is lower, indicating the limitation of carbon and phosphorus on rhizosphere soil microorganisms is more pronounced; 4) Fertilization treatments improved the contents of alkaline hydrolysable N and available P in the rhizosphere and non-rhizosphere soils, especially the availability of N and P in the soils was significantly improved in the N₂P₂ and N₂P₁ treatments; 5) The availability of nitrogen and phosphorus in soil was correlated with the activity of soil-related enzymes and their stoichiometric ratios. The alkaline hydrolysable N content in the non-rhizosphere was significantly and positively correlated with the soil enzyme activities of BG, NAG, LAP and AP (P<0.05), while a significantly negative correlation was detected between the alkaline hydrolysable N content and LAP activity in the rhizosphere (P<0.05). The available phosphorus content in the rhizosphere soil was significantly positively correlated with the activities of BG, NAG and AP enzymes and the stoichiometric ratios of carbon-nitrogen acquisition enzymes and carbon-phosphorus acquisition enzymes (P<0.05), but was extremely significantly negatively correlated with the activity of LAP enzyme and the stoichiometric ratio of nitrogen-phosphorus acquisition enzymes (P<0.001). However, the available P contents in the non-rhizosphere soil are significantly and positively correlated with measured soil enzyme activities and their stoichiometric ratios, except for EA_C:P.

The combined application of N and P influenced the soil enzyme activities and available nutrients in the C. paliurus plantations, while the appropriate application ratios of N and P can promote soil enzyme activity and significantly improve the soil N and P availability.