Effects of N and P addition on soil available nitrogen and pH in a subtropical forest

In order to understand the effects of N and P addition on soil available nitrogen and pH in a subtropical evergreen broadleaved forest dominated by Schima superba, we investigated contents of soil NH₄ ⁺-N, NO₃ ^–-N and pH. Four N and P addition treatments were included as follows: CK (no addition of N and P), N (50 kg N • hm^–2• a^–1), P (15 kg P • hm^–2• a^–1) and NP (100 kg N • hm^–2• a^–1+ 15 kg P • hm^–2• a^–1). The results showed that: 1) The soil NH₄ ⁺-N and N03”-N concentrations were significantly increased after N addition, and the range of stimulation increased with prolonged nitrogen fertilization time. The soil NO₃ ^–-N concentration was significantly decreased by P addition, and there was no significant impact on soil NH₄ ⁺-N concentration under P addition. NP addition significantly increased the soil NO₃ ^–-N concentration, and N addition alone had more obvious effects on the soil NO₃ ^–-N concentration than NP addition. The response of soil NH₄ ⁺-N concentration to NP addition turned from insignificant to significant over time. 2) The soil pH decreased obviously with prolonged N addition and NP addition time, while remained stable under P addition. 3) Relative to the increase of nitrate nitrogen concentration, the same increase of soil ammonium nitrogen concentration resulted in a greater decline in pH value.