2.1 Study site
The experiment sample site is located in the Korean pine plantation in Liangshui National Nature Reserve, Heilongjiang Province, China (128°53′20″E, 47°10′50″N), Natural N deposition is 12.93 kg N ha-1 year-1. The area has a distinct temperate continental monsoon climate with an average annual temperature of -0.3°C, of which the average annual precipitation of 676 mm, a frost-free period of 100-120 d, and an annual snowpack period of 130-150 d. The Korean pine plantation in this test site was established in 1954 after a clear-cutting of the mixed broadleaved-Korean pine forest. The soil is dark-brown forest soil (using the Chinese classification), The dominated species is Korean pine, and undergrowth associated herbs wereOxalis corniculate , Mitella nuda , etc.