4.1 Effect of N addition on soil properties
Soil acidification is caused by nitrogen deposition, and soil pH decreases linearly with increasing N application (Tian & Niu, 2015). In this study, long-term HN treatment (80 kg N ha-1year-1) reduced soil pH, while long-term low and medium N treatments had no significant effect on soil pH. The reason may be that soils in terrestrial ecosystems are more sensitive to N inputs and forest ecosystems are more stable than other types of ecosystems.
MN treatment had a significant effect on Soil TP content, while AP content decreased to varying degrees with N addition. This may be because soil acidification activates aluminum ions and iron ions in the soil, causing more phosphorus to be adsorbed by the soil, resulting in a decrease in AP content. When plants and microorganisms absorb and utilize AP, they get a corresponding supplement from the phosphorus element of other components, which leads to the reduction of AP content in different degrees.
Soil TN content was stable, ammonium content increased linearly with N application, and nitrate content fluctuated with N application. The result of Qin et al. (2022) showed long-term nitrogen application can change the soil ammonium and nitrate content. Besides, total N increased significantly with the increase of N addition. The stability of whole nitrogen content in this experiment may be because the forest ecosystem is more stable and can resist a certain concentration of N addition.
As the gradient of N addition concentration increased, there was a tendency for the easily EE-GRSP content in the in-situ soil to increase, and the T-GRSP content was significantly higher in the HN treatment than CK, probably due to the significant acidification of the soil, and it has been shown that acidic soil had high GRSP content (Agnihotri et al., 2022). On the contrary, short-term N addition reduced the GRSP content in the soil (Huang et al., 2022; Jia et al., 2021), the reason for this contrasting result may be that short-term N additions cannot significantly change soil acidity and alkalinity, but could lead to a decrease in GRSP by changing soil dissolved organic carbon and N/P ratio. The proportion of large soil aggregates and GRSP content was significantly elevated in the HN treatment. Pearson correlation analysis of in-situ soil GRSP content and MWD showed that both EE-GRSP and T-GRSP were significantly and positively correlated with MWD values (Fig.3), which is further evidence of the importance of AM fungi in soil stability.