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.