Impact of nitrogen addition on the chemical properties and bacterial community of subtropical forests in northern Guangxi

From the acquired results, it was demonstrated that nitrogen application led to a significant decrease in soil pH. Compared with CK, the pH of treatment IV decreased by 4.23%, which corresponded to an increase in soil organic carbon and total nitrogen. Moreover, compared with CK, the soil organic carbon of treatment IV increased by 9.28%, and the total nitrogen of treatment IV increased by 19.69%. However, no significant impact on the available nitrogen and phosphorus was detected. The bacterial diversity index first increased and then decreased with the increase of the nitrogen application level. The dominant phylum in the soil was <i>Acidobacteria</i> (34.63-40.67%), <i>Proteobacteria</i>, and <i>Chloroflexi</i>. Interestingly, the abundance of <i>Acidobacteria</i> notably increased with higher nitrogen application levels, particularly evident in the IV treatment group where it surpassed the control group. Considering that nitrogen addition first changes soil nutrients and then lowers soil pH, the abundance of certain oligotrophic bacteria like <i>Acidobacteria</i> can be caused, which showed a first decreasing and then increasing trend. On the contrary, eutrophic bacteria, such as <i>Actinobacteria</i> and <i>Proteobacteria</i>, displayed a decline. From the redundancy analysis, it was highlighted that total nitrogen and pH were the primary driving forces affecting the bacterial community composition.