Effects of experimental N addition on plant diversity in an old-growth temperate forest

Temperate forest ecosystems have experienced mounting negative effects due to increasing levels of nitrogen (N) deposition. We examined the effects of experimental N addition on plant diversity in an old-growth temperate forest to test the following hypothesis: Long-term excessive N addition decreases plant diversity by affecting the growth of plants, which results from changes in the soil nutrient content and a decrease in the soil pH in temperate forests. Experimental N additions were administered at the following levels since 2008: control (0 kg N ha^-1 year^-1), low N (30 kg N ha^-1 year^-1), medium N (60 kg N ha^-1 year^-1), and high N (120 kg N ha^-1 year^-1). Additionally, plant diversity was studied from 2014 to 2016. The results showed that the experimental N additions had significant effects on plant diversity and soil properties in an old-growth temperate forest. The high-N treatment decreased the density, cover, and diversity of understory plants, and some herbs even appeared to undergo premature aging, whereas the species diversity of herbs and ferns in the low-N treatment plots showed a slight increasing tendency. This may have been because the old-growth temperate forest is an N-limited ecosystem, so the moderate N input did not show a large influence on plant diversity. However, the long-term high-N treatment ultimately reduced plant diversity by changing the soil nutrient contents, decreasing the pH values, and damaging plant growth. Our results suggested that the long-term excessive N addition negatively affected the forest ecosystem in an N-limited temperature forest.