Nitrogen uptake by plants may alleviate N deposition-induced increase in soil N2O emissions in subtropical Chinese fir (Cunninghamia lanceolata) plantations.

Background: Continuous nitrogen (N) deposition interferes with soil N cycling in forests, which highly impacts soil nitrous oxide (N₂O) emissions and accelerates global warming. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is one of the most widely planted species in southern China, and is usually located in areas with high N deposition rates. However, the impact of N deposition on soil N₂O emissions in subtropical Chinese fir plantations and the potential risk of increasing N input remain elusive. Methods: Here, we conducted an in situ study in a subtropical Chinese fir plantation at Fengyang Mountain Nature Reserve, China, from 2019 to 2020 with four N addition rates: control (CK: ambient N deposition), low-N (LN: 50 kg N ha⁻¹ yr⁻¹), medium-N (MN: 100 kg N ha⁻¹ yr⁻¹), and high-N (HN: 200 kg N ha⁻¹ yr⁻¹). Results: We found that soil N₂O emission rates increased with N addition rates by 71%, 176%, and 241% under LN, MN, and HN treatment compared to CK, respectively, and reached a significant level only under HN. Soil moisture was significantly reduced together with increased leaf N concentrations under N addition. Meanwhile, the microbial biomass in the middle of the growing season was significantly lower than at the end of the growing season. These results may suggest that N deposition stimulated plants to take up more N and water, which intensified plant-microbe competition and therefore alleviated further increases in N₂O emissions under N deposition, especially under low N inputs. N deposition enhanced the abundance of ammonia oxidizing archaea and bacteria and the accumulation of NO₃⁻-N in the soil but did not affect the abundance of nitrate-reducing bacteria (nirS and nirK). The results likely support that nitrification processes act as the major source of enhanced N₂O emissions under N fertilization. Conclusions: Our study advances our understanding of the impacts of N deposition on the soil N₂O emissions in the Chinese fir plantations and highlights that plant N acquisition needs to be incorporated as an important explanatory variable when predicting N₂O fluxes under global increases in N deposition. [ABSTRACT FROM AUTHOR]