Nitrogen Uptake by Plants May Alleviate N Deposition-induced Increase in Soil N2O Emissions in Subtropical Chinese Fir Plantations

BackgroundContinuous increasing nitrogen (N) deposition interferes with soil nitrogen cycle of forests, which highly impacts soil N2O emissions and accelerates global warming. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is one of the most widely planted species in southern China which locates in the high N deposition area. However, the impact of N deposition on soil N2O emissions in subtropical Chinese fir plantations and the potential risk of increasing N deposition still remain elusive. Here, we conducted an in situ study in a subtropical Chinese fir plantation at Fengyang Mountain Nature Reserve, China, from 2019-2020 with four different levels of N enrichment: 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). ResultsWe found that soil N2O emission rates increased with N enrichment from an average of 5.89 ± 3.66 to 20.11 ± 3.44 μg N m−2 h−1. The N enrichment in general showed no significant effect on the abundance of nitrate-reducing bacteria, but it tended to raise the abundance of ammonia oxidizing archaea and bacteria, and to decrease the abundance of N2O-reducing bacteria, which likely provided the microbial basis for accelerating soil N2O emissions along with increasing N deposition. However, the relationship of soil N2O emissions with N input did not match an exponential increase, but it matched a logarithmic increase, illustrating that the risk of increasing N deposition on soil N2O emissions was attenuated. It is found that N enrichment significantly decreased soil moisture and tended to increase the fir leaf N concentrations and soil CO2 emission rates. Besides, soil microbial biomass was significantly suppressed by N enrichment during the mid-growing season, while not in end of growing season. These may suggest that N enrichment stimulated plant growth with more N and water uptake, which competed with microorganisms for N and therefore alleviated further increasing N2O emissions under N enrichment. ConclusionThis study deepen our understanding of the impacts of increased N deposition on the greenhouse gas (GHG) balance in the Chinese fir plantations, and highlight that plants need to be incorporated as an important explanatory variable when predicting GHG fluxes in the background of global increasing N deposition.