Response of N2O and N2 Emissions in Forest Soils to Temperature Change across China

N₂O and N₂ Emissions from soil in terrestrial ecosystems is a crucial component of the global nitrogen (N) cycle. The response of these two gases emissions from forest soil to temperature change and its underlying mechanisms are essential for predicting N cycle to global warming. Despite the warming-induced effects on soil N cycle is considered to be positive in general, our understanding of temperature sensitivity (Q₁₀) of N₂O and N₂ emissions is rather limited. We quantified the Q₁₀ of N₂O and N₂ emissions in forest soils and explored their major driving factors by conducting an incubation experiment using ¹⁵N tracer (Na¹⁵NO₃) with soil samples from nineteen forest sites from temperate to tropical zones. The environmental conditions largely varied: mean annual temperature (MAT) ranging from -5.4 to 21.5^oC and mean annual precipitation (MAP) ranging from 300 to 2449 mm. The soil pH varied between 3.62 to 6.38. We incubated soil samples under an anaerobic condition with temperature from 5 to 35^oC with an interval of 5^oC for 12 or 24 hours, respectively. Soil temperature strongly affected the production of N₂O and N₂. N₂O and N₂ production rates showed a positive exponential relation with incubate time and temperature for all forest soils. Our results showed that the Q₁₀ values ranged from 1.31 to 2.98 for N₂O emission and 1.69 to 3.83 for N₂ emission, indicating a generally positive feedback of N₂O and N₂ production to warming. Higher Q₁₀ values for N₂ than N₂O implies that N₂ emission is more sensitive to temperature increase. The N₂O/(N₂O+N₂) decreased with increasing temperature in fifteen of nineteen forest soils, suggesting that warming accelerates N₂ emission. Strong spatial variation in Q₁₀ were also observed, with tropical forest soils exhibiting high Q₁₀ values and relatively low Q₁₀ in temperate forest soils. This variation is attributed to the inherent differences in N biogeochemical cycling behavior between the microbial communities among sites. Despite soil temperature primarily controls the N₂O and N₂ emissions, we  explored the effects of other factors such as pH, C/N, DOC and related functional genes. In addition, we partitioned N₂O and N₂ emissions to different microbial processes (e.g., denitrification, co-denitrification and anammox). The results indicated that denitrification was the main pathway of N₂O and N₂ production under anaerobic environment and the contribution increased as temperature rise.

Key words: Temperature sensitivity, N₂O, N₂, Forest soil, Nitrogen cycle, Global warming, Denitrification