Response of N2O and N2 Emissions in Forest Soils to Temperature Change across China
Authors: Yu Haoming , Fang Yunting , Kang Ronghua 
Published: March 2020

&lt;p&gt;N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; 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 (Q&lt;sub&gt;10&lt;/sub&gt;) of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions is rather limited. We quantified the Q&lt;sub&gt;10&lt;/sub&gt; of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions in forest soils and explored their major driving factors by conducting an incubation experiment using &lt;sup&gt;15&lt;/sup&gt;N tracer (Na&lt;sup&gt;15&lt;/sup&gt;NO&lt;sub&gt;3&lt;/sub&gt;) 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&lt;sup&gt;o&lt;/sup&gt;C 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&lt;sup&gt;o&lt;/sup&gt;C with an interval of 5&lt;sup&gt;o&lt;/sup&gt;C for 12 or 24 hours, respectively. Soil temperature strongly affected the production of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt;.&amp;#160;N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; production rates showed a positive exponential relation with incubate time and temperature for all forest soils. Our results showed that the Q&lt;sub&gt;10&lt;/sub&gt; values ranged from 1.31 to 2.98 for N&lt;sub&gt;2&lt;/sub&gt;O emission and 1.69 to 3.83 for N&lt;sub&gt;2&lt;/sub&gt; emission, indicating a generally positive feedback of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; production to warming. Higher Q&lt;sub&gt;10&lt;/sub&gt; values for N&lt;sub&gt;2&lt;/sub&gt; than N&lt;sub&gt;2&lt;/sub&gt;O implies that N&lt;sub&gt;2&lt;/sub&gt; emission is more sensitive to temperature increase. The N&lt;sub&gt;2&lt;/sub&gt;O/(N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt;) decreased with increasing temperature in fifteen of nineteen forest soils, suggesting that warming accelerates N&lt;sub&gt;2&lt;/sub&gt; emission. Strong spatial variation in Q&lt;sub&gt;10&lt;/sub&gt; were also observed, with tropical forest soils exhibiting high Q&lt;sub&gt;10&lt;/sub&gt; values and relatively low Q&lt;sub&gt;10&lt;/sub&gt; 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 N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions,&amp;#160;we &amp;#160;explored the effects of other factors such as pH, C/N, DOC and related functional genes. In addition, we partitioned N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions to different microbial processes (e.g., denitrification, co-denitrification and anammox).&amp;#160;The results indicated that denitrification was the main pathway of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; production under anaerobic environment and the contribution increased as temperature rise.&lt;/p&gt;&lt;p&gt;Key words: Temperature sensitivity, N&lt;sub&gt;2&lt;/sub&gt;O, N&lt;sub&gt;2&lt;/sub&gt;, Forest soil, Nitrogen cycle, Global warming, Denitrification&lt;/p&gt;