Nitrous oxide (N₂O) is a potent greenhouse gas, with a global warming potential 298 times greater than carbon dioxide. Emissions of N₂O from forest soils are particularly concerning, especially in ecosystems under elevated anthropogenic nitrogen (N) deposition. While it's established that higher N availability typically boosts these emissions, the precise mechanisms across different timescales remain elusive. Our research aimed to address this gap by examining the temporal dynamics of N₂O emissions and the underlying microbial and environmental factors in a subalpine forest under controlled nitrogen addition.

Figure 1. A typical landscape of alpine and subalpine ecosystems (Courtesy of Ruiyig Chang).

The Motivation Behind Our Study

Forests play a crucial role in regulating greenhouse gas emissions, with soil N₂O emissions being a key part of this regulation. Much of the existing research has focused on sub/tropical and boreal forests, leaving high-elevation subalpine ecosystems, such as the one we studied, often overlooked.

Our previous observations showed that high N₂O emissions only occurred for a short time following nitrogen addition (N₂O pulse emissions following N addition), with emissions spiking under higher N addition than lower N addition. This is expected because of elevated soil N availability. When the pulse stopped, the N₂O emissions were greater in lower N addition than higher N addition, which is very shocking us. What droves these patterns? Were the mechanisms governing N₂O emissions different at different timescales? These questions formed the core of our inquiry.

To explore this, we undertook high-frequency N₂O gas sampling alongside measurements of nitrogen cycle-related factors. We were intrigued to find that traditional soil environmental variables had little relation to annual N₂O emissions. Instead, microbial functional gene ratios—particularly those related to denitrification—showed a strong linear relationship with annual N₂O emissions. This insight pointed to microbial functional genes as key regulators of long-term emissions.

Once we completed this phase of the study, Professor Chang Ruiying suggested broadening our investigation through a global meta-analysis. By comparing data from natural terrestrial ecosystems, we confirmed that while nitrate (NO₃⁻) was a major controlling factor, the combination of NO₃⁻ levels and microbial functional genes were the most important predictors of nitrous oxide emissions globally.

The Journey Into the Field

Our fieldwork in the subalpine forest was both challenging and rewarding. Located in a remote mountainous region, accessing the site required overcoming significant logistical hurdles. We installed automated N₂O measurement stations capable of capturing emissions data at an hourly resolution, allowing us to track both rapid "pulse" emissions following nitrogen additions and more gradual changes over time. These short-term bursts were critical to understanding how N₂O emissions responded to nitrogen deposition. However, weather conditions often complicated our ability to collect data, so we need to maintain these chambers and machine to ensure the accuracy of data.

Figure 2. The automated N₂O measurement chambers in a subalpine forest (Courtesy of Ruiyig Chang).

In September 2022, a 6.8-magnitude earthquake disrupted our progress, resulting in only two years of continuous data. Power and communication lines were cut, preventing us from accessing the data for several months. 

Figure 3. Challenges of this study: Bad weather affected the performance of the automated chambers, but we quickly performed maintenance to restore functionality (Courtesy of Xinran Wu).

Key Findings: Temporal-dependent-mechanisms of N₂O emissions

Our study revealed two key mechanisms of N₂O emissions across different timescales: pulse emissions and long-lasting effect. Pulse emissions occurred immediately after nitrogen addition, driven by elevated soil nitrogen availability. These pulses were short-term but contributed significantly to total emissions.

The long-lasting effects, however, were a surprise. We found that annual N₂O emissions were not tied to N availability alone. Instead, the ratio of microbial functional genes, particularly those involved in denitrification, played a crucial role in regulating emissions over longer periods. Specifically, the ratios of functional genes responsible for N₂O production and reduction (nirK+nirS/nosZ) determined the variation of soil N₂O emissions under N addition.

Figure 4. Temporal-dependent-mechanisms of N₂O emissions under N addition.

The Role of Microbial Functional Genes

Our discovery that microbial functional genes control long-term N₂O emissions has major implications for understanding nitrogen cycling in forest soils. While nitrogen availability is still a factor, our results emphasize that the composition and activity of microbial communities may be even more important in determining long-term N₂O emissions.

To validate these findings, we conducted a global meta-analysis, which confirmed that microbial functional genes are a critical predictor of N₂O emissions in terrestrial ecosystems worldwide. This highlights the need to incorporate microbial gene data into microbial derived models that forecast N₂O emissions on broader scales under global change.

The Research Group

We are the "Alpine Soils and Global Change Group" from the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, led by Professor Ruiying Chang. Our research focuses on the processes and mechanisms governing alpine soil carbon and nitrogen dynamics under global change. We conduct multi-level nitrogen deposition and soil warming experiments in high-elevation ecosystems, typically ranging from 3000 to 5000 meters above sea level. Our solid-liquid-gas integrated observation platforms enable us to monitor these ecosystems closely, advancing our understanding of how the functional of alpine soils respond to global change.