Overestimated nitrogen loss from denitrification for natural terrestrial ecosystems in CMIP6 Earth System Models.

Denitrification and leaching nitrogen (N) losses are poorly constrained in Earth System Models (ESMs). Here, we produce a global map of natural soil 15N abundance and quantify soil denitrification N loss for global natural ecosystems using an isotope-benchmarking method. We show an overestimation of denitrification by almost two times in the 13 ESMs of the Sixth Phase Coupled Model Intercomparison Project (CMIP6, 73 ± 31 Tg N yr-1), compared with our estimate of 38 ± 11 Tg N yr-1, which is rooted in isotope mass balance. Moreover, we find a negative correlation between the sensitivity of plant production to rising carbon dioxide (CO2) concentration and denitrification in boreal regions, revealing that overestimated denitrification in ESMs would translate to an exaggeration of N limitation on the responses of plant growth to elevated CO2. Our study highlights the need of improving the representation of the denitrification in ESMs and better assessing the effects of terrestrial ecosystems on CO2 mitigation.

The impacts of nitrogen addition on upland soil methane uptake: A global meta-analysis.

Elevated nitrogen (N) addition from anthropogenic activities has great impacts on soil methane (CH4) uptake, which could interrupt the existing global CH4 balance and cause feedbacks to climate and biogeochemical processes. Previous studies have come to inconsistent conclusions on both the quantification of the response of CH4 uptake to N addition and understanding of its underlying mechanisms, probably due to the lack of experimental data. Here, we conduct a broad meta-analysis of 90 papers to quantify the responses of CH4 uptake to N addition in upland soil. The results show that N addition has a significant negative impact on soil CH4 uptake (-19.25%), which is termed the N inhibition effect. Soil pH is identified as the dominant factor, with the other factors affecting the CH4 uptake through the alteration of soil pH. The N inhibition effect is observed to be large and significant in forest and grassland, but small and insignificant in farmland, because of the distinct composition of their methanotrophic communities. A threshold of the N addition level is identified at about 68 kg N ha-1 year-1, which indicates the lowest N inhibition effect. Furthermore, the convex relationship between response ratio of CH4 uptake (negative) and N addition duration indicates that a medium level of N addition duration has the largest N inhibition effect, and longer or shorter durations will both reduce the effect. Our analysis of the N inhibition effect implies that controlling the N addition level could effectively reduce the CH4 concentration in the atmosphere and thus relieve global warming.

Resilience analysis of the nexus across water supply, power generation and environmental systems from a stochastic perspective.

Effects of external disturbances such as the population change on dynamics of water supply, power generation and environmental (WPE) systems have seldom been investigated. Following the WPE nexus profiled in the study of Feng et al. (2016), this study incorporated stochasticity of population, water supply and power generation into the modeling of the dynamical system in the Hehuang region of China, and further quantified resilience measures to understand the system's ability to withstand stochastic disturbances. First, the stochastic differential equations were used to improve the simulation of stochasticity in the WPE nexus. Next, the transient probability distribution functions (pdfs) of system variables, obtained by Monte Carlo simulation, were used to describe the evolutionary process of the system. Finally, the stationary pdfs of variables which reflect stable states of the system were derived to calculate four resilience measures. It is shown that: (1) The system approached a stable state after Year 2400 by calculating the L2 norm of the difference between transient and stationary pdfs. (2) The environmental system was identified as the most vulnerable subsystem because of its long convergence time. (3) The water supply system did not change greatly and it would remain stable at its current low level, i.e., water consumption per capita would be less than 80m3. The method adopted in this study is conducive to avoiding risk and the results provide valuable insights for regional management of a WPE nexus.