Search

Methane (CH4) is the second most important atmospheric greenhouse gas (GHG) and forest soils are a significant sink for atmospheric CH4. Uptake of CH4 by global forest soils is affected by nitrogen (N) deposition; clarifying the effect of N deposition helps to reduce uncertainties of the global CH4 budget. However, it remains an unsolved puzzle why N input stimulates soil CH4 uptake in some forests while suppressing it in others. Combining previous findings and data from N addition experiments conducted in global forests, we proposed and tested a "stimulating‐suppressing‐weakened effect" ("three stages") hypothesis on the changing responses of soil CH4 flux (RCH4) to N input. Specifically, we calculated the response factors (f) of RCH4 to N input for N‐limited and N‐saturated forests across biomes; the phased changes in f values supported our hypothesis. We also estimated the global forest soil CH4 uptake budget to be approximately 11.2 Tg yr−1. CH4 uptake hotspots were predominantly located in temperate forests. Furthermore, we quantified that the current level of N deposition reduced global forest soil CH4 uptake by ∼3%. This suppression effect was more pronounced in temperate forests than in tropical or boreal forests, likely due to differences in N status. The proposed "three stages" hypothesis in this study generalizes the diverse effects of N input on RCH4, which could help improve experimental design. Additionally, our findings imply that by regulating N pollution and reducing N deposition, soil CH4 uptake can be significantly increased in the N‐saturated forests in tropical and temperate biomes. Plain Language Summary: Methane is an important greenhouse gas. Forest soils can absorb methane from the atmosphere and mitigate its warming effect. Meanwhile, forests suffer from high atmospheric nitrogen deposition, yet the effect of nitrogen on the methane uptake by forest soils remains unclear. Using data from global nitrogen addition experiments, we validated a "stimulating‐suppressing‐weakened effect" ("three stages") hypothesis, which could explain the diverse responses of soil methane flux to nitrogen input observed in different forests. On this basis, we quantified that nitrogen deposition decreased global forest soil methane uptake by approximately 3%. Our findings also imply that by regulating nitrogen pollution, soil methane uptake can be significantly increased in the nitrogen‐saturated forests in tropical and temperate biomes, potentially mitigate global warming. Key Points: A "three stage hypothesis" was developed to generalize the diverse responses of forest soil CH4 flux to N inputCH4 uptake by global forest soils was estimated to be 11.2 Tg yr−1, with N deposition suppressing ∼3% of this uptakeEffective regulation to reduce N deposition would promote CH4 uptake by N‐saturated forests and mitigate global warming [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is an influential greenhouse gas (GHG) and an unregulated ozone‐depleting substance. The extent to which N2O emissions from natural forest soils have been enhanced by high atmospheric nitrogen (N) deposition in China during past decades is unclear; however, assessing land‐related N2O emissions for 2060 national "carbon neutrality" goal is urgent. In this study, we proposed a "gray box" conceptual model and deduced a linear relationship between soil N2O emissions and N deposition on a large scale. On this basis, we created a "process‐augmented data‐driven" approach using which we combined experimental N addition data from 38 Chinese forests with N deposition observation data to estimate regional soil N2O emissions. We found that the N2O emission budget of eastern China forest soils averaged 0.24 ± 0.07 TgN yr−1 from 1996 to 2015; of this, ∼36% (0.087 TgN yr−1) was directly induced by N deposition. Soil N2O emission rates (RN2O) fluctuated slightly after 2006, and the RN2Os of different ecoregions were significantly different (p < 0.001). Soil factors and N‐deposition‐related factors dominated the spatial variation of RN2O. Our findings provide country‐specific and ecoregion‐specific emission factors for national GHG inventories in China. Our approach bridges the gap between site‐level experiments and demand for regional N2O emissions, and is applicable for estimating N2O emissions in other countries/regions. Meanwhile, more N addition experiments are needed to comprehensively understand N cycling processes and extend the predictive capability of the approach. Plain Language Summary: Nitrous oxide (N2O) is a greenhouse gas that is around 300 times more powerful than carbon dioxide. As many countries and regions are aiming to achieve zero net carbon emission ("carbon neutrality") by the mid‐21st century, we need to evaluate the emissions of nitrous oxide to build an accurate picture of the current situation. However, it is difficult to estimate the emission of N2O from natural soils because the spatial variation is high. Inspired by the linkage between elemental nitrogen (N) that enters an ecosystem and N that leaves the ecosystem, we proposed a conceptual model and deduced a linear relationship between N deposition and soil N2O emissions. On the basis, we used experimental N addition data from various sites to quantify the linear relationship. Then, the N2O emission and its variation was estimated with dynamic N deposition data. In eastern China forests where the N deposition rate is exceptionally high, around 0.24 million tons of N2O‐N was emitted annually from 1996 to 2015, around 36% of which was directly induced by N deposition. From another perspective, soil N2O emissions in this region are expected to decline owing to pollution control practices and decreasing N deposition. Key Points: Eastern China forest soils emitted 0.24 TgN2O‐N yr−1 from 1996 to 2015N deposition directly induced ∼36% of emitted N2O from eastern China forest soilsDirect contribution of N deposition to soil N2O emissions increased from tropical to boreal forests [ABSTRACT FROM AUTHOR]

Non‐native species have invaded most parts of the world, and the invasion process is expected to continue and accelerate. Because many invading non‐native species are likely to become permanent inhabitants, future consideration of species‐area relationships (SARs) should account for non‐native species, either separately or jointly with native species. If non‐native species occupy unused niches and space in invaded areas and extinction rate of native species remains low (especially for plants), the resultant SARs (with both native and non‐native species) will likely be stronger. We used published and newly compiled data (35 data sets worldwide) to examine how species invasions affect SARs across selected taxonomic groups and diverse ecosystems around the world. We first examined the SARs for native, non‐native, and all species. We then investigated with linear regression analyses and paired or unpaired t tests how degree of invasion (proportion of non‐native species) affected postinvasion SARs. Postinvasion SARs for all species (native plus non‐native) became significantly stronger as degree of invasion increased (r2 = 0.31, p = 0.0006), thus, reshaping SARs worldwide. Overall, native species still showed stronger and less variable SARs. Also, slopes for native species were steeper than for non‐native species (0.298 vs. 0.153). There were some differences among non‐native taxonomic groups in filling new niches (especially for birds) and between islands and mainland ecosystems. We also found evidence that invasions may increase equilibrial diversity. Study of such changing species–area curves may help determine the probability of future invasions and have practical implications for conservation. Article Impact Statement: Increasingly, altered species–area relationships due to non‐native species invasions affect future conservation efforts. [ABSTRACT FROM AUTHOR]