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The effects of exotic plants on soil nitrogen (N) transformations may influence species invasion success. However, the complex interplay between invasive plant N uptake and N transformation in soils remains unclear. In the present study, a series of 15 N-labeled pot experiments were carried out with Solidago canadensis L. (S. canadensis), an invasive plant, and the Ntrace tool was used to clarify the preferred inorganic N form and its effects on soil N transformation. According to the results, nitrate-N (NO 3 - -N) uptake rates by S. canadensis were 2.38 and 2.28 mg N kg -1  d -1 in acidic and alkaline soil, respectively, which were significantly higher than the ammonium-N (NH 4 + -N) uptake rates (1.76 and 1.56 mg N kg -1  d -1 , respectively), indicating that S. canadensis was a NO 3 - -N-preferring plant, irrespective of pH condition. Gross N mineralization rate was 0.41 mg N kg -1  d -1 in alkaline soil in the presence of S. canadensis L., which was significantly lower than that in the control (no plant, CK, 2.44 mg N kg -1  d -1 ). Gross autotrophic nitrification rate also decreased from 5.95 mg N kg -1  d -1 in the CK to 0.04 mg N kg -1  d -1 in the presence of S. canadensis in alkaline soil. However, microbial N immobilization rate increased significantly from 1.09 to 2.16 mg N kg -1  d -1 , and from 0.02 to 2.73 mg N kg -1  d -1 after S. canadensis planting, in acidic and alkaline soil, respectively. Heterotrophic nitrification rate was stimulated in the presence of S. canadensis to provide NO 3 - -N to support the N requirements of plants and microbes. The results suggested that S. canadensis can influence the mineralization-immobilization turnover (MIT) to optimize its N requirements while limiting N supply for other plants in the system. The results of the present study enhance our understanding of the competitiveness and mechanisms of invasion of alien plants., Competing Interests: Declaration of competing interest The authors have no relevant financial or non-financial interests to disclose., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Biochar (BC) has attracted attention for carbon sequestration, a strategy to mitigate climate change and alleviate soil acidification. Most meta-analyses have insufficiently elaborated the effects of BC on soil N transformation so the practical importance of BC could not be assessed. In this study, a 15 N tracing study was conducted to investigate the effects of BC amendment on soil gross N transformations in acidic soils with different land-use types. The results show that the BC amendment accelerated the soil gross mineralization rate of labile organic N to NH 4 + (M Nlab ) (3 %-128 %) which was associated with an increase in total nitrogen. BC mitigated NH 3 volatilization (V NH3 ) (52 %-99 %) in upland and forest soils due to NH 4 + /NH 3 adsorption, while it caused higher gaseous N losses (NH 3 and N 2 O) in flooded paddy soils. An important function was the effect of BC addition on NH 4 + oxidation (O NH4 ). While O NH4 increased (4 %-19 %) in upland soils, it was inhibited (34 %-71 %) in paddy soils and did not show a response in forest soils. Overall, the BC amendment reduced the potential risk of N loss (P RL ), especially in forest soils (82 %-98 %). This study also shows that the BC effect on soil N cycling is land-use specific. The suitability of practices including BC hinges on the effects on gaseous N losses., Competing Interests: Declaration of competing interest The authors declare that the paper is being submitted for consideration for publication in Science of the Total Environment and that the content has not been published or submitted for publication elsewhere, in whole or in part, in any language. All authors have contributed significantly, and all authors are in agreement with the content of the manuscript. The authors have no relevant financial or non-financial interests to disclose., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Nitrous oxide (N 2 O) is an important greenhouse gas and contributes to stratospheric ozone depletion. Increasing temperature generally exerts a positive effect on soil N 2 O production. However, not much is known on the temperature influence on individual N 2 O production pathways. In this study, both laboratory 15 N labelling experiments with an incubation temperature gradient (35 °C, 25 °C, 15 °C, 5 °C) and field 15 N labelling experiments carried out in different seasons were conducted in Korean pine forest (KF) and Redwood coniferous forest (RF) soils. The results showed that the contribution of denitrification was positively correlated with temperature in KF and negatively correlated with temperature in RF, while their N 2 O production rates via denitrification (N 2 O d ) all declined with decreasing temperature. The contribution of autotrophic nitrification in KF ranged from 11% to 21%, while the contribution in RF significantly increased with decreasing temperature (P < 0.05). However, the N 2 O production rates via autotrophic nitrification process (N 2 O a ) were significantly and positively correlated with incubation temperature (P < 0.05). In addition, the contribution of heterotrophic nitrification to N 2 O production showed a negative and positive relation with increasing temperature in KF and RF, respectively. Whereas, the N 2 O production rates via heterotrophic nitrification (N 2 O h ) showed a significantly positive correlation with temperature (P < 0.05), but a negative relation with gross heterotrophic nitrification rates. The results in the field experiments corresponded to the laboratory results, indicating that the methods applied in field experiments were suitable for the estimation and prediction of in situ N 2 O production. The response of calculated N 2 O production rates to seasonal temperature in KF during the year of 2015-2017 also confirmed the suitability of the field research methods. This novel in situ technique to determine N 2 O production in temperate forest soils should be validated for other ecosystems., (Copyright © 2019. Published by Elsevier B.V.)