Effects of Solidago canadensis L. on mineralization-immobilization turnover enhance its nitrogen competitiveness and invasiveness.

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 15N-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 (NO3--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 (NH4+-N) uptake rates (1.76 and 1.56 mg N kg-1 d-1, respectively), indicating that S. canadensis was a NO3--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 NO3--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.

Biochar application can mitigate NH3 volatilization in acidic forest and upland soils but stimulates gaseous N losses in flooded acidic paddy soil.

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 15N 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 NH4+ (MNlab) (3 %-128 %) which was associated with an increase in total nitrogen. BC mitigated NH3 volatilization (VNH3) (52 %-99 %) in upland and forest soils due to NH4+/NH3 adsorption, while it caused higher gaseous N losses (NH3 and N2O) in flooded paddy soils. An important function was the effect of BC addition on NH4+ oxidation (ONH4). While ONH4 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 (PRL), 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.

Dynamics of nitrite in acidic soil during extraction with potassium chloride studied using 15 N tracing.

RATIONALE: Nitrite is well known to be unstable, including during soil extraction with KCl (especially in acidic soils), but the source and fate of NO2 - in the short duration of the extraction process remain unclear. METHODS: A series of 15 N-tracing studies explored NO2 - transformations during KCl extraction in acidic and alkaline soils. Tests considering multiple factors assessed the interactions of such factors as soil sterilization, extraction time, and pH adjustment. After addition of 15 NO2 - , 15 NH4 + , and 15 NO3 - tracers, the concentrations and isotopic compositions of N2 O, N2 , NH4 + , NO3 - , NO2 - , and dissolved organic nitrogen (DON) were measured to investigate the production and consumption of NO2 - . RESULTS: Nitrite was stable in alkaline soils during KCl extraction. In contrast, changes did occur in acidic soils during KCl extraction: NO2 - declined rapidly in the first 10 min of extraction although the subsequent rate of decrease lessened as the extraction time progressed. Significant dilution of 15 NO2 - suggested high rates of NO2 - production and even higher rates of consumption. The soil's organic N was the only source of NO2 - and also its main destination. Soil sterilization showed that NO2 - processes during extraction were chemical, not microbial. The pH adjustment of acidic soil stabilized its NO2 - . CONCLUSIONS: Overall, the pH adjustment of KCl solution appears favorable for investigating NO2 - dynamics. For example, this work recommends an extraction solution comprising a 4:1 mixture of 2.5 M KCl solution and pH 8.4 buffer, which was more convenient to operate than the method reported by Stevens and Laughlin.

Autotoxic Ginsenoside Disrupts Soil Fungal Microbiomes by Stimulating Potentially Pathogenic Microbes.

Autotoxic ginsenosides have been implicated as one of the major causes for replant failure of Sanqi ginseng (Panax notoginseng); however, the impact of autotoxic ginsenosides on the fungal microbiome, especially on soilborne fungal pathogens, remains poorly understood. In this study, we aimed to investigate the influence of the ginsenoside monomers Rg1, Rb1, and Rh1, and that of their mixture (Mix), on the composition and diversity of the soil fungal community, as well as on the abundance and growth of the soilborne pathogen Fusarium oxysporum in pure culture. The addition of autotoxic ginsenosides altered the composition of the total fungal microbiome, as well as the taxa within the shared and unique treatment-based components, but did not alter alpha diversity (α-diversity). In particular, autotoxic ginsenosides enriched potentially pathogenic taxa, such as Alternaria, Cylindrocarpon, Gibberella, Phoma, and Fusarium, and decreased the abundances of beneficial taxa such as Acremonium, Mucor, and Ochroconis Relative abundances of pathogenic taxa were significantly and negatively correlated with those of beneficial taxa. Among the pathogenic fungi, the genus Fusarium was most responsive to ginsenoside addition, with the abundance of Fusarium oxysporum consistently enhanced in the ginsenoside-treated soils. Validation tests confirmed that autotoxic ginsenosides promoted mycelial growth and conidial germination of the root rot pathogen F. oxysporum In addition, the autotoxic ginsenoside mixture exhibited synergistic effects on pathogen proliferation. Collectively, these results highlight that autotoxic ginsenosides are capable of disrupting the equilibrium of fungal microbiomes through the stimulation of potential soilborne pathogens, which presents a significant hurdle in remediating replant failure of Sanqi ginseng.IMPORTANCE Sanqi ginseng [Panax notoginseng (Burk.) F. H. Chen] is geoauthentically produced in a restricted area of southwest China, and successful replanting requires a rotation cycle of more than 15 to 30 years. The increasing demand for Sanqi ginseng and diminishing arable land resources drive farmers to employ consecutive monoculture systems. Replant failure has severely threatened the sustainable production of Sanqi ginseng and causes great economic losses annually. Worse still, the acreage and severity of replant failure are increased yearly, which may destroy the Sanqi ginseng industry in the near future. The significance of this work is to decipher the mechanism of how autotoxic ginsenosides promote the accumulation of soilborne pathogens and disrupt the equilibrium of soil fungal microbiomes. This result may help us to develop effective approaches to successfully conquer the replant failure of Sanqi ginseng.

Temperature effects on N2O production pathways in temperate forest soils.

Nitrous oxide (N2O) is an important greenhouse gas and contributes to stratospheric ozone depletion. Increasing temperature generally exerts a positive effect on soil N2O production. However, not much is known on the temperature influence on individual N2O production pathways. In this study, both laboratory 15N labelling experiments with an incubation temperature gradient (35 °C, 25 °C, 15 °C, 5 °C) and field 15N 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 N2O production rates via denitrification (N2Od) 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 N2O production rates via autotrophic nitrification process (N2Oa) were significantly and positively correlated with incubation temperature (P < 0.05). In addition, the contribution of heterotrophic nitrification to N2O production showed a negative and positive relation with increasing temperature in KF and RF, respectively. Whereas, the N2O production rates via heterotrophic nitrification (N2Oh) 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 N2O production. The response of calculated N2O 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 N2O production in temperate forest soils should be validated for other ecosystems.