Responses of Nitrous Oxide Emissions and Bacterial Communities to Experimental Freeze-Thaw Cycles in Contrasting Soil Types.

Nitrous oxide (N2O) pulse emissions are detected in soils subjected to freeze-thaw cycles in both laboratory and field experiments. However, the mechanisms underlying this phenomenon are poorly understood. In this study, a laboratory incubation experiment that included freeze-thaw cycles (FTC), freezing (F) and control (CK) treatments was performed on three typical Chinese upland soils, namely, fluvo-aquic soil (FS), black soil (BS) and loess soil (LS). A higher similarity in soil properties and bacterial community structure was discovered between FS and LS than between FS and BS or LS and BS, and the bacterial diversity of FS and LS was higher than that of BS. FTC significantly increased the denitrification potential and the proportion of N2O in the denitrification gas products in FS and LS but decreased the denitrification potential in BS. Accordingly, with the increasing number of freeze-thaw cycles, the bacterial community composition in the FTC treatments in FS and LS diverged from that in CK but changed little in BS. Taxa that responded to FTC or correlated with denitrification potential were identified. Taken together, our results demonstrated that the effects of FTC on N2O emissions are soil-type-dependent and that the shift in the microbial community structure may contribute to the elevated N2O emissions.

Nitrate-Nitrite-Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants.

Oxygen (O2) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O2 deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO3-), nitrite (NO2-), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N2O) when supplied with NO3- and NO2-, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N2O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants' survival during O2-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N2O budget.

Plants are a natural source of nitrous oxide even in field conditions as explained by 15N site preference.

Plants are either recognized to produce nitrous oxide (N2O) or considered as a medium to transport soil-produced N2O. To date, it is not clear whether in their habitat plants conduit N2O produced in soil or are a natural source. We aimed to understand role of plants in N2O emissions in field conditions. Therefore, rubber plants (Ficus elastica) were planted in the field; then plant and soil chambers were deployed simultaneously to collect gas samples, and 15N site preference (SP) of N2O was evaluated. The mean SP values of plant and soil emitted N2O were -20.85 ± 2.8‰ and -8.85 ± 1.08‰, respectively, and were significantly different (p < 0.0001); while bulk 15N of plant and soil emitted N2O were -10.83 ± 3.33‰ and -22.56 ± 3.37‰, respectively and were similar (p = 0.06). In the current study, soil always acted as a source of N2O, while plants were both source and sink. Plant and soil N2O fluxes had significant positive exponential relationship with both soil and air temperature. Soil water-filled pore space (WFPS) had significant negative linear relationship with only soil N2O fluxes. Plant N2O fluxes had significant positive linear relationship with plant respiration rates and negative linear relationship with plant surface areas. Based on the relationship between plant respiration rates and N2O fluxes, we suggest that mitochondria are the possible sites of N2O formation in plant cells while the relationship between plant surface areas and N2O fluxes suggests that roots are the parts of its formation in natural and field conditions. Our results suggest that plants are a natural source of N2O even at field conditions and challenge a view that plants are a medium to transport soil-produced N2O into the atmosphere.

Long-term warming and nitrogen fertilization affect C-, N- and P-acquiring hydrolase and oxidase activities in winter wheat monocropping soil.

The enzymatic activities and ratios are critical indicators for organic matter decomposition and provide potentially positive feedback to carbon (C) loss under global warming. For agricultural soils under climate change, the effect of long-term warming on the activities of oxidases and hydrolases targeting C, nitrogen (N) and phosphorus (P) and their ratios is unclear, as well as whether and to what extend the response is modulated by long-term fertilization. A 9-year field experiment in the North China Plain, including an untreated control, warming, N fertilization, and combined (WN) treatment plots, compared the factorial effect of warming and fertilization. Long-term warming interacted with fertilization to stimulate the highest activities of C, N, and P hydrolases. Activities of C and P hydrolase increased from 8 to 69% by N fertilization, 9 to 53% by warming, and 28 to 130% by WN treatment compared to control, whereas the activities of oxidase increased from 4 to 16% in the WN soils. Both the warming and the WN treatments significantly increased the enzymatic C:N ratio from 0.06 to 0.16 and the vector length from 0.04 to 0.12 compared to the control soil, indicating higher energy and resource limitation for the soil microorganisms. Compared to WN, the warming induced similar ratio of oxidase to C hydrolase, showing a comparable ability of different microbial communities to utilize lignin substrates. The relationship analyses showed mineralization of organic N to mediate the decomposition of lignin and enzyme ratio in the long-term warming soil, while N and P hydrolases cooperatively benefited to induce more oxidase productions in the soil subject to both warming and N fertilization. We conclude that coupled resource limitations induced microbial acclimation to long-term warming in the agricultural soils experiencing high N fertilizer inputs.

Nitrogen isotopic signatures and fluxes of N2O in response to land-use change on naturally occurring saline-alkaline soil.

The conversion of natural grassland to semi-natural or artificial ecosystems is a large-scale land-use change (LUC) commonly occurring to saline-alkaline land. Conversion of natural to artificial ecosystems, with addition of anthropogenic nitrogen (N) fertilizer, influences N availability in the soil that may result in higher N2O emission along with depletion of 15N, while converting from natural to semi-natural the influence may be small. So, this study assesses the impact of LUC on N2O emission and 15N in N2O emitted from naturally occurring saline-alkaline soil when changing from natural grassland (Phragmites australis) to semi-natural [Tamarix chinensis (Tamarix)] and to cropland (Gossypium spp.). The grassland and Tamarix ecosystems were not subject to any management practice, while the cropland received fertilizer and irrigation. Overall, median N2O flux was significantly different among the ecosystems with the highest from the cropland (25.3 N2O-N µg m-2 h-1), intermediate (8.2 N2O-N µg m-2 h-1) from the Tamarix and the lowest (4.0 N2O-N µg m-2 h-1) from the grassland ecosystem. The 15N isotopic signatures in N2O emitted from the soil were also significantly affected by the LUC with more depleted from cropland (- 25.3 ‰) and less depleted from grassland (- 0.18 ‰). Our results suggested that the conversion of native saline-alkaline grassland with low N to Tamarix or cropland is likely to result in increased soil N2O emission and also contributes significantly to the depletion of the 15N in atmospheric N2O, and the contribution of anthropogenic N addition was found more significant than any other processes.

Potential Pathway of Nitrous Oxide Formation in Plants.

Plants can produce and emit nitrous oxide (N2O), a potent greenhouse gas, into the atmosphere, and several field-based studies have concluded that this gas is emitted at substantial amounts. However, the exact mechanisms of N2O production in plant cells are unknown. Several studies have hypothesised that plants might act as a medium to transport N2O produced by soil-inhabiting microorganisms. Contrarily, aseptically grown plants and axenic algal cells supplied with nitrate (NO3) are reported to emit N2O, indicating that it is produced inside plant cells by some unknown physiological phenomena. In this study, the possible sites, mechanisms, and enzymes involved in N2O production in plant cells are discussed. Based on the experimental evidence from various studies, we determined that N2O can be produced from nitric oxide (NO) in the mitochondria of plants. NO, a signaling molecule, is produced through oxidative and reductive pathways in eukaryotic cells. During hypoxia and anoxia, NO3 in the cytosol is metabolised to produce nitrite (NO2), which is reduced to form NO via the reductive pathway in the mitochondria. Under low oxygen condition, NO formed in the mitochondria is further reduced to N2O by the reduced form of cytochrome c oxidase (CcO). This pathway is active only when cells experience hypoxia or anoxia, and it may be involved in N2O formation in plants and soil-dwelling animals, as reported previously by several studies. NO can be toxic at a high concentration. Therefore, the reduction of NO to N2O in the mitochondria might protect the integrity of the mitochondria, and thus, protect the cell from the toxicity of NO accumulation under hypoxia and anoxia. As NO3 is a major source of nitrogen for plants and all plants may experience hypoxic and anoxic conditions owing to soil environmental factors, a significant global biogenic source of N2O may be its formation in plants via the proposed pathway.

Distribution Pattern of Gymnosperms' Richness in Nepal: Effect of Environmental Constrains along Elevational Gradients.

Understanding the pattern of species distribution and the underlying mechanism is essential for conservation planning. Several climatic variables determine the species diversity, and the dependency of species on climate motivates ecologists and bio-geographers to explain the richness patterns along with elevation and environmental correlates. We used interpolated elevational distribution data to examine the relative importance of climatic variables in determining the species richness pattern of 26 species of gymnosperms in the longest elevation gradients in the world. Thirteen environmental variables were divided into three predictors set representing each hypothesis model (energy-water, physical-tolerance, and climatic-seasonality); to explain the species richness pattern of gymnosperms along the elevational gradient. We performed generalized linear models and variation partitioning to evaluate the relevant role of environmental variables on species richness patterns. Our findings showed that the gymnosperms' richness formed a hump-shaped distribution pattern. The individual effect of energy-water predictor set was identified as the primary determinant of species richness. While, the joint effects of energy-water and physical-tolerance predictors have explained highest variations in gymnosperm distribution. The multiple environmental indicators are essential drivers of species distribution and have direct implications in understanding the effect of climate change on the species richness pattern.

Nitrous Oxide Emissions from Paddies: Understanding the Role of Rice Plants.

: Paddies are a potential source of anthropogenic nitrous oxide (N2O) emission. In paddies, both the soil and the rice plants emit N2O into the atmosphere. The rice plant in the paddy is considered to act as a channel between the soil and the atmosphere for N2O emission. However, recent studies suggest that plants can also produce N2O, while the mechanism of N2O formation in plants is unknown. Consequently, the rice plant is only regarded as a channel for N2O produced by soil microorganisms. The emission of N2O by aseptically grown plants and the distinct dual isotopocule fingerprint of plant-emitted N2O, as reported by various studies, support the production of N2O in plants. Herein, we propose a potential pathway of N2O formation in the rice plant. In rice plants, N2O might be formed in the mitochondria via the nitrate-nitrite-nitric oxide (NO3-NO2-NO) pathway when the cells experience hypoxic or anoxic stress. The pathway is catalyzed by various enzymes, which have been described. So, N2O emitted from paddies might have two origins, namely soil microorganisms and rice plants. So, regarding rice plants only as a medium to transport the microorganism-produced N2O might be misleading in understanding the role of rice plants in the paddy. As rice cultivation is a major agricultural activity worldwide, not understanding the pathway of N2O formation in rice plants would create more uncertainties in the N2O budget.