Effects of Nitrogen Input on Community Structure of the Denitrifying Bacteria with Nitrous Oxide Reductase Gene (nosZ I): A Long-Term Pond Experiment

Excessive nitrogen (N) input is an important factor influencing aquatic ecosystems and has received increasing public attention in the past decades. It remains unclear, however, how N input affects the denitrifying bacterial communities that play a key role in regulating N cycles in various ecosystems. To test our hypothesis – that the abundance and biodiversity of denitrifying bacterial communities decrease with increasing N – we compared the abundance and composition of denitrifying bacteria having nitrous oxide reductase gene (nosZ I) from sediments (0-20 cm) in five experimental ponds with different nitrogen fertilization treatment (TN10, TN20, TN30, TN40, TN50) using quantitative PCR and pyrosequencing techniques. We found that: 1) N addition significantly decreased nosZ I gene abundance, 2) the Invsimpson and Shannon indices (reflecting biodiversity) first increased significantly along with the increasing N loading in TN10~TN40 followed by a decrease in TN50, 3) the beta diversity of the nosZ I denitrifier was clustered into three groups along the TN concentration levels: Cluster I (TN50), Cluster II (TN40), and Cluster III (TN10-TN30), 4) the proportions of Alphaproteobacteria and Betaproteobacteria in the high-N treatment (TN50) were significantly lower than in the lower N treatments (TN10-TN30). 5). The TN concentration was the most important factor driving the alteration of denitrifying bacteria assemblages. Our findings shed new light on the response of denitrification-related bacteria to long-term N loading at pond scale and on the response of denitrifying microorganisms to N pollution.

Nitrogen use efficiency trends for sustainable crop productivity in Lake Victoria basin: smallholder farmers’ perspectives on nitrogen cycling

Nitrogen Use Efficiency (NUE) is one of the established metrics for benchmarking management of Nitrogen (N) in various systems. Numerous approaches to calculate NUE exist, making it difficult to compare the performances of systems depending on the methodology used. This study adopted the conceptualized framework by European Union Nitrogen Expert Panel (EUNEP) to calculate NUE values for cereal crops to determine future trends for the first time in the Lake Victoria region. Data were collected through in-person interviews among maize and rice smallholder farmers within the Lake Victoria region. A total of 295 observations were recorded. Collected data on yield and N fertilizer were used to make projections on the changes of NUE based on scientific and policy recommendations for Sub-Saharan Africa for 2020 (base year), 2025, 2030, and 2050. Significant differences in maize grain yield for both fertilized and unfertilized farms were observed with very low yields of 2.4 t ha-1 (fertilized) and 1.4 tha-1 (unfertilized). The graphical representation of NUE of both maize and rice showed that most farmers were in the zone of soil N mining. Projected results showed that most maize farmers within Lake Victoria region will continue to experience NUE values >90%, low N inputs <50 kg N ha-1) and less than 5 t ha-1 maize crop yield over the years. For rice farmers, Nyando and Nzoia catchments had surpassed the set target of both yield (6 t ha-1) and N input (50 kg N ha-1). However, NUE values remain higher than the optimal ranges of 50-90% (127.14% -267.57%), indicating risks of depleting soil N status. The unbalanced N fertilization also showed a trend below the linear neutrality option and the average N output for good N management for both crops. Therefore, farmers need to explore various crop management options that could increase N use efficiencies. This should be coupled with policies that promote farmers to access more N input and advocate for optimal management of N and improved quality of the cereals

Growth and Yield Responses of Pot-Grown Long Bean and Luffa to Nitrogen Rates

Optimizing nitrogen (N) input rates for vegetable production is crucial in Florida to reducing production costs and enhancing environmental sustainability. Asian vegetables emerging and expanding in Florida not only increase profit for growers, but also enhance food diversity for consumers. The objective of this study was to gain a better understanding of the partitioning and usage of N and carbohydrates in two Asian vegetable crops: long bean (Vigna unguiculata ssp. sesquipedalis (L.) Verdc.) and angled luffa (Luffa acutangular (L.) Roxb.). Four N rates (0, 0.91, 1.36, and 1.81 g N pot−1) were compared in a high tunnel trial to understand the influence of N fertilization on the two crops. For long bean, plant biomass was highest at the highest N input, and N-fertilized plants had significantly higher leaf greenness than the control at the flower initiation and mid-reproductive stages. However, N inputs had no apparent effect on yield, nitrogen use efficiency (NUE), blade total N concentration, roots (length, volume, dry biomass, and root-to-shoot ratio), or nodules (number plant−1 and biomass). For luffa, the highest N input had significantly greater total yield, fruit number, and leaf greenness at the flower initiation and mid-reproductive stages, although there was no significant difference in shoot biomass, blade total N content, or NUE among treatments. Within the range of these N rates, our results suggest that higher N inputs promoted vegetative growth of long bean, whereas reproductive growth was promoted in luffa. This study highlights differences in the sink–source relationship of N for long bean and luffa production in high tunnel, which can guide N input decisions for these two crops that are rapidly expanding in the USA.

Modelling Impacts of Lateral N Flows And Seasonal Warming on an Arctic Footslope Ecosystem N Budget and N2O Emissions Based on Species-Level Responses

Future Arctic tundra primary productivity and vegetation community composition will partly be determined by nitrogen (N) availability in a warmer climate. N mineralization rates are predicted to increase in winter and summer, but because N demand and –mobility varies across seasons, the fate of mineralized N remains uncertain. N mineralized in winter is released in a “pulse” upon snowmelt and soil thaw, with the potential for lateral redistribution in the landscape. In summer, the release is into an active rhizosphere with high local biological N demand. In this study, we investigated the ecosystem sensitivity to increased lateral N input and near-surface warming, respectively and in combination, with a numerical ecosystem model (CoupModel) parameterized to simulate ecosystem biogeochemistry for a tundra heath ecosystem in West Greenland. Both model and measurements indicated that plants were poor utilizers of increased early-season lateral N input, indicating that higher winter N mineralization rates may have limited influence on plant growth and carbon (C) sequestration for a hillslope ecosystem. The model further suggested that, although deciduous shrubs were the plant type with overall most lateral N gain, evergreen shrubs had a comparative advantage utilizing early-season N. In contrast, near-surface summer warming increased plant biomass and N uptake, moving N from soil to plant N pools, and offered an advantage to deciduous plants. Neither simulated high lateral N fluxes nor near-surface soil warming suggests that mesic tundra heaths will be important sources of N2O under warmer conditions. Our work highlights how winter and summer warming may play different roles in tundra ecosystem N and C budgets depending on plant community composition.

Nitrogen Balance in a Sweet Sorghum Crop in a Mediterranean Environment

Sweet sorghum is a C4 plant with great biomass potential yield in semi-arid environments. Under growing conditions affected by water shortage and nutrient deficiency, the optimal combination of irrigation and nitrogen (N) fertilization rate is a central issue for sustainable farming systems. In this paper, a N balance study was applied to sweet sorghum cv. Keller, managed under three irrigation levels (I0, I50, I100: 0, 50, and 100% crop evapotranspiration—ETc restoration) and four N-fertilization rates (N0, N60, N120, N180: 0, 60, 120, and 180 kg ha−1). The 15N-labelled fertilization technique was used to assess the fate of N fertilizer within the agroecosystem. Dry biomass yield was significantly affected by the irrigation, while N rates had no effect. Across N and irrigation levels, the isotopic composition showed that approximately 34% of N applied by fertilization was used by the crop, 56% remained in the soil at the end of the cropping season, 1.83% was leached as nitrate, and 1.72% was volatilized as ammonia. N-fertilizer uptake was the lowest in I0, while in N0, the soil was strongly N-impoverished since sorghum showed a great aptitude to benefit from the soil N reserve. An even N input/output system (i.e., N-output corresponded to N-input) was observed in the N120 treatment, and the soil N reserve remained unchanged, while the system was N-enriched (positive input/output) in N180. However, although beneficial for crop nutrition and soil N reserve for subsequent crops in rotation, the N180 treatment is unsustainable due to many environmental side effects in the agroecosystem.

Nitrogen Transport in a Tundra Landscape: The Effects of Early and Late Growing Season Lateral N Inputs on Arctic Soil and Plant N Pools and N2O Fluxes

Understanding N budgets of tundra ecosystems is crucial for projecting future changes in plant community composition, greenhouse gas balances and soil N stocks. Winter warming can lead to higher tundra winter nitrogen (N) mineralization rates, while summer warming may increase both growing season N mineralization and plant N demand. The undulating tundra landscape is inter-connected through water and solute movement on top of and within near-surface soil, but the importance of lateral N fluxes for tundra N budgets is not well known. We studied the quantity and fate of lateral N fluxes in the snowmelt period with a shallow thaw layer, and the late growing season with a deeper thaw layer. We used 15N to trace inorganic lateral N movement in a Low-arctic mesic tundra heath slope in West Greenland and to quantify the fate of N in the receiving area. We found that half of the early-season lateral N input was retained by the receiving ecosystem, whereas half was transported downslope. Plants appear as poor utilizers of early-season N, indicating that higher winter N mineralization may influence plant growth and carbon (C) sequestration less than expected. Still, evergreen plants were better at utilizing early-season N, highlighting how changes in N availability may impact plant community composition. In contrast, later growing season lateral N input was deeper and offered an advantage to deeper-rooted deciduous plants. The measurements suggest that N input driven by future warming at the study site will have no significant impact on the overall N2O emissions. Our work underlines how tundra ecosystem N allocation, C budgets and plant community composition vary in their response to lateral N inputs, which may help us understand future responses in a warmer Arctic.