Increased Nitrogen Loading Boosts Summer Phytoplankton Growth by Alterations in Resource and Zooplankton Control: A Mesocosm Study

The effectiveness of controlling nitrogen (N) to manage eutrophication of aquatic ecosystems remains debated. To understand the mechanisms behind phytoplankton growth in shallow lakes (resource and grazing effects) under contrasting N loading scenarios, we conducted a 70-days mesocosm experiment in summer. The mesocosms contain natural plankton communities deriving from a 10-cm layer of lake sediment and 450 L of lake water. We also added two juvenile crucian carp (Carassius carassius) in each mesocosm to simulate presence of the prevailing omni-benthivorous fish in subtropical lakes. Our results showed that N addition increased not only water N levels but also total phosphorus (TP) concentrations, which together elevated the phytoplankton biomass and caused strong dominance of cyanobacteria. Addition of N significantly lowered the herbivorous zooplankton to phytoplankton biomass ratio and promoted the phytoplankton yield per nutrient (Chl-a: TP or Chl-a: TN ratio), indicating that summer N addition likely also favored phytoplankton growth through reduced grazing by zooplankton. Accordingly, our study indicates that summer N loading may boost eutrophication via both changes in resource and grazing control in shallow lakes. Thus, alleviation of eutrophication in shallow eutrophic lakes requires a strategic approach to control both nutrients (N and P) appropriately.

Evaluation of nitrogen loading in the last 80 years in an urbanized Asian coastal catchment through the reconstruction of severe contamination period

Most semi-enclosed seas have experienced severe eutrophication owing to high nutrient loading from rivers during rapid population growth periods. In Japan, the coastal areas of some megacities (e.g., Tokyo and Osaka) experienced considerable economic growth during the 1960s–1970s. Therefore, determining the amount of nutrient loading during this period is essential to undertake measures for the conservation of coastal environments. However, determining the nutrient loading that occurred several decades ago is generally difficult owing to lacking water quality records. In this study, the nitrogen loading in the Yamato River catchment, an urbanized coastal catchment in Asia, for 80 years from the 1940s to the 2010s is reconstructed using the Soil and Water Assessment Tool. We considered factors such as population growth, wastewater treatment plant (WWTP) construction, and changes in land and fertilizer usage in different urbanization stages. Results show that the total nitrogen loading in the catchment peaked in the 1970s at 6616 tons/year owing to untreated wastewater discharge and rapid increase in population growth. By reducing 57% of the nitrogen loading in the 2010s from the catchment, WWTPs have been instrumental in improving the water environment. The decrease in and integration of agricultural land has reduced nitrogen loading attributed to nonpoint sources; however, this reduction was not obvious because of the high fertilizer usage before the 2000s. Overall, the findings of this study provide a comprehensive understanding of the impact of rapid urbanization in an Asian coastal catchment on nitrogen loading during the high economic growth period in the past. This study will be useful for the long-term assessment of nutrient loading in other.

Slow-Nitrogen Releasing Urea-Coated Zinc and Magnesium Doped Hydroxyapatite Nanohybrids for the Wheat Crop Biofortification

Background: Chemical fertilizer is an indispensable component for optimal crop production in agriculture. However, excess urea fertilizer application to the agricultural fields leaves severe environmental deterioration. Researchers are actively seeking safer alternatives or solutions for the implementation of sustainable agriculture practices without compromising the agricultural output. Nano-scale particles, due to their unique properties, are emerging as interesting candidates for agrochemicals, especially nutrient delivery applications. In the present study, three variants hydroxyapatite-urea, magnesium-doped hydroxyapatite-urea, and zinc-doped hydroxyapatite-urea nanohybrids have been synthesized and characterized as slow nitrogen release fertilizers for the wheat crop.Results: The doping of hydroxyapatite with zinc and magnesium instigated structural distortion that assisted relatively higher nitrogen loading and optimal urea release patterns. The nitrogen molecules slowly release from the water incubated nanohybrids, as per the Hixson-Crowell model equation, for up to two weeks in the soil environment. With zinc and magnesium integrated into hydroxyapatite, the synthesized nanohybrids now serve as a multi-nutrient complex of nitrogen, calcium, phosphorus, magnesium, and zinc nutrients. Additionally, iron uptake was increased in nanohybrids treated wheat crop. The results manifest the potency of the 50% nitrogen doses as nanohybrids that maintain the wheat crop yield and nutrient uptake equivalent to the 100% nitrogen doses as urea fertilizer. Higher nitrogen doses as nanohybrids significantly enhanced the wheat growth parameters. The zinc-doped hydroxyapatite-urea nanohybrids performed better among all three variants. Conclusion: The research epitomized the delivery of multiple nutrients to the crops while mitigating ammonia emissions from agricultural fields. The doped and undoped hydroxyapatite-urea nanohybrids can be a revolutionary tool to alleviate the pollution and waste generation arising from agriculture. We present a comprehensive experimental evidence of the design and utilization of biocompatible nitrogen nanohybrids as fertilizer for agricultural production and biofortification while cutting off nitrogen input up to half to mitigate environmental repercussions. This study establishes extensive experimental evidence for manipulating nano-scale materials for nutrient delivery applications to crops and unlocks new paradigms for the design and application of climate-friendly smart fertilizers for sustainable agriculture.

Comammox Nitrospira bacteria outnumber canonical nitrifiers irrespective of electron donor mode and availability

Complete ammonia oxidizing bacteria coexist with canonical ammonia and nitrite oxidizing bacteria in a wide range of environments. Whether this coexistence is due to competitive or cooperative interactions between the three guilds, or a result of niche separation is not yet clear. Understanding the factors driving coexistence of nitrifying guilds is critical to effectively manage nitrification processes occurring in engineered and natural ecosystems. In this study, microcosms-based experiments were used to investigate the impact of electron donor mode (i.e., ammonia and urea) and loading on the population dynamics of nitrifying guilds in drinking water biofilter media. Shotgun sequencing of DNA from select time points followed by co-assembly and re-construction of metagenome assembled genomes (MAGs) revealed multiple clade A2 and one clade A1 comammox bacterial populations coexisted in the microcosms alongside Nitrosomonas-like ammonia oxidizers and Nitrospira-like nitrite oxidizer populations. Clade A2 comammox bacteria were likely the primary nitrifiers within the microcosms and increased in abundance over canonical ammonia and nitrite oxidizing bacteria irrespective of electron donor mode or nitrogen loading rates. This suggests that comammox bacteria will outnumber nitrifying communities sourced from oligotrophic environments irrespective of variable nitrogen regimes. Changes in comammox bacterial abundance were not correlated with either ammonia or nitrite oxidizing bacterial abundance in urea amended systems where metabolic reconstruction indicated potential cross feeding between ammonia and nitrite oxidizing bacteria. In contrast, comammox bacterial abundance demonstrated a negative correlation with that of nitrite oxidizers in ammonia amended systems. This suggests that potentially weaker synergistic relationships between ammonia and nitrite oxidizers might enable comammox bacteria to displace nitrite oxidizers from complex nitrifying communities.

Nitrogen Balance of Dairy Cows Divergent for Milk Urea Nitrogen Consuming Either Plantain or Perennial Ryegrass

Inefficient nitrogen (N) use from pastoral dairy production systems has resulted in environmental degradation, as a result of excessive concentrations of urinary N excretion leaching into waterways and N2O emissions from urination events into the atmosphere. The objectives of this study were to measure and evaluate the total N balance of lactating dairy cows selected for milk urea N concentration breeding values (MUNBVs) consuming either a 100% perennial ryegrass (Lolium perenne L.) or 100% plantain (Plantago lanceolata L.) diet. Sixteen multiparous lactating Holstein-Friesian × Jersey cows divergent for MUNBV were housed in metabolism crates for 72 h, where intake and excretions were collected and measured. No effect of MUNBV was detected for total N excretion; however, different excretion characteristics were detected, per urination event. Low MUNBV cows had a 28% reduction in the concentration of urinary urea nitrogen (g/event) compared to high MUNBV cows when consuming a ryegrass diet. Cows consuming plantain regardless of their MUNBV value had a 62% and 48% reduction in urinary urea nitrogen (g/event) compared to high and low MUNBV cows consuming ryegrass, respectively. Cows consuming plantain also partitioned more N into faeces. These results suggest that breeding for low MUNBV cows on ryegrass diets and the use of a plantain diet will reduce urinary urea nitrogen loading rates and therefore estimated nitrate leaching values, thus reducing the environmental impact of pastoral dairy production systems.