Higher Food Yields and Lower Greenhouse Gas Emissions from Aquaculture Ponds with High-Stalk Rice Planted

Stocking filter-feeder in fed fish aquaculture pond: Unexpected Aggravation of nitrous oxide emission

Intensive farming of fed fish could produce large amounts of uneaten feed and feces, potentially leading to increased nitrous oxide (N2O) emissions. Filter-feeding fish can ingest residual feed and feces, but it is unclear whether introducing them into fed fish farming ponds could reduce N2O emissions. This study employed monoculture of fed largemouth bass (Micropterus salmoides, LB) and polyculture of LB with filter-feeding silver carp (Hypophthalmichthys molitrix, SC) at density ratios of 18:1, 9:1 and 4.5:1 to compare the N2O emission characteristics. The results showed that silver carp could indeed feed on largemouth bass feces, and isotope mixing model indicated that feces was the second largest contributor to the food of silver carp, reaching 14.75 %-15.56 %. However, polyculture of the two species did not or even increased N2O emission flux at water-air interface and its release potential in sediment. Increased mineralization, nitrification and denitrification rates were observed in polyculture systems, particularly at high stocking densities of silver carp. Also, the higher NH4+ accumulation were found across sediment-water interface within polyculture systems. Metagenome revealed that polyculture disturbed the microbial community structure and increased the abundance of Burkholderiales and Steroidobacteraceae. Moreover, polyculture increased the abundance of nitrogen-cycling functional genes, including gdhA, hao, nirB and norB, potentially contributing to the elevated N2O emissions. Structural equation model highlighted that polyculture of largemouth bass and silver carp could drive N2O emissions, mainly through increased sedimental NH4+ concentration and microbial activity. These findings indicate that the introduction of extractive filter-feeding fish into fed fish farming ponds could not reduce N2O emissions, implying the need for optimized management strategies to balance aquaculture productivity with environmental sustainability.

Aquatic plants dominate spatiotemporal dynamics of N2O fluxes in small urban lake by regulating nutrient distribution and emission path

Small urban lakes are recognized as significant sources of nitrous oxide (N2O) to the atmosphere. Despite the crucial role of aquatic plants in landscape construction and pollutant removal within urban lakes, the modulation of N2O emission dynamics and associated mechanisms by these plants remains elusive. This study investigated the N2O concentrations and fluxes from aquatic habitats covered with seven species of aquatic plants in a small urban lake, and estimated the contribution of plant-mediated N2O emissions. Meanwhile, the physicochemical parameters of water and periradicular sediments were measured synchronously to clarify the main controls of aquatic plants in regulating aquatic N2O emissions. N2O concentrations in the surface waters covered by different aquatic plants (0.041-0.659 μmol L-1) exhibit substantial variation, being 1.2-5.4 (mean of 2.8) times higher than those in open water areas (0.015-0.096 μmol L-1). The range of total N2O fluxes was 11.3-1009.0 μmol m-2 d-1, exhibiting significant spatial and temporal variations, with considerable differences observed among various plant-covered areas. Total N2O fluxes from different plant-covered areas were 1.5-16.7 times (average 7.5 times) higher than those in open water areas. It suggests that diverse aquatic plants could observably intensify the spatial variability in N2O emissions within the small urban lake. The estimated plant-mediated fluxes may contribute approximately 21%-66% of total N2O fluxes. Specifically, N2O concentrations in the stem cavities of different plants were generally higher than atmospheric levels, evidencing the mediated effect of aquatic plants on N2O emissions. While aquatic plants reduce the abundance of nutrients in surface water to varying degrees, the accumulations of carbon and nitrogen in periradicular sediments, combined with plant transport, observably enhance N2O emissions in urban lake with low pollution loads. Furthermore, the phenological processes of aquatic plants and seasonal temperature changes were found to co-affect the seasonal dynamics of aquatic N2O fluxes. Varied aquatic plants can significantly dominate spatiotemporal dynamics of the N2O emissions in urban landscape lakes.

Comprehensive assessment of three crayfish culture modes: From production performance to environmental sustainability

Integrated agriculture-aquaculture has emerged as a promising ecological development model. Crayfish, a popular aquaculture species, are traditionally reared either in monoculture ponds (mono-C) or in rice-crayfish polyculture system (poly-RC). In this study, we introduced a novel polyculture system by combining fruit tree with crayfish (poly-FC), aiming to compare these three crayfish culture modes in terms of production performance and ecological sustainability. The results indicated that crayfish reared in the two polyculture modes exhibited significantly higher specific growth rate and condition factor compared to those in mono-C. Crayfish cultured in poly-FC also showed better muscle quality and higher levels of crude fat and flavor or essential amino acids. Isotope mixing model showed that feed and benthic animals were the primary food sources of crayfish in mono-C, whereas aquatic plants, fruit litter or rice contributed more to those in polyculture modes. For greenhouse gas emissions, poly-FC mode emitted almost no CO2 and N2O even favored negative CH4 emission, while poly-RC and mono-C modes showed positive emissions of CH4 and CO2, respectively. Supported by metagenomics, the sink of CH4 in poly-FC was probably due to the lower mcr abundance but the higher pmo abundance in water. The low production and emission of N2O in poly-FC might result from the low-abundant Nitrospirae_bacterium and its coding gene norC in sediment, consistent with the lower denitrification rate but the higher NO3- concentration than mono-C. Overall, our findings reveal the superiority of polyculture of fruit tree with crayfish in terms of production performance and greenhouse gas emissions in the system.

Slowdown in China's methane emission growth

The unprecedented surge in global methane levels has raised global concerns in recent years, casting a spotlight on China as a pivotal emitter. China has taken several actions to curb the methane emissions, but their effects remain unclear. Here, we developed the Global ObservatioN-based system for monitoring Greenhouse GAses for methane (GONGGA-CH4) and assimilate GOSAT XCH4 observations to assess changes in China's methane emissions. We find the average rate of increase in China's methane emissions (0.1 ± 0.3 Tg CH4 yr-2) during 2016-2021 slowed down compared to the preceding years (2011-2015) (0.9 ± 0.5 Tg CH4 yr-2), in contrast to the concurrent acceleration of global methane emissions. As a result, the contribution of China to global methane emissions dropped significantly. Notably, the slowdown of China's methane emission is mainly attributable to a reduction in biogenic emissions from wetlands and agriculture, associated with the drying trend in South China and the transition from double-season to single-season rice cropping, while fossil fuel emissions are still increasing. Our results suggest that GONGGA-CH4 provides the opportunity for independent assessment of China's methane emissions from an atmospheric perspective, providing insights into the implementation of methane-related policies that align with its ambitious climate objectives.

Rainstorm and strong wind weathers largely increase greenhouse gases flux in shallow ponds

Shallow-water ponds represent the hotspots of greenhouse gas (GHG) emissions. Most current studies focus on the temporal dynamics for GHGs in water, with little consideration given to the effects of weather changes. In this study, we measured and compared the concentrations and fluxes of CO2, CH4, and N2O from a pond in Northeast China under different meteorological conditions. Results showed that the rates of CO2, CH4, and N2O emissions from pond into the atmosphere during strong winds were 85.85 ± 7.55 mmol m-2 d-1, 22.05 ± 6.80 mmol m-2 d-1, and 10.87 ± 0.72 μmol m-2 d-1, respectively, significantly higher than those measured during non-rain weather. Among which, over 88 % of CH4 emissions were contributed by ebullition. Meanwhile, the CO2 and N2O flux were also significantly higher during heavy rainfall, reaching 100.05 ± 19.76 mmol m-2 d-1 and 5.90 ± 1.03 μmol m-2 d-1, respectively. Strong winds and precipitation induced sediment disturbances, high gas transport coefficients, reduced photosynthesis and oxygen greatly promoted the GHGs escape evasion. Wind speed, air pressure, solar radiation, and dissolved oxygen in water were important influencing factors. Our results emphasize the importance of capturing short-term weather disturbance events, especially rainstorm and strong winds, to accurately assess the annual GHG budget from these shallow water ecosystems.