Enhancing total nitrogen removal in constructed wetlands: A Comparative study of iron ore and biochar amendments.

Efficient nitrogen removal in constructed wetlands (CWs) remains challenging when treating agricultural runoff with a low carbon-to-nitrogen ratio (C/N). However, using biochar, iron ore, and FeCl3-modified biochar (Fe-BC) as amendments could potentially improve total nitrogen (TN) removal efficiency in CWs, but the underlying mechanisms associated with adding these substrates are unclear. In this study, five CWs: quartz sand constructed wetland (Control), biochar constructed wetland, Fe-BC constructed wetland, iron ore constructed wetland, and iron ore + biochar constructed wetland, were built to compare their treatment performance. The rhizosphere microbial community compositions and their co-occurrence networks were analyzed to reveal the underlying mechanisms driving their treatment performance. The results showed that iron ore was the most efficient amendment, although all treatments increased TN removal efficiency in the CWs. Ammonia-oxidizing, heterotrophic denitrifying, nitrate-dependent anaerobic ferrous oxidizing (NAFO), and Feammox bacteria abundance was higher in the iron ore system and led to the simultaneous removal of NH4+-N, NO3--N, and NO2--N. Visual representations of the co-occurrence networks further revealed that there was an increase in cooperative mutualism (the high proportion of positive links) and more complex interactions among genera related to the nitrogen and iron cycle (especially ammonia-oxidizing bacteria, heterotrophic denitrifying bacteria, NAFO bacteria, and Feammox bacteria) in the iron ore system, which ultimately contributed to the highest TN removal efficiency. This study provides critical insights into how different iron ore or biochar substrates could be used to treat agricultural runoff in CWs.

Effects of substrate improvement on winter nitrogen removal in riparian reed (Phragmites australis) wetlands: rhizospheric crosstalk between plants and microbes.

With continued anthropogenic inputs of nitrogen (N) into the environment, non-point source N pollutants produced in winter cannot be ignored. As the water-soil interface zones, riparian wetlands play important roles in intercepting and buffering N pollutants. However, winter has the antagonistic effect on the N removal. Substrate improvement has been suggested as a strategy to optimize wetland performance and there remain many uncertainties about the inner mechanism. This study explores the effects of substrate improvement on N removal in winter and rhizospheric crosstalk between reed (Phragmites australis) and microbes in subtropical riparian reed wetlands. The rates of wetland N removal in winter, root metabolite profiles, and rhizosphere soil microbial community compositions were determined following the addition of different substrates (gravel, gravel + biochar, ceramsite + biochar, and modified ceramsite + biochar) to natural riparian soil. The results showed that the addition of different substrates to initial soil enhanced N removal from the microcosms in winter. Gravel addition increased NH4+-N removal by 8.3% (P < 0.05). Gravel + biochar addition increased both TN and NH4+-N removals by 8.9% (P < 0.05). The root metabolite characteristics and microbial community compositions showed some variations under different substrate additions compared to the initial soil. The three treatments involving biochar addition decreased lipid metabolites and enhanced the contents and variety of carbon sources in rhizosphere soil, while modified ceramsite + biochar addition treatment had a greater impact on the microbial community structure. There was evidence for a complex crosstalk between plants and microbes in the rhizosphere, and some rhizosphere metabolites were seen to be significantly correlated with the bacterial composition of the rhizospheric microbial community. These results highlighted the importance of rhizospheric crosstalk in regulating winter N removal in riparian reed wetland, provided a scientific reference for the protection and restoration of riparian reed areas and the prevention and control of non-point source pollution.

Assessing wetland nitrogen removal and reed (Phragmites australis) nutrient responses for the selection of optimal harvest time.

Wetlands play an important role in reducing the impact of nitrogen pollution on natural aquatic environments. However, during the plant wilting period (winter) there will inevitably be a reduction in nitrogen removal from wetlands. Understanding optimum harvest time will allow the use of management practices to balance the trade-off between nitrogen removal and the sustainability of wetlands. In this study, we investigated wetland nitrogen removal and reed (Phragmites australis) nutrient responses for two years [first year: influent total nitrogen (TN) 17.6-34.7 mg L-1; second year: influent TN 3.2-10.0 mg L-1] to identify the optimal harvest time: before wilting, mid-wilting, or late wilting. Harvesting decreased wetland nitrogen removal in both years, with later harvest time producing a smaller decrease in TN and ammonium-nitrogen (NH4+-N) removal. In addition to harvest before wilting, aboveground reed harvest at mid-wilting harvested more nutrients [carbon (C) 7.9%, nitrogen (N) 46.6% and phosphorus (P) 43.6%] in the first year, while harvest at late wilting harvested more nutrients (C 4.9%, N 7.8% and P 24.1%) in the second year, although this was not statistically significant. The late wilting harvest caused fewer disturbances to root stoichiometric homeostasis in the first year, while mid-wilting harvest promoted root nutrient availability in the second year. In addition, redundancy analysis (RDA) showed that root stoichiometry was interrelated with wetland nitrogen removal. Our results suggest that optimal harvest time was late wilting on the basis of wetland nitrogen removal, or either mid- or late wilting according to reed nutrient response to influent nitrogen concentration in some years. Our results provide crucial information for winter wetlands management.

Effect of nitrogen application rate on soil fungi community structure in a rice-fish mutualistic system.

Rice-fish mutualistic production systems rationalise the use of water and soil resources in an improved approach to sustainable food production. However, drivers of fungi community structure in paddy soil, including effects of nitrogen (N) application rate, are unclear in these systems. Here, we assessed soil fungi community and soil physicochemical responses in paddy soil to contrasting rates of N application in a rice-fish system. To clarify the mutualistic effects, the rice-fish system was compared with a standard rice monoculture under a 325.5 kg ha-1 N application rate. The results showed that N application rate affected abundance of paddy soil fungi (P < 0.05). Alpha diversity and richness of fungi were lower in the rice-fish system, but evenness increased with a decrease in N application rate, while the rate of N determined diversity of soil fungi in the rice-fish system. Dominant genera in the two systems differed, and soil physicochemical properties were more important drivers of soil fungi community structure in the rice-fish mutualistic system than in rice monoculture. Total N, available N and P regulated the abundance of dominant fungi. Our results indicate that management of soil fungi may contribute to sustainable agricultural production.

Increased planting density of Chinese milk vetch (Astragalus sinicus) weakens phosphorus uptake advantage by rapeseed (Brassica napus) in a mixed cropping system.

Neighbouring plants can affect plant growth through altering root morphological and physiological traits, but how exactly root systems respond to neighbouring plants with varied density, determining nutrient uptake and shoot growth is poorly understood. In a pot-based experiment, rapeseed was grown alone (single rapeseed), or mixed with 3, 6, or 15 Chinese milk vetch plants. As controls, monocropped Chinese milk vetch was grown at the same planting density, 3, 6, or 15 plants per pot. Root interaction between rapeseed and Chinese milk vetch facilitated phosphorus (P) uptake in rapeseed grown with 3 plants of Chinese milk vetch. As the planting density of Chinese milk vetch in mixture increased, there was a decrease in citrate concentration and acid phosphatase activity but an increase in the total root length of Chinese milk vetch per pot, resulting in decreases in rapeseed root biomass, total root length and P uptake when rapeseed was grown with 6 or 15 Chinese milk vetch plants relative to rapeseed grown with 3 plants. These results demonstrate that the enhanced nutrient utilization induced by root interaction at low planting densities was eliminated by the increased planting density of the legume species in rapeseed/Chinese milk vetch mixed cropping system, suggesting that root/rhizosphere management through optimizing legume planting density is important for improving crop productivity and nutrient-use efficiency.

Assessment of eutrophication and water quality in the estuarine area of Lake Wuli, Lake Taihu, China.

Our study assessed the actual water situation in the estuarine area of Lake Wuli, Meiliang Bay, Lake Taihu, China, based on eutrophication levels and status of water quality using the trophic level index (TLI) and water quality index (WQI) methods. In the wet (August 2017) and dry (March 2018) seasons, 22 estuarine areas were tested at 69 sampling sites, which included lake and rivers. Five parameters-chlorophyll a (Chl-a), total phosphorus (TP), total nitrogen (TN), Secchi disk (SD) and permanganate index (CODMn)-were measured to calculate the TLI, and 15 parameters-temperature (T), pH, electrical conductivity (EC), dissolved oxygen (DO), total dissolved solids (TDS), TN, TP, ammonium (NH4-N), nitrate (NO3-N), nitrite (NO2-N), CODMn, calcium (Ca2+), magnesium (Mg2+), chloride (Cl-) and phosphate (PO4-P)-were measured to calculate the WQI. The average TLI and WQI values in the wet season were 61.69 and 60.70, respectively, and the eutrophication level and water quality status were worse than that in the dry season (TLI: 57.40, WQI: 65.74). Significant differences were observed between three parts of Lake Wuli (West, Middle and East). Regardless of wet or dry season, East Wuli had worse eutrophication levels and water quality status than the other parts, whereas West Wuli showed less severe levels. DO, TN and CODMn used in the minimum WQI (WQImin) were the most effective parameters in our study. WQImin had stricter standards than WQI when analyzing water quality in the estuarine area of Wulihu. Factor analysis from principal component analysis (PCA) indicated that N might be the main factor affecting water quality of the most eastern sites in the wet season, and P may be the main factor in the dry season. Our results provide a valuable contribution to inform decision-making for the management of water environments by providing the actual water situation of the estuarine area of Lake Wuli.

Runoff nitrogen (N) losses and related metabolism enzyme activities in paddy field under different nitrogen fertilizer levels.

Nitrogen (N), one of the most important nutrients for plants, also can be a pollutant in water environments. N metabolism is sensitive to N fertilization application and related to rice growth. Different levels of N fertilization treatment (N0, control without N fertilizer application; N100, chemical fertilizer of 100 kg N ha-1; N200, chemical fertilizer of 200 kg N ha-1; N300, chemical fertilizer of 300 kg N ha-1) were tested to investigate N loss due to surface runoff and to explore the possible involvement of rice N metabolism responses to different N levels. The results indicated that N loss through runoff and rice yield was simultaneously increased in response to increasing N fertilizer levels. About 30% of total nitrogen (TN) was lost in the form of ammonium (NH4+) in a rice growing season, while only 3% was lost in the form of nitrate (NO3-). Higher N application increased carbon (C) and N content and increased nitrate reductase (NR) and glutamine synthetase (GS) activities in rice leaves, while it decreased glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) activities. These results suggest that N caused the accumulation of assimilation products in flag leaves of rice and stimulated N metabolic processes, while some protective substances were also stimulated to resist low N stress. This study provides a theoretical basis for improving N fertilizer management to reduce N loss and increase rice yield.

Reducing N losses through surface runoff from rice-wheat rotation by improving fertilizer management.

To better understand N runoff losses from rice-wheat rotation and demonstrate the effectiveness of improved fertilizer management in reducing N runoff losses, a field study was conducted for three consecutive rice-wheat rotations. Nitrogen losses through surface runoff were measured for five treatments, including CK without N application, C200, C300 simulating the conventional practices, CO200, and CO300. Optimum N rate was applied for C200 and CO200, and 30% of chemical fertilizer was substituted with organic fertilizer for CO200 and CO300 with respect to C200 and C300, respectively. Rice season had higher runoff coefficients than wheat season. Approximately 52% of total N was lost as NH4+-N in rice season, ranging from 21 to 83%, and in wheat season, the proportion of NO3--N in total N averaged 53% with a variation from 38 to 67%. The N treatments lost less total N in rice season (1.67-10.7 kg N ha-1) than in wheat season (1.72-17.1 kg N ha-1). These suggested that a key to controlling N runoff losses from rice-wheat rotation was to limit NO3--N accumulation in wheat season. In both seasons, N runoff losses for C200 and CO300 were lower than those for C300. CO200 better cut N losses than C200 and CO300, with 64 and 57% less N in rice and wheat seasons than C300, respectively. Compared with the conventional practices, optimum N inputs integrated with co-application of organic and chemical fertilizers could reduce N runoff losses with a better N balance under rice-wheat rotation.

A two-year field measurement of methane and nitrous oxide fluxes from rice paddies under contrasting climate conditions.

The effects of three irrigation levels (traditional normal amount of irrigation [NA100%], 70%, and 30% of the normal amount [NA70% and NA30%]) and two rice varieties (Oryza sativa L. Huayou14 and Hanyou8) on CH4 and N2O emissions were investigated over two years under contrasting climate conditions (a 'warm and dry' season in 2013 and a normal season in 2014). Hanyou8 was developed as a drought-resistant variety. The mean seasonal air temperature in 2013 was 2.3 °C higher than in 2014, while the amount of precipitation from transplanting to the grain-filling stage in 2013 was only 36% of that in 2014. CH4 emission rose by 93-161%, but rice grain yield fell by 7-13% in 2013, compared to 2014 under the NA100% conditions. Surface standing water depths (SSWD) were higher in Hanyou8 than in Huayou14 due to the lower water demand by Hanyou8. A reduction in the amount of irrigation water applied can effectively reduce the CH4 emissions regardless of the rice variety and climate condition. However, less irrigation during the 'warm and dry' season greatly decreased Huayou14 grain yield, but had little impact on Hanyou8. In contrast, N2O emission depended more on fertilization and SSWD than on rice variety.