Yongqiu Xia

Seasonal variation and controlling factors of anaerobic ammonium oxidation in freshwater river sediments in the Taihu Lake region of China

Anaerobic ammonium oxidation (anammox) has been recently recognized as an important pathway for the removal of fixed nitrogen (N) from aquatic systems. However, the functions of anammox in freshwater river systems remain uncertain. In this study, we evaluated the occurrence of anammox activity in two rivers in the Taihu Lake region in China during a seasonal survey. Homogenized sediments were incubated with (15)N-labeled NO3(-) and NH4(+) amendments to determine the potential importance of the anammox process relative to canonical denitrification. Production of (29)N2 and (30)N2 in slurries was determined using membrane inlet mass spectrometry. Potential anammox rates in the two river sediments ranged from 0.11±0.07 to 6.79±1.28 μmol N m(-2) h(-1) and the remove of N by anammox accounted for 0.8±0.00% to 10.7±0.03% of total N2 production. Potential anammox rates varied spatially and temporally in the two rivers, with the highest and lowest mean anammox rates appearing during summer and early autumn and during winter, respectively. The variation of the percentage of anammox to total N2 production displayed the same trend with potential anammox rates. Water temperature and NO3(-) content in sediments were the main factors affecting anammox activity. Anammox bacteria were detected in sediment samples using barcode pyrosequencing. The 16S rRNA anammox gene sequences in the river sediments were affiliated with Candidatus Kuenenia, Candidatus Jettenia, and Candidatus Scalindua, among which C. Kuenenia dominated the anammox bacterial communities. Our results confirmed the presence of anammox bacteria but their role is relatively small in removing fixed N from freshwater river systems.

Nitrogen Removal Capacity of the River Network in a High Nitrogen Loading Region

Denitrification is the primary process that regulates the removal of bioavailable nitrogen (N) from aquatic ecosystems. Quantifying the capacity of N removal from aquatic systems can provide a scientific basis for establishing the relationship between N reduction and water quality objectives, quantifying pollution contributions from different sources, as well as recommending control measures. The Lake Taihu region in China has a dense river network and heavy N pollution; however, the capacity for permanent N removal by the river network is unknown. Here, we concurrently examined environmental factors and net N2 flux from sediments of two rivers in the Lake Taihu region between July 2012 and May 2013, using membrane inlet mass spectrometry, and then established a regression model incorporating the highly correlated factors to predict the N removal capacity of the river network in the region. To test the applicability of the regression model, 21 additional rivers surrounding Lake Taihu were sampled between July and December 2013. The results suggested that water nitrate concentrations are still the primary controlling factor for net denitrification even in this high N loading river network, probably due to multicollinearity of other relevant factors, and thus can be used to predict N removal from aquatic systems. Our established model accounted for 78% of the variability in the measured net N2 flux in these 21 rivers, and the total N removed through N2 production by the river network was estimated at 4 × 10(4) t yr(-1), accounting for about 43% of the total aquatic N load to the river system. Our results indicate that the average total N content in the river water discharged into Lake Taihu would be around 5.9 mg of N L(-1) in the current situation, far higher than the target concentration of 2 mg of N L(-1), given the total N load and the N removal capacity. Therefore, a much stronger effort is required to control the N pollution of the surface water in the region.

Dissimilatory Nitrate Reduction Processes in Typical Chinese Paddy Soils: Rates, Relative Contributions, and Influencing Factors

Using soil slurry-based (15)N tracer combined with N2/Ar technique, the potential rates of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), and their respective contributions to total nitrate reduction were investigated in 11 typical paddy soils across China. The measured rates of denitrification, anammox, and DNRA varied from 2.37 to 8.31 nmol N g(-1) h(-1), 0.15 to 0.77 nmol N g(-1) h(-1) and 0.03 to 0.54 nmol N g(-1) h(-1), respectively. The denitrification and anammox rates were significantly correlated with the soil organic carbon content, nitrate concentration, and the abundance of nosZ genes. The DNRA rates were significantly correlated with the soil C/N, extractable organic carbon (EOC)/NO3(-) ratio, and sulfate concentration. Denitrification was the dominant pathway (76.75-92.47%), and anammox (4.48-9.23%) and DNRA (0.54-17.63%) also contributed substantially to total nitrate reduction. The N loss or N conservation attributed to anammox and DNRA was 4.06-21.24 and 0.89-15.01 g N m(-2) y(-1), respectively. This study reports the first simultaneous investigation of the dissimilatory nitrate reduction processes in paddy soils, highlighting that anammox and DNRA play important roles in removing nitrate and should be considered when evaluating N transformation processes in paddy fields.

Diurnal pattern in nitrous oxide emissions from a sewage-enriched river

Estimates of N2O emission based on limit measurements could be highly inaccurate because of considerable diurnal variations in N2O flux due to rapid transformation of nutrients and diel change of dissolved oxygen (DO). In the present study, the N2O fluxes, dissolved N2O concentrations, and the controlling variables were measured hourly for 3d and night cycles at five sites on a typically sewage-enriched river in the Taihu region. There were no significant diurnal patterns in N2O emissions and dissolved N2O saturation, with respective mean value of 56.1μg N2O-Nm(-2)h(-1) (range=41.1μg N2O-Nm(-2)h(-1) to 87.7μg N2O-Nm(-2)h(-1)) and 813% (range=597-1372%), though distinct diurnal patterns were observed in DO concentration and river chemistry. However, the mean N2O emissions and the mean dissolved N2O saturation during the day (61.7μgNm(-2)h(-1) for N2O fluxes and 0.52μgNL(-1) for dissolved N2O saturation) were significantly higher than those during the night (50.1μgNm(-2)h(-1)for N2O fluxes and 0.44μgNL(-1) for dissolved N2O saturation). Factors controlling the N2O flux were pH, DO, NH4(+),SO4(2-), air temperature, and water temperature. Sampling at 19:00h could well represent the daily average N2O flux at the studied river.

Greenhouse gas emissions and reactive nitrogen releases during the life-cycles of staple food production in China and their mitigation potential

Life-cycle analysis of staple food (rice, flour and corn-based fodder) production and assessments of the associated greenhouse gas (GHG) and reactive nitrogen (Nr) releases, from environmental and economic perspectives, help to develop effective mitigation options. However, such evaluations have rarely been executed in China. We evaluated the GHG and Nr releases per kilogram of staple food production (carbon and Nr footprints) and per unit of net economic benefit (CO2-NEB and Nr-NEB), and explored their mitigation potential. Carbon footprints of food production in China were obviously higher than those in some developed countries. There was a high spatial variation in the footprints, primarily attributable to differences in synthetic N use (or CH4 emissions) per unit of food production. Provincial carbon footprints had a significant linear relationship with Nr footprints, attributed to large contribution of N fertilizer use to both GHG and Nr releases. Synthetic N fertilizer applications and CH4 emissions dominated the carbon footprints, while NH3 volatilization and N leaching were the main contributors to the Nr footprints. About 564 (95% uncertainty range: 404-701) TgCO2eqGHG and 10 (7.4-12.4) Tg Nr-N were released every year during 2001-2010 from staple food production. This caused the total damage costs of 325 (70-555) billion ¥, equivalent to nearly 1.44% of the Gross Domestic Product of China. Moreover, the combined damage costs and economic input costs, accounted for 66%-80% of the gross economic benefit generated from food production. A reduction of 92.7TgCO2eqyr(-1) and 2.2TgNr-Nyr(-1) could be achieved by reducing synthetic N inputs by 20%, increasing grain yields by 5% and implementing off-season application of straw and mid-season drainage practices for rice cultivation. In order to realize these scenarios, an ecological compensation scheme should be established to incentivize farmers to gradually adopt knowledge-based managements.

Linking river nutrient concentrations to land use and rainfall in a paddy agriculture-urban area gradient watershed in southeast China.

The effects of land use and land-use changes on river nutrient concentrations are not well understood, especially in the watersheds of developing countries that have a mixed land use of rice paddy fields and developing urban surfaces. Here, we present a three-year study of a paddy agricultural-urban area gradient watershed in southeast China. The annual anthropogenic nitrogen (N) input from the agricultural region to the urban region was high, yet the results showed that the monthly nutrient concentrations in the river were low in the rainy seasons. The nutrient concentrations decreased continuously as the river water passed through the traditional agriculture region (TAR; paddy rice and wheat rotation) and increased substantially in the city region (CR). The traditional agricultural reference region exported most of the nutrient loads at high flows (>1mmd(-1)), the intensified agricultural region (IAR, aquaculture and poultry farming) exported most of the nutrient loads at moderate flows (between 0.5 and 1mmd(-1)), and the CR reference area exported most of the nutrient loads under low to moderate flows. We developed a statistical model to link variations in the nutrient concentrations to the proportion of land-use types and rainfall. The statistical results showed that impervious surfaces, which we interpret as a proxy for urban activities including sewage disposal, were the most important drivers of nutrient concentrations, whereas water surfaces accounted for a substantial proportion of the nutrient sinks. Therefore, to efficiently reduce water pollution, sewage from urban areas must be addressed as a priority, although wetland restoration could also achieve substantial pollutant removal.

Nitrate source apportionment using a combined dual isotope, chemical and bacterial property, and Bayesian model approach in river systems

Nitrate (NO3-) pollution is a serious problem worldwide, particularly in countries with intensive agricultural and population activities. Previous studies have used δ15N-NO3- and δ18O-NO3- to determine the NO3- sources in rivers. However, this approach is subject to substantial uncertainties and limitations because of the numerous NO3- sources, the wide isotopic ranges, and the existing isotopic fractionations. In this study, we outline a combined procedure for improving the determination of NO3- sources in a paddy agriculture–urban gradient watershed in eastern China. First, the main sources of NO3- in the Qinhuai River were examined by the dual isotope bi-plot approach, in which we narrowed the isotope ranges using site-specific isotopic results. Next, the bacterial groups and chemical properties of the river water were analyzed to verify these sources. Finally, we introduced a Bayesian model to apportion the spatio-temporal variations of the NO3- sources. Denitrification was first incorporated into the Bayesian model because denitrification plays an important role in the nitrogen pathway. The results showed that fertilizer contributed large amounts of NO3- to the surface water in traditional agricultural regions, whereas manure effluents were the dominant NO3- source in intensified agricultural regions, especially during the wet seasons. Sewage effluents were important in all three land uses and exhibited great differences between the dry season and the wet season. This combined analysis quantitatively delineates the proportion of NO3- sources from paddy agriculture to urban river water for both dry and wet seasons and incorporates isotopic fractionation and uncertainties in the source compositions.

Different effects of biochar and a nitrification inhibitor application on paddy soil denitrification: A field experiment over two consecutive rice-growing seasons

Biochar and nitrification inhibitors are increasingly being proposed as amendments to improve nitrogen use efficiency (NUE). However, their effects on soil denitrification and the major N loss in rice paddies over an entire rice-growing season are not well understood. In this study, using intact soil core incubation combined with N2/Ar technique, the impacts of biochar and a nitrification inhibitor (Ni), 2-chloro-6-(trichloromethyl)-pyridine, on rice yield and soil denitrification, as well as ammonia (NH3) volatilization, were investigated over two rice-growing seasons in the Taihu Lake region of China. Field experiments were designed with four treatments: N0 (no N applied), N270 (270kg N ha-1 applied), N270+C (25tha-1 biochar applied) and N270+Ni (2-chloro-6- [trichloromethyl] -pyridine, 1.35kgha-1N applied). Compared with single application of N fertilizer alone (N270), biochar (N270+C) and Ni (N270+Ni) applications increased rice yields by 4.2-5.2% and 6.2-7.3%, respectively. The cumulative N2-N and NH3-N losses in different treatments varied from 11.9 to 21.8% and from 11.5 to 22.0% of the applied N, respectively. Compared with the single application of N fertilizer, the Ni application increased total NH3 emission by 4.0-20.6% and significantly decreased total N2-N emission by 9.7-19.4% (p<0.05), while the biochar application increased total NH3 and N2-N emissions by 8.6-17.9% and 3.3-9.7%, respectively. Overall, the biochar application resulted in an 11-15% higher net gaseous N than the Ni application. Although the biochar application may increase the rice yield and consequently the plant N uptake, it also promoted N loss more than Ni. Therefore biochar may not be good for maintaining soil fertility over a long period. Instead, applying Ni may be an optimal practice to ensure food security, while decreasing gaseous N loss, for rice production in the Taihu Lake region of China.

Comparison of statistical models for predicting cost effective nitrogen rate at rice–wheat cropping systems

Due to increased economic and environmental concerns, developing statistical models of crop yield has become one of the most important steps in determination of the cost effective rates (CERs) of nitrogen (N) fertilization. Although quadratic models are commonly used to describe wheat and paddy rice yield response to fertilizer rates in the Taihu Lake region of China, few studies have investigated why this model is selected over others. This study evaluated quadratic, exponential and square root models describing the wheat (Triticum aestivum L.) and rice (Oryza sativa L.) yield response to N fertilizer when determining the CERs, while also considering the environmental costs of N losses. All models fit the data almost equally well when evaluated using the variability and standard error statistics. However, there were marked discrepancies among models when calculating the CER of fertilization and the economic returns form Z-test. The quadratic model had a greater CER value (194 kg N ha–1 for rice and 185 kg N ha–1 for wheat) averaged over all sites than the exponential and square root models. The residuals obtained from the quadratic models were closer to a normal distribution than those of the other two models, indicating a less systematic bias. The mean economic uncertainties resulting from the quadratic model were more dependable than the other two models evaluated. These results show that the quadratic model best describes the rice and wheat yield responses and tends to indicate the optimal rates of fertilization while considering the environmental and economic effects of over fertilization for rice and wheat in the Taihu Lake region.

Effects of long-term pig manure application on antibiotics, abundance of antibiotic resistance genes (ARGs), anammox and denitrification rates in paddy soils

Previous studies of long-term manure applications in paddy soil mostly focused on the effects on denitrification, occurrence of antibiotics and antibiotic resistance genes (ARGs) without considering the effects on anaerobic ammonium oxidation (anammox). Here, we investigated the potential rates of anammox and denitrification, occurrence of antibiotics and AGRs in response to three fertilization regimes (C, no fertilizer; N, mineral fertilizer; and NM, N plus pig manure) in six long-term paddy experiment sites across China. The potential rates of anammox (0.11-3.64 nmol N g-1 h-1) and denitrification (1.5-29.05 nmol N g-1 h-1) were correlated with the abundance of anammox genes (hzsB) and denitrification functional genes (narG, nirK, nirS and nosZ), respectively. The anammox and denitrification rates were affected by soil organic carbon (SOC) and significantly (p < 0.05) increased in NM treatments relative to those in N treatments. Although pig manure application increased antibiotic concentrations and abundance of ARGs compared with N treatments, the increased antibiotics did not directly affect the anammox and denitrification rates. Our results suggested that long-term pig manure application significantly increased antibiotic concentrations, abundance of ARGs, and rates of anammox and denitrification, and that the effects of pig manure-derived antibiotics on anammox and denitrification were marginal. This is the first report that investigates the effects of long-term pig manure application on anammox in paddy soils. More attention should be paid to the potential ecological risk of increased ARGs caused by pig manure application in paddy soils.