Kewei Yu

Nitrous oxide and methane emissions from soil-plant systems

The closed chamber method was used to measure the N2O and CH4 emissions from rice, maize, soybean and spring wheat fields in Northeast China. Rice field almost did not emit or deposit N2O in total during flooding period, whereas N2O was substantially emitted during non-flooding period. The annual emission amount of N2O was 1.70 kg N2O ha-1, but that in flooding period was only 0.04 kg N2O ha-1. Daily average and seasonal total CH4 emission in rice field were 0.07 and 7.40 g CH4m-2, respectively. A trade-off between N2O and CH4 emissions from rice field was found. The growth of Azolla in rice field greatly stimulated both N2O and CH4 emissions. Total N2O emissions (270 days) from maize and soybean fields were 7.10 and 3.12 kg N2O ha-1, respectively. The sink function of the uplands monitored as the atmospheric CH4 was not significant.

Nitrous oxide and methane transport through rice plants

Abstract The separate closed chamber technique was used to study the potential of rice plants for transporting N2O and CH4 produced in soil to the atmosphere. The results indicate that N2O produced in soil can be conducted to the atmosphere via rice plants similarly to CH4 transport. More than 80% of both N2O and CH4 was emitted through rice plants. The rest was emitted through the soil/water/atmosphere interface by ebullition and diffusion. Nitrate addition increased the total N2O emission rate substantially but decreased the total CH4 emission. Nitrate addition did not change the CH4 emission ratio through rice plants, but lowered the percentage of N2O emission through rice plants. The results suggest that rice plants serve not only as a conduit for most of the CH4 leaving the soil but also for the N2O produced in the soil.

Comparison of monometal and multimetal adsorption in Mississippi River alluvial wetland sediment: Batch and column experiments

Monometal and multimetal adsorption of selected heavy metals in a sediment from a coastal Louisiana forested swamp used for wastewater treatment was studied. Results from the batch experiments show that the maximum adsorption capacities of the metals by the sediment were in the order of Pb>>Hg>Cr>CdCuZn>>As based on monometal adsorption isotherm, and Hg>Cr>CuCd approximately Pb>>As approximately Zn based on multimetal adsorption isotherm, respectively. Batch experimental data best fit the Langmuir model rather than the Freundlich isotherms. In the column experiments, the maximum adsorption capacities of the metals were in the order of Pb>>Hg>Cr>Cd>Cu>Zn>>As in monometal conditions, and Hg>>Cr>>Pb>CuZn approximately Cd>As in multimetal conditions. The metals became more mobile in multimetal than in monometal conditions. Results from both the batch and column experiments show that competitive adsorption among metals increases the mobility of these metals. Particularly, in this study, Pb in multimetal conditions lost it adsorption capacity most significantly. In both monometal and multimetal conditions, the maximum adsorption capacity of the metals in the column experiments was higher than that in the batch experiment indicating other metal retention mechanisms rather than adsorption may be involved. Therefore, both column and batch experiments are needed for estimating retention capacities and removal efficiencies of metals in sediments.

Reduction of global warming potential contribution from a rice field by irrigation, organic matter, and fertilizer management

The major objective of this study is to find a feasible management practice to mitigate the cumulative global warming potential (GWP) from CH4 and N2O emission in an irrigated rice field. Nonflooding (but wet) conditions reduced CH4 emission by 79 and 71% from the fields with and without organic matter (OM) addition, respectively. This was mainly due to the desirable soil redox status in the nonflooded fields with less CH4 production and more CH4 oxidation when CH4 diffused up the soil profile. Increase in N2O emission from the nonflooded fields offset part of the reduction in CH4 emission, especially when OM was not added. Thus the nonflooding treatment reduced the cumulative GWP by 72% in the OM-added field but only 46% in the field without OM addition. Under flooding conditions, no OM addition reduced CH4 emission by 57%, but rice yield was decreased by 16% in comparison with the OM-added fields. The best management practice proposed from this study is to keep the fields nonflooded but wet with OM addition, which largely reduced the GWP from the fields with no decrease in rice yield.

Influence of salinity level on sediment denitrification in a Louisiana estuary receiving diverted Mississippi River water

The Mississippi River water containing elevated nitrate is being diverted into Louisiana coastal estuaries to abate wetland deterioration attributed to lack of sediment and nutrients, rapid subsidence and accompanying salt water intrusion. In this study effect of salinity change on sediment denitrification at a Mississippi River freshwater diversion site (Davis Pond, Louisiana) was determined. Results show that the denitrification potential of the sediment was highest under fresh water condition (salinity close to 0‰). Addition of sea water immediately inhibited the denitrification activity of the sediment. Further analysis, by separate treatment of NaCl and K2SO4 addition, revealed that inhibition of the denitrification of the sediment by sea water was mainly caused by NaCl content in sea water. Denitrification activity of the sediment was not significantly affected by the sulfate content in sea water. Salinity increase seems a primary reason for the sediment denitrification rate decrease. A significant inverse relationship of denitrification rate and salinity was obtained [denitrification rate (mg N kg−1 day−1) = −0.20 × salinity(‰) + 10.41, R2 = 0.91]. Under highest sea water condition (salinity = 36‰), denitrification rate of the sediment would be 30.8% of its original activity (salinity = 0‰).

Effects of ferric iron reduction and regeneration on nitrous oxide and methane emissions in a rice soil

A laboratory soil slurry experiment and an outdoor pot experiment were conducted to study effects of ferric iron (Fe(III)) reduction and regeneration on nitrous oxide (N(2)O) and methane (CH(4)) emissions in a rice (Oryza sativa L.) soil. The anoxic slurry experiment showed that enhancing microbial Fe(III) reduction by ferrihydrite amendment (40 mol Fe g(-1)) transitionally stimulated N(2)O production and lowered CH(4) production by 16% during an initial 33-day incubation. Increased regeneration of Fe(III) through a 4-day aeration period in the Fe-amended slurry compared to the control slurry reduced CH(4) emission by 30% in the subsequent 15-day anaerobic incubation. The pot experiment showed that ferrihydrite amendment (63 micromol Fe g(-1)) stimulated N(2)O fluxes in the days following flooding. The Fe amendment suppression on CH(4) emission was obscured in the early season but became significant upon reflooding in the mid- and late-seasons. As a result, seasonal CH(4) emission in Fe-amended pots was 26% lower than the control with a single 2-day drainage and 69% lower with a double 2-day drainage. The reduction in CH(4) emission upon reflooding from the Fe-amended pots was mainly attributed to the increased Fe(III) regeneration during drainage showing a mechanism of Fe(III) regeneration in mitigating CH(4) emission by short-term drainage in flooded soils.

Incomplete Acetylene Inhibition of Nitrous Oxide Reduction in Potential Denitrification Assay as Revealed by using 15N-Nitrate Tracer

One lake sediment and three soils for rice production were used to test the effectiveness of inhibiting of nitrous oxide (N2O) reduction to dinitrogen gas (N2) by acetylene (C2H2) using 15N tracer. Regardless of the sources of the samples, results show that in presence of C2H2, significant isotopic enrichment of 15N of N2 was found at end of a typical denitrification assay. The δ15N of N2 value increased from 0‰ to 7.8–19.3‰ and 7.5–10.6‰ for the treatment with addition of 0.05 and 0.2 mg 15N nitrate, respectively. Such 15N enrichment can be interpreted as N2 formation accounting for 15.3% and 2.5% of the total added N in these two treatments, respectively. Nitrous oxide accumulation in presence of C2H2 could not account for the total added N. The result indicates incomplete inhibition of N2O reduction to N2 by C2H2 in denitrification when N2O reduction enzyme is developed.

Nonpoint Source of Nutrients and Herbicides Associated with Sugarcane Production and Its Impact on Louisiana Coastal Water Quality

A watershed analysis of nonpoint-source pollution associated with sugarcane (Saccharum officinarum L.) production was conducted. Runoff water samples following major rainfall events from two representative sugarcane fields (SC1 and SC2) were collected and analyzed. The impact of runoff on two receiving water bodies, St. James canal (SJC) and Bayou Chevreuil (BC) in a drainage basin (Baratarian Basin), was studied. Results show that runoff flow/rainfall ratios at the SC1 were significantly higher (P < 0.0001, n = 14) than at the SC2, probably mainly due to higher sand content and higher infiltration rate of surface soil at the SC2. In runoff water samples, total suspended solids (TSS) showed a significant correlation with the concentrations of N and P. Sugarcane runoff showed a direct impact on the SJC and BC locations where seasonal variations of pollutant concentrations in the waters followed the patterns of runoff loadings. Swamp forest runoff (SFR) location showed a buffering effect of forested wetlands on water quality with the lowest measured pollutant concentrations. The ratios in total N/total P and in inorganic N/organic N in runoff waters indicated that fertilization in spring greatly contributed to the temporal increase of N loadings, especially in forms of inorganic N. Isotope signature of (15)N-nitrate in the water samples verified that the nitrate was derived from fertilizers and was consumed during transportation. Both N and P concentrations in the receiving water bodies were above the eutrophic level. During the study period, herbicide concentrations in the receiving water bodies rarely exceeded the drinking water standards.

Nematode and copepod diversity (2012) from Louisiana near the Deepwater Horizon oil spill

Abstract Meiofauna were sampled from the NOAA ship Gordon Gunter during Fall 2012 off the coast of Louisiana. At five locations near the Deepwater Horizon drilling site (located 54–115 km away) box core samples and Shipek® grab samples were collected for subsurface meiofauna and sediment analysis. The goals of this study were to: 1) perform a taxonomic analysis of the meiofauna groups Nematoda and Copepoda, 2) perform statistical analyses of animal densities and sediment characteristics, and 3) compare sampling results using two different collection devices. Nematodes were the most abundant animals recovered, ranging from 88–791 animals per 10 cm2 area. Nematodes were represented by 60 genera in 23 families. The nematode community was dominated by one genus at the deepest location. Cluster analysis showed that there were two major groups for the five sites; nematodes from two shallow sites 81 and 84, and those from two shallow sites 85 and 86 along with deep site 82. Copepods were represented by 35 species from six families, with no animals identified at the deepest location (site 82). Cluster analysis also demonstrated two major groups similar to the two nematode groups but without site 82 present. Spearman correlation analysis revealed positive correlations among nematode, copepod, polychaete, and kinorhynch densities, and no correlations among the meiofauna densities and sediment chemistry values (metals and polycyclic aromatic hydrocarbon [PAH] concentrations). Nickel concentrations varied from 3.1–30.0 mg/kg, vanadium from 5.5–71.6 mg/kg, and PAHs from 94–395 ppb. Statistical comparison (Mann-Whitney U-test) of the box corer and Shipek® sampling equipment, using animal abundance, heavy metal analysis, and PAH data, revealed no difference between the two samplers.

Impact of exposure of crude oil and dispersant (COREXIT® EC 9500A) on denitrification and organic matter mineralization in a Louisiana salt marsh sediment

In response to the 2010 oil spill from the explosion of the Deepwater Horizon oil rig in the Gulf of Mexico, this experiment aims to study the ecological impact of the crude oil and dispersant (COREXIT® EC 9500A) in a coastal salt marsh ecosystem. The marsh sediment was incubated under an anaerobic condition with exposure to the crude oil or/and dispersant. The experiments were conducted in two continuous phases of nitrate addition to study denitrification potential using acetylene blockage technique and organic matter mineralization potential indicated by CO2 production in the sediment. Results show that the oil slightly (with no statistical significance p>0.05) increased both the denitrification and organic matter mineralization activities, likely due to oil components serving as additional organic matter. In contrast, the dispersant significantly (p<0.05) inhibited denitrification, but stimulated organic matter mineralization activities in the sediment due to unknown mechanisms. As a consequence, redox potentials (Eh) were much lower in the dispersant treated systems. The ecological impacts from the dispersant exposure may come from two fronts. First, loss of organic matter from the coastal marsh will threaten the long-term stability of the ecosystem, and the decrease in denitrification activity will weaken the N removal efficiency. Secondly, more reducing conditions developed by the dispersant exposure will likely preserve the oil in the ecosystem for an extended period of time due to weaker oil biodegradation under anaerobic conditions.