Syam K. Dodla

Denitrification potential and its relation to organic carbon quality in three coastal wetland soils

Capacity of a wetland to remove nitrate through denitrification is controlled by its physico-chemical and biological characteristics. Understanding these characteristics will help better to guide beneficial use of wetlands in processing nitrate. This study was conducted to determine the relationship between soil organic carbon (SOC) quality and denitrification rate in Louisiana coastal wetlands. Composite soil samples of different depths were collected from three different wetlands along a salinity gradient, namely, bottomland forest swamp (FS), freshwater marsh (FM), and saline marsh (SM) located in the Barataria Basin estuary. Potential denitrification rate (PDR) was measured by acetylene inhibition method and distribution of carbon (C) moieties in organic C was determined by 13C solid-state NMR. Of the three wetlands, the FM soil profile exhibited the highest PDR on both unit weight and unit volume basis as compared to FS and SM. The FM also tended to yield higher amount of N2O as compared to the FS and SM especially at earlier stages of denitrification, suggesting incomplete reduction of NO3(-) at FM and potential for emission of N2O. Saline marsh soil profile had the lowest PDR on the unit volume basis. Increasing incubation concentration from 2 to 10 mg NO3(-)-N L(-1) increased PDR by 2 to 6 fold with the highest increase in the top horizons of FS and SM soils. Regression analysis showed that across these three wetland systems, organic C has significant effect in regulating PDR. Of the compositional C moieties, polysaccharides positively influenced denitrification rate whereas phenolics (likely phenolic adehydes and ketonics) negatively affected denitrification rate in these wetland soils. These results could have significant implication in integrated assessment and management of wetlands for treating nutrient-rich biosolids and wastewaters, non-point source agricultural runoff, and nitrate found in the diverted Mississippi River water used for coastal restoration.

Fundamental and molecular composition characteristics of biochars produced from sugarcane and rice crop residues and by-products

Biochar conversion of sugarcane and rice harvest residues provides an alternative for managing these crop residues that are traditionally burned in open field. Sugarcane leaves, bagasse, rice straw and husk were converted to biochar at four pyrolysis temperatures (PTs) of 450 °C, 550 °C, 650 °C, and 750 °C and evaluated for various elemental, molecular and surface properties. The carbon content of biochars was highest for those produced at 650-750 °C. Biochars produced at 550 °C showed the characteristics of biochar that are commonly interpreted as being stable in soil, with low H/C and O/C ratios and pyrolysis fingerprints dominated by aromatic and polyaromatic hydrocarbons. At 550 °C, all biochars also exhibited maximum CEC values with sugarcane leaves biochar (SLB) > sugarcane bagasse biochar (SBB) > rice straw biochar (RSB) > rice husk biochar (RHB). The pore size distribution of biochars was dominated by pores of 20 nm and high PT increased both smaller and larger than 50 nm pores. Water holding capacity of biochars increased with PT but the magnitude of the increase was limited by feedstock types, likely related to the hydrophobicity of biochars as evident by molecular composition, besides pore volume properties of biochars. Py-GC/MS analysis revealed a clear destruction of lignin with decarboxylation and demethoxylation at 450 °C and dehydroxylation at above 550 °C. Overall, biochar molecular compositions became similar as PT increased, and the biochars produced at 550 °C demonstrated characteristics that have potential benefit as soil amendment for improving both C sequestration and nutrient dynamics.

Characterization of labile organic carbon in coastal wetland soils of the Mississippi River deltaic plain: Relationships to carbon functionalities

Adequate characterization of labile organic carbon (LOC) is essential to the understanding of C cycling in soil. There has been very little evaluation about the nature of LOC characterizations in coastal wetlands, where soils are constantly influenced by different redox fluctuations and salt water intrusions. In this study, we characterized and compared LOC fractions in coastal wetland soils of the Mississippi River deltaic plain using four different methods including 1) aerobically mineralizable C (AMC), 2) cold water extractable C (CWEC), 3) hot water extractable C (HWEC), and 4) salt extractable C (SEC), as well as acid hydrolysable C (AHC) which includes both labile and slowly degradable organic C. Molecular organic C functional groups of these wetland soils were characterized by (13)C solid-state nuclear magnetic resonance (NMR). The LOC and AHC increased with soil organic C (SOC) regardless of wetland soil type. The LOC estimates by four different methods were positively and significantly linearly related to each other (R(2)=0.62-0.84) and with AHC (R(2)=0.47-0.71). The various LOC fractions accounted for ≤4.3% of SOC whereas AHC fraction represented 16-49% of SOC. AMC was influenced positively by O/N-alkyl and carboxyl C but negatively by alkyl C, whereas CWEC and SEC fractions were influenced only positively by carboxyl C but negatively by alkyl C in SOC. On the other hand, HWEC fraction was found to be only influenced positively by carbonyl C, and AHC positively by O/N-alkyl and alkyl C but negatively by aromatic C groups in SOC. Overall these relations suggested different contributions of various molecular organic C moieties to LOC in these wetlands from those often found for upland soils. The presence of more than 50% non-acid hydrolysable C suggested the dominance of relatively stable SOC pool that would be sequestered in these Mississippi River deltaic plain coastal wetland soils. The results have important implications to the understanding of the liability and refractory character of SOC in these wetlands as recent studies suggest marsh SOC to be an important C source in fueling hypoxia in the northern Gulf of Mexico.

SOIL SILICON EXTRACTABILITY WITH SEVEN SELECTED EXTRACTANTS IN RELATION TO COLORIMETRIC AND ICP DETERMINATION

The importance of silicon (Si) in nutrition is currently being recognized by its beneficial effects in many crops. At present, various procedures are being used to extract Si and to determine soil Si status. This study was undertaken to evaluate the relationships among various extractants and to compare inductively coupled plasma (ICP) Si measurement with the molybdenum blue colorimetric method for possible incorporation of Si determination into existing routine soil and water testing procedures. Seven extractants were evaluated on 30 Louisiana soils. These were: deionized (D.I.) water, 0.5 M acetic acid, 1 M sodium acetate buffer (pH 4.0), 0.5 M ammonium acetate (pH 4.8), 0.1 M hydrochloric acid, 0.5 M citric acid, and Mehlich III. Soil-extractable Si determination by ICP correlated highly with that by colorimetric analysis in HCl, citric acid, acetic acid, acetate buffer, and ammonium acetate extractions (R2 ≥ 0.972, P < 0.001). The correlation between ICP and colorimetric determinations was low for Mehlich III (R2 0.762) and for H2O (R2 = 0.576) extractions. The latter has significant implications for interpreting Si status in soil water extracts or surface water samples. The amount of extractable Si resulting from use of different extractants was in the order of Mehlich III > citric acid > HCl > acetic acid > acetate buffer > NH4OAc > D.I. water, as determined by colorimetry. Silicon extracted by different extractants was well correlated among citric acid, HCl, acetic acid, acetate buffer, and NH4OAc (R2 0.611, P < 0.001). Water and Mehlich III showed poor correlations with other extractants (R2 ≤ 0.430). The results suggest that these seven extractants characterize different pools of Si-supplying capacity of the soil: extractable by water, extractable by any of HCl, citric acid, acetic acid, acetate buffer, and NH4OAc, and extractable by Mehlich III.

Effect of biochar amendment on tylosin adsorption-desorption and transport in two different soils.

The role of biochar as a soil amendment on the adsorption-desorption and transport of tylosin, a macrolide class of veterinary antibiotic, is little known. In this study, batch and column experiments were conducted to investigate the adsorption kinetics and transport of tylosin in forest and agricultural corn field soils amended with hardwood and softwood biochars. Tylosin adsorption was rapid at initial stages, followed by slow and continued adsorption. Amounts of adsorption increased as the biochar amendment rate increased from 1 to 10%. For soils with the hardwood biochar, tylosin adsorption was 10 to 18% higher than that when using the softwood biochar. Adsorption kinetics was well described by Elovich equation ( ≥ 0.921). As the percent of biochar was increased, the rates of initial reactions were generally increased, as indicated by increasing α value at low initial tylosin concentration, whereas the rates during extended reaction times were generally increased, as indicated by decreasing β value at high initial tylosin concentration. A considerably higher amount of tylosin remained after desorption in the corn field soil than in the forest soil regardless of the rate of biochar amendment, which was attributed to the high pH and silt content of the former. The breakthrough curves of tylosin showed that the two soils with biochar amendment had much greater retardation than those of soils without biochar. The CXTFIT model for the miscible displacement column study described well the peak arrival time as well as the maximum concentration of tylosin breakthrough curves but showed some underestimation at advanced stages of tylosin leaching, especially in the corn field soil. Overall, the results indicate that biochar amendments enhance the retention and reduce the transport of tylosin in soils.

Application effects of coated urea and urease and nitrification inhibitors on ammonia and greenhouse gas emissions from a subtropical cotton field of the Mississippi delta region.

Nitrogen (N) fertilization affects both ammonia (NH3) and greenhouse gas (GHG) emissions that have implications in air quality and global warming potential. Different cropping systems practice varying N fertilizations. The aim of this study was to investigate the effects of applications of polymer-coated urea and urea treated with N process inhibitors: NBPT [N-(n-butyl)thiophosphoric triamide], urease inhibitor, and DCD [Dicyandiamide], nitrification inhibitor, on NH3 and GHG emissions from a cotton production system in the Mississippi delta region. A two-year field experiment consisting of five treatments including the Check (unfertilized), urea, polymer-coated urea (ESN), urea+NBPT, and urea+DCD was conducted over 2013 and 2014 in a Cancienne loam (Fine-silty, mixed, superactive, nonacid, hyperthermic Fluvaquentic Epiaquepts). Ammonia and GHG samples were collected using active and passive chamber methods, respectively, and characterized. The results showed that the N loss to the atmosphere following urea-N application was dominated by a significantly higher emission of N2O-N than NH3-N and the most N2O-N and NH3-N emissions were during the first 30-50 days. Among different N treatments compared to regular urea, NBPT was the most effective in reducing NH3-N volatilization (by 58-63%), whereas DCD the most significant in mitigating N2O-N emissions (by 75%). Polymer-coated urea (ESN) and NBPT also significantly reduced N2O-N losses (both by 52%) over urea. The emission factors (EFs) for urea, ESN, urea-NBPT, urea+DCD were 1.9%, 1.0%, 0.2%, 0.8% for NH3-N, and 8.3%, 3.4%, 3.9%, 1.0% for N2O-N, respectively. There were no significant effects of different N treatments on CO2-C and CH4-C fluxes. Overall both of these N stabilizers and polymer-coated urea could be used as a mitigation strategy for reducing N2O emission while urease inhibitor NBPT for reducing NH3 emission in the subtropical cotton production system of the Mississippi delta region.

Carbon gas production under different electron acceptors in a freshwater marsh soil

Dynamics of carbon (C) gas emission from wetlands influence global C cycling. In many freshwater systems such as Louisiana freshwater marsh, soil contents of NO3(-) and SO4(2-) have increased due to nutrient loading and saltwater intrusion. This could affect C mineralization and the emission of the major greenhouse gases carbon dioxide (CO2) and methane (CH4). In this investigation, a laboratory microcosm study was carried out to elucidate the effects of NO(3)(-) and SO4(2-) on CO2 and CH4 production from a freshwater marsh soil located in the Barataria Basin of Louisiana coast, which has been subjected to the Mississippi River diversion and seawater intrusion. Composite soil samples were collected from top 50 cm marsh profile, treated with different levels of NO3(-) (0, 3.2 and 5mM) or SO4(2-) (0, 2, and 5mM) concentrations, and incubated for 214d under anaerobic conditions. The results showed that the presence of NO3(-) (especially at 3.2mM) significantly decreased CO2 productions whereas SO4(2-) did not. On the other hand, both NO(3)(-) and SO4(2-) treatments decreased CH4 production but the NO3(-) almost completely inhibited CH4 production (>99%) whereas the SO4(2-) treatments reduced CH4 production by 78-90%. The overall C mineralization rate constant under the NO3(-) presence was also low. In addition, the results revealed that a large proportion (95%) of anaerobic carbon mineralization in the untreated freshwater soil was unexplained by the reduction of any of the measured major electron acceptors.

Effect of redox potential and pH status on degradation and adsorption behavior of tylosin in dairy lagoon sediment suspension.

Veterinary antibiotics are the most heavily used pharmaceuticals in intensive animal farming operation. Their presence in the environment through application of manure and lagoon water as fertilizer in agricultural fields has generated a growing concern in recent years due to potential threat to the ecosystem and the risk they pose to human and animal health. Among the antibiotics, tylosin, a macrolide class of antibiotics, has been widely used for disease prevention and growth promotion in swine, cattle/dairy, and poultry production. To understand degradation and sorption behavior of tylosin A, a laboratory microcosm incubation study was conducted on dairy lagoon sediments suspension under different pH (5.5, 7.0, 8.5) and redox potentials (Eh at -100 mV, 0 mV, +250 mV, +350 mV). Sorption and degradation of tylosin was strongly influenced by sediment pH and redox conditions. Under acidic (pH 5.5) and reduced (Eh -100 mV) condition, tylosin persisted in the solution phase of dairy lagoon sediment suspension much longer with resident time of 77 d. Under oxidized (Eh +350 mV) condition, microbial degradation was much greater causing 68-75% of tylosin loss from the solution at pH 5.5 and 32-75% at pH 7.0 during the 20 d incubation. At pH 8.5, abiotic transformation of tylosin A into unknown degradates rather than sediment adsorption and microbial degradation was the major mechanism controlling tylosin disappearance from the solution regardless of the status of redox potentials. Overall, the results suggested that under reduced condition with low pH, tylosin will be persisted in the lagoon effluents and residue of tylosin may enter agricultural fields through the application of lagoon slurry as fertilizer.

Soil Gas Efflux in Perennial Bioenergy and Conventional Agricultural Crops in the Lower Mississippi Alluvial Valley

ABSTRACT The Lower Mississippi Alluvial Valley (LMAV) has favorable attributes for producing biofuels. Two study sites were established on retired agricultural fields in the LMAV to explore switchgrass (SWITCH) and eastern cottonwood (CTWD) as biofuel feedstocks. A soybean-sorghum rotation (CROP) was also established as a conventional cropping system. Soil efflux gas (carbon dioxide [CO2], methane [CH4], and nitrous oxide [N2O]), microbial biomass carbon (Cmic) and dehydrogenase activity were measured for two years. Cumulative growing-season soil CO2 efflux of SWITCH exceeded that of CROP; SWITCH had higher daily CO2 efflux than CTWD and CROP in some months. SWITCH and CTWD had greater Cmic than CROP at both sites. Soil CH4 and N2O efflux rates were low for much of the study, with only short-term differences in soil CH4 observed. Converting these retired agricultural sites to SWITCH increased soil CO2 efflux relative to CROP, with increases attributable to greater plant and microbial respiration.