Event-scale hysteresis metrics to reveal processes and mechanisms controlling constituent export from watersheds: A review✰.

Due to the increased availability of high-frequency measurements of stream chemistry provided by in situ sensors, researchers have gained more access to relationships between stream discharge and constituent concentrations (C-Q relationships) at event-scales. Existing studies reveal that event-scale C-Q relationships are mostly non-linear and exhibit temporal lags between peaks (or troughs) of hydrographs and chemographs, resulting in apparent hysteresis effects. In this paper, we summarize and introduce tools and methods in hysteresis analysis, especially the history and progresses of metrics to quantify hysteresis patterns. In addition, this paper provides a typical workflow to conduct event-scale hysteresis analysis, such as how to obtain the access to high-frequency measurements, existing methods to delineate storm events, approaches to classify and quantify hysteresis patterns, possible features/properties controlling hysteresis patterns, statistical methods to identify features at play, and strategies to deliver the inferences from hysteresis analysis. Lastly, we discuss some potential limitations that arise in the workflow and possible future work to address the challenges, including the development of advanced quantitative hysteresis metrics, generalized and standardized tools to delineate events and the integration of hysteresis analysis with numerical modeling. This paper aims to provide a critical overview of technical approaches for hysteresis analysis for researchers and hopefully foster their interests to advance our understanding of complex mechanisms in event-scale hydro-biogeochemical processes.

Impacts on soil nitrogen availability of converting managed pine plantation into switchgrass monoculture for bioenergy.

Biofuels derived from lignocellulosic materials is one of the options in addressing issues on climate change and energy independence. One of the most promising bioenergy crops is switchgrass (Panicum virgatum L.), particularly in North America. Future advancement in large-scale conversion of lignocellulosic feedstocks and relatively more competitive price for biomass and other economic advantages could lead to landowners opting to venture on switchgrass monoculture (SWITCH) in lieu of loblolly pine monoculture (PINE). Therefore, we investigated the conversion of previously managed loblolly pine stand into SWITCH in eastern North Carolina, U.S.A. on soil N availability. Treatments included PINE, SWTICH, and mature loblolly pine stand (REF). Each treatment was replicated three times on 0.8 ha plots drained by open ditches dug 1.0-1.2 m deep and spaced at 100 m. Rates of net N mineralization (Nm) and nitrification (Nn) at the top 20 cm were measured using sequential in-situ techniques in 2011 and 2012 (the 3rd and 4th years of establishment, respectively) along with a one-time laboratory incubation. On average, PINE, SWITCH, and REF can have field net Nm rates up to 0.40, 0.34 and 0.44 mg N·kg soil-1·d-1, respectively, and net Nn rates up to 0.14, 0.08 and 0.10 mg N·kg soil-1·d-1, respectively. Annually, net Nm rates ranged from 136.98 to 167.21, 62.00 to 142.61, and 63.57 to 127.95 kg N·ha-1, and net Nn rates were 56.31-62.98, 16.45-30.45, 31.99-32.94 kg N·ha-1 in PINE, SWITCH, and REF, respectively. Treatment effect was not significant on field Nm rate (p = 0.091). However, SWITCH significantly reduced nitrate-N production (p < 0.01). Overall, results indicated that establishment of SWITCH on poorly drained lands previously under PINE is less likely to significantly impact total soil N availability and potentially has minimum N leaching losses since soil mineral N under this system will be dominated by ammonium-N.

Drying-Rewetting Cycles Affect Nitrate Removal Rates in Woodchip Bioreactors.

Woodchip bioreactors are widely used to control nitrogen export from agriculture using denitrification. There is abundant evidence that drying-rewetting (DRW) cycles can promote enhanced metabolic rates in soils. A 287-d experiment investigated the effects of weekly DRW cycles on nitrate (NO) removal in woodchip columns in the laboratory receiving constant flow of nitrated water. Columns were exposed to continuous saturation (SAT) or to weekly, 8-h drying-rewetting (8 h of aerobiosis followed by saturation) cycles (DRW). Nitrate concentrations were measured at the column outlets every 2 h using novel multiplexed sampling methods coupled to spectrophotometric analysis. Drying-rewetting columns showed greater export of total and dissolved organic carbon and increased NO removal rates. Nitrate removal rates in DRW columns increased by up to 80%, relative to SAT columns, although DRW removal rates decreased quickly within 3 d after rewetting. Increased NO removal in DRW columns continued even after 39 DRW cycles, with ∼33% higher total NO mass removed over each weekly DRW cycle. Data collected in this experiment provide strong evidence that DRW cycles can dramatically improve NO removal in woodchip bioreactors, with carbon availability being a likely driver of improved efficiency. These results have implications for hydraulic management of woodchip bioreactors and other denitrification practices.

Temporal variations and controlling factors of nitrogen export from an artificially drained coastal forest.

Nitrogen losses in drainage water from coastal forest plantations can constrain the long term sustainability of the system and could negatively affect adjacent nutrient sensitive coastal waters. Based on long-term (21 years) field measurements of hydrology and water quality, we investigated the temporal variations and controlling factors of nitrate and dissolved organic nitrogen (DON) export from an artificially drained coastal forest over various time scales (interannual, seasonal, and storm events). According to results of stepwise multiple linear regression analyses, the observed large interannual variations of nitrate flux and concentration from the drained forest were significantly (p < 0.004) controlled by annual mean water table depth, and annual drainage or precipitation. Annual precipitation and drainage were found to be dominant factors controlling variations of annual DON fluxes. Temporal trends of annual mean DON concentration could not be explained explicitly by climate or hydrologic factors. No significant difference was observed between nitrogen (both nitrate and DON) export during growing and nongrowing seasons. Nitrate exhibited distinguished export patterns during six selected storm events. Peak nitrate concentrations during storm events were significantly (p < 0.003) related to 30-day antecedent precipitation index and the minimum water table depth during individual events. The temporal variations of DON export within storm events did not follow a clear trend and its peak concentration during the storm events was found to be significantly (p < 0.006) controlled by the short-term drying and rewetting cycles.

DRAINMOD-FOREST: Integrated Modeling of Hydrology, Soil Carbon and Nitrogen Dynamics, and Plant Growth for Drained Forests.

We present a hybrid and stand-level forest ecosystem model, DRAINMOD-FOREST, for simulating the hydrology, carbon (C) and nitrogen (N) dynamics, and tree growth for drained forest lands under common silvicultural practices. The model was developed by linking DRAINMOD, the hydrological model, and DRAINMOD-N II, the soil C and N dynamics model, to a forest growth model, which was adapted mainly from the 3-PG model. The forest growth model estimates net primary production, C allocation, and litterfall using physiology-based methods regulated by air temperature, water deficit, stand age, and soil N conditions. The performance of the newly developed DRAINMOD-FOREST model was evaluated using a long-term (21-yr) data set collected from an artificially drained loblolly pine ( L.) plantation in eastern North Carolina, USA. Results indicated that the DRAINMOD-FOREST accurately predicted annual, monthly, and daily drainage, as indicated by Nash-Sutcliffe coefficients of 0.93, 0.87, and 0.75, respectively. The model also predicted annual net primary productivity and dynamics of leaf area index reasonably well. Predicted temporal changes in the organic matter pool on the forest floor and in forest soil were reasonable compared to published literature. Both predicted annual and monthly nitrate export were in good agreement with field measurements, as indicated by Nash-Sutcliffe coefficients above 0.89 and 0.79 for annual and monthly predictions, respectively. This application of DRAINMOD-FOREST demonstrated its capability for predicting hydrology and C and N dynamics in drained forests under limited silvicultural practices.

Isolation and identification of triglycerides and ester oligomers from partial degradation of potato suberin.

Suberized cell walls from wound-healing potato tubers ( Solanum tuberosum ) were depolymerized under mild conditions using methanolic potassium hydroxide in order to investigate the chemical linkages present in this protective plant biopolymer. Analysis of the resulting soluble oligomeric fragments with HPLC, 1D and 2D NMR, LC/MS, and MS(n) methods allowed identification of several novel compounds: a family of homologous triglycerides, a family of homologous aliphatic ester trimers, and an ether-linked phenylacetic acid dimer. These findings illustrate the diversity of rigid and flexible molecular linkages present in both poly(aliphatic) and poly(aromatic) domains of potato suberin, and they point toward architectures that may account for its function as a potent hydrophobic barrier to water, thermal equilibration, and microbial pathogens.

Isolation and identification of oligomers from partial degradation of lime fruit cutin.

Complementary degradative treatments with low-temperature hydrofluoric acid and methanolic potassium hydroxide have been used to investigate the protective biopolymer cutin from Citrus aurantifolia (lime) fruits, augmenting prior enzymatic and chemical strategies to yield a more comprehensive view of its molecular architecture. Analysis of the resulting soluble oligomeric fragments with one- and two-dimensional NMR and MS methods identified a new dimer and three trimeric esters of primary alcohols based on 10,16-dihydroxyhexadecanoic acid and 10-oxo-16-hydroxyhexadecanoic acid units. Whereas only 10-oxo-16-hydroxyhexadecanoic acid units were found in the oligomers from hydrofluoric acid treatments, the dimer and trimer products isolated to date using diverse degradative methods included six of the seven possible stoichiometric ratios of monomer units. A novel glucoside-linked hydroxyfatty acid tetramer was also identified provisionally, suggesting that the cutin biopolymer can be bound covalently to the plant cell wall. Although the current findings suggest that the predominant molecular architecture of this protective polymer in lime fruits involves esters of primary and secondary alcohols based on long-chain hydroxyfatty acids, the possibility of additional cross-linking to enhance structural integrity is underscored by these and related findings of nonstandard cutin molecular architectures.

Magic-angle spinning NMR studies of cell wall bound aromatic-aliphatic biopolyesters associated with strengthening of intercellular adhesion in potato (Solanum tuberosum L.) tuber parenchyma.

Intercellular adhesion strengthening, a phenomenon that compromises the texture and the edible quality of potatoes (Solanum tuberosum L.), has been induced reproducibly by exposure to low-pH acetic acid solutions under tissue culture conditions. The resulting parenchyma tissues have been examined by solid-state nuclear magnetic resonance (NMR) in order to characterize the biopolymer(s) thought to be associated with this syndrome. Cross polarization-magic angle spinning (CPMAS) (13)C NMR has been used to establish the presence of a polyphenol-suberin-like aromatic-aliphatic polyester within an abundant cell wall polysaccharide matrix in potato tubers that exhibit hardening due to strengthened intercellular adhesion. Dipolar dephasing and CP chemical shift anisotropy experiments suggest that the aromatic domain is composed primarily of guaiacyl and sinapyl groups. Two-dimensional wide-line separation experiments show that the biopolymer associated with parenchyma hardening contains rigid polysaccharide cell walls and mobile aliphatic long-chain fatty acids; (1)H spin diffusion experiments show that these flexible aliphatic chains are proximal to both the phenolics and a subpopulation of the cell wall polysaccharides. Finally, high-resolution MAS NMR of parenchyma samples swelled in DMSO in conjunction with two-dimensional through-bond and through-space NMR spectroscopy provides evidence for covalent linkages among the polysaccharide, phenolic, and aliphatic domains of the intercellular adhesion-strengthening biopolymer in potato parenchyma tissue.

Synthesis and structure identification of thiol conjugates of (-)-epigallocatechin gallate and their urinary levels in mice.

(-)-Epigallocatechin gallate (EGCG), the most abundant and most biologically active compound in tea, has been proposed to have many beneficial health effects. The metabolic fate of EGCG, however, is not well understood. In the present study, we found that EGCG can be oxidized by peroxidase and hydrogen peroxide and then reacted with cysteine or glutathione to form conjugates. The structures of the cysteine and glutathione conjugates of EGCG were identified using 2D NMR and MS. Two thiol conjugates of EGCG (2'-cysteinyl EGCG and 2' '-cysteinyl EGCG) were identified by ESI-LC-MS/MS analysis from the urine samples of mice administered 200 or 400 mg/kg EGCG, i.p. These conjugates were not found in urine samples of mice after receiving EGCG at 50 mg/kg i.p., or 2000 mg/kg i.g., or in human urine following consumption of 3 g of decaffeinated green tea solids (containing 333 mg EGCG). At high doses, EGCG is believed to be oxidized to form EGCG quinone, which can react with glutathione to form the thiol conjugates. These results suggest that detectable amounts of thiol conjugates of EGCG are formed only after rather high doses of EGCG are given to the mice.

New dibenzotropolone derivatives characterized from black tea using LC/MS/MS.

Theaflavins and thearubigins are major pigments in black tea, and it is generally accepted that they are produced by oxidation of flavan-3-ols (catechins) during tea fermentation. In the course of studies on the oxidation mechanism of tea polyphenols, especially the formation of thearubigins, a method combining the enzymatic synthesis and LC/ESI-MS/MS analysis was developed to search for new higher molecular weight polymers from black tea. Three new dibenzotropolones, theadibenzotropolone A, B, and C, together with one new tribenzotropolone, theatribenzotropolone A, were formed by the reaction of theaflavins and tea catechins with horseradish peroxidase in the presence of H(2)O(2). The structures of these new benzotropolone derivatives were elucidated on the basis of MS and 2D NMR spectroscopic analyses. The existence of these compounds in black tea was characterized by LC/ESI-MS/MS. Theadibenzotropolone A and B were the first benzotropolone-type trimers of catechins found in the black tea extract. The observation that galloyl ester groups of theaflavins can be oxidized to form di- or tri-benzotropolone skeletons strongly implied that this type of oxidation is an important pathway to extend the molecular size of thearubigins.

Enzymatic synthesis of tea theaflavin derivatives and their anti-inflammatory and cytotoxic activities.

Derivatives based on a benzotropolone skeleton (9-26) have been prepared by the enzymatic coupling (horseradish peroxidase/H2O2) of selected pairs of compounds (1-8), one with a vic-trihydroxyphenyl moiety, and the other with an ortho-dihydroxyphenyl structure. Some of these compounds have been found to inhibit TPA-induced mice ear edema, nitric oxide (NO) synthesis, and arachidonic acid release by LPS-stimulated RAW 264.7 cells. Their cytotoxic activities against KYSE 150 and 510 human esophageal squamous cell carcinoma and HT 29 human colon cancer cells were also evaluated.

Chemical studies of the antioxidant mechanism of tea catechins: radical reaction products of epicatechin with peroxyl radicals.

Tea catechins, an important class of polyphenols, have been shown to have antioxidant activity and are thought to act as antioxidants in biological systems. However, the mechanisms of their antioxidant reactions remain unclear. The objective of this study was to characterize the reaction products of epicatechin with peroxyl radicals generated by thermolysis of the azo initiator azo-bisisobutyrylnitrile (AIBN). Structural elucidation of these products can provide insights into specific mechanisms of antioxidant reactions. Eight reaction products were isolated and identified using high-field 1D and 2D NMR spectral analysis. The observation of these compounds confirmed that the B-ring is the initial site for formation of reaction products in the peroxyl radical oxidant system.

Unlocking the molecular structure of fungal melanin using 13C biosynthetic labeling and solid-state NMR.

Melanins are enigmatic pigments found in all biological kingdoms that are associated with a variety of functions, including microbial virulence. Despite being ubiquitous in nature, melanin pigments have long resisted atomic-level structural examination because of their insolubility and amorphous organization. Cryptococcus neoformans is a human pathogenic fungus that melanizes only when provided with exogenous substrate, thus offering a unique system for exploring questions related to melanin structure at the molecular level. We have exploited the requirement for exogenous substrate in melanin synthesis as well as the capabilities of high-resolution solid-state nuclear magnetic resonance (NMR) to establish the predominantly aliphatic composition of l-dopa melanin and to introduce (13)C labels that permit the identification of proximal carbons in the developing biopolymer. By swelling solid melanin samples in organic solvents and using two-dimensional heteronuclear NMR in conjunction with magic-angle spinning, we have identified chemical bonding patterns typical of alkane, alkene, alcohol, ketone, ester, and indole functional groups. These findings demonstrate the feasibility of a novel approach to determining the structure of melanin using metabolic labeling and NMR spectroscopy.