Browsing by Author "Wei Shi, Committee Member"

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Cultural and Biological control methods for Phytophthora root rot in Fraser fir
(2009-11-16) Richter, Brantlee Spakes; Wei Shi, Committee Member; D. Michael Benson, Committee Co-Chair; Kelly L. Ivors, Committee Co-Chair; David Shew, Committee Member
Phytophthora root rot of Fraser fir caused by Phytophthora cinnamomi and several other Phytophthora spp. is a severe problem in Christmas tree production. Since fungicides are ineffective in disease control and host resistance is not yet available, cultural control methods are under investigation as a means of reducing disease pressure on infested production sites. Mulching systems with raised beds of pine bark, wood chips, or wood chips blended with compost were tested, along with compost or sulfur as soil amendments, at five sites spanning the western North Carolina growing region. Microbial populations and activity in soils and mulches were characterized over a two year period, using dilution plating with calculation of a log series diversity index for counts of bacteria, fungi, and cellulose-degrading micoorganisms, analysis of fluorescein diacetate hydrolysis for estimation of total microbial activity, and quantification of reducing sugars after incubation with carboxymethyl-cellulose for estimation of total cellulase enzyme activity. Bacterial and fungal counts, microbial activity, and cellulase activity were higher in mulch than in soil at all sites and times (P<0.01), and generally did not differ among mulch types nor among soils. Treatments significantly affected disease ratings and tree survival at three of five sites, with one or more mulch treatments yielding lower disease ratings and greater survival than controls. Tree mortality at each time point varied significantly with cellulase activity in the upper portion of the root zone (P=0.005). Other biological variables did not show significant relationship with disease ratings or mortality. To further investigate the role of cellulase enzymes in suppression of P. cinnamomi, a commercial formulation of cellulase was used to generate a standard curve which could be used to correlate cellulase activity levels in field samples with the enzyme unit concentrations commonly used in laboratory studies. Two isolates of P. cinnamomi were exposed to a range of enzyme concentrations, and data were collected on biomass and sporangia production. Cellulase exposure reduced sporangia production but did not reduce biomass within the range analogous to that observed in field-applied mulch. In a bioassay with lupine, cellulase applied to soil containing infested root fragments did not reduce disease progress. Container trials were also used to assess the impacts of a wide range of organic and inorganic amendments on Phytophthora root rot in Fraser fir seedlings, and to examine the contributions of compost, microbial inoculants, and isolates of cellulytic fungi to disease suppression in wood chip mulches. In trials with wood chips blended into soil and trials with seedlings planted directly into wood chip mulch, seedling survival was greater when wood chips were amended with compost, a soil inoculant, or a biocontrol agent. If wood chips were amended with compost, the addition of a cellulytic fungus, including a known biocontrol agent, did not further enhance plant survival, and in most cases did not significantly increase cellulase activity over compost amended wood chips alone.
Establishment and Stabilization of pH in Container Root Substrate.
(2010-07-23) Jeong, Ka Yeon; Paul Nelson, Committee Chair; Dean Hesterberg, Committee Member; Brian Whipker, Committee Member; Wei Shi, Committee Member; Markus Peterson, Committee Member
Hydrologic Effects on Subsurface Fates and Transport of Contaminants
(2009-06-19) Abit, Sergio Manacpo, Jr.; Aziz Amoozegar, Committee Co-Chair; Michael Vepraskas, Committee Co-Chair; Wei Shi, Committee Member; Owen Duckworth, Committee Member; William Showers, Committee Member
Concerns over contamination of ground water (GW) and its subsequent effect on surface water quality underscore the need for an improved understanding of the fate and transport of the contaminants in the subsurface. Among the contaminants that are harmful to humans and the environment are nutrient pollutants [e.g., nitrogen (N) and phosphorus (P)] and microbes. The general goal of this research was to evaluate the subsurface fates and transport of contaminants in a vadose zone-GW continuum under various simulated hydrologic conditions through a series of laboratory-scale studies. The first study, which aimed to visually evaluate the effects of GW velocity and water table (WT) fluctuation on the fate and extent of horizontal transport of solutes and microbes in the capillary fringe (CF) and GW, was conducted in a sand-packed flow cell. Subsurface transport of surface-applied solutes and microbes tended to be isolated in the CF at a higher pore-water velocity. A rise in WT resulting from surface recharge of contaminated water occurred without the contaminants reaching the GW. Subsequent drainage did not effectively leach contaminants that were initially in the CF into the GW. The second study assessed the effect of pore-water velocity on the development of reduced conditions in a vadose zone-GW continuum. Reduction potential (Eh) was monitored at various locations in flow cells packed with Ponzer (Terric Haplosaprists), Lynchburg (Aeric Paleaquult), and Leon (Aeric Alaquod) soil materials that were subjected to different lateral pore-water velocities. Regardless of organic carbon (OC) content of the soil materials (12.4 to 195 g kg-1), locations close to the WT became reduced within 14 days. In contrast, the upper portions of the CF remained oxic. Increasing the pore-water velocity also slowed the development of reducing conditions especially in soils with low OC content. The third study was conducted to evaluate the effect of pore-water velocity on the fate and transport of nitrate (NO3-) in a simulated vadose zone-GW continuum. This was conducted in flow cells packed with soils of various OC content (0.3 to 35 g kg-1) that were subjected to different horizontal-water velocities. Nitrate and bromide (Br) concentrations as well as Eh at various locations along the flow path of an applied NO3- and Br- solution were monitored. Results show that in the presence of sufficient OC, NO3- was lost under reducing conditions below the WT but persisted while in transport in aerobic regions in the CF. Increasing GW flow pore-water velocity from 3.5 to 28 cm d-1 reduced the degree of NO3- removal from solution. High flow velocity also tended to limit the horizontal transport of surface-applied NO3- only in the upper regions of the CF. The fourth study was conducted to evaluate the dissolution of phosphorus (P) in pore-water flowing through the vadose zone-GW continuum. Distilled water was allowed to flow horizontally at different pore-water velocities through flow cells packed with an organic soil material (from Ponzer series). Extensive P dissolution was detected below and just above the WT. Phosphorus dissolution at the upper portion of the CF was relatively limited. These results suggest the following: a) the non-detection of contaminants below the WT down-gradient from a source does not definitively indicate that contaminants are not being transported horizontally in the subsurface as they can be transported in the CF, b) collection of samples from the CF should be considered when monitoring the subsurface transport of contaminants, and c) the hydrology of a system could be managed to improve nitrate removal from solution or to limit P dissolution.
Impact of Management and Texture on Soil Organic Matter Fractions
(2007-12-20) Gruver, Joel Brooks; Michael Wagger, Committee Co-Chair; Shuijin Hu, Committee Co-Chair; Dean Hesterberg, Committee Member; Wei Shi, Committee Member
Growing concerns about elevated levels of atmospheric CO2 and associated climate change have increased interest in soil C. While general increases in the adoption of conservation management practices may result in C sequestration, efficient utilization of soil as a C sink will require identification of soils with high potential for sequestration and improved methods of monitoring soil C. The objectives of this research were to: 1) evaluate the historical roots, experimental validation and subsequent impact of the C saturation relationships proposed by Jan Hassink, 2) evaluate the effects of management and texture on aggregation and C fractions using soil from two long term experiments, 3) develop new methods of structural disruption and physical fractionation that address shortcomings in existing methods, 4) evaluate the impact of antecedent C on C and aggregate dynamics and 5) evaluate the simplified MnoxC method proposed by Weil et al. (2003). Collectively, the literature we reviewed did not support broad application of simple C saturation relationships such as those proposed by Hassink but did support selective use of fine mineral content as an indicator of C storage capacity. Results from two incubation experiments demonstrated the modulating effect of antecedent C on soil C and aggregate dynamics following structural disruption and residue addition. Positive effects of residue and structural disruption on aggregation were greatest in soil with low antecedent C. Residue decomposed more rapidly in soil with high antecedent C but had a greater priming effect in soil with low antecedent C. Addition of a 15N labeled nitrate source revealed that immobilization of nitrate-N within microaggregates is a minor process irrespective of structural disruption and antecedent C. Carbon contained in microaggregates within stable macroaggregates from an organic transition experiment was sensitive to C input regime but unrelated to fine mineral content. Strong tillage system effects on C fractions, aggregation and texture (tillage intensity↑ = ↓C, aggregate stability and sand content) were identified in soil from a long term tillage system study. Permanganate oxidizable C (Weil method) was found to be a sensitive indicator of management effects on soil C particularly after correction for non-linearity.
Managing nitrogen from swine and poultry manure in North Carolina.
(2004-01-08) Allen, Mark Benjamin; Philip W. Westerman, Committee Member; Michael G. Wagger, Committee Member; Wei Shi, Committee Member; Robert L. Mikkelsen, Committee Chair
With increasing pressure to regulate land application of animal manure, North Carolina faces a difficult dilemma, given the number of large-scale animal production facilities currently in operation. Poultry and swine industries in the state generate large volumes of animal manure that must be properly managed in order to avoid loss of N to ground water and surface water bodies. Using swine manure as an N source for soybean production is not commonly practiced due to soybean's ability to fix N, but recent research suggests that soybean may be a suitable receiver crop of anaerobic swine lagoon effluent. The objectives of this research were twofold: (1) determine the quantity of swine effluent-derived N taken up by soybean and estimate the degree of inhibition of symbiotic N-fixation and (2) determine how soil pH affects N mineralization, nitrification and immobilization when soil is amended with broiler litter. Swine effluent was spiked with (15NH4)2SO4 in order to attain a mean final 15N enrichment of 5.765 atom % 15N. The enriched effluent was applied 6 times at weekly intervals to nodulating and nonnodulating soybean growing in one-meter deep lysimeters at a rate of 185 kg PAN ha-1. Additional lysimeters with nodulating and nonnodulating soybean received no applications of effluent. Leachate was collected on a weekly basis and analyzed for 15N and total N. Soybean were harvested near maturity and analyzed for 15N and total N. Biological N-fixation in soybean was not completely inhibited when swine effluent was added and accounted for 55% of the total N in the shoot. Nodulated and nonnodulated soybean shoots recovered similar amounts of effluent N (36.6% and 33.4%, respectively). The addition of effluent and nodulation were both important sources of N for soybean growth, although the results suggest that nodulating and nonnodulating soybean behaved differently when they received effluent additions, as indicated by significant interactions. The experimental data showed that less than 1% of the added effluent N was accounted for in the leachate. An N budget of the plant-soil-water system showed that, of the effluent N added to nodulated soybean, 37% remained in the soil after the soybean were harvested, while 33% remained in the effluent-treated nonnodulated soybean. These results suggest that soybean can serve as an N receiver crop when swine effluent is the N source. To determine the effects of soil pH on N transformations in broiler litter amended soils, Wagram loamy sand with a pH of 4.4 was collected from a forested area near Clayton, NC, and sub-samples were limed to pH 4.8, 5.3, 5.8, 6.4, and 7.0. Broiler litter was added at a rate of 155 kg PAN ha-1 to the limed soils and incubated at 25°C and 60% of field capacity for 112 d. Total inorganic N was measured at 0, 7, 14, 28, 56, 77, and 112 d. Cumulative net N mineralized was fitted to a first order model to determine potentially mineralizable N. Although nitrification rates increased as soil pH increased, there were significant inverse relationships between soil pH and net N mineralized, as well as soil pH and potentially mineralizable N. Isotope dilution measurements showed that gross and net mineralization rates were equivalent, refuting the notion that relatively more NH4 immobilization had occurred in the high pH soils. The results indicate that N mineralization was enhanced at low soil pH, a phenomenon that presently is not fully understood.
Nitrogen Relations in Bermudagrass During Growth and Dormancy Cycles
(2007-12-19) Wherley, Benjamin George; Thomas Rufty, Committee Chair; Daniel Bowman, Committee Co-Chair; Fred Yelverton, Committee Member; Wei Shi, Committee Member; Art Bruneau, Committee Member
Use of recycled water for turfgrass irrigation is increasing in the southeastern U.S. because of population growth and interest in protecting water quality. Turfgrass systems are perceived to be well suited for effluent dispersal due to their proximity to waste treatment facilities, in-ground irrigation systems, and ability to efficiently absorb (i.e. filter) nutrient contaminants when actively growing. However, effluent generation is continuous and bermudagrass growth is seasonal in the southeastern U.S. Clearly, there is a need to more thoroughly understand the capacity of bermudagrass, the turfgrass most often involved with effluent dispersal, for receiving effluent irrigation. This series of experiments was designed with the overall intent of examining the capacity of a bermudagrass turf⁄soil system for handling effluent applications. Experiments involved 1) characterizing seasonal changes in nitrate assimilation efficiency of the system, 2) determining the effects of prolonged soil saturation on nitrate uptake efficiency, and 3) characterizing internal nitrogen relations during the spring emergence period. While it is difficult to extend the results of these experiments, quantitatively, to situations where effluent is being applied in the field, the evidence does support a few basic observations. Bermudagrass appears to be capable of assimilating large amounts of N when growing, an ability that may well extend into transition months when little vertical shoot growth is occurring. Furthermore, although reduction in quality occurred, shoot growth and nitrate uptake efficiency of bermudagrass and centipedgrass was relatively unaffected by prolonged saturated soil conditions, a condition that may be likely with effluent irrigated sites.
Phosphate Sorption and Reductive Dissolution in Aluminum/Iron-Hydroxide Co-Precipitates.
(2010-08-16) Liu, Yu-Ting; Dean Hesterberg, Committee Chair; Wayne Robarge, Committee Member; James Martin, Committee Member; Wei Shi, Committee Member; Mari Chinn, Committee Member
Phosphorus Release Mechanisms in Rhizospheres of Wetland Trees.
(2010-07-27) Moorberg, Colby; Michael Vepraskas, Committee Chair; Wei Shi, Committee Member; Stephen Broome, Committee Member; Daniel Richter, Jr., Committee Member
Quantifying the effects of organic residues on soil nitrogen and phosphorus availability
(2008-04-21) Tisdale, Jessica Linn; H. Lee Allen, Committee Co-Chair; Jennifer N. Phelan, Committee Co-Chair; Wei Shi, Committee Member; Christopher Maier, Committee Member
Relationships Between Soil Biological and Physical Properties in a Long-term Vegetable Management Study
(2005-11-29) Overstreet, Laura Flint; Shuijin Hu, Committee Member; Greg D. Hoyt, Committee Chair; Michael Wagger, Committee Member; Wei Shi, Committee Member
Agricultural management decisions that influence biological activity and diversity include tillage, fertilizer and pest-control inputs, and crop rotations. Our research objective was to characterize relationships between biological and physical properties resulting from long-term agricultural management decisions. A nine-year old factorially-designed field experiment was used to examine the effects of tillage (moldboard plow or strip-tillage), input (synthetic fertilizers and pesticides or inputs approved for organic certification programs), and crop rotation (continuous staked tomatoes or 3-year vegetable rotation) on a suite of biological and physical soil parameters. Biological measurements included microbial, nematode, and earthworm community composition, soil respiration and N mineralization potential, enzyme activity, and microbial biomass. Physical property measurements included aggregate stability, bulk density, and pore-size distribution. Biological properties generally responded to all treatment combinations, but tillage provided the strongest treatment effect in most cases. Compared to strip-tillage, tillage consistently yielded significantly lower values for the following biological measurements: total C and N, above-ground biomass, microbial biomass, enzyme activity, soil respiration, N mineralization, some nematode trophic groups, and earthworms. Compared with organic inputs, synthetic inputs consistently induced significantly lower values for the following biological measurements: microbial biomass, enzyme activity, some nematode trophic groups, and soil respiration. An examination of relationships between biological and physical parameters using redundancy analysis revealed that microporosity was the physical property that was most strongly correlated with most biological parameters. Soil organisms responded to our treatments in the following order: tillage > input > rotation.
Soil Properties Affect Simazine and Saflufenacil Fate, Behavior, and Performance.
(2008-04-15) Hixson, Adam Charles; Fred H. Yelverton, Committee Co-Chair; Jerome B. Weber, Committee Co-Chair; Thomas W. Rufty, Committee Member; Wei Shi, Committee Member
Soil Reduction Rates under Water Saturated Conditions in Relation to Soil Properties.
(2008-08-16) Zelasko, Amanda Jean; Michael J Vepraskas, Committee Co-Chair; Dean Hesterberg, Committee Co-Chair; Wei Shi, Committee Member; Daniel Richter, Committee Member
The success of wetland restoration projects depends in part on the length of time that a soil is in a reduced redox state. The length of time that a soil is reduced depends on how quickly reduction occurs following saturation with water. The relationship between reduction rate and various soil chemical and mineralogical properties is poorly understood, but such properties might be manipulated to improve the success of wetland restoration projects. The goals of this research were to determine soil properties that predict the rate at which soils undergo reduction when saturated, and to determine the roles of electron donors and acceptors on reduction rates. Sixteen soil samples were collected at various depths from two wetland sites, a Carolina bay (Juniper Bay) and a wetland catena (Frog Level). Soils were incubated in specially designed redox incubators to monitor reduction rates, changes in soil properties, and soil solution chemistry. Soil samples were subjected to three cycles of oxidation and reduction during the course of 36 d. Soil reduction rates were determined from the slopes of linear regression models fit to data for redox potential (Eh) over time. Reduction rates varied among soils from 1.2 to 46.2 mV h-1, and were significantly greater (p-value < 0.05) for soils with total organic carbon (TOC) > 10 g kg-1 than in soils with TOC < 10 g kg-1. Increasing amounts of dissolved Fe(II) were found at Eh values below 500 mV for pH between 4.5 and 5.1. Mineral soils with total reduction rates > 10 mV h-1 released significantly more Fe(II) into solution than mineral soils with reduction rates < 10 mV h-1 (p-value < 0.05). Regression results indicated that organic carbon, an electron donor, was the dominant factor controlling reduction rates up to 10 mV h-1, and an electron acceptor Fe(III) was the dominant factor controlling reduction rates > 10 mV h-1. For wetland restoration purposes multiple linear regression models based on our results that include TOC concentration and pH can be used along with hydrologic data to predict reduction rates in saturated soils.