Jac J. Varco

Rhododendron Decline in the Great Smoky Mountains and Surrounding Areas: Intensive Site Study of Biotic and Abiotic Parameters Associated with the Decline

Abstract Rhododendron dieback was continuously observed with increasing frequency on Rhododendron maximum (Rosebay Rhododendron) during the last 20 years in the southern Appalachian Mountains. The dieback was especially evident following several years of drought from 2004 to 2008 recorded in Great Smoky Mountains National Park (GRSM). With the concern that a disease epidemic could occur, a holistic study evaluated site factors including tree health, number of clonal units, aspect, slope, depth to bedrock, and rhizosphere microbes. This study was conducted at two locations: Laurel Falls in GRSM and Albert Mountain in Nantahala National Forest (NNF). Yearly sampling for nematodes showed no differences in frequencies across or between years. A total of 11 species were identified from replicated healthy and dieback plots with no significant trends observed. Criconemella xenophus, Helicotylenchus sp., and Meloidogyne sp. were the species most commonly found. Belonolaimus sp. occurred at the NNF site at below 1% of the total nematode population identified, but this nematode species is considered damaging to crops and forest nursery seedlings even at low numbers. Fungal/Oomycota diversity and densities were determined from roots and rhzosphere soil samples using three identification methods. The results ranged from 110 species of fungi to 0 for Oomycota. Of 110 fungi isolated, one putative root pathogen was identified, and the saprophytic species Mycena silvae-nigrae (unknown Basidiomycota 1) was the most common match using the GenBank database. Elevation at NNF was significantly greater than at GRSM, with significantly greater dieback levels at the higher elevation. Furthermore, greater dieback ratings were associated with significantly greater tree diameters. No trends were observed for percent slope or nutrient levels when compared between healthy and dieback sites or locations. Site factors such as aspect, elevation, associated nematode species, and a putative root pathogen may form a disease complex resulting in Rosebay Rhododendron dieback.

Comparability of anaerobic swine lagoon effluent to commercial fertilizer on soil nutrient dynamics

The effects of swine effluent relative to commercial fertilizer on nutrient dynamics in two soils were studied. Swine effluent and commercial fertilizer were applied to established bermudagrass [Cynodon dactylon (L.) Pers.] on a slightly acid Vaiden silty clay (very fine, montmorillonitic, thermic, Aquic Hapludalfs) and to johnsongrass [Sorghum halepense (L.) Pers.] on a slightly alkaline Okolona silty clay (fine, montmorillonitic, thermic, Typic Chromuderts) from 1994 to 1996. Treatments were multiple effluent irrigations resulting in 4 nitrogen (N) rates from 0 to 665 kg N ha−1 yr−1. Treatments were arranged as randomized complete blocks and replicated four times. Commercial fertilizer treatments were applied at equivalent nitrogen–phosphorus–potassium (N–P–K) rates. The extractable soil P, K, calcium (Ca), magnesium (Mg), copper (Cu), and zinc (Zn) at the top 5 cm of soil profile increased with increasing swine effluent application rates. For both soils, the high effluent and medium fertilizer treatments resulted in similar NO3‐N concentrations when compared to lower application rates. Excessive soil NO3‐N was observed with the high fertilizer treatments with concentrations of 50 mg kg−1 or more for some depths of both soils. No significant differences in extractable soil K and P were observed between swine effluent and commercial fertilizer at the low and medium rates. However, at the high rate, P and K contents were greater for swine effluent than for fertilizer. Swine effluent applied at a rate equivalent to approximately 448 kg N ha−1 y−1 minimized soil residual and salt content, and appears to be the optimal rate under the conditions of this study.

Development of sensor based detection of crop nitrogen status for utilization in variable rate nitrogen fertilization

Early season detection of corn (Zea mays L.) and cotton (Gossypium hirsutum L.) N status as an in-field indicator of spatial N availability and fertilizer N demand has been difficult with proximal sensors. The objective of this research was to evaluate selected vegetation indices (VI) for their ability to detect early season crop N status in cotton and corn. A tractor mounted sensor was used to acquire canopy reflectance at an oblique angle and selected VIs were related to cotton leaf N during early flower bud formation, and corn leaf and whole plant tissue N concentrations and whole plant N content at 6 fully collared leaves. Indices utilizing a red edge component were the most consistent and resulted in stronger relationships to plant tissue N concentration for early season sampling.

Nitrogen availability from alfalfa suppressed or killed for no‐till production

Abstract There is interest in intercropping perennial legumes with com (Zea mays L.) under no‐tillage soil management. Evaluation of N availability by measuring plant N uptake in field research trials with such systems is often complicated by competition for water. We monitored soil inorganic N (ammonium and nitrate) levels at 14‐day intervals for 42 days at 0–10 cm and 10–20 cm soil depths after an alfalfa (Medicago sativa L.) sod was subjected to four different suppression treatments: a) cut and remove, b) cut and return, c) above‐ground kill with paraquat, and d) complete kill with glyphosate. The initial (14 day) release of N was similar in all treatments where residues were left or returned, but alfalfa regrowth immobilized much of the N mineralized in all treatments, except where alfalfa was killed. The greatest quantity of soil inorganic N was found on day 28 where alfalfa was killed, equal to nearly 72% of the N contained in the alfalfa topgrowth. Soil nitrate N concentrations, averaged to a depth...

All Taxa Biodiversity Inventory Survey of Select Soil and Plant Ecological Parameters Associated with Rhododendron Decline in the Great Smoky Mountains and Surrounding Areas

Abstract As part of the All Taxa Biodiversity Inventory (ATBI) of Great Smoky Mountains National Park (GRSM), select site parameters associated within upland sites were measured within variable sized pockets of stressed and dead Rhododendron maximum (Great Rhododendron). During the last 20 years, Great Rhododendron, an important shrub in the southern Appalachian Mountains, has been dying in small to larger areas from an unknown cause. With increased visibility of dieback, the study (2006–2009) was conducted at 10 sites in GRSM and 1 in Nantahala National Forest (NNF). Eleven nematode species were identified, with no specific trends across locations or by plot treatments in frequencies of occurrences. Exceptions were Criconemella xenoplax, which was found at all 11 locations and was generally significantly greater in healthy or control than dieback plots, whereas Helicotylenchus sp. was more frequent in dieback plots. Meloidogyne spp., known parasitic nematodes of woody plants and agricultural crops, occurred in over 50% of the locations; Hoploliamus sp. occurred at 40%; and Belomolaimus sp., which are very destructive to root systems, were found at low levels at the NNF site. Also, Heterodera sp. occurred in control and dieback plots in 10 of the locations. Dieback ratings were not significantly correlated to stem diameter; although the finding was not statistically significant, six of seven sites with dieback plots had numerically greater-sized stems than in the controls. No other parameters such as number of clonal units, site aspect, percent slope, or elevation showed any trends at both sites. Nutrient data did not indicate any specific relationships to plot damage or health. This study provides the first comprehensive reporting of nematode species that occur in Great Rhododendron and associated riparian and upland sites. With concerns of global climate impacts, this research provided additional baseline data for the ATBI of GRSM and NNF.

Effects of Swine Lagoon Effluent and Commercial Fertilizer Applications on Phosphorus Status of an Acid and Alkaline Soil

Abstract Two separate field experiments were conducted to evaluate the effects of swine lagoon effluent relative to inorganic fertilizer at equivalent rates on phosphorus (P) status of an acidic Vaiden (very fine, montmorillonitic, thermic, Aquic Dystrudert) and an alkaline Okolona (fine, montmorillonitic, therimic, Typic Chromudert) silty clay soil. In each site, a randomized complete block design with a factorial arrangement of treatments was used. Treatments were replicated four times. Cumulative swine lagoon effluent P application rates for the year 1994 through 1996 were 0, 59, 121, and 175 kg P ha−1 on the Vaiden soil and 0, 72, 148, and 223 kg P ha−1 on the Okolona soil. In each replication, commercial fertilizer P at rates equivalent to swine effluent P application were also included. For both sites, soil P concentration increased with increasing swine effluent and commercial fertilizer P applications. No significant difference in soil P level was observed between two P sources. At high application rate, desorbed P was 1.20 and 0.59 mg P kg−1 in the Okolona and Vaiden soil respectively. In the Vaiden soil, P adsorption approached the maximum for equilibrium P concentration greater than 600 mg L−1. However, Okolona soil displayed a linear adsorption potential with application of swine effluent P. Among P fractions, NH4Cl‐P and HCl‐P concentrations increased the most compared to the check in both Okolona and Vaiden soils. Results indicated that P status differs between the soils, but no significant differences in P concentration were obtained between swine lagoon effluent and commercial fertilizer, suggesting that both P sources had similar effect on soil P after 3 years of application.