This paper uses a refined soil gradient method to estimate soil CO2 efflux. Six different models are used to determine the relative gas diffusion coefficient (ξ). A weighted harmonic averaging is used to estimate the soil CO2 diffusion coefficient, yielding a better estimate of soil CO2 efflux. The resulting soil CO2 efflux results are then compared to the soil CO2 efflux measured with a soil chamber. Depending on the choice of ξ model used, the estimated soil CO2 efflux using the gradient method reasonably approximates the efflux obtained using the soil chamber method. In addition, the estimated soil CO2 efflux obtained by this improved method is well described by an exponential function of soil temperature at a depth of 0.05 m with the temperature sensitivity (Q10) of 1.81 and a linear function of soil moisture at a depth of 0.12 m, in general agreement with previous findings. These results suggest that the gradient method is a practical cost‐effective means to measure soil CO2 emissions. Results from the present study suggest that the gradient method can be used successfully to measure soil CO2 efflux provided that proper attention is paid to the judicious use of the proper diffusion coefficient.

No abstract

Abstract. Continuous measurements of net ecosystem CO 2 exchange (NEE) using the eddy-covariance method were made over an agricultural ecosystem in the southeastern US. During optimum environmental conditions, photosynthetically active radiation (PAR) was the primary driver controlling daytime NEE, accounting for as much as 67 to 89% of the variation in NEE. However, soil water content became the dominant factor limiting the NEE-PAR response during the peak growth stage. NEE was significantly depressed when high PAR values coincided with very low soil water content. The presence of a counter-clockwise hysteresis of daytime NEE with PAR was observed during periods of water stress. This is a result of the stomatal closure control of photosynthesis at high vapor pressure deficit and enhanced respiration at high temperature. This result is significant since this hysteresis effect limits the range of applicability of the Michaelis-Menten equation and other related expressions in the determination of daytime NEE as a function of PAR. The systematic presence of hysteresis in the response of NEE to PAR suggests that the gap-filling technique based on a nonlinear regression approach should take into account the presence of water-limited field conditions. Including this step is therefore likely to improve current evaluation of ecosystem response to increased precipitation variability arising from climatic changes.

Recent peanut cultivar releases are trending to a larger seed size, but have great resistance to tomato spotted wilt virus (TSWV). Larger-seeded cultivars cost more to plant than smaller at an equivalent population. Reduced seeding rates could save growers on seed costs and impede the spread of southern stem rot, but can reduce plant stands which can lower yields and increase TSWV incidence. Therefore, the objectives of this experiment were to compare seven peanut cultivars (Georgia Green, Georgia-06G, AT 3085RO, Florida-07, Tifguard, AP-3, and Georgia-03L) in single and twin row patterns at three seeding rates (17, 20, and 23 seed/m) on a sandy loam soil at Plains, GA for disease incidence, agronomic, and economic performance. Measured variables included yield and grade, plant height and stand, TSWV and southern stem rot incidence, and adjusted net revenue in 2008 and 2009. Twin rows outperformed single rows whenever differences occurred. The only factors consistently affected by reducing seeding rate were plant height and stand, both decreased at the lowest seeding rate. There was a trend toward lower yields (approximately 6% reduction) at the 17 seed/m rate in twin row pattern, although net returns were not diminished compared to the higher seeding rates since lower seed costs offset yield reductions. The cultivars Georgia-06G and Florida-07 had the highest yield and adjusted net revenue among the seven cultivars in both years. Tifguard and Georgia Green had lowest overall yields and would not be preferred cultivars in sandy loam soils. This study demonstrates that twin rows have higher yield, plant stands, and net revenue, plus reduced TSWV incidence than single row pattern, and a reduction in seeding rate to 17 seed/m can be made without serious risk of lost revenue. However, benefits of reducing seeding rate in twin rows were not as pronounced as they were for single rows, and exhibited a greater potential for lower yield. A grower planting in single rows would likely have the most to gain from planting fewer seed, especially under heavy southern stem rot pressure, but planting in twin rows would still be a preferred option over single rows.

Three peanut cultivars, Georgia Green, NC-V11, and ANorden, were grown using production practices that encouraged the development of Tomato spotted wilt virus (TSWV). The progression of TSWV infection was examined through the season using enzyme-linked immunosorbent assay (ELISA) tests on different tissue types [roots, leaves, pegs (pod attachment stem structures) and pods] and the effect of TSWV infection on physiological functions was examined at three harvest dates. Plants were classed into three severity categories: (i) no TSWV symptoms or previous positive ELISA tests; (ii) less than 50% of leaf tissue exhibiting TSWV symptoms; and (iii) greater than 50% of leaf tissue affected. TSWV showed a slow rate of infection at the beginning of the season and a greater percentage of infection of the roots than in the leaves. Photosynthesis was reduced in virus-affected infected plants by an average of 30% at the mid-season harvest and 51% at the late season harvest compared with virus-free plants across all three cultivars. Leaf tissue with symptoms had lower photosynthetic rates than healthy leaves. There were small differences among cultivars, with cv. ANorden maintaining higher average photosynthetic levels than cv. Georgia Green and higher transpirational levels than cv. NC-V11. The ability to maintain high assimilation physiology in the presence of the virus may help cultivars withstand TSWV infection and maintain final yields.

Field experiments were conducted at Gainesville and Marianna, FL in 2004 and 2005 in which severity of spotted wilt, caused by Tomato spotted wilt virus, and pod yield were compared in six peanut (Arachis hypogaea) cultivars. The six cultivars included the moderately field resistant cultivars ANorden, C-99R, and Georgia Green; the highly field resistant cultivars AP-3 and DP-1; and the susceptible cultivar SunOleic 97R. There were four trials at each location, with four planting dates that ranged from late March to early June. Tomato spotted wilt severity in moderately resistant and susceptible cultivars was lower at Gainesville than at Marianna in both years in moderately resistant and susceptible cultivars. Trends in incidence for the two locations were less evident for AP-3 and DP-1. At Gainesville, there were few differences in tomato spotted wilt severity, and severity ratings were similar for Georgia Green and SunOleic 97R in two of four trials in 2004 and across all trials in 2005. At Marianna, severity ratings were lower for Georgia Green than for SunOleic 97R in six of the eight trials, and severity of tomato spotted wilt was lower for AP-3, C-99R, and DP-1 than for Georgia Green in all eight trials. In 2004, there was a trend toward decreasing severity ratings for Georgia Green and SunOleic 97R with later planting dates, but not for AP-3 or DP-1 at Marianna. Split-plot field experiments were also conducted at Tifton, GA in 2005 through 2007 in which incidence of tomato spotted wilt and pod yield were compared for peanut cultivars AP-3 and Georgia Green across planting dates ranging from late April through late May. Incidence of tomato spotted wilt was lower for AP-3 than for Georgia Green within each planting date of all years, and planting date effects were smaller in AP-3, if observed at all, than in Georgia Green. In most planting dates of all three trials, yields were higher for AP-3 than for Georgia Green. The relationships between yield and planting date were not consistent. These results indicate that the level of field resistance in AP-3 and DP-1 cultivars is sufficient to allow planting in late April without greatly increasing the risk of losses to tomato spotted wilt.

Tomato spotted wilt virus (TSWV), a member of the genus Tospovirus (family Bunyaviridae), is an important plant virus that causes severe damage to peanut (Arachis hypogaea) in the southeastern United States. Disease severity has been extremely variable in individual fields in Georgia, due to several factors including variability in weather patterns. A TSWV risk index has been developed by the University of Georgia to aid peanut growers with the assessment and avoidance of high risk situations. This study was conducted to examine the relationship between weather parameters and spotted wilt severity in peanut, and to develop a predictive model that integrates localized weather information into the risk index. On-farm survey data collected during 1999, 2002, 2004, and 2005 growing seasons, and derived weather variables during the same years were analyzed using nonlinear and multiple regression analyses. Meteorological data were obtained from the Georgia Automated Environmental Monitoring Network. The best model explained 61% of the variation in spotted wilt severity (square root transformed) as a function of the interactions between the TSWV risk index, the average daily temperature in April (TavA), the average daily minimum temperature between March and April (TminMA), the accumulated rainfall in March (RainfallM), the accumulated rainfall in April (RainfallA), the number of rain days in April (RainDayA), evapotranspiration in April (EVTA), and the number of days from 1 January to the planting date (JulianDay). Integrating this weather-based model with the TSWV risk index may help peanut growers more effectively manage tomato spotted wilt disease.

Experiments conducted from 2007 to 2009 evaluated germination of 11 peanut runner-type cultivars. Germination was evaluated in Petridishes incubated over a thermal gradient ranging from 14 to 30°C at 1.0 C increments. Beginning 24 hr after seeding, peanut was counted as germinated when radicles were greater than 5 mm long, with removal each day. Germination was counted daily for seven days after seeding. Growing-degree day (GDD) accumulation for each temperature increment was calculated based on daily mean temperature for that Petri dish. Two indices were obtained from a logistic growth curve used to elucidate seed germination by cultivar: (1) maximum indices of germination and (2) GDD value at 80% germination (Germ80), an indication of seed vigor the lower the Germ80value, the greater the seed lot vigor. Based on the two indices, seed lots “AT 3081R”, “AP-3”, “GA-06G”, and “Carver” had the strongest seed vigor (Germ8026 to 47 GDD) and a high maximum incidence of germination rate (80 to 94%). Seed lots of “C99-R”, “Georgia-01R”, “Georgia-02C”, and “Georgia-03L” had inconsistent seed performance, failing to achieve 80% germination in at least two of three years.