A gronomy J our n al • Volume 10 0 , I s sue 3 • 2 0 0 8 551 ABSTRACT To improve site-specifi c N recommendations a more complete understanding of the mechanisms responsible for synergistic relationships between N and water is needed. Th e objective of this research was to determine the infl uence of soil water regime on the ability of corn (Zea mays L.) to use N derived from fertilizer and soil. A randomized split-block experiment was conducted in 2002, 2003, and 2004. Soil at the site was a Brandt silty clay loam (fi ne-silty, mixed, superactive frigid Calcic Hapludoll). Blocks were split into moderate (natural rainfall) and high (natural + supplemental irrigation) water regimes. Nitrogen rates were 0, 56, 112, and 168 kg urea-N ha -1 that was surface applied. Water, soil N, and N fertilizer use effi ciencies were determined. Plant utilization of soil N was determined by mass balance in the unfertilized control plots and by using the δ 15 N approach in fertilized plots. Findings showed that: (i) plants responded to N and water simultaneously; (ii) N fertilizer increased water use effi ciency (170 kg vs. 223 kg grain cm -1 in 0 and 112 kg N ha -1 treatments, respectively); and (iii) water increased the ability of corn to use N derived from soil (67.7 and 61.6% effi cient in high and moderate water regimes, respectively, P = 0.002) and fertilizer (48 and 44% effi cient in high and moderate water regimes, respectively, P = 0.10). Higher N use effi ciency in the high water regime was attributed to two interrelated factors. First, total growth and evapotranspiration (ET) were higher in the high than the moderate water regime. Second, N transport to the root increased with water transpired. For precision farming, results indicate that: (i) the amount of N fertilizer needed to produce a kg of grain is related to the yield loss due to water stress; and (ii) the rate constant used in yield goal equations can be replaced with a variable.

Techniques for measuring soil organic C (SOC) turnover in production fields are needed. The objectives of this study were to propose and test nonisotopic and 13 C stable isotopic techniques for assessing SOC turnover. Based on SOC equilibrium and mass balance relationships, an equation was derived: NHC/SOC initial 5 [1/(SOC 3 k NHC )] (dSOC/dt) 1 k SOC /k NHC , where dSOC/dt is the annual change in SOC, NHC is nonharvested C returned to soil, k SOC is the annual mineralization rate of SOC, and k NHC is the annual mineralization rate of NHC. This equation was used to calculate maintenance rates. An isotopic approach based on simultaneously solving the equations was developed to determine C budgets:and (iv) SOC final 5 SOC retained 1 PCR incorp , where e is the Rayleigh fractionation constant, PCR incorp is the amount of NHC incorporated into SOC final with D PCR being the associated 13 C discrimination (D) value, and SOC retained is the amount of SOC initial retained in the soil after mineralization with D SOC retained being the associated D value.Isotopic and nonisotopic approaches were tested on a production field where aboveground corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields were measured with a yield monitor and soil samples collected from a slightly offset grid (30 m) in 1995 and 2003 were analyzed for organic C and D. The nonisotopic approach showed that maintenance rates increased with SOC and that an accurate measure of NHC was required to calculate maintenance requirements. Sensitivity analysis of the isotopic approach showed that calculated budgets were sensitive to 13 C discrimination during SOC mineralization. If 13 C discrimination during SOC and NHC mineralization did not occur (e 5 0), then 14.9 and 7.6% of the SOC measured in 1995 (SOC initial ) was mineralized, and 7420 and 2780 kg C ha 21 of NHC were incorporated into SOC final in the 523.4-to 527.3-and 527.3-to 529.2-m elevation zones, respectively. If 13 C discrimination occurred (e 5 22.52%) during SOC mineralization, then the calculated amount of SOC mineralized and the amount of new C incorporated into SOC were reduced.

Competition mechanisms among adjacent plants are not well understood. Th is study compared corn growth and yield responses to water, N, and shade at 74,500 plants ha -1 (1×) with responses to water and N when planted at 149,000 plant ha -1 . Plant biomass, leaf area, chlorophyll content, refl ectance, and enzyme expression (transcriptome analysis) were measured at V-12. Grain and stover yields were measured with grain analyzed for 13 C isotopic discrimination (Δ) and N concentration. At V-12, 60% shade plants had increased chlorophyll and reduced leaf area and height compared to full sun plants. In the 2× treatment, plants had 11% less chlorophyll than 1× plants with leaf area and height similar to 60% shade plants. At harvest, plants in the 2× treatment were smaller, had increased water and N use effi ciency, and an 11% per hectare yield increase compared with the 1× unstressed treatment. Per-plant yields from 60% shade and 2× treatments were 50% less than 1× unstressed treatment. Yield reduction in shaded plants was attributed to light stress. Lower yield in the 2× treatment was attributed to a population-density induced 20% decrease in the red/near-infrared (NIR) ratio, which resulted in downregulation of C 4 carbon metabolism enzymes (phosphoenolpyruvate carboxykinase, phosphoenolpyruvate carboxylase, and pyruvate orthophosphate dikinase). Although the net impact of high plant density and shade stress on per-plant yield were similar, the stress compensation mechanisms diff ered.

A NASA sounding rocket launched from Wallops Island, VA (37.84 N, 75.48 W) on 1 July 2003 at 2:50 EST made the first in situ measurement of the relative concentrations of Fe+, Mg+, O2+, and NO+ within a nighttime intermediate layer below 140 km altitude. Ion composition measurements were made from 80–220 km altitude and included observations of three separate regions having high concentrations of metallic ions: the intermediate layer at 118 km, a sporadic‐E layer at 105 km, and a third layer above 160 km altitude. These observations demonstrate that metallic ions may be a significant source of ionization in the nighttime E and F region ionosphere at midlatitudes.

No abstract