Conventional steam‐distillation techniques for concentrating inorganic N in soil extracts prior to 15N analysis are time‐consuming and labor‐intensive procedures. Diffusion procedures have been proposed as alternatives to steam distillation, but incomplete recovery and isotope dilution have been reported. We evaluated a procedure for diffusing low masses of 15NH4‐N from 2 M KCl solution to determine the effects of NH4 and 15N concentrations on recovery and isotope‐ratio analysis of diffused N. A series of solutions were prepared containing from 0.5 to 2.5 mg N L−1 at 1.1461 atom % 15N. Forty‐milliliter aliquots were placed in plastic containers, equipped with acidified glass‐fiber disks, and allowed to diffuse for either 6 d at room temperature or 5 d at 55 °C. Nitrogen recovery in glassfiber disks averaged 100.5% of the added N. Quantitative recovery of added N was also obtained in a second study in which 30‐mL aliquots of standard solutions containing from 25 to 100 µg N at 15N concentrations varying from 0.1030 to 1.1461 atom % 15N were diffused for 10 d at 55 °C. Significant dilution of 15N concentration for low‐mass N samples occurred. Differences in 15N concentrations between diffused and nondiffused samples were attributed to background N at a 15N concentration of natural abundance. Close agreement was obtained between diffused and nondiffused samples when an isotope‐dilution equation was used to calculate the atom % 15N concentration of the sample N. These studies indicate that the diffusion method is suitable for preparing low‐mass N samples for 15N analysis if appropriate blanks are determined and used to correct for background N.
Tall ironweed is a troublesome perennial weed that infests cool-season grass pastures in Kentucky. Field experiments were conducted in 2000 through 2003 to evaluate the efficacy of fall-applied herbicides on established tall ironweed following a midsummer mowing. Triclopyr-containing treatments showed the greatest suppression of tall ironweed 12 mo after treatment (MAT), across all years. With triclopyr at 0.56 and 0.63 kg/ha, tall ironweed control was 80% or greater in 2 of the 3 yr. Dicamba initially provided 87% control 8 MAT in 2 of 3 yr and declined to less than 60% 12 MAT. Tall ironweed shoot density was also reduced 66% or more 12 MAT with fall-applied triclopyr-containing treatments. In contrast, tall ironweed density increased approximately twofold in dicamba-treated plots between 8 to 12 MAT in all 3 yr. The impact of herbicide treatment on dry matter (DM) yield of spring-seeded red clover (Trifolium pratenseL.), tall ironweed, and forage grasses was also evaluated. Red clover DM yield in the herbicide-treated plots in 2002 showed no significant differences from the untreated control. However, red clover DM yield in 2003 was lowest for the two triclopyr + clopyralid treatments, indicating a decrease in DM production compared with that of the nontreated control. Results indicated that fall-applied triclopyr-containing herbicides following a midsummer mowing is an effective program for removing tall ironweed from grass pastures, but further research is needed to evaluate the establishment of red clover following herbicide treatment.
No‐till soft red winter wheat (Triticum aestivum L.) production reduces soil erosion and increases the opportunity for timely planting. The purpose of this study was to determine if the elimination of tillage in winter wheat production influences plant populations, disease incidence, and grain yield response to N fertilization. A 3‐yr study was conducted on a Loradale silt loam (fine, mixed, mesic Typic Argiudolls) using two cultivars. No‐tillage (NT) was compared to more conventional (CT) systems involving either a moldboard or chisel plow. Fertilizer application rates were 0, 70, and 140 lb N/acre. Plant stands for the cultivar ‘Caldwell’, measured 3 wk after planting, were 14.7% lower in NT than in CT in 1982–1983. Plant stands for both cultivars were 12% lower in the NT system in 1983–1984. The incidence of powdery mildew (Erysiphe graminis) was higher under NT in 1982–1983, while septoria glume blotch (Septoria nodorum) infection was not affected by tillage. Powdery mildew infection was greatest for the cultivar ‘Wheeler’ while septoria glume blotch infection was greatest for Caldwell. Lodging was greater in the NT system in 1982–1983. Lodging was higher for Wheeler in NT than in the CT system in 1982–1983. Flag leaf and grain N concentrations were not influenced by tillage system in any season. Grain yields responded positively to N application 2 of the 3 yr, but were not affected by tillage system in any year. A tillage × N rate interaction for grain yield was not observed. Efficiency of fertilizer N removal by grain (corrected for controls) averaged 43% at 70 lb N/acre and 29% at 140 lb N/acre.
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