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Modern rice (Oryza sativa L.) varieties produce very high yields under high input and favorable environments. Limited work has been done to develop plant types suitable for less than optimal environments at moderate N management levels. The objectives of this study were: 1) to examine the relationship between the uptake of N and different leaf characteristics, sink size, and grain yield of three morphologically distinct rice varieties; 2) to examine the CO2 exchange rates (CER) of rice varieties in relation to light, N management, water use efficiencies (WUE), and N use efficiency (NUE): and 3) to determine characteristics of rice varieties that are associated with productive potentials under low N management levels. A greenhouse experiment was conducted in 1989 at Virginia Polytechnic Institute and State University, Blacksburg, VA and a field experiment was conducted in Sri Lanka at two locations in 1990. In the greenhouse experiment three varieties (a traditional, an intermediate, and a modem variety) were grown under four N management (rate and time) treatments: viz 1) a 0 N control, 2) 200 mg N kg⁻¹ of soil split into two applications, 3) 200 mg N kg⁻¹ of soil split into three applications, and 4) 400 mg N kg⁻¹> of soil split into three applications. Competitive use of fertilizer N for the development of either large leaf area or high leaf N content per unit leaf area (LNLA) varied with the rate and timing of N fertilizer application and rice variety. When N supply was limited and early N applications were restricted, the intermediate variety increased LNLA in the flag leaf with little increase in total leaf area of the plant. Specific leaf weight did not differ with N management except for the low value of the N control treatment. Increased N applications reduced stomatal density. Total stomatal number leaf⁻¹ varied little within cultivars indicating that N fertilization enables the leaf to increase leaf area and thereby disperses the fixed number of stomates. Carbon dioxide exchange rates were higher in the flag leaf than lower leaves and were directly correlated to LNLA. / Ph. D.

A field experiment was conducted in the Ridge and Valley region of Virginia near Blacksburg during the 1989 through 1991 corn-rye growing seasons. The treatments in this experiment consisted of varying amounts of ¹⁵N-depleted fertilizer N applied to corn (Zea mays L.) at planting followed by a winter rye (Secale cereale L.) cover crop treatment. The research was divided into four studies. The first study was conducted to evaluate an analytical procedure that could be used for the diffusion of low masses of ¹⁵N-labeled NH₄ in 2M KCI and subsequent analysis for N recovery and ¹⁵N concentrations in soil by direct combustion mass spectrometry. Diffusion was found to be a suitable technique for preparing low-mass N samples for automated ¹⁵N analysis by Automated Nitrogen-Carbon Analysis/Mass Spectrometry (ANCA/MS). Recoveries of low masses of added N were quantitative, and accurate ¹⁵N concentrations were obtained when the results were corrected for isotope dilution due to background or contaminant N. The second study was conducted to determine if ¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn-winter rye crop rotation. Fertilizer-derived N in the soil NO₃-N fraction following corn harvest was clearly detectable and distinguishable from natural abundance to a 90-cm depth. Detection of fertilizer N in the total N pool below the 30-cm depth was not reliable, particularly at the lower N rates. Clay-fixation of fertilizer N measured at corn harvest was not detected by ¹⁵N analysis. Inconclusive results indicate that further research is needed to determine the feasibility of using depleted material for measuring clay-fixation of fertilizer-derived NH₄⁺-N. Nitrogen uptake by a winter rye cover crop reduced soil NO₃-N levels below that required for accurate isotope-ratio analysis. Following winter fallow (approx. 1 yr after fertilizer application) residual ¹⁵N-depleted fertilizer N was still detectable in plant tissue and the soil NO₃-N fraction. The objectives of the third study were to measure plant uptake and soil distribution of fertilizer N applied to corn at varying N rates and to determine the relationships between economic optimum N rate, fertilizer-use efficiency, and potential leaching loss of residual fertilizer N to groundwater. Plant recovery of fertilizer N in 1989 ranged from 33 to 47% even though no grain yield and fertilizer N uptake response resulted from N fertilization. Greatest accumulation of residual fertilizer N was found in the surface 30-cm both years following corn harvest. The economic optimum N rate for 1990 corn planted into a rye mulch (218 kg N ha⁻¹) corresponded closely with the rate (224 kg N ha⁻¹) resulting in the highest fertilizer-use efficiency. Low levels of residual fertilizer-derived NO₃ in the 60-90-cm depth following the 1990 corn harvest provides evidence to support the use of the economic optimum N rate concept from both economic and environmental viewpoints. The fourth study was designed to measure the effectiveness of a winter rye cover crop for recovering residual fertilizer N from the previous application of varying N rates to corn. Recovery of fertilizer N by winter rye increased with increasing N rate applied to the previous corn crop and ranged from 3.5 to 35.9 kg N ha⁻¹ in 1990 and 2.3 to 25.7 kg N ha⁻¹ in 1991. Residual fertilizer N recovery in 1991 was higher in rye plots where the previous corn crop had been planted no-till into rye stubble as compared to corn planted no-till into rye mulch. Little or no fertilizer-derived mineral N was measured in the soil to a final depth of 90-cm following a winter rye cover crop. Amounts of fertilizer-derived mineral N increased with depth and previous fertilizer N rate applied to corn following winter fallow. These results provide evidence to support the use of a winter rye cover crop on a silt loam soil to recover residual fertilizer-derived mineral N that might otherwise be lost to groundwater. / Ph. D.

Two grazing trials and a feedlot trial were conducted with crossbred lambs to evaluate 'Quickstand' bermudagrass (Cynodon dactylon [L.] Pers.) in a cool temperate environment. Four stocking rates and two N fertilization rates (4 x 2 factorial) were replicated three times on .11-ha paddocks. Paddocks were continuously stocked with two, four, six, or eight lambs (light, moderate, heavy, and very heavy stocking rate, respectively) in grazing trial 1. Stocking rates were increased to 4, 8, 12, or 16 lambs per paddock, respectively, in grazing trial 2. In both grazing trials, paddocks were fertilized with 114 (low N) or 340 (high N) kg N ha-1yr-1 in split applications. / Master of Science

’Kentucky 31’ endophyte-free tall fescue fertilized with 160 kg N ha⁻¹yr⁻¹ was compared to similar fescue grown with ’Cimarron’ alfalfa in a randomized block pasture experiment with four replications. Quality and yield of stockpiled forages and performance and serum minerals of grazing steers were investigated during 1991-92 and 1992-93. Effects of grazing and timing of initiation of stockpiling forages were investigated during autumn of 1992. Rate and extent of release of Ca, Mg, P, S, Cu, and Fe from stockpiled forages were determined in a dacron bag study in 1992-93. In 1991-92, stockpiled N-fertilized fescue improved steer performance over stockpiled fescue-alfalfa (P<0.05); in 1992-93, this result was reversed (P<0.05) due to forage availability. Blood urea nitrogen was higher (P<0.07) and serum Ca and S were higher (P<0.05) in steers which grazed stockpiled fescue-alfalfa. September stockpiling of fescue-alfalfa improved botanical composition, yield, and forage quality, compared to August stockpiling. September stockpiling of N-fertilized fescue improved quality but lowered yield, compared to August stockpiling. Fescue-alfalfa had higher (P<0.05) yield and improved botanical composition when grazing occurred. Nitrogen- fertilized fescue had higher (P<0.05) yield when mechanically harvested. Alfalfa released P, Ca, Mg, S, and Fe to a greater extent and rate than either type of fescue at 24 h and P, Mg, S, and Fe at 72 h (P<0.05). Fescue grown with alfalfa released S to a greater extent and rate than N-fertilized fescue, Fescue-alfalfa produces animal performance as good or better than N-fertilized tall fescue, while eliminating need for N-fertilization. / Master of Science