Graduate Thesis Or Dissertation
 

Nitrogen available to winter wheat as influenced by previous crop in a moist xeric environment

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  • Rotating wheat with other crops is a common practice in the Willamette Valley of western Oregon. Depending upon previous crop and soil type, current N fertilizer recommendations for wheat in the Willamette Valley vary widely. Excessive fertilizer poses environmental risk, whereas lower N inputs than required by the crop represent economic losses to growers. Growers and their advisors face the challenge to minimize the environmental risk, and at the same time to maintain or increase economic returns. Questions are often raised concerning the efficient use of N fertilizer and accurately predicting the amount of N needed by wheat following different crops. The first study measured growth, N uptake and N use efficiency (NUE) of winter wheat grown after either a legume or oat for three years. In all three growing seasons, winter wheat showed higher biomass, N uptake and NUE when grown after a legume than after oat. The contribution of legume was evident before the wheat was fertilized in spring, indicating that legume N had mineralized in fall or winter. Contribution of soil N to wheat suggested that fertilizer N can be reduced by 44 kg N ha⁻¹ if a legume is grown previously. Nitrogen use efficiency estimated 50 to 70 days after N application by isotopic method (24 to 94%) was comparable with that estimated simply by difference (21 to 94%) at the same time. The second study predicted gross mineralization rates using analytical models. Comparable N mineralization was predicted by a model assuming remineralization and a model assuming no remineralization, suggesting that remineralization was negligible. In the spring, mineralization-immobilization turnover was at a lower pace than expected in both rotations. In two growing seasons, gross mineralization rates were higher where the previous crop was legume (0.37 to 0.74 kg⁻¹ ha⁻¹ day⁻¹) as compared to where oat was grown previously (0.14 to 0.6 kg⁻¹ ha⁻¹ day). Negative net mineralization indicated that fertilizer N was immobilized in the oat-wheat rotation. The third study evaluated calibration and digestion techniques used to determine elemental concentration in grasses. Use of a dry ashed standard to calibrate the ICP spectrometer generated highly variable calibration curves and was not a viable calibration method. Good agreement was found between chemical and microwave digested standards. Dry ashing resulted in considerable S and Mn losses, whereas, perchloric acid digestion and microwave digestion showed similar results. Our study suggests that if routine analysis are to be performed for macro nutrients or involve trace level work, the best method is microwave digestion with chemical standard calibration of ICP spectrometer.
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file details QureshiMaqsoodHassan1999.pdf 2017-08-15 Público Baixar