Date of Graduation

5-2014

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Trenton Roberts

Committee Member

Jason Kelley

Second Committee Member

Lawton Nalley

Third Committee Member

Nathan Slaton

Keywords

Crop Residue, Fertilizer, Nitrogen, N-STaR, Wheat

Abstract

Soil testing methods such as the Illinois Soil Nitrogen Test (ISNT) and Direct Steam Distillation (DSD) have been developed which measure alkaline hydrolyzable-N (AH-N) as a means of estimating potentially mineralizable-N. Crop residues play an important role in N cycling. However, the ability of the ISNT and DSD methods to determine AH-N within crop residues is unknown. Therefore, the first objective of this study was to determine the ability of the ISNT and DSD to quantify potentially mineralizable-N within five different crop residues common to Arkansas. Corn (Zea mays L.), soybean (Glycine max, L.), wheat, rice (Oryza sativa, L.), and grain sorghum (Sorghum bicolor, L.) residues were labeled with 15N using 10 atom% 15N labeled-urea. A 0.2 g subsample of residue was subjected to both the DSD and ISNT. Hydrolyzed-N was captured and analyzed for atom % 15N to compare fertilizer atom % 15N to that of the original residue. Total N was quantified to establish percent recovery. Analysis of variance for percent N recovery showed a significant residue by method interaction (p<0.0001) indicating that the two methods recovered varying amounts of N based on the type of residue. Atom % 15N recovered from the soybean residue as AH-N was significantly lower than what was quantified in the plant tissue. Conversely, atom % 15N recovered from the rice residue as AH-N was significantly greater than that which was quantified in the original plant tissue. Comparison of atom % 15N in the residue and recovered AH-N suggested that certain crop species partition fertilizer N differently. The final objective of this study was to determine the influence of N rate and application time on fertilizer N uptake efficiency (FNUE) for winter wheat on a poorly-drained silt loam soil. Six different fertilizer N-rates were applied by hand ranging from 0 to 225 kg N ha-1 at three different times: Early-single, Late-single, and Split applications in 1.5 x 1.74 m microplots using 2.65 atom% 15N-labeled urea. There was a significant application time by rate interaction (p<0.0408). The greatest FNUE was achieved with the Early-single and Split applications at the 90 kg N ha-1 rate, and were 80.1% and 83.1%, respectively. The minimum yield-maximizing, N-rate was determined to be 135 kg N ha-1 applied as an Early-single or Split application. The Late-single application across all N-rates resulted in lower FNUE and yield. Soil N uptake was not significantly different for any of the treatments that received fertilizer regardless of N rate or timing of application, but were significantly higher than soil N uptake where no fertilizer was applied. Total N uptake by the wheat was directly related to fertilizer N uptake with the Early-single and Split application tending to have higher TN uptake than the Late-single application. Results of both the TN uptake and FNUE support the yield data obtained in these trials and indicate that current N rate recommendations for wheat produced in the delta region of Arkansas optimize fertilizer N inputs while maintaining high yields. These results highlight the importance of proper rate and application time for maximizing FNUE and yield in winter wheat production.

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