Joshua Nasielski - Final Oral Examination for PhD


Interested Members of the University Community are invited to attend the Final Oral Examination for the Degree of Doctor of Philosophy of Joshua Nasielski of the Department of Plant Agriculture

Date: Wednesday, April 3
Time: 1:00 PM
Location: Room 202, Crop Science Building

Thesis Title: Late nitrogen (N) fertilizer applications in maize: investigating N fluxes at the field and plant scale

Advisory Committee
Dr. Bill Deen, Advisor
Dr. Tom Bruulsema
Dr. Hugh Earl
Dr. Manish Razaida                                                 

Examination Committee
Dr. Max Jones, Chair
Dr. Tony Vyn, External Examiner
Dr. Claudia Wagner-Riddle
Dr. Bill Deen
Dr. Hugh Earl


With the increasing availability of high clearance equipment, there is growing interest in late nitrogen (N) management strategies where a certain proportion of fertilizer N is applied much later in the season than is traditionally done. A suite of experiments was conducted to understand the physiologic response of maize to late N applications.

A field experiment was conducted to understand how yield formation in maize is affected by late N management strategies. It was found that the bulk of N fertilizer can be delayed until 9-11 days before silking (V13) without any yield penalty. Mechanistically, this is because maize yield potential established just after silking is maximized at relatively low initial N rates. Low N rates become suboptimal in terms of yield only after silking. Thus, on soils which supply at least moderate amounts of indigenous N via mineralization, the bulk of fertilizer N can be applied much closer to silking than traditionally done without yield penalty.

A greenhouse study found that the accumulation of luxury N prior to silking can help buffer grain yield against N stresses experienced after silking, and also elucidated the mechanisms responsible for this protective effect. To the extent that late N applications reduce luxury N uptake prior to silking, they may cause maize crops to be less resilient to shortfalls in post-silking N availability. 

Late N applications presumably increase post-silking N uptake due to greater late-season soil N availability. A greenhouse study was conducted to better understand how exogenous soil N supply and maize N demand interact to regulate post-silking N uptake in maize. The data supports a model that characterizes post-silking N uptake as a function of plant source-sink ratio, rather than solely a function of grain yield or soil N supply.

Using the DNDC model, the effects of 16 different N management strategies, including late N applications, on the environmental performance of economically optimum N rates (EONR) was assessed. Late N applications reduced leaching N losses at the EONR modestly (~7 kg N ha-1) but did not reduce yield-scaled N losses. It was found that N source and N placement decisions more strongly affect the agronomic and economic performance of the EONR relative to N timing.


\Nasielski, J., Deen, B., & Earl, H. (2019). Luxury vegetative nitrogen uptake in maize buffers grain yield under post-silking water and nitrogen stress: a mechanistic understanding. Frontiers in Plant Science, 10, 318.