Nitrogen efficiency in our N systems study fields Peter Scharf, Newell Kitchen, and Ken Sudduth University of Missouri and USDA-ARS

Nitrogen efficiency Lots of definitions Each with a different purpose Lots of scales Within-field… … to global

N efficiency: system definition Production system N inputs N removed In grain Efficiency = N removed in grain / N inputs

Slide from Cliff Snyder, PPI

What is the highest N efficiency we can achieve? Assume that soil organic matter is at steady state i.e., we’re not mining organic N to support crop growth or losing it in any other way Then the highest efficiency that we can achieve is defined by crop N removal If N supplied (fertilizer, manure, forage legumes) equals crop removal, then there is no N leakage from the system as a whole But leakage is inevitable in row-cropping systems

What is the highest N efficiency we can achieve? With corn, N removal is about 0.75 to 0.8 lb N/bu We’re applying 1 lb fertilizer N per bu to supply that 0.75 to 0.8 lb i.e., 75 to 80% of what we apply as fertilizer equals grain removal If we ignore manure and forage legumes, this would imply 75 to 80% efficiency of our N fertilizer Maximum efficiency is: 1 / 0.8 = 1.25 to 1 / 0.75 = 1.33 bu/lb N supplied

What is the highest N efficiency we can achieve? In a corn/soybean system, efficiency must be lower than the 1.25 to 1.33 bu/lb N ceiling Soybean is a net sink for N Supplied from soil organic matter Low residue does not replace lost organic matter Long-term continuous soybean mines organic matter and results in lower soil organic matter To have steady-state soil organic matter, the organic N lost in the soybean year must be replaced by N in corn residue Thus N supply to corn must exceed N removed in grain, just to keep soil organic matter at steady state

Slide from Cliff Snyder, PPI

Increasing N efficiency We’re already doing a pretty good job But there is room for improvement Role of sensors: identify areas where the soil is supplying a substantial proportion of the crop’s N requirement In these areas, efficiency of fertilizer N use is low Cutting back on N in these areas will increase field-average N efficiency Also, N timing with sensors is close to period of rapid crop uptake—the chance of N loss to air or water is reduced

N efficiency in out N systems study fields 8 field-scale experiments from 2000 to 2002 Three soil regions Claypan Deep loess Mississippi delta

Claypan 2000: map of N efficiency at EONR (kg grain/kg N) 70 kg grain contains about 1 kg N

What do we see in this map? Nitrogen efficiency was spatially variable Possibly a connection to landscape Over much of the field, N removed in grain was about the same as N applied (at EONR) In areas that appeared pink or red, N efficiency was 70 to 130%

Total N efficiency Kg grain/kg N at EONR

Total N efficiency Kg grain/kg N at EONR Red = more N removed in grain than applied in fertilizer

Efficiency at EONR In 5 of 8 fields, over most of the field we were removing more N in grain at EONR than we were applying This is not sustainable But it’s desirable in fields that have a rich organic N supply, as many fields have that have a history of generous N applications Overall, N efficiency at EONR was excellent except in a few fields with wet soils and probable N loss

N efficiency: EONR vs. 165 kg N/ha Deep loess 2000 N effic. at EONR (variable) N effic. at 165 Red >70 kg/kg Pink <70 kg/kg

N efficiency: EONR vs. 165 kg N/ha Variable-rate application of N rates near EONR can increase N efficiency relative to uniform application of typical N rates

Summary Harvested corn grain in the U.S. contains about 75 to 80% as much N as is applied to corn in fertilizer (we’re doing a good job!) We can do better. In the long-term we may be able to increase this to as much as 90% of the total N supplied to corn (fertilizer, manure, & forage legumes) We probably cannot exceed this limit due to: Inevitable N loss from row crop systems Soil organic N depletion during soybean years

Summary At economically optimal N rates in our 8 field- scale experiments, N efficiency was high: more N was removed in grain than was applied as fertilizer (>100% efficient!) Variable-rate N that matches or approaches EONR patterns can increase efficiency relative to conventional uniform N rates

8 Maps of partial N efficiency (kg grain yield increase/kg N) at EONR

Nitrogen inputs (fertilizer, manure, forage legumes) Soil Crop Soil org. matter Water Air (NH 3, N 2 ) Increasing N efficiency = reducing N loss