Gurian-Sherman and Gurwick: Nitrogen Use Efficiency in Farming
When nitrogen is added to farm fields as fertilizer, some of it inevitably flows off the land, polluting streams and coastal waters.
In a study of planetary boundaries published in Nature this fall, the authors find that human nitrogen use is far beyond safe levels. Atmospheric nitrogen is converted, primarily by bacteria, from an inert to a usable form as part of the nitrogen cycle, but human conversion of nitrogen now exceeds that of all other natural processes. The boundaries study recommends reducing human use to a quarter of current levels.
A recent Union of Concerned Scientists (UCS) paper called "No Sure Fix" by Doug Gurian-Sherman and Noel Gurwick looks at nitrogen use efficiency (NUE). NUE is a way to measure effectiveness by calculating crop yield per unit of added nitrogen.
Key questions about NUE include the potential for precision farming, meaning the application of fertilizers on an as-needed, where-needed basis, and mechanisms for encouraging greater use of cover crops, which can remove excess nitrogen from the soil. No Sure Fix touches on these questions as well as the potential for genetically engineering (GE) and traditional plant breeding to improve NUE.
On precision farming:
More effective synchronization between crop growth and the amount and timing of nitrogen application therefore requires the calibration of indigenous soil nitrogen, crop variety, weather, and other factors that may affect growth rates. Measurements of soil nitrogen show considerable variation, even on a scale as small as a few meters. ...
For example, variable fertilizer application rates have been adjusted on a per-meter basis by using tractor-mounted sensors that measure light reflectance from plant leaves (Raun et al. 2002). Although some improvements in NUE have been demonstrated in experiments using such methods, nitrogen measurements must be calibrated for each location.
On cover crops:
The seed, planting, and incorporation of cover crops into the soil involve expenses and farming challenges that must also be taken into account. Incorporating the cover crop at the appropriate interval before cash crop planting, for instance, can sometimes be a problem.
On GE and traditional breeding:
Approval has been given for approximately 125 field trials of NUE GE crops in the United States (primarily corn, soybeans, and canola), mostly in the last 10 years. ... About half a dozen genes (or variants of these genes) appear to be of primary interest. ... No GE NUE crop has been approved by regulatory agencies in any country. ...
Improvements in NUE for experimental GE crops, mostly in controlled environments, have typically ranged from about 10 to 50 percent for grain crops, with some higher values. There have been few reports of values from the field, which may differ considerably from labbased performance.
By comparison, improvement of corn NUE through currently available methods has been estimated at roughly 36 percent over the past few decades in the United States. Japan has improved rice NUE by an estimated 32 percent and the United Kingdom has improved cereal grain NUE by 23 percent.
The report concludes:
[T]he opportunities to address the problems caused by the overuse of synthetic nitrogen in agriculture are considerable. But achieving the degree of improvement in NUE needed over the coming years will require increased public investment and a commitment to move beyond our current fixation on industrial agriculture methods such as precision farming and GE.
There is also a good FAQ on the UCS website.
