Nitrogen use efficiency and minimization of N losses to the environment is an on-going economic and environmental concern of nutrient management for crop production. How are we doing with N management for corn production? What are the opportunities for improvement?
Nitrogen Use Efficiency and Partial Factor Productivity
Partial factor productivity (PFP) is a measure of efficiency of input use. There are other measures applied to N use efficiency including agronomic efficiency, recovery efficiency, N harvest index, and others, but these require more data to calculate. The PFP is used for assessing system efficiency, such as comparing past and current efficiency or continuous corn compared with rotated corn.
PFP is commonly expressed as yield per unit input, e.g. bushels of corn per pound of fertilizer N applied (bu/lb N). PFP can be adapted to units of nutrient removed in grain harvest to units of nutrient applied, such as corn N harvested relative to fertilizer N applied (PFPN, lb/lb). The PFPN might be better visualized for a fertilizer N rate in consideration of Table 1. The math displayed shows highest PFPN with low N rates, but in reality the N rates generally need to be high enough for maximum profitability.
|200 bu/ac||260 bu/ac||200 bu/ac||260 bu/ac|
|PFPN, lb/lb||bu /lb N|
The PFPN used for the analysis in this article was derived from growers’ practices statewide with the assumption that growers’ N use was aimed at profit maximization. The average PFP of fertilizer N for corn in Nebraska was estimated to average 1.16 bu/lb N in 2012 compared to 0.57 bu/lb N in 1965 (Ferguson, 2014).
This represents a doubling in PFP for fertilizer N applied to corn. The trend of increase was linear from 1965 to 2012. Assuming a grain N concentration of 1.2% at 84.5% dry wt. or 0.67 lb N/bu, the PFPN converts to 0.79 lb of grain N per lb of fertilizer N applied in 2012 compared with 0.38 lb/lb in 1965.
It is recognized that available soil N for corn production comes from other sources in addition to fertilizer N. These include applied manure, ammonium deposition, irrigation water, and mineralized soil organic N. These need to be credited when determining fertilizer N rate at the field level. However, these N sources are recycled statewide over-time and not considered in calculation of PFPN with exceptions.
One exception is N from biological N fixation that is not removed in the previous harvest, such as N fixation by free-living bacteria during the corn-growing season. This is probably less than 10 lb/ac per year of N.
A second exception is nitrate produced in the atmosphere with lightning and deposited to the soil by rainfall. A common concentration of this N is 0.15 ppm, implying about 1.1 lb N/ac per year with 10 inches of rainfall.
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A third exception is the net mineralization of organic N. This N source is highly variable within and between fields and across years with no net N contribution for some fields but substantial contribution for other fields, depending on the likelihood of the soil to gain or lose soil organic matter.
Nitrogen supplied to corn from these three sources of non-recycled N have been excluded from this analysis because of the small or inconsistent N supply. There is at least partial compensation for these N supplies as the analysis also does not consider N harvest by grazing (removal in animal and NH3volatilization loss from urine) or baling corn stalks.
In Nebraska, about 60% of corn follows soybean in rotation and 40% follows corn or another crop. A higher PFP is expected with corn following soybean compared with other crops due to less fertilizer N application following soybean and often increased corn yield. Allocation of the overall PFPN of 0.793 lb/lb requires some assumptions. If average PFPN is 0.700 for corn following corn or another crop, then PFPN for corn following soybean is about 0.855 with only the corn component considered.
Soybean, on average, gets about 45% of its N from the soil and 55% from biological N fixation. Nitrogen removed in soybean grain harvest exceeds the N from biological N fixation. Soybean yields of 50 and 65 bu/ac of soybean grain harvest removes about 85 and 110 lb N/ac of soil N, respectively, assuming 3.8 lb N/bu. This results in PFPN of greater than 1.0 lb/lb for the corn:soybean rotation. Overall PFPN is very high for the corn-soybean rotation.
Leaching and volatilization may each account for about 30% of total N loss in Nebraska while attributing 22% to erosion and runoff, 12% to denitrification, and 6% to N2O and NO emission. Much variation in each of these estimates occurs from field to field. More volatilization is expected with high pH soil and with broadcast application to high residue fields. Most denitrification is expected on poorly drained bottomlands while most erosion and runoff loss is expected from highly erodible lands.
Emission of N2O and NO, relative to the applied N rate, may vary little across Nebraska corn land, but is expected to be greater with higher annual averages of ammonium and nitrate N in the surface soil. Decision tools such as Maize N and the Nitrogen Loss Assessment Tool (NebGuide G2249) can be used to assess risks of N loss.
There is, however, much field-to-field difference (disproportionality) in N loss, especially due to leaching and denitrification “hotspots” for loss (Figure 1). Well over 50% of the N loss to leaching likely may occur from only 20% of the corn land.
The “hotspots” for leaching loss are partly indicated by high nitrate-N in groundwater and associated with sand content of soil and past and current N and irrigation practices.
Nitrogen leaching hotspots are expected to occur more frequently
- with continuous corn compared with corn-soybean rotation,
- with sands compared with silt loam or finer texture soils,
- with furrow irrigation compared with pivot irrigation or rainfed, and
- with pre-plant time application of all fertilizer N compared with 50% or more applied in-season.
Practices to reduce N loss with corn production in Nebraska need to be well-targeted to the specific N loss process and situation. Rate of fertilizer N application and the time of application should get the greatest consideration. A significant reduction in N rate often results in little or no profit loss and can be a highly cost-effective means to reduce N loss.
Application of most of the fertilizer N in-season, especially if in response to crop N need, can be highly effective for situations with high potential for leaching and denitrification loss. Changing from continuous corn to a corn-soybean rotation and adding alfalfa may help reduce N loss.
Slow release fertilizer N products and N loss inhibitors can be cost-effective if well targeted to high loss situations and if used at the right time. Good irrigation management is important to reduced leaching loss. Planting of cover crops can be beneficial to reduced N loss if there is sufficient growth and if terminated early enough to avoid the need for additional fertilizer N.
Adequately crediting manure N, such as by applying only 60 lb/ac fertilizer N pre-plant with the remaining applied in-season according to crop need, is a great opportunity to reduce N loss for manured fields.
Charles Wortmann – Extension Soil and Nutrient Management Specialist
Richard Ferguson – Extension Soil Fertility Specialist
Jim Schepers – Emeritus Professor of Agronomy and Horticulture
Bijesh Maharjan – Extension Soil and Nutrient Management Specialist
Brian Krienke – Soils Extension Educator