Illinois Fertilizer
Conference Proceedings
January 27-29, 1992
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R.J. Lambert, R.G. Hoeft, R.D. Seif, and J.E. Harper1
Application of nitrification inhibitors (NI's) to N fertilizers used in maize production has been proposed as a strategy for improving nitrogen use efficiency and minimizing the risk of groundwater nitrate contamination. Enhancing the supply of ammonium-N relative to NO3-N benefits maize growth in gravel culture and hydroponic studies.
In the field, NH4-N is commonly supplied to maize as a band of injected anhydrous ammonia. Typically, this band is spring applied in a mid-row position, 36-46 cm from maize plants. As fertilizer N diffuses and soil temperatures increase, NI effectiveness declines. By the time roots reach a mid-row fertilizer N zone significant nitrification may have occurred. Potential benefits of enhanced ammonium supply could be lost. However, harmful effects are risked by bringing the band closer to germinating seed. Germination and growth of maize has been shown to decline with proximity and rate of a band applied directly below the seed. Roots would not grow into a soil layer containing 1485 mg ammoniacal-N. One approach to resolving this dilemma is to identify seedling genotypes with superior response to rates of ammoniacal-N placed in close proximity. Such differences could result from variation in preference for the NH4-N form or tolerance to NH3-N, or both.
The objective of this study was to compare maize hybrids and inbreds for their
response to increasing levels of ammoniacal-N applied in a band placed close
to germinating
A greenhouse study was conducted to screen ten corn inbreds for the effects
of N form and NH4OH rates on shoot dry weight accumulation and root development.
Three injected N treatments were used: 80 ppm NO3-N, 80 ppm NH3-N and 160 ppm
NH3-N. Plants were harvested 28 days after germination. Shoot dry weights were
recorded and 120 cm3 cores were taken to a depth of 15 cm to assess root length
density in fertilized and unfertilized zones.
Ammonium N supplied at 80 and 160 ppm tended to produce slightly larger shoot dry weights than NO3 treated pots, but the differences were not significant. However, the interaction of genotype by N regime was significant at p=0.08. Three different patterns of response to N regimes among the 10 inbreds screened were apparent, although many of the within-line differences were not significant (Table 1). Inbreds LH74, C103, and LH51 tended to prefer 80 ppm NO3-N over 80 ppm NH4-N. A preference for NH4 over NO3 at the 80 ppm N rate was observed for 38-11, L317, B68, and B14, but growth was suppressed where NH4OH rate was doubled. These four genotypes showed a preference for EAS over the predominantly NO3-N regime, but were not tolerant of the higher ammoniacal-N placed in close proximity to the seedling. Inbreds WF9, LHE136, and Oh545 preferred the EAS regime and tended to show greater tolerance to the high NH4OH rate. Shoot dry weights averaged across N treatments tended to be small in the first group and large in the last group, particularly in Oh545.
Shoot N concentrations were affected by genotype, N regime and their interaction (Table 2). When averaged across genotypes, plants supplied NO3 had significantly higher shoot N concentrations than plants receiving NH4OH. This effect was pronounced in the more rapidly growing seedling genotypes that tended to respond to EAS (Oh545, LHE136, 38-11) and therefore may represent a dilution affect. Shoot N accumulation varied among genotypes, but not due to lv regime or genotype x N regime interactions (date not shown).
When averaged across N regimes, root length densities differed among inbreds in both the fertilized and unfertilized zones. As expected, 160 ppm N of NH4OH & significantly reduced root length density in the fertilized zone compared to either N source at 80 mg N kg-1. Root length density in the unfertilized zone was not affected by N regime (Table 3).
Root length data suggested some explanations of the dry weight observations (Table 1). Genotypes with slow growth rates (LH74 and LH51) may have avoided detrimental effects of the high NH4OH rate because root growth to this zone was slow. Inbred 38-11 may have suffered its severe growth reduction at the high NH4OH rate because it produced an unusually high root length density in that zone. The larger genotypes which were tolerant or even responsive to the high NH4OH rate (WF9, LHE136, Oh545) did not have particularly low root length densities in the fertilized zone, suggesting they may possess physiological traits conferring tolerance to high ammoniacal-N supply.
The data support the concept of genotypes (inbreds) capable of producing high
root length densities (Oh545, LHE136, B68) in soils fertilized with 80 ppm NH4-N
rate will respond to enhanced ammonium supply.
Table 1: The influence of genotype and N regime on shoot dry matter accumulation
Table 2: The influence of genotype and N regime on shoot N concentration
1R.J. Lambert, R.G. Hoeft, and R.D. Seif
are Professors of Agronomy, University of Illinois, and J.E. Harper is Research
Leader USDA/ARS and Professor of Agronomy, University of Illinois
