Illinois Fertilizer
Conference Proceedings
January 24-26, 2005
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F.E. Below, J.R. Seebauer, and M. Uribelarrea
1
Numerous studies have shown that continuous corn requires more fertilizer N to achieve maximal yields than does rotated corn, and often the top yields achieved with continuous corn are lower. Therefore, a so-called ‘soybean N credit‘ is commonly used in the Midwest in order to adjust fertilizer N recommendations downward when corn is grown following soybean. For example, the fertilizer recommendation system used in Illinois adjusts the required N rate downward by 40 lbs/acre if corn follows soybean. Our previous work has shown that this reduction in fertilizer N usage is not due to the soybean crop adding N to the soil, but rather is the result of corn residue immobilizing more soil N than soybean residue (Gentry et al., 2001; Bergerou et al., 2004). Regardless of the exact cause of the soybean N credit, the net result is that more fertilizer N is needed for crops of continuous corn.
Because all N must enter the plant through the roots, changes in the architecture and/or activity of the root system can have a profound impact on a plant's N acquisition. The process of N acquisition is commonly referred to as uptake efficiency, and is defined as the percentage of fertilizer-applied N found in the plant at maturity. Although poorly understood physiologically, the efficiency of N uptake undoubtedly involves one of two major aspects associated with root growth including: 1) the architecture and morphology of the root system; and 2) the activity of permease carriers in the root plasma membrane which are responsible for N uptake. Each of these aspects can also be influenced by cultural practices, especially the level of N fertilization; as well as environmental conditions, particularly those associated with N availability and N loss. Thus, differences in N uptake capability could arise as a result of changes in root architecture, root physiological activity, or some combination of both.
The degree of genetic variability for root factors and N uptake among maize genotypes is largely unknown. Ideally, corn hybrids with a specific root architecture, or with superior uptake capabilities, should be best suited to continuous corn production. Two maize inbreds have been developed at the University of Illinois that impart marked differences in N uptake capabilities to their hybrids. These inbreds, IHP derived from the Illinois High Protein strain and ILP derived from Illinois Low Protein represents the known genetic extremes for grain protein concentration with accompanying changes in N uptake and metabolism by the plant. Our previous experiments have revealed that IHP is vastly superior to ILP and other corn genotypes in its ability to absorb N from the soil (see Below et al., 2004), which appears to be due to changes in both root architecture and root activity. Using these inbreds and hybrids derived from them, we evaluated how the root system, the previous crop, and the N supply interact to determine plant N uptake. We anticipate that such information will be useful in planning strategies to improve fertilizer N management and productivity of continuous corn.
The objective of this research is to understand how the roots effect differences in N use between continuous corn and corn rotated with soybean. This year, our goals were to:
Root sampling, separation, and quantification equipment and software were obtained in late June 2004. The root equipment and software required some practice and calibration, which we conducted using three corn inbreds with known differences in their use of N (Below, et al 2004). At the V8 and V12 stages of corn, when N uptake is the greatest, root samples were acquired using a 2" split-core sampler, which opens up to allow access to the intact sample. Soil cores were taken at two distances perpendicular to the planting row, 4" and 12". We sampled to a depth of 12", then subdivided the core into three equal depths, accounting for compaction. The majority of corn roots are found within this sampled area (Eghball, et al 1993). We will use the results obtained to focus our samplings in 2005 to highlight differences in root parameters that may be related to N uptake.
A typical way to compare the effects of crop rotation (corn-soybean vs. corn-corn) is the center-line approach which consists of applying the same treatments (e.g. hybrids, and seeding or fertilizer rates) and experimental design on both sides of a center line that divides the two crop rotations (Gentry, et al. 2001; Bergerou, et al. 2004). Despite its ease in implementation, the center-line approach has the disadvantage that rotation effects are confounded with the respective side of the field. To overcome this issue, we established the Crop Rotation Field in 2004 shown in Figure 1. The corn crop was unfertilized to utilize the residual N in the soil and thereby enhance the potential N response in subsequent years. In 2005, treatments (hybrids × N rate) will be arranged in a split plot design with four blocks (the field will be blocked in the N–S direction), where the main plots will consist of the previous crop and the sub-plots will be the factorial combination of hybrids and N rates. With this design, the effect of the crop rotation on the agronomic and physiological parameters can be better evaluated statistically.
Concurrent with the establishment of The Crop Rotation field in 2004, we also conducted preliminary experiments in a separate field using the center–line approach. Three corn hybrids which were known to differ in their N uptake efficiency (B73 × Mo17, B73 × IHP, and B73 × ILP) were grown in adjacent field areas, which were either corn or soybean the previous year. Nitrogen fertilizer was applied as ammonium sulfate at the V3 growth stage at rates of 0, 75, 150, 225, and 300 lb N/acre. The experiment was arranged as a split−plot design with four blocks. Whole plant samples were collected at V8, anthesis and physiological maturity, and all ears in one 17.5 foot row of each plot were sampled for final yield.
From this preliminary root quantification, we see a strong varietal influence for most root parameters (Table 1). Total root length doubled in ILP and B73 when plants were grown with additional N. However, IHP roots were of similar length and volume regardless of N supply. Mean root diameter was similar across all inbreds at this early sampling time. At high N, root length of B73 was much greater than either of the protein strains. Taking into account the root distribution, IHP had the greatest amounts of total root length in this soil section nearest the planted seed when supplied with N. In IHP, this increase in "proximal" roots may be due to more new roots able to accumulate N. While both IHP and B73 had an increased percentage of "proximal" roots with N, plants of the ILP had similar percentage of "proximal" roots regardless of N supply. In contrast to the other inbreds, ILP does not extend it’s roots under conditions of low N. In the field, ILP plants are easily pulled up out of the soil, possibly becase of this preference for "proximal" roots. Analysis of the hybrid roots grown with the crop rotation and varying N rates is in progress.
Grain yield of the hybrids was highly influenced by the genotype, the N rate, and the previous crop (Fig. 2, Table 2). Yield of corn following corn had a larger response to N supply increasing from an average of 60 bushels acre–1 without supplemental N up to 160 bushels acre-1 with the application of 300 lb N acre–1 (Figure 2). On the other hand, corn following soybean exhibited a higher yield without N application (80 bushels acre–1), but only increased in yield by 40 bushels acre–1 with additional N (Table 2).
B73 × IHP was the lowest yielding hybrid regardless the cropping history of the field or the level of N supply. However, yield of B73 × IHP exhibited a larger response to N supply when the plots followed corn than when they followed soybean (Figure 2, Table 2). Continuous corn yield showed an interaction between genotype and N rate with B73 × Mo17 and B73 × IHP exhibiting similar responses (linear and quadratic terms) (Table 2). On the other hand, yield of B73 × ILP had the largest response to N of the whole experiment since it yielded only 43 bushels acre–1 without supplemental N but yield increased to 179 bushels with the application of 300 lb N acre–1 .
In 2004, we were able to establish a crop rotation field to be used for studies in 2005 and beyond. In the meantime, we were able to collect some preliminary samples and measurements using a centerline approach.
Overall, the continuous corn rotation yielded more than the corn after soybean rotation this year, possibly due to the western corn rootworm beetle laying it’s eggs in the previous soybean crop. If the rootworm larvae are the cause for the loss of yield, we should be able to detect a difference in the root characteristics between crop rotations. The IHP strain has almost three–quarters of its roots in the zone nearest the planted seed at V8 in response to N, possibly indicating a abundance of new roots able to aquire N. We are currently analyzing roots of the hybrids grown under the different rotations for possible clues leading to a characteristic which allows the corn plant to grow in continuous corn with less N fertilizer in the future.
1 F.E. Below is a professor, J. R. Seebauer is a senior research specialist, and M. Uribelarrea is a research assistant, Dept. of Crop Sciences, University of Illinois, Urbana, IL
Below, F.E., Seebauer, J.R., Uribelarrea, M., Schneerman, M.C., Moose, S.P. 2004 Physiological changes accompanying long-term selection for grain protein in maize. Plant Breeding Reviews 24:133–151.
Bergerou, J.A., L.E. Gentry, M.B. David, and F.E. Below. 2004. Role of N2 fixation in the soybean N credit in maize production. Plant Soil 262:383–394.
Eghball, B., Settimi, J.R., Maranville, J. W., and A. M. Parkhurst. 1993. Fractal analysis for morphological description of corn roots under nitrogen stress. Agron. J. 85:287–289.
Gentry, L.E., F.E. Below, M.B. David, and J.A. Bergerou. 2001. Source of the soybean N credit in maize production. Plant Soil 236:175–184.
