Illinois Fertilizer
Conference Proceedings
January 26-28, 2006
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A.W. Becker, M. Ruffo, and F.E. Below 1
It has been well established that corn grown in rotation with soybean requires less fertilizer N to reach maximum yields than does continuous corn. As such, a "soybean N credit" is incorporated into N fertilizer recommendations throughout the Midwest for fields where corn follows a soybean crop. For example, in Illinois, a 40 lb/acre credit is subtracted from the N fertilizer recommendation. One explanation for this credit was that soybean added N to the soil that the succeeding corn crop would utilize. However, as our previous studies have shown, the resulting soybean "N credit" is more accurately a rotation credit resulting mostly from soybean residue having less immobilizing capacity than corn residue (Gentry et al., 2001; Bergerou et al., 2004). Others have focused on the amount of soil organic C added by corn root biomass through rhizodeposition, where corn roots and the associated rhizodepostion contribute 1.7 to 3.5 times the amount of C to soil organic matter than corn stover (Allmaras et al., 2004; Wilts et al., 2004). This assumption further supports the contention that continuous corn needs more fertilizer N to overcome the rotation penalty of corn as the previous crop.
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. A plant acquires N by roots coming in contact with a N03- or NH4+ ion in the soil matrix and subsequent transportation of the ion across the Casparian boundary layer into the xylem of the root. Thus, for greater N uptake efficiency, plants must have a root system that exploits a large surface area. A number of factors can affect root architecture, but cultural practices and environmental conditions seem to play a particularly large role. Karlen (1990) and Box (1996) noted that cover crops have the capability to positively influence root growth in subsequent crops by increasing biopore density, reducing the bulk density of the soil, and by increasing soil organic matter with the higher amount of plant residue available for breakdown. Leaching and N immobilization in the microbial biomass are two factors that largely affect the availability of N to crops. Since additional N seems to help overcome the residue penalty, a larger root surface area and an increase in lateral growth could play an important role in reducing the amount of additional N needed for continuous corn.
Although root growth undoubtedly plays a major role, the degree of genetic variability for root factors and N uptake among maize genotypes is largely unknown. Our preliminary measurements of roots from contrasting hybrids suggests that large differences exist in rooting characteristics, and these differences might be related to their N uptake (see 2004 Proceedings). The objectives of our 2005 work was to see how root architecture, previous crop, and N supply impact N uptake efficiency in corn. Twenty two commercial hybrids were evaluated for grain yield response, and two hybrids were selected for detailed root and growth measurements.
The 2005 trial was carried out on the Management Evaluation Nursery (MEN), located at the Crop Sciences Research and Education Center, Champaign, IL. This site was designed specifically to evaluate the impact of previous crop and N fertilization. Unfertilized corn and soybean were grown in 2004 to avoid residual fertilizer effects and to help assure responsiveness to N in 2005. The soil type of the MEN is a Drummer/Flanagan silty clay loam with a high productivity index, but which we have previously shown to be responsive to N. A similar design has been established in a different location for 2006.
Treatments consisted of the hybrids, the previous crop (soybean and 3rd year corn), and the N rates (0, 30, 60, 120, 180, and 240 lbs/acre). The experimental design was a split-split-plot arrangement of treatments in a randomized complete block design with 4 replications. Previous crop was the main plot, N rate was the sub-plot, and hybrid was the sub-sub plot. An experimental unit was a 4 row plot (rows 15' long, spaced 30" apart) where the outer two rows were used for destructive sampling and the inner rows for yield determination. The field was overplanted on April 30 and plots were thinned to a 31,000 plants/acre stand. The N treatments were applied as granular ammonium sulfate at the V3 growth stage and incorporated with field cultivation.
Roots were sampled at two growth stages, V8 and V12, when plant N uptake is in the linear phase. To sample the roots, a 2" split-core sampler was driven into the ground at two distances; 4" perpendicular to the plant at V8 and 12" perpendicular to the plant at V12. The cores were taken to a depth of 12", then subdivided into three equal depths. Roots were then extracted from the soil, cleaned, and analyzed by WinRhizo, a computer/scanning program. In conjunction with the root samplings, whole shoots were sampled for N analysis and biomass accumulation. Four plant aggregate samples were obtained, oven dried, weighed dry, and ground for N analysis. Plots were harvested for grain yield from September 21-23 by a plot combine.
The 2005 growing season was exceptionally dry during the early part of the season, with one of the 5 driest springs since the end of the 19th century. May precipitation was 65% less than average, while June totals were 39% shy of the monthly average (Data from the Illinois State Water Survey). Visual indications of water stress were evident in leaf rolling during vegetative growth, and in average grain yields that were nearly 30 bu/acre lower than the 5-year average for this field. Despite this stress, both N rate and the previous crop significantly impacted grain yield for all hybrids (Fig. 1). Interestingly, the magnitude of yield increase attributed to N (~25 bu/acre) was similar to the yield difference from previous crop. Both previous crop situations also exhibited the same response to N rate, and required the same amount of N to maximize yield (~72 lbs/acre). In contrast to many previous studies, additional N did not help overcome the yield penalty associated with continuous corn in our study. This response was consistent for all of the 22 hybrids, as there was no hybrid x previous crop or hybrid x previous crop x N rate interactions (data not shown).
Differences in above-ground plant growth attributed to previous crop were detected as early as V8, when the dry weight of rotated plants was 1.2 grams more (on average) than continuous corn (Fig. 2A). Oddly, plants with either previous crop exhibited trends towards lower weight with increasing N, especially for continuous corn. These data clearly show that continuous corn negatively impacts plant growth at an early stage, and that this growth impairment cannot be overcome with additional N.
Associated with the improved plant growth of the rotated corn was an increase in the concentration of shoot N (Fig. 2B). The concentration of shoot N ranged from 2.4-2.6% for rotated corn compared to 2.2-2.4% for continuous corn. In contrast to dry weight, the concentration of shoot N increased with N rates (Fig. 2B). The combination of higher shoot biomass and higher N concentrations for rotated plants indicates that they accumulated substantially more N than did plants of continuous corn. Expressed on a land area basis (lbs N/acre), this difference was 30% (11.7 lbs/acre for rotated vs. 9 lbs/acre for continuous corn). Since the same amount of fertilizer N was applied to both previous crop situations, our data suggest that more of it must be unavailable under continuous corn.
Previous crop had a large effect on root architecture as roots of continuous corn plants were double the length and twice the surface area of rotated plants (Table 1). Thus, the lower N accumulation of continuous corn plants does not appear to be the result of insufficient root absorbing area, but rather the result of low N availability. Interestingly, adding N had no apparent effect on root architecture at this growth stage (Table 1).
Previous crop and N rate were both big factors affecting corn yield in 2005, with each independently altering grain yield by ~25 bu/acre. The lower yield of continuous corn was evident as early as V8 when plants were smaller in size and lower in shoot N concentration than rotated plants. Neither the lower grain yield, nor the smaller V8 plant characteristics could be overcome in continuous corn with additional N. Despite the lower accumulation of N, and poorer growth of continuous corn plants, they had a much more extensive root system than rotated plants. The changes in shoot/root growth observed with previous crop are indicative of N sufficient (rotated corn) and N deficient (continuous corn) conditions. Why the fertilizer N additions could not overcome this deficiency for continuous corn is not clear, but it could be related to the time of fertilizer application (V3 growth stage), and the dry spring conditions. Alternatively, our data might be demonstrating the powerful immobilization potential of corn residue, which will be challenging to manage by conventional means. Understanding how to manage this residue challenge is the subject of our ongoing research.
1 A. W. Becker is a Research Assistant, M. Ruffo is a Postdoctoral Research Associate, and F.E. Below is a Professor, Dept. of Crop Sciences, University of Illinois, Urbana IL
Allmaras, R.R., D.R. Linden, and C.E. Clapp. 2004. Corn-residue transformations into root and shoot carbon as related to nitrogen, tillage, and stover management. Soil Science Society of America Journal. 68:1366-1375.
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.
Box, J.E., Jr. 1996. Modern methods of root investigation. pp. 193-237. In Y. Waisel et al. (ed) Plant roots: The hidden half. 2nd ed. Marcel Dekker, New York.
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.
Karlen, D.L. 1990. Conservation tillage research needs. Journal of Soil and Water Conservation. 45:365-369.
Wilts, A.R., D.C. Reicosky, R.R. Allmaras, and C.E. Clapp. 2004. Long-term corn residue effects: Harvest alternatives, soil carbon turnover, and root-derived carbon. Soil Science Society of America Journal. 68:1342-1351.
