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Agronomy Journal 93:396-403 (2001)
© 2001 American Society of Agronomy

SOYBEAN

Effect of Soybean Plant Populations in a Soybean and Maize Rotation

Stella A. Ennina and Max D. Cleggb

a Crops Res. Inst., P.O. Box 3785, Kumasi, Ghana
b Dep. of Agron., Univ. of Nebraska, Lincoln, NE 68583-0817

Corresponding author (mclegg1{at}unl.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant population of soybean [Glycine max (L.) Merr.] may influence the residual N contribution to a cropping system and yield benefits to following cereals. Field studies were conducted from 1994 to 1996 on a N-depleted Sharpsburg silty clay loam soil at Mead, NE to: (i) determine soybean yield at different plant populations; (ii) investigate residual N, chlorophyll–N–yield relations, and yield benefits from these different soybean populations to a following maize (Zea mays L.) crop; (iii) and compare N credits from soybean assessed with fallow and cereal plots as references. Eight soybean populations from14 000 to 544 000 plants ha-1 in narrow 50-cm rows, a fallow plot, and a maize plot were followed by maize in a rotation study. Soybean yield was highest at populations of >=129000 plants ha-1. Maize grain yields were highest following fallow and soybean populations <20000 plants ha-1, intermediate following higher soybean populations, and least in continuous maize. This is most likely due to N uptake as indicated by chlorophyll and N accumulation of maize. Nitrogen credits to maize were 16 to 46 kg N ha-1 when calculated as Nitrogen Fertilizer Replacement Values (NFRVs). This is probably overestimating the potential N contribution from soybean because N credits from soybean populations assessed with fallow instead of maize as references were negative. A net positive N balance due to soybean reached a maximum of 17 kg N ha-1, but soil N was depleted at populations <20000 plants ha-1. We conclude that yield increases of maize in rotation with soybean may be due to N from reduced N immobilization, N added to the soil from N2 fixation, and possibly from non-N rotation effects such as water use efficiency.

Abbreviations: DAS, days after sowing • NFRV, N fertilizer replacement value • TDM, total dry matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
RELATIVE TO CONTINUOUS PRODUCTION, cereal yield benefits are realized when cereals are planted in rotation with legumes (Clegg, 1992; Mohammed and Clegg, 1993; Copeland et al., 1993), and these benefits are achieved at a lower optimum N application rate (Kurtz et al., 1984; Bundy et al., 1993). Due to the symbiotic association between soybean and Bradyrhizobia, soybean can convert atmospheric N2 to NH3 in its root nodules. Amounts converted in commercial soybean production in the USA range from 75 to 300 kg N ha-1 (Keyser and Li, 1992). Nitrate N contents in the soil profile have been much higher after soybean production than after cereal production (Peoples and Herridge, 1990; Mohammed and Clegg, 1993). These observations suggest that yield benefits of nonlegumes following soybean may be due to N contributions from biological N2 fixation. However, other reports support findings that, when available, soil N is the main source of N for soybean growth rather than N fixation (Herridge and Brockwell, 1988). Thus, growing soybean can result in a net depletion of soil N (Zapata et al., 1987).

High amounts of N are removed by harvested soybean seeds (144 and 169 kg N ha-1, Clement et al., 1992; 150–200 kg N ha-1, Varvel and Peterson, 1992). This can result in a net N balance (total N fixed - total N removed by harvested seeds) ranging from -74 to +80 kg N ha-1 for soybean and from -44 to +136 kg N ha-1 in other legumes (Peoples and Herridge, 1990; Beck et al., 1991).

Plant population is one factor that may influence how much residual N soybean is contributing to the cropping system. Bello et al. (1980) reported that doubling the population of determinate, semideterminate, and indeterminate soybean from 380000 to 760000 plants ha-1 increased average N2 fixation from 44 to 69 kg N ha-1. A corresponding increase was not observed in total N accumulation, seed N, or seed yield. Estimated N2 fixation of determinate soybean was approximately 200 to 280 kg N ha-1 when plant population was increased from 48500 to 194000 plants ha-1 without a change in seed yield (Nelson and Weaver, 1980). Chen et al. (1992) reported increased nodule fresh mass per unit area with increased soybean plant populations. Blumenthal et al. (1988) reported that the highest soil N occurred in fallow plots. Thus, for wheat (Triticum aestivum L.), N accumulation was the highest when grown on fallow, second highest following soybean, and lowest following maize. From these results, the authors concluded that soybean depleted soil N but to a lesser extent than maize.

There is interest in planting soybean in narrow rows to increase light interception for higher yields (Board and Harville, 1993). This is especially true with some of the determinate soybean cultivars that tend to be short statured. Thus, evaluation is needed of the N contribution of soybean at varying plant populations in narrow-row culture to accurately predict application of supplementary N fertilizer for cereal production in soybean–cereal rotations. The objectives of the study were to: (i) determine soybean yield at different plant populations; (ii) investigate residual soil N, chlorophyll–N–yield relations, and yield benefits to a following maize crop from these different soybean populations; and (iii) compare N credits from soybean assessed with fallow and cereal plots as references. We hypothesized that soybean grown at higher plant populations would not rely as much on soil N due to greater intraplant competition for N, resulting in increased N2 fixation that would more than offset increased N removal in higher yields at the higher populations. This would leave more soil N for maize in addition to any contribution from soybean residue.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field studies of soybean populations and maize following soybean at different populations were carried out at the University of Nebraska Agricultural Research and Development Center near Mead, NE in 1994, 1995, and 1996. The soil was a Sharpsburg silty clay loam (fine montmorillonitic, mesic Typic Argiudoll). Soil N was reduced by continuous maize (1994 site) or sorghum [Sorghum bicolor (L.) Moench] (1995 site) production without fertilizer application for 6 yr before the study. Thus, the yield response on fallow would indicate the N mineralization potential of this soil. Soybean seeds were coated with a commercial Bradyrhizobia inoculant, Nitragin, before planting. Planting was done mechanically using a commercial planter. Weeds were controlled by postemergence application of Pursuit herbicide mixture (240 g L-1 imazethapyr {(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H- imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid}) at 0.29 L ha-1, urea ammonium nitrate (28% N) at 4.6 L ha-1, and surfactant at 1 L 400 L-1 total mixture, followed by hand weeding during the growing season. Five centimeters of supplementary irrigation water was applied during the prolonged drought in 1995 when soybean was at full bloom stage (R2) (Fehr et al., 1971), and a second 5 cm was applied when soybean was setting pods (R3–R4) and maize was at 50% silking.

Soybean Population
Studies of soybean plant populations were established using an early Maturity Group 3 determinate soybean variety, Hobbit 87. A determinate variety was selected to diminish plant response to space. Eight soybean plant populations were established on 17 May 1994 in rows that were 50 cm wide and were repeated on 25 May 1995 at a different site. Target plant populations were 10, 50, 100, 150, 200, 300, 500, and 600 thousand plants ha-1. A medium-maturing hybrid maize plot (Pioneer 3417IR at 55000 plants ha-1 with no N) and a fallow plot were included for a total of 10 treatments. The experimental design was a randomized complete block with six replications per year. The fallow plot was kept weed free by regular hand weeding during the growing season. Stand counts and thinning as needed were done at the second node stage (V2) of soybean (Fehr et al., 1971). Soybean populations were 14, 51, 67, 81, 121, 221, 339, and 423 thousand plants ha-1 in 1994 and 17, 48, 69, 129, 168, 253, 424, and 544 thousand plants ha-1 in 1995. Plot size was six 9-m-long rows. Soils were sampled to five depths at planting (0–15, 15–30, 30–60, 60–90, and 90–120 cm) and analyzed for inorganic N (NH4–N and NO-3–N), P, K, organic matter, and pH by the Soil and Plant Analytical Laboratory of the University of Nebraska-Lincoln Department of Agronomy. At physiological maturity, 116 d after sowing (DAS), plants in two 1-m-long central rows were cut from each plot at ground level and oven-dried at 80°C to a constant weight. The total dry matter (TDM) was weighed and recorded. Plant samples were mechanically ground and 50 g was kept for analysis. Nitrogen concentrations were determined for seeds and plant residue by the Soil and Plant Analytical Laboratory. Yield was determined from plants harvested later from two 4.5-m-long central rows. Harvesting was accomplished manually, and plants were threshed mechanically. The threshed grain was weighed, and total yield was calculated. One hundred seeds from each plot were oven-dried at 80°C for 48 h and weighed for seed weight.

Maize following Soybean
The entire soybean plant residue in the soybean population experiment was left on each plot, and the field was disked to a depth of 30 cm at the beginning of the following growing season. The plots were planted to maize cultivar 3417IR at 55000 plants ha-1 on 19 May 1995 and 21 May 1996. At emergence, N as ammonium nitrate [NH4NO3] (34% N) was applied at three rates (0, 80, and 160 kg N ha-1). The experimental design was a split plot with N rate as the main plot and the previous cropping system as subplots. The addition of three levels of N as the main plot on the soybean population experiment resulted in two replications of each N level. Main plots and subplots were randomized. At planting, soil samples from the population study were taken from each plot at two depths (0–15 and 15–30 cm) to determine residual mineral N. Chlorophyll content of the ear leaf was measured nondestructively with a chlorophyll meter to give an indication of plant N status (Wood et al., 1992). At midsilking stage, measurements were made on the ear leaf of 15 random plants in the two central rows of each plot. Plant height, aboveground dry matter at physiologic maturity (138 DAS), grain yield, number of plants and ears harvested, 100-seed weight, and N content of grain and aboveground plant residue were determined. Harvest index is the ratio of grain yield to TDM. Nitrogen fixation was estimated by the difference method: N uptake by soybean minus N uptake by maize. This assumes that soybean (without nodules) and maize crops take up about the same amount of soil N (Fujita et al., 1990). Nitrogen fertilizer replacement value of a legume is equivalent to the amount of N fertilizer required to produce a similar yield in a nonlegume monoculture (Hesterman, 1988). This was determined using both continuous maize (maize–maize) and fallow as references for the system.

Statistical Analyses
Analysis of variance and correlation and regressions were performed using SAS (SAS Inst., 1988). Treatment mean comparisons were made using orthogonal contrasts following methods described by Steel and Torrie (1980). A probability level <=0.05 was considered significant. Orthogonal polynomial (Steel and Torrie, 1980) and quadratic plus plateau models (Cerrato and Blackmer, 1990) were fitted to the data.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil and Climatic Data
Total monthly precipitation and mean maximum monthly temperatures were very different for the 3 yr of the study (Fig. 1). This would have a great impact on soil N cycling and plant response. In 1995, to save the crops, 5 cm irrigation was applied to the experiments during full flowering (R1–R2) of soybean along with a second application in August at 50% silking of maize and at pod set (R3 and R4) of soybean.



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  		Fig. 1. Total monthly precipitation and mean maximum monthly temperatures during the growing seasons of 1994, 1995, 1996 and the long term averages at Mead, NE

 
Soil analyses before planting soybean were similar in 1994 and 1995. Nitrogen ranged from 0.9 to 4.9 mg kg-1 NO3–N and from 7.2 to 16.0 mg kg-1 NH4–N. Phosphorous (35.5 mg kg-1), K (344–436 mg kg-1), and organic matter (24–30 g kg-1) levels were high in the 0- to 30-cm profile. Soils were slightly acidic (pH of 5.6–6.4) within the 120-cm profile.

Data on total soil N (30-cm depth) before planting of maize in 1995 showed that the highest soil mineral N was in plots that were previously planted to soybean at 51000 plants ha-1 (Table 1). Soil N availability decreased below and above 51000 plants ha-1. Also, soil N availability in the fallow plots was high. Soil N availability of plots previously planted to maize (no N) was similar to that of the soybean plots with higher plant populations. In 1996, soil N availability up to a 30-cm depth was low at the time of maize planting with a mean of 23 mg kg-1 N. No differences between the previous cropping system were detected (P < 0.05). Availability of soil mineral N probably reflects the soil moisture and temperature conditions before sampling because N availability is dependent on soil microbiological activity.


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  		Table 1. Average total soil N (NH4NO3) within a 30-cm depth for the cropping systems fallow, soybean plant populations, and maize at maize planting, Mead, NE, 1995 and 1996

 
Soybean Population
Dry Matter, Residue, and Seed Yield
Total dry matter, residue, and seed yield of soybean at varying plant populations followed quadratic plus plateau models in both 1994 and 1995 (Fig. 2). An increase of these variables was observed as soybean population increased up to 51000 plants ha-1 in 1994. No further increase was observed (P < 0.05) with a further increase in plant population. A similar response was observed in 1995, but differences among populations ceased at 129000 plants ha-1. Thus, a plant population beyond 129000 plants ha-1 might not be necessary to maximize grain yield for Hobbit 87 in 50-cm rows. Nelson and Weaver (1980) also reported a lack of increase in seed yield of determinate soybean at higher populations. They found no difference in grain yield of determinate soybean at 48500 and 194000 plants ha-1. Bello et al. (1980) also did not find an increase in grain yield of determinate, semideterminate, and indeterminate soybean by doubling plant population from 380000 to 760000 plants ha-1. Blumenthal et al. (1988) reported optimum population of soybean cultivar Ransom to be 200000 plants ha-1 in a study of soybean populations ranging from 50000 to 400000 plants ha-1. This suggests that maximum yields could still be obtained using fewer seeds, thus reducing seed costs.



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  		Fig. 2. Mean response of total aboveground dry matter, residue, and seed yield of determinate soybean variety Hobbit 97 to plant populations at Mead, NE (1994 and 1995)

 
Total dry matter produced is determined not only by the amount of solar radiation intercepted but also by the efficiency with which intercepted radiation is converted into dry matter production (Shibles and Weber, 1966). At the low plant populations of determinant soybean, interception of total solar radiation would be low because of the reduced number of plants even though interception efficiency by a plant may increase. Thus, the very low TDM produced at the lowest population coupled with the higher partitioning to seeds left little plant residue on the soil (Fig. 2). Residue dry matter reached a maximum at 51000 plants ha-1 in 1994 and 129000 plants ha-1 in 1995. Beyond these plant populations, no differences were observed.

Nitrogen Fixation and Net Nitrogen Balance
Nitrogen fixation per unit area, estimated by the difference method (Fujita et al., 1990), showed a significant (P < 0.05) positive cubic response to increasing plant population (Table 2). The lowest fixation occurred with the lowest plant population. Fixation increased with increased populations for both years. These fixation values are within the values estimated for commercial soybean production (75–300 kg N ha-1) reported under low available soil N and effective Bradyrhizobia strains (Keyser and Li, 1992). Increasing the plant population of determinate soybean from 48500 to 194000 plant ha-1 also resulted in increased N fixation from 200 to 280 kg N ha-1 (Nelson and Weaver, 1980). Although we found a decline in N fixation beyond 424000 plants ha-1 in 1995, Bello et al. (1980) reported an increase in mean N fixation when plant population was doubled from 380000 to 760000 plants ha-1 for soybean of varying growth habits. It is possible that limited moisture in 1995 could have amplified the effect of population that was above 424000, preventing an increase in N fixation. The amount of N fixed by plants at the low population may be underestimated by the N difference method, which assumes similar soil N uptake by soybean and maize. Determination of N on the soybean plots at maturity would have given a more accurate N fixation amount.


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  		Table 2. Estimated means of N fixation, seed N yield, and net N balance of soybean at different plant populations and maize entry, Mead, NE, 1994 and 1995

 
Net N balance is the potential N contribution of a legume to the environment (Sangakkara, 1995). Values in this study ranged from -48 to +17 kg N ha-1 (Table 2). Despite the high amounts of N fixation, net N balance was low or negative due to the high amounts of N removed by the harvested seeds. The N removed by harvested soybean seeds at the lowest soybean population was significantly higher than the N removed by maize seeds (Table 2), ranging from 95 to 212 kg N ha-1 in 1994 and from 24 to 111 kg N ha-1 in the drier year of 1995. These values are similar to those reported by Varvel and Peterson (1992). Availability of soil mineral N with an associated reduction in reliance on N fixation may have been one factor that resulted in the negative N balance of soybean (Peoples and Herridge, 1990). This could be the reason for the negative net N balance at lower soybean populations. At very low soybean plant populations, e.g., <20000 plants ha-1, soybean would be a net exporter of soil N into harvested seeds. At higher plant populations, there would be more intraplant competition, and soybean would have less soil N per plant, and thus depend on N2 fixation. Nodule number is inversely related to the amount of N available (Beard and Hoover, 1971). The potential N contribution of soybean to the cropping systems was low with a maximum value of 17 kg N ha-1.

Maize following Soybean
Leaf Chlorophyll Content
Chlorophyll content of the maize ear leaf was determined at midsilking to give an indication of plant N status. There was no N x previous cropping system interaction for chlorophyll content of maize in both 1995 and 1996. There was a significant response of leaf chlorophyll content of maize to the previous cropping system (Fig. 3 and 4). The highest chlorophyll content in 1995 was measured in the leaves of plants from fallow plots, and the next highest chlorophyll content was in the leaves of plants from plots with the lowest soybean population, 14000 plants ha-1. No differences occurred in ear leaf chlorophyll content of maize plants from plots with plant populations between 51000 and 423000 plants m-2. The leaves of plants from the maize–maize plots had the lowest value (P < 0.05). There was a strong positive correlation between ear leaf chlorophyll content of maize at midsilking and total N accumulation of aboveground tissue. In 1996, the highest chlorophyll content was again in the leaves of plants from fallow plots. Maize leaves of plants from the population treatments that had higher chlorophyll contents than continuous maize were the lowest population, 17000 plants ha-1, and the higher populations of 253000 and 544000 plants ha-1 (Fig. 4). Although soybean in the both the low and higher plant populations depletes soil N, almost all of the N in the soybean residue is mineralized for a following crop (Power et al., 1986). Thus, there is N available for uptake by the sorghum crop. A linear response of ear leaf chlorophyll content and N application rate occurred in both years . This is expected because N forms a major component of chlorophyll, and a strong positive correlation has been reported between leaf chlorophyll content and leaf N concentrations at 10-leaf and midsilk stages of maize (Wood et al., 1992).



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  		Fig. 3. Mean ear leaf chlorophyll and maize grain yield as influenced by previous cropping systems involving soybean plots at different plant populations and fallow and maize plots with no N application, Mead, NE (1995). Maize was at 55000 plants ha-1. SE indicates 0.05 level of probability

 


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  		Fig. 4. Mean ear leaf chlorophyll and maize grain yield as influenced by previous cropping system involving soybean plots at different plant populations and fallow and maize plots with no N application, Mead, NE (1996). Maize was at 55000 plants ha-1. SE indicates 0.05 level of probability

 
Grain Yield
Main effects of N application rate and the previous cropping system significantly (P < 0.05) influenced maize grain yields. There was no yield interaction between N fertilizer and the previous cropping system. This might be expected because of low available N of the systems. Grain yield was greatest when maize followed fallow and soybean at a low population (14000 plants ha-1) in 1995 (Fig. 3). In 1996, grain yield was greatest when maize followed fallow and soybean at the low population (17000 plants ha-1), but the intermediate populations, 168000 and 253000 plants ha-1, were included (Fig. 4). In both years, the lowest maize grain yields (P < 0.05) were obtained from maize–maize plots.

One basis for using fallow is to increase water availability for a following crop. In a 12-yr study of water relations of continuous barley (Hordeum vulgare L.) and fallow–barley systems, Orphanos and Metochis (1994) found significant water storage in 3 of the 12 yr. Only once was the fallow–barley grain yield increased. Similarly, our study suggests that N availability is important and probably can be enhanced with water that is stored in the previous year if rainfall during the growing season is poor, such as in 1995. The increases in grain yield of cereal in a legume–cereal system compared with a continuous cereal system is consistent with the results of several studies and review papers (Kurtz et al., 1984; Peoples and Herridge, 1990; Mohammed and Clegg, 1993; Copeland et al., 1993; Vanotti and Bundy, 1995; Kessavalou and Walters, 1997).

Nitrogen Accumulation in Aboveground Biomass by following Maize
In both 1995 and 1996, no differences were observed between the harvest index of all the treatments, indicating similar partitioning of the maize crop (Tables 3 and 4). Also, no interactions (P < 0.05) occurred between N rate applied to maize and the previous cropping system for total N accumulation of aboveground maize. In 1995, the total aboveground N accumulated by maize plants also was not affected by N application rates. Differences (P < 0.05) were found in the effect of the previous cropping system on total N accumulation of maize (Table 3). Nitrogen accumulation in maize was higher on plots that had been under fallow the previous growing season than on those planted to soybean or maize, indicating N mineralization. Previous soybean (all populations) did not differ in its effect on total N accumulation of maize. Its effect on maize N accumulation was intermediate between fallow and continuous maize. Blumenthal et al. (1988) reported similar results with wheat following fallow, soybean, and maize. They concluded that soybean depleted soil N but to a lesser extent than maize.


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  		Table 3. Mean response (over N rates) of maize harvest index, N concentrations, dry matter, and N accumulation after previous cropping systems involving bare fallow, soybean plant populations, and continuous maize, Mead, NE, 1995

 

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  		Table 4. Mean response of maize grain yield to N applied to maize following soybean, fallow, and maize (0 N) and N credits to maize from soybean plant populations using maize and fallow as references, Mead, 1996

 
Total N accumulated by maize followed a similar trend as TDM accumulation. Grain N concentrations and N accumulations were also significantly (P < 0.05) greater for maize grown on fallow plots. Although N concentrations in the grain of the maize–maize system were higher than those of the soybean–maize system, total N accumulated by the maize–maize system was the least due to the low TDM accumulated (Table 3). In contrast to higher N concentration in the maize–maize system than in the soybean–maize system, Copeland and Crookston (1992) reported an increase in maize concentrations of N, P, and K in the soybean–maize system compared with the maize–maize system.

Even though the fallow plots had no addition of N, greater N accumulation occurred in maize grown in a fallow–maize system than in a soybean–maize or maize–maize system. Soil water availability as a result of water storage during the fallow period has been attributed to better performance of crops on fallow (Gibson et al., 1992; Schillinger and Bolton, 1993). But greater N uptake and higher chlorophyll in our study indicated that more N was available. More N was available to maize when it followed fallow than when it followed soybean at all populations, and least amount of N was available when maize followed maize (no applied N). We conclude that N availability to maize was highest when it followed fallow due to mineralization, intermediate when it followed soybean at varying populations due to intermediate microbial immobilization, and lowest in a continuous maize cropping system due to higher microbial immobilization.

Applied Nitrogen and Nitrogen Credits
Maize yield response was different to applied N in the various cropping systems for the 2 yr. Yield was increased from 2.1 (0 N) to 3.1 Mg ha-1 with either 80 or 160 kg ha-1 applied N (P < 0.05) in 1995. The drought in 1995 probably caused the lack of response to the higher level of N. In 1996, the response to N application rate by maize yields from previous cropping systems followed quadratic models (P < 0.05) (Table 4). Fertilizer N replacement value showed varying N contributions to following maize, ranging from 16 kg N ha-1 from a soybean population of 129000 plants ha-1 to 46 kg N ha-1 from the lowest soybean population, 17000 plants ha-1. Nitrogen contribution was highest at 55 kg N ha-1 when maize followed fallow (Table 4).

Vanotti and Bundy (1995) grew continuous maize for 2 yr after a 15-yr rotation involving soybean, maize, and oat (Avena sativa L.) and reported N credits (NFRVs) of 75 kg N ha-1 when maize followed soybean and 153 kg N ha-1 when maize followed alfalfa. Mohammed and Clegg (1993) measured a N credit of 45 kg N ha-1 when millet (Panicum miliaceum L.) followed soybean. As a result of earlier N credits studies, when maize follows legumes, N recommendations for maize production are adjusted by 22 to 45 kg N ha-1 following soybean and 54 to 112 kg N ha-1 following alfalfa (Kurtz et al., 1984).

Although N credits indicated that N fertilizer savings ranged from 16 to 46 kg N ha-1 when maize followed soybean rather than maize (Table 4), the N contribution of soybean estimated by NFRV may overestimate the actual N contributed by N2 fixation of soybean. Other scientists have also noted this (Peoples and Herridge, 1990; Lory et al., 1995). We made two observations that seem to support this view. First, the N credit values estimated (Table 4) were much higher than the potential N added to the soil environment by N2 fixation of soybean, which was measured by the net N balance (Table 2). Secondly, although there was no N fixation in the fallow plot, it resulted in the highest N credit to maize. We used fallow rather than maize as reference for reassessment of N credits by deducting N credit of the fallow–maize system from N credit by the soybean populations. This method gave negative N values for soybean (Table 4). Nitrogen credit, determined by NFRV, may not therefore represent the actual amount of N added to the soil by legumes in the rotation.

The importance of reduced use of soil N by legumes for increasing N availability to nonlegumes in cropping systems has been emphasized (Danso et al., 1993). Green and Blackmer (1995) attributed the N credit value when maize succeeds soybean to the difference in immobilization rates during residue decomposition of soybean and maize rather than the mineralization of N from biological N2 fixation by soybean. Nitrogen credits to cereals assessed by NFRV would represent a total yield benefit due to N added to the soil from N2 fixation, residue decomposition dynamics, and other non-N rotation effects when cereals are grown in rotation with legumes.

Also of importance is the increase of crop productivity by N with greater water availability. Copeland et al. (1993) found increased seasonal water uptake by maize of 16 mm more in a soybean–maize system compared with a maize–maize system. Varvel (1994) reported a greater mean precipitation use efficiency of 101.8 kg ha-1 cm-1 for maize in an 8-yr soybean–maize rotation compared with 83.6 kg ha-1 cm-1 for continuous maize.

Correlations
Pearson's correlation analysis in 1995 showed positive and similar correlation (P < 0.01) between maize grain yield and ear leaf chlorophyll content at midsilking and between maize grain yield and total N accumulation of aboveground tissue . The correlation between maize grain yield and ear leaf chlorophyll was higher in 1996 . No correlation was found between maize grain yield and previous soybean grain yield, soybean residue, or total mineral N of soil (30-cm depth) at maize planting. The number of seeds m-2 was the only yield component that was affected by the previous cropping system, and it had a strong positive correlation with maize grain yield. Maize grain yield was also correlated with number of ears m-2. The number of seeds m-2 and 100-seed weight were the yield components that both responded to N application rate with linear responses (, respectively).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Determinate soybean in narrow-row culture may be maintained at lower plant populations (129000 plants ha-1) than normally recommended for soybean and still maintain soybean seed yield and a yield benefit to a following maize crop. Maize grain yield was associated with N availability. Available mineral N appears to be from the low potential for N immobilization. No residue (fallow) or high N residue (soybean plots) would favor low immobilization, whereas low N residue (maize plots) would favor high immobilization. Because of this, NFRV appears to overestimate the potential contribution from N2 fixation. Also, due to the partitioning of greater amounts of N into the harvested soybean seeds, net N balance data showed that soybean contributed <=17 kg ha-1 residual N and may have depleted soil N, especially at the lowest soybean populations.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contrib. of the Nebraska Agric. Res. Division. Nebraska Journal no. 12678. Work supported in part by the Canadian Int. Dev. Agency (CIDA).

Received for publication July 20, 1999.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome