Eur. J. Agron., 1994, 3(2), 93-100

The contribution of nitrogen from legume cover crops double-cropped with winter wheat to tilled and non-tilled. maize

Z. Dou 1 and R. H. Fox

Department of Agronomy, The Pennsylvania State University, University Park, PA 16802, USA

Received 4 January 1994 ; accepted 25 January 1994

1 To whom correspondence should be addressed.

Abstract A three-year experiment was conducted to determine the contribution of nitrogen (N) from red clover (Trifolium pratense L.) and hairy vetch (Vicia villosa Roth) green manures double-cropped with winter wheat (Triticum aestivum L.) to two succeeding maize (Zea mays L.) crops under conventional tillage (CT) and no-till (NT). With maize under no-till, the vetch crop was suppressed by herbicide spray or by mowing and the clover crop by herbicide spray only. Total N contents in the legumes (tops plus roots) were 134 kg N ha- 1 in the clover and 151 kg N ha- 1 in the vetch when legume growth was terminated in order to plant maize. The first maize crop following red clover and hairy vetch produced grain yields of 10.68 and 9.89 t ha- 1 in the CT, and 7.89 and 8.01 t ha- 1 in the NT with herbicides, comparable to the maximum of 10.73 (CT) and 9.08 (NT) t ha- 1 in fallow-fertilizer treatments (150 and 200 kg N ha- 1). Maize following mowed vetch produced only 4.62 t ha- 1• With an addition of 75 kg N ha- 1 fertilizer, the second maize crop after the legumes yielded 7.57 to 9.78 t ha- 1• Estimated fertilizer N equivalences in the two successive maize crops after clover and vetch were 182 and 133 kg N ha- 1 under CT, and 186 and 164 kg N ha- 1 under NT with herbicides, with about 82 per cent occur­ring in the first year. The mowed vetch had a fertilizer N equivalence of 105 kg N ha- 1 as a two year total. The contribution of N from the legumes to maize crops is also discussed in terms of accumula­tion and apparent recovery.

Key-words : maize, cover crop, hairy vetch, N contribution, red clover.

INTRODUCTION

Nitrogen (N) management in high-N demanding crops such as maize is often complicated by inade­quate supply, high losses, low efficiency, and the potential for polluting water resources. Since the early 1980s there has been a renewed interest in the incor­poration of legumes in cropping systems, particularly as winter cover/green manures. One of the primary benefits from using legumes as winter cover/green manure is the significant contribution of N from the legumes to subsequent non-legume crops.

Hairy vetch (Vicia villosa Roth) grown as a winter cover/green manure has been widely reported as con­tributing substantial amounts of N to succeeding crops (e.g. Utomo et al., 1990; Mitchell and Teel, 1977). Red clover (Trifolium pratense L.), when used as a perennial forage legume in crop rotations, can provide

N for near maximum yield of maize (Fox and Piekielek, 1988), but may be unsuitable as a winter cover/green manure in severe winters in the northeast­ern USA (Holderbaum et al., 1990).

In the northern USA, the short growing season and severe winters generally preclude seeding a legume after maize harvest. However, it may be possible to obtain a substantial contribution of N from a legume used as a green manure if the legume is over-seeded into a small grain crop or planted immediately after the harvest of a small grain crop. In field studies con­ducted by Hesterman et al. (1992) at three locations in Michigan, red clover produced an average of 138 kg N ha- 1 when over-seeded into winter wheat and 37 kg N ha- 1 when inter-seeded with oats. In Pennsylvania, Craig (1987) found an N contribution of 20 to 80 kg N ha- 1 from red clover to maize crops when the clover was drilled with spring oats or over­seeded into winter wheat. Govere (1989) obtained

ISSN ll6!-030I/94!02/$ 4.001 © Gauthier-Villars - ESAK

94 Z. Dou and R. H. Fox

maize grain yields of 9.3 and 7 .6 t ha- 1, compared to the control of 5.1 t ha- 1, when the maize followed hairy vetch and red Clover which were planted after harvest of an oat crop.

This paper reports a study of the contribution of N from legume cover crops to tilled and non-tilled maize crops. We conducted the experiment in central Pennsylvania by over-seeding red clover into winter wheat and planting hairy vetch after the harvest of the wheat. These two green manure crops were then followed by two successive maize crops. For compar­ison, two crops of maize following fallow after the wheat harvest, fertilized at rates of 0, 50, 100, 150, and 200 kg N ha- 1 were included in the study. Specific objectives were to (i) examine the drymass production of the legumes and their N contents, (ii) estimate the effect of the legumes on the yields of tilled and non-tilled maize, and (iii) evaluate the efficiency with which residual N from the legumes was used by the maize crops.

MATERIALS AND METHODS

The field experiment was conducted between 1990 and 1992 at The Pennsylvania State University's R. E. Larson Agricultural Experiment Station on a Murrill silt loam (fine-loamy, mixed, mesic Typic Hapludult). The soil was maintained in the optimum range of available P, K, and Mg and at a pH (I : l, soil : H20) of 6.7. The soil's plough layer (0 to 25 cm) contained 26 g kg- 1 organic matter and had a cation exchange capacity (CEC) of 11.8 cmolc kg- 1•

Red clover was broadcast into a winter wheat crop at a seeding rate of 9 kg ha- 1 on 18 Apri I 1990, and hairy vetch was planted with a no-till drill at a rate of 22 kg ha- 1 on 9 August 1990 after the wheat har­vest. Fallow after the wheat harvest was included as a comparison. The legumes were allowed to grow until 16 May 1991 when tillage treatments were applied, followed by maize planting. With maize under CT, the legumes were incorporated into the soil by mouldboard plough. With maize under NT, the legumes were sprayed with 1.75 kg ha- 1 of Dual [2­chloro-N-(2-ethy 1-6-methylphenyl)-N-(2-methyoxy- l­methylethyl) acetamide], 1.75 kg ha- 1 of Gramoxone [l,l'-dimethyl-4,4'-bipyridinium ion (dichloride salt)], and 0.58 kg ha- 1 of 2,4-D (2,4-dicholorophenoxy-ace­tic acid). An additional NT treatment consisted of suppressing the vetch by mowing instead of by her­bicides. Fallow in the NT received a herbicide appli­cation of Dual and Gramoxone at the same rate as for the legumes. The maize crop (cv. Doebler's 61X) was planted on 17 May 1991 with final populations from 53000 to 59000 plants ha- 1• The second maize crop (cv. Pioneer 3527) was planted on 6 May 1992 with final populations from 48000 to 57000 plants ha- 1•

Tillage was the same as in 1991.

The experimental design was a randomized com­plete block, split-plot design with four replications. The main plots consisted of seven treatments that were the combination of cover crops and tillage meth­ods : maize following fallow or clover with conven­tional tillage and no-till herbicide, and maize follow­ing vetch with conventional tillage, no-till herbicide (NTh), or no-till mowed (NTm). The subplots, 4.56 by 15.2 m in size, received ammonium nitrate ferti­lizer at the following rates : 0, 50, 100 kg N ha- 1 for the first year maize and 0, 75, 150 kg N ha- 1 for the second year maize after the legumes, and 0, 50, 100, 150, 200 kg N ha- 1 for maize after fallow in both years. At planting, 10-30-10 (N-P20 5-K20) starter fer­tilizer was banded below and to the side of the seed at a rate of 112 kg ha- 1 in both 1991 and 1992. Ammonium nitrate was broadcast by hand within a week after planting each year.

To determine drymass production and N content, the legumes were sampled twice, once in the seeding year before frost killing (28 September 1990) and again the next spring ( 14 May 1991) prior to maize planting, by harvesting the above-ground portion from an area of 20 x 60 cm in each subplot, which gave a total of 24 samples for clover and 36 samples for vetch each time. At the spring sampling time, samples of legume roots, five for each species, were collected by digging out a volume of soil in 20 x 60 x 20 cm (width x length x depth) from five randomly selected locations and washing away the soil in the laboratory.

At maturity, maize ears were hand-harvested from 14 m of the two centre rows in each subplot. Six stalks were taken randomly from the harvested section and chopped. After weighing, subsamples of ears and stover were taken, and dried at 60 °C. Subsamples of maize grain and stover as well as legume tissues were ground to pass a 1 mm screen and micro-Kjeldahl digests of these were analyzed for total N by a Technicon automated analyzer.

The N content in maize grain and stover for indi­vidual treatments were calculated from the N concen­tration in plant tissue and the corresponding dry mat­ter yields. Fox and Piekielek (1983) determined that, on average, maize cobs plus husks contained five per cent of the stover plus grain N content. Therefore, total N uptake was calculated by multiplying the stover plus grain N content by 1.05.

Analyses of variance were computed for drymass and N content of the legumes, yields of grain and above-ground dry matter of maize, and total N con­tent of maize in 1991 and 1992. Least significant dif­ferences (LSD) were calculated from these analyses of variance to make comparisons among treatments. Each year, the response of maize grain yield to ferti­lizer N rate in the fallow treatment was fitted to a quadratic function with a linear plateau using the SAS NLIN procedure (SAS Institute, 1985). These func­tions were used to determine the fertilizer N equiv­alences for the legumes.

95 The contribution of nitrogen from legume cover crops double-cropped

RES UL TS AND DISCUSSION

Legume drymass and N content

During the period from planting to the autumn sampling in the seeding year (about 7 weeks), hairy vetch produced 1.89 t drymass ha- 1, which contained 59 kg N ha- 1• Red clover had more than double the drymass (4.21 t ha- 1) and N content (118 kg N ha- 1)

of hairy vetch, mainly due to earlier planting and lon­ger growth period (Table I).

The above-ground portion of the legumes was killed by frost at the end of September. The follow­ing winter (1990/91) had monthly average tempera­tures of 1.8, - 1.8, 1.1, and 4. 7 °C during December to March. Regrowth of both legume species was observed by the end of March in the spring of 1991. We observed very rapid growth of the legumes dur­ing the first two weeks in May, though no quantita­tive assessment was made for that period. By mid­May 1991 when the legume growth was terminated in order to plant maize, red clover had produced a dry­mass (tops plus roots) of 3.59 t ha- 1 and hairy vetch 3.65 t ha- 1, with total N content of 134 kg N ha- 1 in the clover and 151 kg N ha- 1 in the vetch. In an ear­lier study conducted in the same area by Govere (1989), hairy vetch and red clover planted after har­vest of an oat crop (late July and early August) pro­duced a drymass of 4.48 t ha- 1 and total N content of 198 kg N ha- 1 in the vetch, and 2.58 t ha- 1 and 91 kg N ha- 1 in the clover by mid-May in the fol­lowing spring. This shows that substantial drymass and N accumulation can be obtained in this climatic zone by double-cropping the legumes with small grain crops and allowing the legumes to grow to mid-May. The drymass and N content of the legumes in this study were also within the reported ranges in the lit­erature where the legumes were used as winter cover/green manures under various weather and soil conditions.

Calculated drymass of roots was 12 per cent of the total drymass for the clover but only 6 per cent for the vetch (Table 1). These values could be under-esti­mated because of incomplete recovery during root collection due to the fineness and branched nature of the root system. Govere (1989) reported 21 and 7 per cent for red clover and hairy vetch though Kroontje and Kehr (1956) obtained only 2 per cent for hairy vetch.

Maize yields

The 1991 season was very dry with a total preci­pitation of 448 mm between April and November, 170 mm less than the long-term average in the area (618 mm). The field was irrigated once in early June with an estimated 4 to 5 cm of water. With this one irrigation, maximum grain yield of maize in the fal­low fertilizer treatment under conventional tillage exceeded the normal maximum of 10 t ha- 1 typical of the area. We feel that the delay of maize planting in order to allow substantial legume growth did not significantly affect maize production. The normal planting dates for maize in this area range from the first to the fifteenth day of May. Our planting (17 May in 1991) was only a few days late.

The maize grain (Table 2) and above-ground dry matter (Table 3) yields were affected similarly by the treatments. Both yields were significantly reduced by no-till, with an overall mean grain yield of 9.78 t ha- 1

in the CT vs. 7.50 in the NT, and above-ground dry matter 14.15 t ha- 1 in the CT vs. 11.29 in the NT. In addition to reduced mineralization in NT, which was observed in this experiment (Dou, 1993) and has been reported by others and summarized by Fox and Bandel (1986), weed competition was another factor contributing to the yield reduction in the no-till system, especially for the low N rate treatments. For example, the fallow 0 N NT treatment had less than half of the yield in the fallow 0 N CT treatment.

Table 1. Dry matter yield and N content of red clover and hairy vetch sampled in autumn 1990 and spring 1991.

Legume species Autumn 1990 Spring 1991 Total N 1

Dry mass N concentration N content Drymass N concentration N content t ha-I g kg-I kg N ha- 1 t ha-I g kg-I kg N ha- 1 kg N ha- 1

Red clover 134 above-ground 4.21 28 118 3.17 39. J 124 root 0.42 23.5 JO Hair~ vetch 151 above-ground 1.89 31 59 3.42 42.9 147 root 0.23 17.8 4 LSD (p = 0.05) 0.42 5 10 0.30 2.0 14

0.10 2 6.3 2

1 Total N content in legume tops plus roots at the time of growth termination on 14 May 1991. 2 LSD for roots.

Vol. 3, n° 2 - 1994

96 Z. Dou and R. H. Fox

Table 2. Grain yield of maize as affected by tillage, cover Where maize followed fallow, nitrogen fertilizer crop, and fertilizer N rate.

Year Treatment ' Grain yield, t ha- 1

N rate, kg N ha- 1

0 50(75) 2 100 150 200

1991 Fallow, CT 7.83 8.99 9.84 10.73 9.62 Clover, CT 10.68 10.37 8.57 Vetch, CT 9.89 10.93 10.18 Fallow, NT 2.94 4.41 6.84 8.16 9.08 Clover, NT 7.89 7.58 9.05 Vetch, NTh 8.01 8.93 9.58 Vetch, NTm 4.62 6.91 8.06

LSD (p = 0.05) 3 = 1.43, 1.49

1992 Fallow, CT 6.71 8.23 9.48 9.57 9.86 Clover, CT 7.66 9.39 10.42 Vetch, CT 8.12 9.78 10.34 Fallow, NT 4.62 6.07 8.05 8.13 8.70 Clover, NT 5.71 7.82 8.91 Vetch, NTh 5.14 7.57 7.87 Vetch, NTm 6.58 7.64 8.42

LSD (p = 0.05) 3 = 1.04, 1.10

1 See text for details. 2 The N rate for the second maize crop (1992) after legumes was 75 kg N ha- 1• 1 LSDs for differences between subplot treatments (N rate) for the same main-plot treatment (tillage and cover crop) and for different main-plot treatments, respectively.

Table 3. Above-ground dry matter of maize as affected by tillage, cover crop, and fertilizer N rate.

Year Treatment 1 Above-ground dry matter, t ha - I

N rate, kg N ha- 1

0 50(75) 2 100 150 200

1991 Fallow, CT 12.37 13.56 13.89 14.81 13.88 Clover, CT 14.71 15.32 13.16 Vetch, CT 13.53 15.67 14.72 Fallow, NT 5.61 8.38 10.96 12.09 13.69 Clover, NT 12.34 11.36 13.05 Vetch, NTh 12.73 13.47 14.25 Vetch, NTm 7.91 10.65 11.59

LSD (p = 0.05) 3 = 2.03, 2.17

1992 Fallow, CT 11.20 13.18 14.58 16.08 16.62 Clover, CT 12.46 15.00 16.02 Vetch, CT 12.96 15.43 16.26 Fallow, NT 8.85 12.40 13.13 13.99 14.91 Clover, NT 10.44 12.92 15.57 Vetch, NTh 10.37 12.94 13.90 Vetch, NTm 12.12 13.42 14.00

LSD (p = 0.05) 3 = 1.73, 1.78

increased the yields of maize under both conventional tillage and no-till (Tables 2 and 3). A quadratic-linear plateau model was fitted to the data of grain yield versus fertilizer N rate. This model has been used in Pennsylvania by Fox and Piekielek (1983) for ten years and was found by Cerrato and Blackmer (1990) to be the best among several commonly used models for describing maize grain yield response. Using this model, the following response functions were obtained for the fallow treatments : Y = 7.79 + 3.03 x 10-2 N - 9.77 x 10-s N2 (CT) (1) Y = 2.76 + 4.47 x 10-2 N - 6.31x10-5 N2 (NT) (2) where Y =grain yield in t ha- 1 and N =fertilizer rate in kg N ha- 1•

Where the legumes were incorporated into the soil, grain yields reached 10.68 t ha- 1 and 9.89 t ha- 1 fol­lowing clover and vetch, with no broadcast fertilizer (Table 2). There was no significant response of grain or above-ground dry matter to fertilizer N where maize followed legumes under CT. The incorporated legume residues provided an adequate amount of N for near-maximum maize production.

However, in the treatments of maize following legumes under no-till with herbicides, grain yields were 7.89 t ha- 1 after the clover and 8.01 t ha- 1 after the vetch, significantly lower than those in the corre­sponding CT treatments. Maize under no-till responded to fertilizer N as the yields at the JOO N rate were significantly greater than the vetch 50 N and clover 0 N treatments (Table 2). This suggests that N supply was insufficient for near-maximum grain yields in the treatment of maize following legumes under NT with herbicides. Additional evi­dence supporting this conclusion is that estimated net N mineralization four weeks after maize planting in the legume with no-till was about half of that in the conventional tillage treatments and that after account­ing for the contents of total N in weeds and nitrate N in soil, the availability of N in the overall soil-crop system was still significantly lower in the no-till than in the conventional tillage (Dou, 1993). It is likely that the extremely dry weather prevailing during the 1991 growing season desiccated the surface-applied legume residues to the extent that mineralization was greatly retarded compared to the conventional tillage system.

The mowed vetch treatment produced much lower yields than the herbicide spray treatment, especially at the low fertilizer N rates (Tables 2 and 3). An additional problem with this treatment was that the vetch crop was not successfully suppressed by mow­ing and a regrowth occurred which, in turn, resulted

1 See text for details. in poor emergence and a low population of maize. 2 The N rate for the second maize crop (1992) after legumes was The yields of maize in this treatment were signifi­75 kg N ha- 1• cantly increased with fertilizer N addition.

1 LSDs for differences between subplot treatments (N rate) for the same main-plot treatment (tillage and cover crop) and for different main- The second maize crop (1992) had less weed com­plot treatments, respectively. petition, which contributed to the smaller difference

Eur. J. A11ron.

--

97 The contribution of nitrogen from legume cover crops double-cropped

in yields between the two tillage methods in the fal­low with low rates of fertilizer, compared to 1991 (Tables 2 and 3). Using the same quadratic-linear pla­teau model, the following functions describing grain yield response to fertilizer N rate in the fallow ferti­lizer treatments were obtained :

N2Y =6.68 + 3.93 x 10-2 N - 12.60 x 10-5 (CT) (3)

Y =4.55 + 4.14 x 10-2 N - 10.50 x 10-5 N2 (NT) (4)

As expected, the yields of this second maize crop after the legumes responded to fertilizer N addition in both tillage treatments, since much of the labile N in the legume residues would have been mineralized and used by the maize in the first season. Nevertheless, there was an overall tendency for greater yields in the legume 0 N treatments compared to yields in the fal­low 0 N, although not all differences were statistically significant. With an application of 75 kg N ha- 1 as broadcast fertilizer, the grain yields after legumes were all close to the maximum yields obtained in the fallow treatments. The additional N in the 150 N treatment did not significantly increase grain yields compared to the 75 N treatment except with the NT clover treatment, although there was an overall ten­dency of yield increase in both grain and above­ground dry matter with the additional fertilizer N (Tables 2 and 3).

Legume N contribution

Fertilizer N equivalence (FNE)

Fertilizer N equivalence, defined as the fertilizer N required to achieve the same yield in maize follow­ing fallow as was attained by non-N-fertilized maize that followed a legume (Fox and Piekielek, 1983), has been commonly used to estimate the credit of legumes when fertilizer recommendations are made for succeeding non-legume crops. This has also been referred to as 'inorganic nitrogen equivalence' (Stickler et al., 1959) and 'fertilizer-N-replacement value' (Hesterman et al., 1987).

In this study, the FNEs of red clover and hairy vetch were calculated using function ( 1) for the CT and (2) for the NT in 1991, and functions (3) and ( 4) in 1992, by substituting Y in the function by grain yield of maize in the treatment of legume 0 N and calculating the N rate that would have produced that yield.

In 1991, when the grain yield in the clover 0 N CT treatment was greater than the plateau yield in the fallow treatment, the N rate at the plateau initiation (155 kg N ha- 1) was arbitrarily chosen as a minimum estimate of the FNE for this treatment. Calculated FNE for the hairy vetch under CT in 1991 was 103 kg N ha- 1, about two thirds of that for the clover. However, we observed that there was more

total available N throughout the season in the vetch plots than in the clover plots (total available N was defined as the amount of nitrate-N in the 0 to 45 cm soil profile plus the N contained in the above-ground portion of maize and weeds : see Dou, 1993 for details). This contradiction could be partially due to the variation in maize yields at harvest. One of the replications in the vetch 0 N CT treatment had a much lower yield than the other three (7.40 vs. 11.03, 11.88 and 9.25 t ha- 1), but it was retained, based on the Q­test for outlier significance (Allmaras, 1965) when calculating the treatment average. Due to the flatness of the yield response curve (derived from function 1), the slightly lower average yield (9.89 t ha- 1) for this treatment, compared to 10.68 t ha- 1 in the clover 0 N CT treatment, resulted in a large difference in ferti­lizer N equivalence. This suggests that there are prob­lems using the fertilizer N equivalence method to evaluate legume N supplying capacity, although it is probably the best way to provide information for fer­tilizer N recommendation purposes at present.

The calculated FNE for the vetch 0 N treatment under no-till with herbicides (149 kg N ha- 1) was much greater than that under conventional tillage (103 kg N ha- 1) (Table 4). This is surprising since the

Table 4. Calculated fertilizer N equivalences for the legumes with different tillage methods.

Treatment 1 Fertilizer N equivalence, kg N ha- 1

1991 1992 Total

Clover, CT 155 27 182 Clover, NT 144 42 186 Vetch, CT 103 30 133 Vetch, NTh 149 15 164 Vetch, NTm 48 57 105

1 See text for details.

yields of maize in the two treatments show the oppo­site trend: 8.01 t ha- 1 in the NT vs. 9.89 t ha- 1 in the CT (Table 2). The higher FNE for NT vetch com­pared to CT vetch is due to the difference in the yield response functions (functions 1 and 2) for the two tillage systems. This is another example illustrat­ing the weakness of using FNE values calculated in this manner.

The two year total FNEs for the clover CT and NT, vetch CT and NT with herbicides were 182, 186, 133, and 164 kg N ha- 1, respectively, with an aver­age of 82 per cent available during the first year. For the mowed vetch treatment, calculated FNE in 1991 (48 kg N ha- 1

) was smaller than that in 1992 (57 kg N ha- 1

). The low population of maize as well as severe weed competition in this treatment in 1991, as

Vol. 3, n° 2 - 1994

98 Z. Dou and R. H. Fox

mentioned earlier, probably left more residual N in the soil profile to be used by the second maize crop.

Accumulation and apparent recovery

The accumulation and apparent recovery of N in successive crops from sources of legumes have also been used by several researchers to estimate N cred­its for the legumes (e.g. Holderbaum et al., 1990 and Hesterman et al., 1987). In this study, the accumula­tion of N in maize from the clover and vetch was calculated by subtracting the total N uptake in the above-ground portion of maize at harvest in the fal­low 0 N treatment from that of legume 0 N with the same tillage system (Table 5). The apparent recovery of N in maize from legumes was derived by dividing the accumulation value by the total N content of the legumes prior to maize planting in 1991.

Calculated accumulations ranged from 25 to 76 kg ha- 1 in the first year maize and from 71 to 105 kg ha- 1 in the two year total (Table 5). There were no significant differences between any two treatments, except for the treatment of mowed vetch, though total N uptake in maize showed significant differences between CT and NT in 1991 (163 vs. 108 for the clover 0 N, and 169 vs. 113 for the vetch, see Table 5). Because of severe weed competition, total N uptake in maize in the fallow 0 N NT treatment in 1991 was very low compared to the fallow 0 N CT treatment (37 vs. 95 kg ha- 1 in Table 5). The extremely low N uptake in the 0 N NT maize would under-estimate the soil's N supplying capacity which, in tum, would result in an over-estimated accumulation of N from legumes for the legume 0 N NT treatments because the latter had a relatively less severe weed problem. Therefore, there are also potential problems when using this dif­

ference method to estimate N credits for legumes under different tillage and management techniques.

The derived apparent recoveries of N from legumes averaged 51 per cent after the first maize crop and 66 per cent for the two year total, excluding the mowed vetch treatment (Table 5). The average recovery in the first maize crop was very close to the average recov­ery of 55 per cent from fertilizer N sources at the economic optimum N rate observed by Fox and Piekielek (1983) in Pennsylvania. It is higher than most of the reported values in the literature. For example, Hesterman and associates (1987) found that the apparent recovery of legume N in published sources ranged from 10 to 34 per cent, and 43 per cent in their own maize-alfalfa rotation trial. Stickler et al. (1959) reported a range of 18 to 47 per cent for 19 legume species grown as green manures in an ear­lier study. However, Varco et al. (1989) obtained a recovery of 54 per cent by subsequent maize from a hairy vetch N source. Differences in legume species, soil characteristics, weather conditions, and manage­ment practices could all contribute to the variation of reported apparent recovery found in the literature.

It is likely that there was an unknown amount of legume residue N in fall 1990 that contributed to the successive maize crops, but was not accounted for when calculating the apparent recovery. Most work cited in the literature uses spring samples of legumes to estimate total N returned to the subsequent crop. However, Stickler et al. (1959) used autumn samples, and Hesterman et al. (1992) used autumn plus spring samples. Theoretically, using 15N could help reveal the fate and behavior of legume N ; however, valid interpretation of N recovery of the added 15N labelled legume can be difficult due to an incomplete under­standing of the mineralization-immobilization turnover interactions of the added N (Jenkinson et al., 1985).

Table 5. Accumulation and apparent recovery of N in maize from legumes as affected by legume species and tillage method.

Treatment 1 Total N in maize Accumulation of N Apparent recovery 3

from legumes 2

kg N ha- 1 kg N ha- 1 per cent

1991 1992 1991 1992 Total 1991 1992 Total

Fallow, CT 95 95 Fallow, NT 37 76 Clover, CT 163 113 68 18 86 51 13 64 Clover, NT 108 95 71 19 90 53 14 67 Vetch, CT 169 126 74 31 105 49 21 70 Vetch, NTh 113 93 76 17 93 50 11 61 Vetch, NTm 62 122 25 46 71 17 30 47 LSD (p = 0.05) 21 26 26 ns 4 27

1 See text for details. 2 Total N content of maize in the legume treatment minus that in the fallow treatment. ' Accumulation of N in maize from legume divided by total N in legume drymass at maize planting in 1991 (134 kg N ha- 1 in the clover and 151 kg N ha- 1 in the vetch) x 100. 4 F test not significant.

Eur. J. Agron.

99 The contribution of nitrogen from legume cover crops double-cropped

Residual nitrate in soil

The availability of N from legume cover crops to non-legume crops has often been estimated based on one successive crop (e.g. Utomo et al., 1990 ; Sarrantonio and Scott, 1988; Govere, 1989). To bet­ter estimate the contribution of N from the source of legumes, Sutherland et al. (1961) as well as Huntington et al. (1985) used an N balance concept by summing total N uptake in maize plus the left­over inorganic N in soil at harvest after subtracting the equivalent fraction in the control treatment.

In this study, residual nitrate in the upper 45 cm soil profile after maize harvest in 1991 was 17 kg N ha- 1 in the clover, 0 N CT treatment, 53 kg N ha- 1 in the vetch, 0 N CT, and negligible for all legume 0 N NT treatments, after subtracting the equivalent frac­tions of soil nitrate in the fallow 0 N treatment. This left-over nitrate would be available to a second crop in the following year only if it remained in the soil profile over winter.

SUMMARY AND CONCLUSION

Under the conditions of this study, red clover and hairy vetch double-cropped with winter wheat were successfully established in the seeding year, survived well during the winter, and produced a substantial amount of drymass and N by mid-May when maize was planted. The legumes provided sufficient N under conventional tillage but insufficient N under no-till for maximum yields of the first successive maize crop. In the second year, residual N from the legumes plus 75 kg N ha- 1 as ammonium nitrate produced grain yields close to the maximum obtained in the fallow fertilizer treatment. Mowing vetch was the least successful in maize production.

Fertilizer N equivalence, the approach used most often to estimate the contribution of N from legume N source to a successive non-legume crop, is very specific to experimental conditions, and may produce misleading information such as the lower fertilizer N equivalence value calculated for the CT vetch than for the NT vetch, even though maize yield in the CT vetch was higher than in the NT vetch. The accumu­lation and derived recovery of N in maize from legumes provide an estimated N use efficiency which is only 'apparent'. This method has the potential for biases such as over-estimating accumulation of N in the first year for the no-till herbicide treatment because of an extremely low N uptake in maize in the control due to weed competition. The amount of residual nitrate in soil from legume sources after har­vest of the first maize crop was appreciable only in the CT treatments, which might provide additional information about potentially available N to the fol­lowing year crop.

REFERENCES

Allmaras R.R. (1965). Bias. In: C. A. Black et al. (Ed.). Methods of soil analysis. Part. I. American Society of Agronomy, 24-42.

Cerrato M. E. and Blackmer A. M. (1990). Comparison of models for describing corn yield response to nitro­gen fertilizer. Agron. J., 82, 138-143.

Craig P. H. (1987). Effect of soil tillage on residual nitrogen contribution by a red clover green manure crop to subsequent corn crops. M. S. Thesis, Dept. of Agronomy, Penn State University, University Park, PA.

Dou Z. (1993). Nitrogen dynamics in conventional and no-tillage corn production following legume cover crops. Ph.D. Thesis. Dept. of Agronomy, Penn State University, University Park, PA, USA.

Fox R. H. and Bandel V. A. (1986). Nitrogen utilization with no-tillage. In : Sprague M. A. and Triplett G. B. (Eds.). No-tillage and surface-tillage agriculture : the tillage revolution. New York: John Wiley and Sons, 117-148.

Fox R.H. and Piekielek W. P. (1983). Response of com to nitrogen fertilizer and the prediction of soil nitro­gen availability with chemical tests in Pennsylvania. Bulletin 843. The Penn State University, College of Agriculture, Agricultural Experiment Station, University Park, PA.

Fox R.H. and Piekielek W. P. (1988). Fertilizer N equivalence of alfalfa, birdsfoot trefoil, and red clover for succeeding corn crops. J. Prod .. Agric., 1, 313-317.

Govere E. M. (1989). Nitrogen contribution of red clover and hairy vetch planted as a doublecrop after small grain harvest to no-till corn. M. S. Thesis, Dept. of Agronomy, Penn State University University Park, PA.

Hesterman 0. B., Russelle M. P., Sheaffer C. C. and Heichel G. H. (1987). Nitrogen utilization from fer­tilizer and legume residues in legume-corn rotations. Agron. J., 79, 726-731.

Hesterman 0. B., Griffin T. S., Williams P. T., Harris G. H. and Christenson D.R. (1992). Forage legume­small grain intercrops : nitrogen production and response of subsequent com. J. Prod. Agric., 5, 340-348.

Holderbaum J. F., Decker A. M., Meisinger J. J., Mulford F. R. and Vough L. R. (1990). Fall-seeded legume cover crops for no-tillage corn in the humid East. Agron. J., 82, 117-124.

Huntington T. G., Grove J. H. and Frye W.W. (1985). Release and recovery of nitrogen from winter annual cover crops in no-till corn production. Commun. Soil Sci. Plant Anal., 16, 193-211.

Jenkinson D.S., Fox R.H. and Rayner J. H. (1985). Interactions between fertilizer nitrogen and soil nitro­gen-the so called 'priming effect'. J. Soil Sci., 36, 425-444.

Vol. 3, n° 2 - 1994

100

Kroontje W. and Kehr W.R. (1956). Legume top and root yields in the year of seeding and subsequent bar­ley yields. Agron. J., 48, 127-131.

Mitchell W. H. and Teel M. R. (1977). Winter-annual cover crops for no-tillage corn production. Agron. J., 69, 569-573.

Sarrantonio M. and Scott T. W. (1988). Tillage effects on availability of nitrogen to corn following a win­ter green manure crop. Soil Sci. Soc. Am. J., 52, 1661-1668.

SAS Institute (1985). SAS user's guide : Statistics. 5th ed. SAS Institute, Cary, NC.

Stickler F. C., Shrader W. D. and Johnson I. J. (1959).

z. Dou and R. H. Fox

Comparative value of legume and fertilizer nitrogen for corn production. Agron. J., 51, 157-160.

Sutherland W. N., Shrader W. D. and Pesek J. T. (1961 ). Efficiency of legume residue nitrogen and inorganic nitrogen in corn production. Agron. J., 53, 339-342.

Utomo M., Frye W.W. and Blevins R. L. (1990). Sustaining soil nitrogen for corn using hairy vetch cover crop. Agron. J. 82, 979-983.

Varco J. J., Frye W.W., Smith M. S. and MacKown C. T. (1989). Tillage effects on nitrogen recovery by corn from a nitrogen-15 labeled legume cover crop. Soil Sci. Soc. Am. J., 53, 822-827.

Eur. J. Agron