The nitrogen- and non-nitrogen-contribution effect of ploughed grass leys on the following arable forage crops: determination and optimum use

European Journal of Agronomy 16 (2002) 57 – 74 www.elsevier.com/locate/eja

The nitrogen- and non-nitrogen-contribution effect of ploughed grass leys on the following arable forage crops: determination and optimum use Frank Nevens *, Dirk Reheul Ghent Uni6ersity, Department of Plant Production, Coupure Links 653, 9000 Gent, Belgium Received 15 August 2000; received in revised form 20 June 2001; accepted 20 June 2001

Abstract From 1990 to 1998, we studied the N release from ploughed 3-year-old grazed grasslands in the subsequent three seasons of forage crops on a sandy loam soil. Silage maize in the ley – arable rotation outyielded continuous maize on permanent arable plots by 85, 21 and 2% at mineral N fertilization rates of respectively 0, 75 and 180 kg N ha − 1. This decreasing yield effect with increasing N fertilization indicated that the ley– arable rotation effect was mainly a N-contribution effect. The N release was highest during the first year; it decreased during the second and third year following the grassland ploughing. Economically optimum N fertilization rates for silage maize in these years were respectively 2, 139 and 154 kg N ha − 1. Simultaneously, on permanent arable plots this was respectively 152, 191 and 183 kg N ha − 1. This resulted in comparable yields (19.75 Mg DM ha − 1 year − 1) but with a possible saving of 231 kg of mineral N fertilizer ha − 1 in a 3-year silage maize period following the ploughed leys compared with continuous silage maize. The N uptake by silage maize on temporary arable plots following grasslands was higher than on permanent arable plots, owing to the higher yields but also to an increased N concentration in the crop on the temporary arable plots. Starting the arable forage crop sequence with fodder beet following the grassland ploughing and adjusting the N fertilization to the enhanced N release minimized the risks on high amounts of residual soil N and hence N leaching losses. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Economically optimum N fertilization rate; Fodder beet; Ley– arable rotation; Permanent arable plots; Residual mineral soil N; Silage maize

1. Introduction For a long time in the history of agriculture, grasslands (with or without legumes) were consid* Corresponding author. Tel.: + 32-9-264-9062; fax: + 329-264-9097. E-mail address: [email protected] (F. Nevens).

ered of vital importance in crop rotations for building up reserves of organic matter and nitrogen in the soil (Clement and Back, 1969; Lindemans, 1952). After World War II however, the plentiful and inexpensive availability of mineral fertilizers and pesticides enabled the development of monocultures, ‘without loss of yield’. Nevertheless, these

1161-0301/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 1 6 1 - 0 3 0 1 ( 0 1 ) 0 0 1 1 5 - 0

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systems often induced a decrease in soil organic matter content, a degradation of the soil structure, increased needs for external inputs and higher risks of surface and groundwater contamination (Karlen et al., 1994; Bullock, 1992). Recent (often legislative) restrictions on the use of N fertilizers and a renewed interest in organic farming or more integrated, sustainable farming systems in general, restored the attention to crop rotations and more specifically to the role of grass/legume leys in these rotations (Baars, 1998; Philipps et al., 1998; Scholefield and Smith, 1996; Younie and Hermansen, 2000). Under grass leys, organic matter accumulates in roots, stubble, litter and soil organic matter (Vaidyanathan et al., 1990; Whitehead et al., 1990). Ploughing the grassland results in the mineralization of this organic matter in the upper soil layer (Bergstro¨ m, 1986; Francis et al., 1992; Greenland and Ford, 1964; Machet and Mary, 1989; Stopes et al., 1988; Strebel et al., 1988; Watson et al., 1997). For arable crops following the grassland, this results in higher yields (Lyon, 1927) and/or a lower N fertilization requirements to reach optimum yields (Johnston, 1990; Van Dijk, 1998, 1999). Moreover, this effect continues in subsequent years (Fassbender, 1998; Linden and Wallgren, 1993; Johnston et al., 1994; Adams et al., 1970) in a decreasing way (Barber, 1972). The size of this N contribution effect depends on a number of factors. The amount of released N (‘‘N credit’’) increases with the age of the ploughed sward (Froment et al., 1999; Young, 1986; Hassink, 1996; Whitehead et al., 1990; Whitmore et al., 1992). Johnston et al. (1994) stated that the accumulation of almost maximum soil N occured by a 3-year-old ley. Since after ploughing such a ley, the amount of mineralized N decreases rapidly over the following years, they recommended that in a ley–arable rotation both the ley and arable intervals should not exceed 3 years. Studdert et al. (1997) concluded that rotations including a minimum of 3 years of pasture maintain essential soil properties such as soil organic matter within acceptable levels, also preventing soil erosion (Onofrii et al., 1996).

Compared with cut grassland, grazed grassland accumulates more subsoil organic matter and N and hence more N is released post ploughing (Clement and Back, 1969; Clement and Williams, 1962, 1967; Davies, 1996; Davies et al., 1997; Garwood et al., 1972; Hassink and Neeteson, 1991; Johnston, 1973; Whitehead et al., 1990; Williams and Clement, 1965). The fertilization level of the grassland has a relatively small effect on the amount of accumulated organic matter and on the N release following the grassland ploughing (Garwood et al., 1972; Hassink and Neeteson, 1991; Arden-Clarke and Hodges, 1987; Hassink, 1992; Watson et al., 1997. Loiseau et al. (1992a,b) found that only previous organic N fertilization (slurry) affected the amount of mineralized N following the ley ploughing, mineral N fertilization did not. Clement and Back (1969) indicated that a higher N fertilization of the grass ley only affected the following arable crop supplementary when the herbage was grazed. Besides the important N contribution effect, a rotation effect that does not relate to N occurs in arable crops following grassland ploughing. This non-N effect is observed and estimated as the yield (or other parameter) advantage for the crop in the rotational system under non-N-limiting circumstances (e.g. at high mineral N fertilizer rates). In the case of silage maize, the nature of such a non-N effect was often ascribed to healthier plant roots (Schro¨ der et al., 1991; Van Dijk et al., 1996, 1997). Ploughing grass or grass/legume leys to allow arable production is often mentioned as a potentially major contribution to nitrate pollution of drinking water resources (Cameron and Wild, 1984; Eriksen et al., 1999). The possible extensive N leaching losses are due to the enhanced mineralization after ploughing and the inadequate use of this released N by the following crops (Bergstro¨ m, 1986; Cameron and Wild, 1984; Clement and Williams, 1962, 1967; Garwood et al., 1972; Hassink, 1996; Linden and Wallgren, 1993; Loiseau et al., 1992a; Machet and Mary, 1989; Philipps and Stopes, 1995; Stopes et al., 1988; Strebel et al., 1988; Vaidyanathan et al., 1990; Watson et al., 1993).

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Possible measures to control these N losses are a time delay in the ploughing of the ley from early to late autumn or from autumn to spring (Djurhuus and Olsen, 1996; Høgh-Jensen, 1996; Philipps and Stopes, 1995; Philipps et al., 1998), reducing the intensity of soil tillage (frequency and depth) (Ko¨ pke, 1995), the cessation of grazing for a short period before sward incorporation (Davies et al., 1997) and installing (catch)crops after autumn ploughing (Fassbender, 1998; Latus et al., 1995). Since the largest amount of mineralized N is released during the first year after ploughing the leys and less N is mineralized from the second year on (Johnston et al., 1994; Stopes et al., 1988; Van Dijk, 1999), much attention should be drawn to the optimization of the ley/first arable crop interface and of the first arable crop season. The challenge is to select a first arable crop which is profitable and which utilizes the released N efficiently (Morvan et al., 2000). Moreover, the immobilization/mineralization processes following the grass ploughing are very variable (Shepherd, 1993) and N fertilizer in the following arable crops should be applied considering the enhanced mineralized N from the ploughed down grass ley (Eriksen and Søegaard, 2000; Van Dijk, 1999). Reliable data from fertilizer experiments are needed to produce soil type specific guidelines in order to reduce nitrate leaching after ploughing leys (Philipps et al., 1998) and to help farmers manage a ley–arable rotation adequately. A first aim of our research was to quantify both the N-contribution effect and the non-N effect of 3-year grazed grassland breaks on a subsequent 3-year period of arable forage crops. We considered the non-N effect to be observable at a mineral N fertilization rate of 180 kg ha − 1. The N-contribution effect of the ploughed leys was budgeted using two systems: the ‘‘N fertilizer replacement value’’ versus the ‘‘difference’’ method (Pare´ et al., 1993). Additional N fertilization levels of 0 and 75 kg N ha − 1 enabled us to draw yield response curves, to calculate economic optimal N fertilization rates and to formulate specific fertilization guidelines.To control the risks on excessive nitrate leaching following the ploughing of the leys, and

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knowing that arable crops with long-lasting nitrogen uptake may be good choices as subsequent crops following ley incorporation (Ko¨ pke, 1998; Morvan et al., 2000; Torstensson, 1998), we compared fodder beet and silage maize as arable openers following the grassland break. We studied their respective DM yields, N uptakes and post harvest residual mineral soil N.

2. Materials and methods In 1966, the basic experiment was established on a sandy loam soil of the experimental farm of Ghent University at Melle (50°59% N, 03°49% E, 11 m above sea level). The clay (B 2 mm), silt (2–20 mm), fine sand (20– 200 mm) and coarse sand (200–2000 mm) content at Melle is 86, 116, 758 and 40 g kg − 1. In a 4 × 4 Latin square design, the following ‘‘treatments’’ were established (also see Table 1): LA1: Ley–arable rotation: 3 years of grass ley (TG) –3 years of temporary arable land with forage crops (TA)–3 years of grass ley (TG). LA2: Arable– ley rotation: 3 years of temporary arable land with forage crops (TA)–3 years of grass ley (TG), etc.–3 years of temporary arable land with forage crops (TA), etc. PA: permanent arable land with forage crops. PG: permanent grass ley. (LA1 and LA2 are similar objects but with a mutual time shift of 3 years) The individual plot size was 750 m2. The grass leys were cut during spring, at a yield of 3000–5000 kg DM ha − 1. Dairy heifers rotationally grazed the aftermath. From 1987 on, a ‘‘high’’ N fertilization (N+ ) was applied on two out of the four replicate plots of the permanent as well as the temporary grasslands: 350 kg N ha − 1 year − 1. The other two plots received a ‘‘low’’ N fertilization (N− ): 240 kg N ha − 1 year − 1. Yearly phosphorus and potassium fertilization was overall 26 kg P ha − 1 and 42 kg K ha − 1. Only

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mineral fertilizers were used: ammoniumnitrate 27%, triple superphosphate 43% and muriate of potash 40%. No pesticides were used on the grassland plots. New temporary leys were established during the autumn (September– October), with recently released Belgian perennial ryegrass (Lolium perenne L.) cultivars. The ploughing of the 3-year-old grasslands was conducted during the spring (March– April). Since the first following arable crops were silage maize or fodder beet, this spring ploughing was feasible. From 1981 on, all the arable land plots were split in two sub-systems: a silage maize (Zea mays

L.) monoculture (Mm) and a crop rotation (Cr) with silage maize, fodder beet (Beta 6ulgaris subsp. 6ulgaris L.) and field beans (Vicia faba L.). An overview of the rotations in the period of 1987–1999 is represented in Table 1. In this paper we consider the arable periods between 1990 and 1999. During this period, three rates of mineral N fertilization (0, 75 or 180 kg mineral N ha − 1) were applied on subplots of 50 m2 each to determine the silage maize or fodder beet yield response curves. To avoid shortages, P and K fertilizations were rather high (Table 2). Only mineral fertilizers were applied: ammoniumnitrate 27%, triple superphosphate 43% and muriate of potash 40%.

Table 1 Applied crop rotation on the experimental field from 1987 to 1999a Main treatment

Year 1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

LA1

Cr Mm

G G

G G

G G

Fb M

M M

Vi M

G G

G G

G G

Fb M

M M

M M

G G

LA2

Cr Mm

Fb M

M M

Vi M

G G

G G

G G

M M

M M

Vi M

G G

G G

G G

Fb M

PA

Cr Mm

Fb M

M M

Vi M

Fb M

M M

Vi M

M M

Fb M

Vi M

Fb M

M M

M M

Fb M

G

G

G

G

G

G

G

G

G

G

G

G

G

PG a

Cr, crop rotation; Mm, maize monoculture; G, grass ley; M, silage maize; Fb, fodder beet; Vi, field beans.

Table 2 Main agronomic data of the silage maize and fodder beet crops (1990–1999) Year

Cultivar

kg P ha−1

kg K ha−1

Sowing date

Harvesting date

Vegetation perioda

Silage maize

1990 1991 1992 1993 1994 1995 1996 1997 1998

Aladin Frida Aladin Kalif Banguy Banguy LG2243 LG2243 Elita

65 65 65 65 44 44 44 44 44

249 249 249 166 249 332 249 249 249

24/04 25/04 24/04 23/04 28/04 27/04 18/04 23/04 08/05

11/09 11/09 23/09 21/09 19/09 12/09 08/10 23/09 25/09

139 138 151 150 143 137 172 152 139

Fodder beet

1990 1996 1999

Bolero Apex Cesar

65 44 44

166 332 332

02/04 09/04 29/04

07/11 17/10 19/10

218 190 171

a

Number of days between sowing and harvesting.

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More agronomic data for the considered maize and fodder beet crops are summarized in Table 2. On these arable plots weeds were killed with the appropriate herbicides. At the end of each growing season, subplots of 12 m2 were harvested manually. The silage maize plant parts (leaves and stalks) and the ears were weighed separately. The plant parts were chopped and dried (for 12 h at 80 °C), the ears were dried unchopped (for 12 h at 80 °C, then for 4 h at 105 °C). The beet roots and the beet tops were separately weighed and sampled (ten beet roots and corresponding ten beet tops per subplot). To determine the DM content of the beet tops, we dried them at 75 °C for 8 h. The DM content of the fodder beet roots was calculated after drying representative beet sectors cut to particles of 9 1 cm3 (at 75 °C for 8 h, subsequently at 105 °C for 4 h). We determined the total N content (Kjehldahl method) of bulked samples (per treatment). The total N uptake was calculated by multiplying the DM yields by the nitrogen contents. A quadratic model was used to describe the DM yield response to applied mineral N. The marginal yield response to applied mineral N fertilization was determined by the first derivative of the yield response curve (Bullock and Bullock, 1994; Clow and Urquhart, 1984; Neeteson and Wadman, 1987).We determined the cost/value ratio (cvr) as the ratio of the cost of 1 kg of mineral fertilizer N to the purchase price of 1 kg DM of crop yield (silage maize or fodder beet). In Flemish agricultural practice conditions, this resulted in: Cost of 1 kg of mineral fertilizer N Cvr = Purchase price of 1 kg of DM yield 0.75 Euro =10 0.075 Euro According to Neeteson and Wadman (1987), we considered the economically optimum application rate of fertilizer N (Nopt) or the ‘‘maximum profit rate’’ to be reached when the marginal yield response dropped to this critical cost value ratio. The yield at Nopt was then calculated by using the quadratic yield response curve.The N-contribution effect of the ploughed leys (the release of useful N, ‘‘N credit’’) was determined in two =

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ways. The traditional method of nitrogen fertilizer replacement values (NFRV) (Bullock, 1992; Pare´ et al., 1993) or fertilizer equivalent, FE (Jokela, 1992) determines the amount of mineral fertilizer N that should be applied on a permanent arable land to reach an equal DM yield as on an unfertilized temporary arable land (illustrated in Fig. 1). The found NFRV or FE provides a quantitative estimate of the amount of efficient N the ploughed ley supplied to the following arable crop. The NFRV was determined by solving for x in the permanent arable land yield response curve, at y equalling to the maize yield on N-unfertilized temporary arable land. The second method (‘‘difference method’’) considers the difference between the Nopt (optimum N fertilization rate) on the temporary arable plots and the Nopt on the permanent arable land during the same growing season to be an accurate estimate of the N credit (Pare´ et al., 1993). We considered the non-N effect to be the yieldor N uptake advantage of the silage maize on the temporary compared with the permanent arable plots at the highest N fertilizer dose of 180 kg N ha − 1. At the end of the growing seasons 1994–1999, the mineral N content of the soil profile was determined (on 29 September 1994, 13 September 1995, 8 November 1996, 7 October 1997, 24 October 1998 and 9 November 1999). Using a gauge auger, four at random spots were sampled on each individual subplot, a bulked sample was composed per treatment, separately for the horizons of 0–30, 30–60 and 60–90 cm of soil depth. We extracted the 0–30 cm samples with KCl (1 N) and determined N (ammonia and nitrate) colorimetrically with a continuous flow analyzer. The deeper soil samples were extracted with a 1% KAl(SO4)2-solution; nitrate was then measured using a nitrate-specific electrode. Between sampling and analysis the soil samples were kept deep frozen; they were not dried before the extraction. The amounts of residual mineral soil N were compared with the Flemish legislative limit: no more than 90 kg NO3-N ha − 1 should be observed in the soil profile up to 90 cm of depth (measured between 1 October and 15 November) (Vlaamse Regering, 2000).

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Fig. 1. Illustration of the determination of the nitrogen fertilizer replacement value (NFRV) or fertilizer equivalent (FE) of the ploughed grassland, using the maize yield response curves on the permanent and the temporary arable land plots (yield data of 1994).

Fig. 2. Precipitation during the growing seasons of 1987 –1999 and the 38-year average (1962 – 1999) at the experimental site of Melle.

For the considered growing seasons, the monthly precipitation and the accumulated effective temperatures measured at the meteorological station at Melle, are summarized in Fig. 2 and

Fig. 3. An average of 38 years of observation (1962– 1999) is also represented. According to Bloc and Gouet (1977), the accumulated effective temperature for silage maize was

F. Ne6ens, D. Reheul / Europ. J. Agronomy 16 (2002) 57–74

determined by adding the positive daily values of: Teffective =

(Tmax +Tmin) −Tbase 2

where: Tmax is the maximum daily temperature (°C); Tmin is the minimum daily temperature (°C); and Tbase = 6 °C.

3. Results

3.1. Dry matter yields and yield responses to mineral N fertilization The observed silage maize DM yields obtained from the temporary and the permanent arable plots (1990–1998) are summarized in Table 3. During the considered 9-year experimental period, the silage maize on temporary arable land significantly outyielded maize on permanent arable plots, by 64.4, 28.8 and 3.4 Mg DM ha − 1 at N fertilization rates of respectively 0, 75 and 180 kg ha − 1 year − 1. The corresponding relative yield advantages were respectively 85, 21 and 2%. Without N fertilization, silage maize on temporary arable land yielded as much as on permanent arable plots receiving 736 kg N ha − 1 over the

63

9-year period (82 kg N ha − 1 year − 1). The 9-year total DM yield on these zero N temporary arable plots was 79% of the yield observed on permanent arable land during the same period and at a fertilization rate of 180 kg N ha − 1 year − 1. During the first year after ploughing the 3-yearold grasslands, the maize yields were high and no significant yield differences between the N fertilization rates were observed: even without N fertilization the maximum DM yield was reached. During the second and third year after ploughing the grasslands, temporary arable plots still outyielded the permanent ones. However, the benefit had decreased and a yield response to applied N fertilization already appeared during the second year following grassland ploughing. This response increased during the third year after ploughing. The permanent arable–temporary arable land effect, as well as the N fertilization rate effect were significant, but so was the interaction between these factors (Table 3): the marginal yield responses to applied nitrogen on the permanent arable land were always higher than those on the temporary arable land (Table 4), the difference decreased from the first to the second and the third year following the grass ley.

Fig. 3. Accumulated effective temperatures (Tbase = 6 °C) during the growing seasons 1987 – 1999 and the 38-year average (1962– 1999) at the experimental site of Melle.

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Table 3 Yields (Mg DM ha−1) of the silage maize on the temporary and the permanent arable plots (1990, 1993, 1996 — first year arable land following the ploughing of 3-year-old grass leys) Year

Mineral N fertilization (kg N ha−1)

Significance

Permanent arable land (PA)

Temporary arable land (TA)

PA/TA

N fertilization

Interaction

0

0

75

180

75

180

1990 1991 1992 Total Rel.

11.6ba 6.6e 9.5e 27.7d 46.5

16.2a 12.1d 17.3c 45.5c 76.5

16.3a 18.4a 22.2a 56.9ab 95.6

17.5a 13.0c 14.5d 45.0c 75.6

17.5a 17.0b 19.8b 54.3b 91.3

17.9a 19.0a 22.6a 59.5a 100.0

* *** *** ***

*** *** *** ***

*** *** ** ***

1993 1994 1995 Total Rel.

6.7c 7.9c 8.6c 23.2e 38.0

14.6b 16.2b 13.5b 44.3d 72.5

22.2a 21.0a 15.9ab 59.1ab 96.7

19.3a 15.6b 13.4b 48.3c 79.0

20.3a 21.9a 19.9a 21.3a 16.3ab 17.9a 56.5b 61.1a 92.4 100.0

** *** * ***

*** *** *** ***

*** *** *** ***

1996 1997 1998 Total Rel

6.8c 11.6c 6.6e 24.9d 41.3

15.0b 17.8b 15.0c 47.8c 79.2

17.1ab 18.5ab 11.3d 46.9c 77.6

17.1ab 17.0ab 21.8ab 23.4ab 16.8b 20.0ab 55.7b 60.4a 92.2 100.0

** * ** **

*** *** *** ***

*** *** *** ***

Total 1990–1998 Rel.

75.8

137.6

177.5

140.2

166.4

180.9

41.9

76.1

98.1

77.5

92.0

100.0

a

17.8a 22.9a 20.7a 61.5a 101.9

Values within the same row with different letters are significantly different at h = 0.05 (Test Newman–Keuls).

Table 4 Silage maize yield responsesa (kg DM kg−1 N) on the permanent (PA) and temporary (TA) arable plots Years (after grass ley)

Object

Mineral N fertilization (kg N ha−1) 0

30

60

90

120

150

180

1990+1993+1996 (first)

PA TA PA/TA

116.5 3.4 34.3

97.0 4.0 24.3

77.6 4.7 16.5

58.1 5.3 11.0

38.6 6.0 6.4

19.2 6.7 2.9

−0.2 7.3

1991+1994+1997 (second)

PA TA PA/TA

105.0 66.2 1.6

92.4 54.4 1.7

79.8 42.6 1.9

67.1 30.8 2.2

54.5 19.0 2.9

41.8 7.2 5.8

29.2 −4.6

1992+1995+1998 (third)

PA TA PA/TA

115.4 76.3 1.5

98.0 63.9 1.5

80.6 51.6 1.6

63.2 39.2 1.6

45.8 26.9 1.7

28.4 14.6 1.9

11.0 2.2 5.0

1990–1998

PA TA PA/TA

112.3 48.6 2.3

95.8 40.8 2.3

79.3 33.0 2.4

62.8 25.1 2.5

46.3 17.3 2.7

29.8 9.5 3.1

13.3 1.7 7.8

a

Value of the first derivative of the yield response curves (Fig. 1).

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Fig. 4 summarizes the DM yields of silage maize and fodder beet in the first arable seasons following the 3-year grazed grassland breaks (1990, 1996 and 1999). On permanent as well as temporary arable plots, fodder beet almost always outyielded the silage maize, by on average 4.1 Mg DM ha − 1 in 1990 and by 5.2 Mg DM ha − 1 in 1996. Only in 1999, the silage maize yields were exceptionally high and kept up with the fodder beet: the yield differences were low and not significant.

3.2. Nopt and Yopt for silage maize The observed differences in yield response of silage maize to applied N fertilization (Table 4) resulted in different levels of Nopt on permanent and temporary arable plots. The calculated rates of the economically optimal N fertilization rate (Nopt) and the corresponding silage maize yields (Yopt) are summarized in Table 5. In case where Nopt was beyond the applied fertilization rates (up 180 kg N ha − 1), it was indicated as \180 kg N ha − 1. Nopt during the first year after ploughing the grassland was 0 kg N ha − 1 in 1990 and 1996, only

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7 kg N ha − 1 in 1993. During the corresponding seasons, Nopt was remarkably higher on the permanent arable plots, on average 152 kg N ha − 1. In the second and third year after ploughing the grassland, Nopt increased substantially but the optimal N rate on permanent arable land was still higher. On average over the 9-year period, (and using an arbitrary value of 200 kg N ha − 1 when Nopt exceeded 180 kg N ha − 1), Nopt was 175 kg N ha − 1 on permanent arable land and 98 kg N ha − 1 on the temporary arable land. However, the corresponding Yopt were comparable: respectively 19.8 and 19.7 Mg DM ha − 1. In the first, second and third silage maize season following the grassland ploughing, Nopt was on average respectively 2, 139 and 154 kg N ha − 1. On the permanent arable plots the Nopt were respectively 152, 191 and 183 kg N ha − 1.

3.3. Crop N uptake The silage maize N uptakes corresponding with the DM yields of Table 3 are represented in Table 6. Over the 9-year trial period, silage maize on

Fig. 4. DM yields of silage maize and fodder beet ( 9 S.D.) in the first arable season following 3-year grassland (TA) and on permanent arable plots (PA).

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Table 5 Economically optimal application rates of fertilizer N (Nopt, kg N ha−1) and corresponding silage maize yields (Yopt, Mg DM ha−1) on the permanent and temporary arable plots Year

Year after grassland ploughing

Temporary arable land

Permanent arable land

Nopt

Yopt

Nopt

Yopt

1990 1991 1992 avg. 1990–1992

1 2 3

0 148 156 101

17.5 18.8 22.4 19.6

112 \180 \180 171a

17.0 18.4 22.2 19.2

1993 1994 1995 avg. 1993–1995

1 2 3

7 130 140 92

19.4 21.3 17.6 19.4

\180 173 150 174a

22.2 20.9 15.7 19.6

1996 1997 1998 avg. 1996–1998

1 2 3

0 139 165 101

17.1 23.2 19.8 20.0

144 \180 \180 181a

18.0 22.9 21.0 20.6

1 2 3

2 139 154 98

18.0 21.1 19.9 19.7 99.3

A6erages 1990+1993+1996 1991+1994+1997 1992+1995+1998 1990–1998 Relative a

152 191 183 175

19.1 20.7 19.6 19.8 100

If Nopt\180 kg N ha−1 occurred, an arbitrary value of 200 kg N ha−1 was used to calculate the average.

temporary arable plots exported 758, 577 and 252 kg more N ha − 1 compared with permanent arable plots at mineral N fertilization rates of respectively 0, 75 and 180 kg ha − 1 year − 1. The corresponding relative advantages were respectively 155, 56 and 15%. During this period of 9 years, above ground silage maize parts on N unfertilized temporary arable land took up as much as 74% (1246 kg N ha − 1) of the amount N exported by silage maize on permanent arable plots with a yearly N dose of 180 kg N ha − 1 (1684 kg N ha − 1). In the first year after ploughing the grasslands, N-unfertilized silage maize exported as much or even more N as maize on corresponding permanent arable plots receiving 180 kg N ha − 1. Considering the DM yield range between 11 and 24 Mg DM ha − 1, the export of N from temporary arable plots is on average 20.4% higher than from permanent arable plots, at equal yield levels (Fig. 5). At the Yopt level (19.75 Mg DM ha − 1) silage maize originating from temporary arable land con-

tained 9.82 g N kg − 1 DM compared with 8.36 g N kg − 1 DM for maize from permanent arable land. During the first arable season following the grassland ploughing, a fodder beet crop (including the beet tops) exported significantly more N than silage maize did (Fig. 6). Only in 1999, when silage maize yields kept up with fodder beet yields, N uptakes were comparable. The N uptake was also significantly higher on temporary arable plots compared with permanent arable parcels and this extra N export from temporary arable plots was higher when beet rather than silage maize was grown.

3.4. Non-N-contribution effect of the ley–arable rotation Table 7 summarizes the relative gains in DM yield and N uptake of silage maize on temporary arable plots when compared with permanent arable plots, at a fertilization rate of 180 kg N ha − 1. At this highest dose of mineral N fertiliza-

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Table 6 Nitrogen uptakes (kg N ha−1) of the maize crops on the temporary and the permanent arable plots (1990, 1993, 1996 — first year after ploughing grass leys) Year

Mineral N fertilization (kg N ha−1)

Significance

Permanent arable land (PA)

Temporary arable land (TA)

PA/TA

N fertilization

Interaction

0

0

75

180

75

180

1990 1991 1992 Total Rel.

81.3da 44.1f 68.3e 193.7d 31.7

130.6c 122.7d 133.4c 386.7c 63.4

161.7b 141.1c 223.6a 526.4b 86.3

168.7b 111.3e 118.4d 398.4c 65.3

187.6ab 165.4b 180.6b 533.6b 87.4

205.5a 172.8a 231.9a 610.2a 100.0

** *** *** ***

*** *** *** ***

*** *** *** ***

1993 1994 1995 Total Rel.

38.9d 52.6e 45.3d 136.8e 20.3

95.3c 122.4d 83.7c 301.4d 44.8

202.9b 223.7b 139.9b 566.5b 84.2

207.6b 130.1d 91.8c 429.5c 63.8

228.5ab 255.5a 190.6c 254a 134.0b 163.2a 553.1b 672.7a 82.2 100.0

*** *** ** ***

*** *** *** ***

*** ** *** ***

1996 1997 1998 Total Rel

44.7c 70.7d 42.9c 158.3f 24.3

126.8b 119.7c 102.4b 348.9e 53.5

199.1a 205.3a 186.7a 591.1b 90.6

184.4a 154.8b 79.3c 418.5d 64.1

193.2a 204.1a 130.4b 527.7c 80.9

*** ** *** ***

*** *** *** ***

*** *** NS ***

Total 1990–1998 Rel.

488.8

1037.0

1684.0

1246.4

1614.4

1935.5

25.3

53.6

87.0

64.4

83.4

100.0

a

201.4a 232.7a 218.5a 652.6a 100.0

Values within the same row with different letters are significantly different at h = 0.05 (Test Newman–Keuls).

Fig. 5. N yields versus DM yields on permanent arable plots (PA) and on temporary arable plots (TA) following grassland cultivation (data from 1990 to 1998).

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Fig. 6. N uptake of silage maize and fodder beet ( 9 S.D.) grown in the first arable season following 3-year grassland (TA) and on permanent arable plots (PA). Table 7 Relative increase (%) in DM yield and N uptake of silage maize on temporary plots compared with permanent plots, at a fertilization level of 180 kg N ha−1

DM yield N uptake

1990

1991

1992

1993

1994

1995

1996

1997

1998

Entire period

10.0 27.1

3.2 22.5

1.7 3.7

−1.4 25.9

1.7 13.5

12.3 16.7

−4.9 1.2

2.1 13.3

−3.6 17.0

1.9 14.9

tion, the relative yield effect of the ley– arable rotation was small, only +1.9% over the 9-year experimental period. We also notice that in three out of the 9 years the DM yields at 180 kg N ha − 1 were higher on the permanent arable plots. When considering N uptake, the advantage of the rotation plots was higher and consistently positive. The overall advantage was 14.9% (varying from +1.2% in 1996 to + 27.1% in 1990).

3.5. N-contribution effect: nitrogen fertilizer replacement 6alue (NFRV) The calculated NFRV of the ploughed grassland, derived from the maize yield response curves are given in Table 8. On average over the three rotation periods, the values decreased from 124 to 81 and 52 kg N

ha − 1 respectively in the first, second and third year after ploughing. This resulted in an estimated amount of at least 257 kg of extra N supply during the 3 years of arable land, or 771 kg N ha − 1 over a 9-year period. The NFRV values found on arable plots following N+ grasslands (350 kg fertilizer N ha − 1 year − 1) and N− grasslands (250 kg fertilizer N ha − 1 year − 1) were quite comparable. No clear differences in N credit following both grassland mineral fertilization managements were observed (Fig. 7).

3.6. N-contribution effect: the ‘‘difference method’’ The calculated N credit on the arable plots following the grassland ploughing (Nopt on per-

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manent arable plots minus Nopt on temporary arable plots) are also summarized in Table 8.

3.7. Residual mineral soil N The amounts of residual mineral soil N are summarized in Fig. 8. Compared with the plots following the ploughed grassland, at the end of the growing seasons there was always less N left in the soil under the permanent arable plots. On

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the latter plots, apart from 1995, the residual soil N never exceeded the limit of 90 kg ha − 1. In a first arable season following the grassland ploughing, the risk of excessive nitrate leaching was substantial: in the autumn of 1996, the post harvest residual mineral N amount in the soil profile (0–90 cm) exceeded the limit of 90 kg N ha − 1 when maize had been grown as the first crop following the ley incorporation, even when no mineral N fertilization had been applied. No less

Table 8 N Fertilizer replacement values (kg N ha−1) of the ploughed 3-year-old grassland on the arable land following ita Period 1

Period 2

Period 3

Avg.

NFRV First year after ploughing Second year after ploughing Third year after ploughing Total of 3 years

1990 1991 1992

130 89 44 263

1993 1994 1995

133 68 72 273

1996 1997 1998

110 85 39 234

124 81 52 257

Mineral N credit First year after ploughing Second year after ploughing Third year after ploughing Total of 3 years

1990 1991 1992

112 52 44 208

1993 1994 1995

193 43 10 246

1996 1997 1998

144 61 35 240

150 52 29 231

a

Mineral N credit (kg N ha−1) of the ploughed 3-year-old grassland, determined by means of the ‘‘difference method’’.

Fig. 7. N fertilizer replacement values of ploughed grasslands during the three following silage maize seasons (N + is preceding grassland fertilization: 350 kg N ha − 1 year − 1; N − is preceding grassland fertilization: 250 kg N ha − 1)

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Fig. 8. Post harvest residual mineral soil N (0 – 90 cm) following silage maize or fodder beet on permanent arable plots (PA) or arable plots in the first (I), second (II) or third (III) year following a 3-year grazed grassland break.

than 210 kg of mineral N remained in the soil profile (0–90 cm), 136 kg N already present in the deep 60– 90-cm horizon. A supplementary fertilization of 180 kg N ha − 1 pushed the residual soil N to an extremely high amount of 340 kg N ha − 1 (0 –90 cm), 258 kg N already below 30 cm of soil depth. Following the fodder beet of 1996 however, no excessive quantities of residual N were found (maximum: 44 kg N ha − 1, 0 – 90 cm), regardless of the applied N fertilization rate. Also the soil profile below 30 cm was well scavenged. During November 1999, there was one overshoot of the 90 kg N ha − 1 value in the soil of the first year arable plots: following maize with a mineral N fertilization of 180 kg ha − 1. Fig. 8 also shows that following the second (1994 and 1997) growing season after a grassland ploughing too high amounts of residual soil N ( \90 kg N ha − 1) were found when the maize had received N fertilization at a rate exceeding Nopt (the Nopt levels for silage maize a second year following the grassland ploughing was on average 139 kg N ha − 1, Table 5). With fodder beet as arable openers in 1996, following the grassland ploughing during the au-

tumn of 1995, the amount of residual soil N under silage maize was below 90 kg ha − 1 following the second growing season (1997) at all three N fertilization rates.Following the third arable season 1995, quite exceptional, high residual amounts of soil mineral N following silage maize were observed, not only on the temporary arable plots but also on the permanent ones. The 1995 growing season was marked by a severe shortage of precipitation (Fig. 2), the amount of rainfall from April to August was only 58% of the 38-year average. As a result, yields and N uptake were low, much N was left unused in the soil profile. During 1998, also a third year arable season following ley incorporation, climatic circumstances were more favourable for production and N uptake, resulting in low residual N amounts (less than 90 kg N ha − 1).

4. Discussion Silage maize following the 3-year grazed grassland breaks significantly outyielded the maize on permanent arable land. The reduction of the yield benefit with increasing N fertilization rate confi-

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rms the findings of Linden and Wallgren (1993), Van Dijk et al. (1996) and indicates a predominating N-contribution effect of the ley– arable rotation. The still positive yield effect at the highest N dose (180 kg N ha − 1) also reveals a non-N effect or ‘‘crop rotation effect’’ in restricted sense (Bullock, 1992). After 4-year leys on sandy soils, Van Dijk et al. (1996) found these ‘‘crop rotation effects’’ in a range of 3– 23%. However, in our experiments this non-N effect was small and nonsignificant when DM yield is considered (on average only + 1.9%). This result also seems unreliable since the amount of 180 kg N ha − 1 appears to be sub optimal for the silage maize on permanent arable land in 5 of the 9 years (Table 5) and because in 3 of the 9 years we even observe a yield advantage for the permanent arable plots. This fact emphasizes that the enhanced N contribution is the principal causal agent of the positive ley– arable rotation effect on DM yields. With the observed DM yields, we also demonstrate that the release of N from the preceding grassland is very high in the first season after the spring ploughing and that it decreases in the second and third year. On our sandy loam soil we obtained optimum yields without any N fertilization in the first arable season following the grassland. During the corresponding seasons, 112 up to more than 180 kg N ha − 1 had to be applied on permanent arable plots to obtain optimum yields. Also Viaux et al. (1999) concluded that maize following a grass ley did not need any supplementary N fertilization. Johnston et al. (1994) found that in the first wheat season after ploughing leys (even up to 6 years old), Nopt was indeed lower but an amount of 50–100 kg N ha − 1 was still necessary to reach optimum yields. This N demand could be accounted for by the summer ploughing of the ley followed by wheat: N offtake in autumn was rather small, lots of N originating from the ploughed grass were leached during the winter season. In our rotation, grasslands were ploughed early in the spring, immediately followed by the maize growing season and hence N uptake. In the second and third season following the grass ploughing, Nopt increased but remained below the Nopt of permanent arable plots. Compared

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with simultane permanent arable plots, in a 3-year temporary arable period one needed a considerably smaller amount of mineral fertilizer N. Based on the 9-year averages, we can formulate a N fertilizer advice of 0, 139 and 154 kg N ha − 1 respectively in the first, second and third year following the grassland ploughing. Compared with permanent arable plots this is 231 kg N ha − 1 less, split as savings of 150, 52 and 29 kg N ha − 1 respectively in the first, second and third year after the grassland ploughing (resulting in comparable silage maize yields of on average 19.8 Mg DM ha − 1). This ‘‘N credit’’ corresponds with the ‘‘N-contribution effect’’ of the ploughed grassland according to the difference method. The calculated N fertilizer replacement values result in an average 3-year N credit of 257 kg kg N ha − 1 (124 kg during the first, 81 kg during the second and 52 kg during the third season) following the ley cultivation. According to Lory et al. (1995a,a) discrepancy between N credit determined by the traditional NFRV method and by the ‘‘difference method’’ may occur. This is definitely the case in the presence of ‘‘y-shift’’ effects on or interaction effects between the compared yield response curves. In our trials, the y-shift is small (non-N-contribution effect) but a significant interaction effect on yield response was clearly present: silage maize yield on temporary arable land responded differently to fertilizer N than maize yield on permanent arable plots did (Table 4). In accordance with Lory et al. (1995b), we observed that over the 3-year arable periods the traditional NFRV method tended to overestimate the N credit, compared with the difference method. We especially notice that for period 2 (1993–1995) and period 3 (1996–1998) the NFRV method underestimated the N credit during the first season after the grassland ploughing, the most decisive and critical season for the N release from the ploughed grassland (Table 8). For these reasons, when drawing conclusions, we follow Lory et al. (1995a) who stated that the difference method rather than the NFRV method should be used. Another reason to choose for this method is that it generates useful fertilization guidelines (Nopt).

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The uptake of N by silage maize on the arable plots after ploughing grass was significantly higher than the N uptake from the permanent arable plots, the difference decreased with increasing N fertilization. In contrast with the DM yield observations, at the highest rate of N fertilization (180 kg N ha − 1), the advantage of the rotational system is higher (on average 14.9% more N uptake compared with permanent arable plots) and significant. This indicates that a pronounced nonN-contribution effect of the rotation is actually observed when considering N uptake. The different response curves of relative DM yield and of relative N yield indicate that the N concentration in silage maize following grass leys is higher than in maize on permanent arable plots. This is confirmed by the amounts of N in silage maize on both types of plots and at the equal yield levels (Fig. 5). Silage maize with a higher N content and hence a higher N uptake at a reference yield level is another positive consequence of the ley– arable rotation. Fodder beet proved to be potentially better arable openers following the grassland break. Owing to their deeper rooting and longer growing season (Table 2) they outyielded silage maize and took up more nitrogen. As a result, we observed a substantial reduction of the post harvest residual mineral soil N following fodder beet and hence of the risks on nitrate leaching in the first arable year after the grassland break.

5. Conclusions Three-year grazed grassland breaks offer a valuable potential for a significant reduction of fertilizer-N use in arable forage crops. Positive yield as well as quality effects occur. Applying an adapted forage crop succession and an adjusted N fertilization, the risks on high N losses can be minimized.

Acknowledgements Our research was supported and financed by the Belgian Ministerie voor Landbouw en Mid-

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