1 CETIOM, BP 4, F-78850 Thiverval Grignon
2 ITCF, Le Magneraud, F-17700 Saint Pierre d'Amilly
3 Chambre d'agriculture, 3 rue Volta, F-18022 Bourges
Field experiments were carried out in two sites with a stony soil in the middle of France : Le Magneraud (oilseed rape-wheat rotation) and La Fringale (oilseed rape-wheat-barley rotation). Six cropping systems were assessed through nitrate leaching measured from ceramic cups. The main factors of variation of nitrate leaching were rainfalls, N uptake before winter, N balance of the previous crop and autumn N application. With early sowing date for oilseed rape and nil autumn N fertilization, the environmental weakness of this rotation comes after oilseed rape under a wheat crop.
KEYWORDS : nitrate leaching, ceramic cups, cropping systems, fertilization
In Western Europe with non irrigated cropping systems, nitrate leaching from the root-zone to aquifers takes place mainly between autumn and spring, when rainfalls exceed evapotranspiration (Vos, 1994) : for winter oilseed rape and winter wheat, it occurs mainly at the beginning of the crop cycle, before the first spring N fertilizer application.
Most studies about nitrate leaching are based on the inorganic N remained in the soil. At harvest it depends on N fertilization : small effects of sub-optimal N fertilizations, large increases in supra-optimal N fertilizations (Shepherd and Sylvester-Bradley, 1996). From post-harvest measures, MacDonald et al (1997) concluded that the risk of nitrate leaching in the following winter is similar for oilseed rape and winter wheat, lower for sugarbeet and higher for potatoes and spring beans. However, this conclusion does not take into account the mineralization after harvest (higher N incorporation with the oilseed rape residues) and the N uptake by the crop (lower absorption with winter wheat).
Fields experiments were carried out in two sites in the middle of France, with quite similar soils (rendzina with 30-35% of clay) : Le Magneraud (Charente Maritime department) and La Fringale (Cher department).
The Le Magneraud main rotation was a two year winter oilseed rape-wheat rotation. Six treatments were assessed :
- system n°1 (optimized fertilization) : usual tillage and N fertilization according to decision support systems ;
- system n°2 (reduced fertilization) : usual tillage and reduction of N fertilization (40 to 50 kg.ha-1 N less than the rate calculated from the balance sheet method) ;
- system n°3 (minimum tillage) : minimum tillage where wheat straw is baled ;
- system n°4 (first legume rotation) : a three year rotation clover-wheat-oilseed rape, where clover is not harvested ;
- system n°5 (second legume rotation) : a three year rotation clover-oilseed rape-wheat, where clover is not harvested.
This experiment began in 1994 and you will find below the first three year results. Sowing dates were rather early : 94/11/2, 95/10/26 and 96/10/23 for wheat, and 94/9/18, 95/8/25 and 96/8/28 for oilseed rape.
The La Fringale rotation was a three year winter oilseed rape-wheat-barley rotation. Three treatments were assessed :
- system n°1 (optimized fertilization) : usual tillage and N fertilization according to local advice ;
- system n°2 (reduced fertilization) : usual tillage and reduction of N fertilization (70 % of the N rate applied to system n°1) ;
- system n°6 (excessive fertilization) : usual tillage and excess of N fertilization (40 kg.ha-1 N autumn fertilization without reduction of spring N rate).
From this five year period, we are presenting the last three year results : 1992-1993, 1993-1994 and 1994-1995. Sowing dates were rather late for oilseed rape : first fortnight of September (October for wheat).
In both experiments, after harvest, the residues were incorporated into the soil and the plots were left as fallow.
Because of stones in both soils, nitrate leaching was measured using lysimeters and ceramic cups : 9 ceramic cups put 100 cm deep in the soil for each treatment, lysimeters for only some treatments in order to estimate water percolation for the different crops. Measures with ceramic cups were made every 50 mm in the lysimeters. Nitrate losses results were cumulated from the sowing date of a crop to the sowing date of the following one.
Water percolation was quite dependent on the climatic year. Under oil seed rape, it reached 253 mm, 483 mm, 541 mm respectively in 1992-1993, 1993-1994, 1994-1995 for La Fringale, and 397 mm, 127 mm, 194 mm respectively in 1994-1995, 1995-1996, 1996-1997 for Le Magneraud. High percolations were related to high rainfalls. Water percolation was very similar between wheat and oil seed rape. Water percolation took place mainly from November to February.
Nitrate losses are presented in Figure 1, giving the cumulative nitrate losses as a function of water percolation in the system 3 of Le Magneraud. For oilseed rape, cumulative nitrate losses were increased with water percolation so long percolation remained below 200 mm. Then the increase of nitrate losses was very poor, indicating water with very low nitrate concentration. For wheat, the curve slope decreased slowly when water percolation was over 200 mm.
Figure 1 : Cumulative nitrate leaching in function of water percolation in system 3 (Le Magneraud)
Total nitrate losses were 33 kg N.ha-1, 1 kg N.ha-1, 25 kg N.ha-1 for oil seed rape and 67 kg N.ha-1, 27 kg N.ha-1, 36 kg N.ha-1 for winter wheat respectively for 1994-1995, 1995-1996 and 1996-1997 in Le Magneraud with system n°1. In La Fringale, total nitrate leachings with the same cropping system were 169 kg N.ha-1, 79 kg N.ha-1 for oil seed rape in 1993-1994 and 1994-1995 ; 94 kg N.ha-1 and 72 kg N.ha-1 for winter wheat respectively in 1992-1993 and 1994-1995 ; 102 kg N.ha-1 and 104 kg N.ha-1 for winter barley respectively in 1992-1993 and 1993-1994.
On the rotation scale, average nitrate losses for the system n°1 were 34 kg N.ha-1.year-1 and 103 kg N.ha-1.year-1 respectively in the two year rotation of Le Magneraud and in the three year rotation of La Fringale. From this global point of view, the differences between the different treatments in Le Magneraud are slight (Figure 2). Nevertheless there is a trend to heavier niterate losses in the systems with clover : for systems n°4 and 5. In La Fringale, nitrate leachings were sharply increased with supra-optimal fertilization with autumn application (system n°6).
Figure 2 : Nitrate leaching in Le Magneraud and La Fringale:
There are more important variations in nitrate losses between years for these rotations (Reau, 1995).
First of all, the main factor of variation in nitrates losses is rainfall through water percolation : generally, nitrate losses were heavier with high water percolation (1994-1995).
Nevertheless, nitrate losses are often lower under oilseed rape than under wheat. The second factor responsible for the variation is the N uptake before winter : with an early sowing date like in Le Magneraud, oilseed rape could uptake more nitrogen before winter than wheat, particularly when rather high temperatures during autumn allowed rapid growth and N uptake. For instance, in Le Magneraud for system n°1, nitrate losses under oil seed rape were only 1 kg.ha-1 in 1995-1996 with high temperatures, and reached 25 kg.ha-1 in 1996-1997 with low temperatures. Under similar conditions, nitrate losses under wheat were respectively 27 and 36 kg N.ha-1. When oilseed rape sowing dates were later like in La Fringale, nitrate leachings were heavier, so that there were no differences between oilseed rape and wheat. With the 1994-1995 climate, under wheat nitrates losses were quite similar between both experiments for system n°1 : 83 and 72 kg N.ha-1 respectively in La Fringale and Le Magneraud, while they were very different under oilseed rape : respectively 67 and 34 kg N.ha-1. Early sowing dates are necessary to reduce nitrate leachings with oilseed rape (Dejoux, 1999).
The third factor of variation is the N balance of the previous crop : with non harvested clover and supra-optimal fertilization nitrate losses were more important. N fertilization applications before winter without early sowing date are very risky for water quality.
From the point of view of nitrate leachings, the performance of these cropping systems depends on the capacity to take up N from the soil before the period of water discharge. The results show the capacity of winter oilseed rape with early emergence to reduce or to eliminate the risk of nitrogen leaching below oilseed rape crop.
The environmental weakness of theses rotations is situated during the autumn after oilseed rape, and during the winter under a winter wheat crop. There was often a high nitrate content in the soil after oilseed rape, and the N uptake capacity of wheat was low. After wheat and under oilseed rape crop, N leaching was often very low, according to the important N uptake of oilseed rape in the autumn and the low inorganic N content after the cereal harvest. Nevertheless early emergence and nil autumn N fertilization are required for oilseed rape.
We thank the staff of the experimental stations of Le Magneraud headed by G. Arjauré (CETIOM) and A. Bouthier (ITCF) and La Fringale headed by J. Dupont (Chambre d'agriculture du Cher). The experiment of Le Magneraud is supported by the Ministry of Agriculture and Agrice.
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5. Vos G. J. M., Bergevoet I. M. J., Védy J. C. and Neyroud J. A., 1994. The fate of spring applied fertilizer N during the autumn-winter period: comparison between winter-fallow and green manure cropped soil. Plans and soil, 160, pp 201-213.