ABSTRACT

An experiment was conducted to investigate the spring translocation of the amount of N absorbed during the autumn and winter period in winter oilseed rape, using labelled 15N-ammonium nitrate. The nitrogen was applied on field plots, at plant growth stage ``second true leave expanded''. Three rates of fertilization were tested : 0, 50 and 100 kgN/ha. At the end of the winter, a few days before the beginning of the stem extension, some plants of each treatment were pricked out in pots, in a soil without 15N, including carefully the rooting system with the lowest field soil residues as possible. From this moment and untill maturity, the experiment continued under grennhouses. Three levels of spring N fertilisation were applied (no labelled) : X (optimal rate determine using a balance method), X/2 and 1.5X. At maturity, the quantity of 15N and 14N was checked by mass spectrometry in each kind of plant organs : roots, stems (main stem + branches), leaves (fallen + non fallen), pod walls and seeds.

The results show that, in our conditions, the greater part (82%) of N absorbed during the autumn and winter period (mainly in the leaves) remain in the whole plant at maturity ; 57% of those amounts were located in the seeds. The N losses are particularly related to the leaves fall. These results do not depend upon the spring N fertilization rate. Therefore, N absorbed during the autumn and winter is greatly involved in the metabolism in spring, and is to be integrated as a full part of the plant N-requierement at spring.

INTRODUCTION

French farmers are suppose to use the balance method for nitrogen application on different crops as set up by HEBERT (1969) ; REMY (1974) and REMY & HEBERT (1977). This balance takes into account the full N-requirements for the crop and the soil contribution (i.e., mineralization, fertilizers and /or organic application - sluggs or slorries). In the case of winter oilseed rape, and from 1993, this balance had been improve in France by checking the N absorption during falls and winter (REAU and al., 1995). Those amounts are discount from the full crop requirements, due to the hypothesis that this nitrogen is a full part of the whole plant requirement according to the yield goal.

In the report presents here, we want to set up the hypothesis that the N absorption during the autumn/winter time is mainly translocated to the new plant organs developped from regrowth until maturity.The experiment was conducted under field conditions at autumn and winter ; plants were provided with labbelling nitrogen 15N. before regrowth, plant were pricked out in pots under greenhouses on soil without any trace of 15N. By using the labelled nitrogen, the translocation during winter had been checked in the different parts of the plant until maturity. The effet of the N amount absorbed by the plant and of the N spring fertilizer had also been evaluated.

MATERIAL AND METHODS

Experimental design

From sowing until the end of winter, the experiment was conduct under field conditions on the cultivar Goeland. Sowing occurs august 22th on a deep lime-clay soil on plot size of 10 sqm. In order to get different levels of growth and various amount of N absorption, 3 levels were applied for N fertilizer : nil, 50 and 100 kg/ha ( refered as 0Na, 50Na, 100Na) using labelled ammonitrate form (enrichment = 2 % of 15N). The application occurs at 2 leaves growth stage using liquid form.

The 22 of december, homogeneous plants (growth stage, vigor, number of leaves) have been sampled for each N fertilization level and pricked out in pot (2 plants per pot, volume 12 l, substrate mixt between soil and sand to get a low level of N residues). Before, as far as possible, the roots had been cleanned from field soil polluted by 15N. Then the experiment goes on under greenhouses. Spring fertilization was applied at 3 levels : 0.3, 0.6 and 0.9 g of N per plant combined with the 3 winter treatments. The fertilisation was split into 3 applications resp. 31 / 51 / 55 (BBCH scale). The others parameters (nutients, excepted N, crop protection, watering) were opimized. We used a bi-factoriel design for the experiment and the 4 replications were fully randomized.

Observations

The accumulation of dry matter (DM), N content (in % of DM) had been controlled at pricking out and maturity for the different plant organs: roots, main stem+ branches, leaves, pod walls and seeds). When sampled in the field at winter, only the leaves attached to the plants were taken into account. At maturity, all the leaves (still attach or fall) were collected. The isotopic ratio (14N /15N) and the excess of 15N compare to the natural abundance had been checked by mass spectrometry (spectrograph FISONS / ISOCHROM - EA)

Calculation

The amount of 15N absorb (15Nabs) by one organ (i) is obtained as following :

15Nabs(i) = Nabs(i) x excess15N (i) (%) / 100 , where Nabs(i) = DM (i) x N (i) (%) / 100 (Nabs is the total amount of N absorption : 14N + 15N).

The real use ratio (RUR) for the nitrogen bring by N fertilizer at autumn is obtained as :

RUR= 15Nabs(whole plant) / 15N fertilizer x 100, where 15N fertilizer (g/plant) = ((N fertilizer (g/m2) / plant density) x excess 15N fertilizer) / 100.

15N fertilizer is the quantity of 15N bring by the fertilizer, N fertilizer the whole N quantity bring by the fertilizer ; plant density express as the number of plant par sqm.

The quantity of absorbed N coming from fertilizer at automn (Naut.fert) is obtained as : RUR x N fertilizer. At pricked out, the quantity of absorbed N coming from the soil during the autumn (Naut.soil) is Naut.total minus Naut.fert. wher Naut.total is the quantity of total N absorbed at pricked out.

RESULTS

Growth parameters before pricking out

Results are summarized in table 1. The plot for treatment 50Na had a plant density lower compared to the others. The winter fertilization treatments have a greater biomass compared to the control ; this lead to increase the difference between the amounts of N uptakes between the treatments : the N quantity in the fertilizated plants were double compare to the control without N. Due to a lower plant density, greater amount of N were encountered in 50Na treatement compared to 100Na.

Table 1 : Quantities of dry matter, total N absorption (N abs) and 15N absorption (15Nabs) in the whole plant before pricking. Values for the RUR of the fertilizer form.

N fertilizer at automn	Plant density	Dry matter	N content	N abs	15N abs.	RUR	N provide by soil	N provide by fertilizer
	(/m2)	(g/plant)	(% DM)	(g/plant)	(mg/plant)		(%Nabs)	(%Nabs)
0Na	52	4.39	2.23	0.098	--	--	100	--
50Na	44	6.27	3.47	0.217	0.405	21.4	89	11
100Na	54	5.11	3.31	0.169	0.695	22.6	75	25

The RUR is about 22 % without any great fluctuation between the treatments. So we can concluded that, in the case of 50Na, 11 % of the total N absorption came from fertilizer and resp. 25 % in the case of 100Na.

N absorption at maturity (figure 1)

The interaction between the winter and the spring treatments is never significative for N uptake at maturity. A positive effect of the winter treatment is observed on the leaves, the seeds and the whole plant. The spring fertilization lead to significative increase in N absorption for all the treatments.

Figure 1 : Detail by organ of the N absorption at maturity (Nabs) and of the total N absorption

(NS : non significative ; * / *** : statistical diff. resp. at 5% and 1% threshold).

Regarding the response curve for N fertilization, the plateau is never reached, leading to suppose that in all the situations (inc. the max. rate fertilization), we are still under the optimum value for nitrogen. The maximum observed in the experiment is 1.16 g/plant.

The N partionning between the different organs is not related to the N rates of fertilization. At maturity stage, around 6 % of the whole N absorbed by the plant is located in the roots, 12 % in the stems, 9 % in the leaves, 7 % in the pod wall and 67 % in the seeds.

Partitioning of N absorbed at winter time

We observe frequently losses for the 15N detected in the plants : this could be related to losses of organs (flowers..), to volatilization or to the heterogeneity between plants and pots.

On figure 2, it is clear that the N rate fertilization at automn had a significative effect on the 15N absorption at maturity. In a less extend, spring fertilization lead also to the same effect. Othervise, the partionning of the labelled nitrogen between the organs does not vary very much and does not interact with the autumn or spring N treatments.

At harwest, all conditions included (Table 2) , more than 50 % of the labelled nitrogen (issue from winter absorption) is located in the seeds (57 % in average). 18 % were detected in the leaves. The other organs represented 10 %.

Figure 2 : Quantity of 15N absorbed (15Nabs) at maturity per organ and total 15N absorption.

(NS : non significative ; * / *** : statistical diff. resp. at 5% and 1% threshold).

Some differences are observed among organs in the ratio of total N versus 15N fraction. In the roots as well as in the leaves, the part of 15N is greater than the part of total N. On the opposite, as far as the reproductive organs are concerned, the total N is greater. This allowed us to propose that the spring redistribution of the 15N fraction (i.e., N fraction issue from autumn/winter absorption) from the roots and leaves towards the stems and later towards the reproductive organs is a little bit less efficient compare to the N absorbed at spring.

Table 2 : Partition (in %) of the total nitrogen (Nabs.) and of the 15N (15Nabs.) in the different plant organs (each value is the average of the automn treatment 50Na and 100Na).

Growth stage	Organ	Nabs (%)	Nabs (%)	15Nabs(%)	15Nabs (%)
		mini - maxi	Average	mini - maxi	Average
Pricking	Roots	18 - 19	18	18 - 19	18
	Leaves	81 - 82	82	81 - 82	82
Maturity	roots	5 - 7	6	8 - 10	9
	Stems	11 - 13	12	9 - 11	10
	Leaves	8 - 10	9	14 - 22	18
	Pods	6 - 8	7	5 - 7	6
	Seeds	64 - 69	67	54 - 61	57

CONCLUSION

As we can set up the hypothesis that there is not a big gap between the capability of 15N versus 14N to be translocated in the plant, we can conclude that most of the nitrogen absorbed at autumn and winter (mainly located in leaves and roots) is still in the plant at maturity : in our experiment 82 % of those amount were still detected in the plants. Only 18 % were lost (due mainly to leaf fall).

Among the quantity of nitrogen absorbed at winter, 57 % is translocated to the reproductive organs.

The N fraction issue from winter absorption takes a significative and efficient part to the whole plant metabolism at spring. This is one more argument for the estimation of those amounts before regrowth and to consider them as a whole part of the full plant requirements. Nevertheless, the spring absorption seems to be more efficiently redistributed compared to the winter absorption fraction.

References

1. BBA, BSA, IGZ, IVA, AGREVO, BASF, BAYER, NOVARTIS (1997) : - Compendium pour l'identification des stades phénologiques des espèces mono- et dicotylédones cultivées ; Echelle BBCH améliorée. 2ème édition.

2. HEBERT J. (1969) : - La fumure azotée du blé tendre d'hiver. Bull. Tech. Inf., 244, 757-766.

3. REAU R., SAUZET G., WAGNER D. (1995) : - A provisional nitrogen balancemethod for winterrapeseed spring fertilization. 9th International Rapeseed Congres, Cambridge, UK, 4-7 july, 1, 317-319.

4. REMY J.C. (1974) : - Pas de recette passe-partout pour la fertilisation azotée du blé. Fermes modernes, n° hors série : Le blé, céréale d'avenir, 89-96.

5. REMY J.C., HEBERT J. (1977) : - Le devenir des engrais azotés dans les sols. Compte Rendu de l'Académie Agronomique de France, 63, 700-710.