MATERIALS AND METHODS

A field experiment was carried out in 2014 at the INRA experimental site of Bretenière (Eastern France). Ten legume crops were sown either in March (faba bean, common vetch, lentil, lupin, pea) or in May (fenugreek, chickpea, common bean, soybean, Narbonne vetch) depending on the physiology of each species. All seeds were

inoculated at sowing with species-specific strains of N2-fixing bacteria. Non-fixing reference crops were also sown:

barley in March and sorghum in May. At sowing, inorganic N content in the first 60 cm of the soil were 69 and 84

kg N ha-1 _{in March and May, respectively. Different levels of N fertiliser (0, 50, 150 and 300 kg N ha}-1_{) were applied}

at both sowing dates, in the form of NH4NO3. To estimate SNF, the 15N isotope dilution technique was used, with

barley or sorghum as reference crops. 5 kg N ha-1 _{of NH}

4NO3 labelled with 15N (1 % 15N enrichment) was additionally

spread on all crops at each sowing date. Shoots were harvested at physiological maturity and dry matter (DM), N

concentration (% N), 15_{N enrichment were measured for each crop and % Ndfa was calculated.}

RESULTS AND DISCUSSION

The amount of shoot N accumulated over the whole growth cycle greatly varied among species. When no N

fertilizer was applied (0N treatment), shoot N ranged from 94 to 360 kg N ha-1_{. For common vetch, lentil, lupin,}

fenugreek, Narbonne vetch, and soybean the level of N fertilizer did not significantly affect the shoot N amount. For common bean, faba bean, pea, and chickpea, the 300N treatment differed from the 0N treatment, with the amount of shoot N compared to the 0N treatment being increased by 11, 17, 33, and 57 %, respectively.

For both sowing dates, a negative relationship between % Ndfa and the amount of inorganic N available at sowing was observed for all species but with great differences in the response intensity among species. As an indicator of this response intensity, the amount of inorganic N available at sowing, for which % Ndfa was equal to 50 %, was

calculated. It ranged from 82 to 251 kg N ha-1 _{for common bean to faba bean, respectively the most and least}

responsive to soil N (Figure 1). This indicator was positively related to mean shoot N amounts at maturity, as averaged for the four levels of N fertilization. However, great differences were observed among species according to their ability to uptake soil inorganic N. As an indicator of plant ability to uptake soil N, theoretical N uptake efficiency of inorganic N available at sowing (NUEt) was calculated. This is the ratio between the amount of total inorganic N uptake by the legumes when SNF was equal to 50 % (corresponding to half of the total amount of

shoot N at maturity), and the amount of inorganic N available at sowing required to inhibit SNF by 50 % of total shoot N (Figure 1). Faba bean, lupin, fenugreek and Narbonne vetch had a NUE value lower than 0.5, indicating that the amount of inorganic N they retrieved was twice less than the amount available at sowing. In contrast, chickpea, common bean, common vetch, lentil, pea, soybean had NUE equal or higher than 0.5. Common bean was the most efficient to uptake inorganic N (NUE = 0.94), meaning that almost all the inorganic N available at sowing was retrieved by the plant.

Figure 1. Relationships between (i) the mean shoot N amounts measured at maturity and (ii) inorganic N available at sowing for which % Ndfa was equal to 50 % (calculated using four level of N fertilization), measured for ten legumes species sown in

a single field experiment, either in March 2014 () and May 2014 (). Cb: common bean, Ch: Chickpea, Cv: Common vetch,

Fa: Fababean, Fe: Fenugreek, Le: Lentil, Lu: Lupin, Nv: Narbonne vetch, Pe: Pea, So: Soybean. The curves represent iso-

efficiency uptake of inorganic N available at sowing.

CONCLUSION

While N fertilization had little effect on the amount of shoot N, SNF decreased with the level of inorganic N available at sowing with great differences among the ten legume species. Our field experiment allowed us to classify ten legume species according to their inorganic N uptake efficiency. We suggest that this ability to retrieve inorganic N might be a key element to understand the differences of SNF apparent inhibition by soil inorganic N. A better understanding of plant traits leading to a better NUE of inorganic N is crucial to avoid N losses, leaching being often observed in legume based rotations (Plazza-Bonilla., 2015). To go further, comparison of root architecture of the ten legume species is currently studied.

Acknowledgements: M. Guinet PhD work was granted by INRA and the Ministry in charge of Agriculture. This experimental work was supported by the ANR LEGITIMES and Bourgogne-Franche-Comté Region (France). REFERENCES

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