ALBERT

Notes: The effect of level of nitrogen application upon the dynamics of herbage growth in a continuously grazed sward of tall fescue was investigated during two successive years. In order to obtain a large range of sward structural conditions, the experiments were carried out with two contrasting cultivars: cv. Clarìne and cv. Barcel, and, in Year 2, with two different sward heights or leaf area indices (LAIs). During each of five experimental periods (2-3 weeks), swards received either optimum (N2) or deficient (N1) N applications, were maintained at their target LAI, and leaf growth was measured on labelled tillers. With continuously defoliated tillers, N-shortage had only a small effect on the leaf elongation rate compared with tillers protected by cages. The leaf production per tiller was only slightly reduced by N shortage, and it was mainly by the means of a reduction in tiller density that the N deficiency resulted in reduced herbage growth per hectare. These results indicate that, in continuously grazed swards, in contrast with results previously found in intermittently defoliated swards, leaf elongation is not the only important component of difference in herbage growth and that the promotion of tillering rate is an additional pathway for N response in such management regimes.

Notes: Two types of diagnostics are used for N management in grasslands: diagnostics based on N concentration of shoots and diagnostics based on soil mineral N. The Nitrogen Nutrition Index (NNI) is an example of the first type. However, its evaluation requires the determination of shoot dry weight per unit area and, thus, constitutes a practical limit to its utilization in the context of farm studies. In order to simplify its evaluation, a method based on the N concentration of the upper sward layer (Nup) has been proposed. The objectives of this study were to test the relationship between NNI and Nup in the context of permanent grassland and to examine the relationship between Nup and soil mineral status. The study was conducted as two experiments, one on small cut-plots receiving contrasting rates of mineral N fertilization, and a second on plots of an existing field-scale lysimeter experiment. In each plot and at several dates, shoot biomass within quadrats was measured, N concentration was determined on the upper leaves and on the entire shoots, and mineral nitrogen of the soil below the vegetation sampled was determined. N concentration of the upper lamina layer of the canopy was linearly related to the NNI determined on the entire shoots. Therefore, determining N concentration in leaves at the top of canopy appears to be an alternative means to evaluate NNI without having to measure shoot biomass. The absence of an overall significant correlation between soil mineral N content and sward N index, observed over the two studies, indicates that each of these two indicators has to be considered specifically in relation to the objective of the diagnostic procedure. As sward N index may vary independently of soil mineral N content, the sward N indicator does not appear to be a suitable indicator for diagnosis of environmental risks related to nitrate leaching. However, soil mineral N content does not allow the prediction of sward N status and thus is not a suitable indicator of sward growth rate. Although soil mineral N content is an important environmental indicator for nitrate-leaching risks during potential drainage periods, it has a limited diagnosis value with respect to the herbage production function of grasslands.

Notes: Abstract. The present study investigates the relationships between nitrogen uptake, transpiration, and carbon assimilation. Plants growing on nutrient solution were enclosed for 10–16 d in a growth chamber, where temperature, photon flux density, vapour saturation deficit and CO2 concentration were controlled. One of these factors was modified every 4 to 5 d. Shoot photosynthesis and root and shoot respiration were recorded every half-hour. Nitrogen uptake from the root medium and plant transpiration were measured daily. In most cases, an increase in photon flux density led to increases in transpiration, net daily carbon assimilation, and nitrogen uptake. By modifying transpiration rate without changing photosynthesis (varying vapour saturation deficit), or by modifying transpiration and carbon assimilation in opposite ways (varying CO2 air concentration), it was shown that nitrogen uptake does not follow transpiration, but is linked to the carbon uptake of the plant. When light was increased from low to intermediate levels, the N uptake/C assimilation ratio remained constant. At higher photon flux density, this ratio declined markedly. It is proposed that in the first case, growth is limited by carbohydrate availability, thus any increase in carbon assimilation leads to a proportional increase in nitrogen uptake, in contrast to the second situation where carbohydrates may accumulate in the plant without further nitrogen requirement.