Contribution of previous year's leaf N and soil N uptake to current year's leaf growth in sessile oak

Stephane Bazot, Chantal Fresneau, Claire Damesin, Laure Barthes
2016 Biogeosciences  
<p><strong>Abstract.</strong> The origin of N which contributes to the synthesis of N reserves of in situ forest trees in autumn and to the growth of new organs the following spring is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20-year-old sessile oaks were <sup>15</sup>N labelled by spraying <sup>15</sup>NH<sub>4</sub><sup>15</sup>NO<sub>3</sub>: (i) on leaves in May, to label the N pool remobilized in the autumn
more » ... lized in the autumn for synthesis of reserves, (ii) on soil in the autumn, to label the N pool taken up from soil and (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of <sup>15</sup>N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of <sup>15</sup>N into the soil–tree system; more than 30<span class="thinspace"></span>% of the administered <sup>15</sup>N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage <sup>15</sup>N (73<span class="thinspace"></span>%) and to a lesser extent from soil <sup>15</sup>N (27<span class="thinspace"></span>%). The following spring, <sup>15</sup>N used for the synthesis of new leaves came first from <sup>15</sup>N stored during the previous autumn, mainly from <sup>15</sup>N reserves formed from foliage (95<span class="thinspace"></span>%). Thereafter, when leaves were fully expanded, <sup>15</sup>N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60<span class="thinspace"></span>%).</p>
doi:10.5194/bg-13-3475-2016 fatcat:kgu2svvu3zbo3ccdrsquiorxfi