The contribution of acorn and soil N to early development of Chinese cork oak (Quercus variabilis Blume) seedlings under contrasting soil fertility conditions

Several studies have addressed the role of soil fertility on acorn N remobilization during seedling growth, but have focused on very early development stages or have assessed remobilization at a coarse grain ontogenetic scale making it difficult to know the precise time when seedlings switch from acorn N to soil N use. We cultivated Quercus variabilis seedlings under two distinct soil N fertility and assessed their growth, acorn N remobilization, and absorption of soil N at five distinct development stages, spanning from the incipient shoot emergence to the completion of the second flush of growth. Acorn N contributed more to seedling N content than soil N at all development stages. Seedlings began to uptake substantial amounts of soil N after the completion of leaf expansion during the first shoot flush of growth, coinciding with a fine root area that reached 50% of the maximum value observed at the end of the study. Roots became less dependent on acorn N before shoots. Soil fertility, rather than seedling growth rate, determined soil N uptake after the completion of leaf expansion in the first shoot flush of growth. We conclude that the acorn is the primary N source for Q. variabilis seedlings until the completion of the first shoot flush of growth. Soil fertility does not significantly affect either the amount of N remobilized from acorns or the switch from acorn N to massive soil N use, suggesting a minimal effect of forest microhabitat fertility on acorn N utilization by Q. variabilis seedlings.