N uptake, soil retention and loss of soil-applied ~(15)NH_4~(15)NO_3 in young Fuji/M.26 apple trees with different N status

摘要

Nitrogen (N) uptake, soil retention and loss of soil-applied N were studied in young apple trees with different N backgrounds. Bench-graft potted Fuji/M.26 (Malus domestica Borkh) trees were fertigated with 5,10 or 20 mM N twice a week from June to Au gust, and the trees were removed from soil and bare-root stored in a 2°C cold room in December of the first season. In April of the second season, the trees were washed and replanted in containers with a N-free medium (perlite:vermiculite=l:l v), and re ceived 500 ml Hoagland's nutrient solution without N weekly through the experiment. The trees received ~(15)NH_4~(15)NO_3 at 1g per plant or no N on June 21. Four trees from each treatment were harvested at one, two and four weeks after ~(15)N applicatio n, and ~(15)N and total N in plant tissues and soil were analysed. N fertigation rates during the first growing season increased tree growth and N reserve levels, and N content in trees in the second year. New shoot and leaf growth in the following seaso n was positively related to reserve N. ~(15)N uptake increased during the four weeks after ~(15)N application while soil ~(15)N retention decreased. There was no significant difference in the total ~(15)N uptake per tree. However, trees with the lowest N contents at the end of the first growing season had the highest rate of ~(15)N uptake per unit root dry weight. Four weeks after application of ~(15)N, tree uptake of N accounted for about 60% of applied ~(15)N, while about 20% of the N still remained i n the soil, and another 20% of the N was lost. Our results suggested that trees with lower N status are more efficient in N uptake from soil.