Combined effects of irrigation amount and nitrogen load on growth and needle biochemical traits of Cryptomeria japonica seedlings

Key message Growth of C. japonica under highly irrigated and N-loaded soil is noticeable because of N-induced increase of needle mass and alleviation of high irrigation-induced reduction in photosynthesis. To clarify the combined effects of change in precipitation amount and increasing nitrogen load on Japanese forest trees, 2-year-old seedlings of Cryptomeria japonica were treated with combinations of four irrigation levels (80, 100, 120, or 140% of annual precipitation) and three nitrogen loads (15, 50, or 100 kg ha −1 year −1 ). The 100% of annual precipitation corresponded to 1520 mm year −1 which was the average annual precipitation in the habitats of C. japonica in Japan. High irrigation amount significantly enhanced the whole-plant dry mass of the seedlings. The N load significantly enhanced the extent of high irrigation-induced increase in needle dry mass. As a result, the irrigation-induced increase in the whole-plant dry mass was greater in the treatment with the highest N. However, increase in needle dry mass induced by high irrigation enhanced plant water consumption and caused dehydration and oxidative damage in needles, thus resulting in reduced net photosynthetic rate. The high irrigation-induced reduction in net photosynthetic rate was alleviated by N load. This alleviation was not caused by N load-induced changes in antioxidative capacity but by a N load-induced increase in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. These results indicate that an increase in the whole-plant dry mass of C. japonica seedlings grown in highly irrigated and N-loaded soil was attributed to N load-induced enhancement of high irrigation-induced increase in needle dry mass and alleviation of high irrigation-induced reduction in net photosynthetic rate.