High nitrogen deposition may enhance growth of a new hybrid larch F₁ growing at two phosphorus levels

A recently developed hybrid larch F₁ (F₁: Larix gmelinii var. japonica × L. kaempferi) is being planted widely in re- and afforestation projects in northeast Asia. Nitrogen (N) deposition to forest ecosystems has been rapidly increasing in this region, due mainly to industrialization and overuse of N fertilizer. Together with excess N, phosphorus (P) is considered to be the key determinant of tree growth in northeast Asia, because most soils have originated from immature volcanic ash. To predict the response of the F₁ to increasing N deposition and its relation with soil P availability related to immature volcanic ash soil in northern Japan, planting stocks of F₁ were grown in potted brown forest soil and categorised into eight treatments, comprising four N treatments covering the amount of N deposition observed and predicted in northeast Asia in combination with two P levels. N application increased the biomass and the light-saturated net photosynthetic rate (A _sat) of the F₁ at all concentrations. Despite expectations, P did not have any effect on these parameters. As N application increased the content of potassium (K), magnesium (Mg) and chlorophyll (Chl) in needles, a positive correlation was found between the content of N, K, P and A _sat. These results suggest that N deposition improves the growth of the hybrid larch F₁ at least by improving the needle N condition, as well as the concentration of other macronutrients in the initial stage of plantation.