Temporal changes in nitrogen acquisition of Japanese black pine (Pinus thunbergii) associated with black locust (Robinia pseudoacacia)

The alien woody legume, black locust (Robinia pseudoacacia), has invaded Japanese black pine (Pinus thunbergii) forests located in Japan's coastal plain and hill regions where gaps are formed in pine forests after nematode infestation. Nitrogen fixation by legumes accelerates N cycling in forest ecosystems. We studied temporal change in the annual tree-ring resolution N stable isotope composition (δ¹⁵N, a per mil deviation of δ¹⁵N/¹⁴N ratio, relative to atmospheric N₂δ¹⁵N=0‰) at two natural locations of Japanese black pine forest with black locust that differed in the time since black locust establishment (Shohnai in northeast and Kita-Kyushu in southwest Japan). Analyzed tree-rings covered the period from 1990/1992 to 2009. N acquisition by Japanese black pine from black locust N input to the soil was evidenced by temporal shifting of N stable isotope composition on the annual pine tree rings. With progressive development of the forest stand, δ¹⁵N values of earlier tree-rings δ¹⁵N of -5‰) from black pine associated with black locust shifted towards values similar to those of black locust δ¹⁵N values nearly to -1‰), which suggests acquisition of N by N₂ fixation (Shohnai site). In contrast, in a forest where black locust had settled for two or three generations, in a black pine stand (Kita-Kyushu site), longer periods of N enrichment in the soil were reflected in the elevated tree-ring δ¹⁵N values of newly established black pine trees. Based on tree-ring δ¹⁵N data from the Shohnai site, we determined that about 10 years after black locust establishment, soil N had already been enriched by black locust N, this, in turn, contributed to N fertilization of surrounding trees in mixed stands.