Nitrogen is a key factor influencing ecosystem structure and function in reforestation, but knowledge of ecosystem nitrogen accumulation through reforestation with mixed species is limited. Especially in the dry-hot valley of the Jinsha River, no studies cover total ecosystem nitrogen accumulation in mature plantations and its allocation for difficulty in collecting tree roots and deep soil from dry red soil. In this study, nitrogen accumulation of seven mixed plantations in the dry-hot valley in the Jinsha River was studied after thirty years of reforestation with an analogous sites method. The results were as follows: (1) Soil nitrogen stocks decreased with depth in the soil profile. Deep soil nitrogen storage (20-80 cm) was significantly correlated with stand age (R2 = 0.752, p = 0.000; n = 7), accounting for 56-63% of total soil nitrogen storage and 43-47% of soil nitrogen accumulation in the dry-hot valley. (2) Total biomass nitrogen stock of the 30-year-old plantation was 1.22 t ha-1, 61 times that of degraded wasteland and 7.6 times that of natural recovery shrub grassland, and it recovered to the reference level of natural forest following 30 years of reforestation. (3) Total ecosystem nitrogen stock in the 30-year-old plantation was 12.72 t ha-1, 1.4 times the reference wasteland and 1.19 times the natural recovery shrub grassland. The contribution of soil nitrogen to ecosystem nitrogen storage and accumulation was 90% and 67%, respectively. Litter nitrogen accounted for 1.6% ecosystem nitrogen storage. It indicated that reforestation with mixed plantation of Leucaena leucocephala and other species greatly facilitated more ecosystem nitrogen accumulation, especially soil nitrogen (including deep compartment). Secondary biomass nitrogen, especially litter, could not be overlooked. This study filled the gap of ecosystem nitrogen storage and distribution during reforestation in the dry-hot valley. Mixed plantation with legume species such as L. leucocephala and other species and an important role of secondary biomass, especially litter in nitrogen accumulation, provided a reference for the strategy formulation of reforestation and forest nitrogen management in the dry-hot valley and other semi-arid or arid regions.
Keywords: afforestation; allocation; deep soil; ecosystem nitrogen.