Objective This paper explores the conservation effects of different tree species of soil and water conservation forests on slope soil nitrogen in black soil region of northeastern China, so as to provide reference for the selection of prevention and control measures for nutrient loss on black soil slopes.

Method In the Keshan Farm, northwest of Heilongjiang Province of northeastern China, soil and water conservation forest of Populus spp. and water conservation forest of Pinus sylvestris var. mongolica and cultivated land with the similar slope, aspect, slope length, and forest age were selected as the research area. In the three sites, we set three lines of 315, 319, and 323 m along the downhill slope from top to bottom, respectively. Along the lines, 10 sample points were set at equal intervals with a distance of 30 m from the top of the slope. Soil samples of 0−15 cm and 15−30 cm were collected from each sample point for the measure of soil physical characteristics, total nitrogen, ammonium nitrogen, nitrate nitrogen, and particulate organic nitrogen. Cultivated land was analyzed as a control.

Result (1) The soil physical characteristics of soil and water conservation forests were better than those of cultivated land. (2) The total nitrogen content of soil and water conservation forest of Populus spp. (1.28−2.54 g/kg) was higher than that of soil and water conservation forest of Pinus sylvestris var. mongolica and cultivated land. The soil total nitrogen content of soil and water conservation forest of Pinus sylvestris var. mongolica (1.04−1.92 g/kg) was higher than that of cultivated land (0.62−1.63 g/kg). The distribution of ammonium nitrogen and particulate organic nitrogen in the soil of three sites was the same as that of total nitrogen, and the content of soil and water conservation forest of Populus spp. was the highest, while that of cultivated land was the lowest.

Conclusion The soil nitrogen content of 0−15 cm in the two forests is higher than that of 15−30 cm, and the soil nitrogen content in the forests is higher than that in the cultivated land. Soil nitrogen content in the soil and water conservation forest of Populus spp. is higher than that in the forest of Pinus sylvestris var. mongolica, and the conservation effect of soil and water conservation forest of Populus spp. on the slope in black soil area is better than that of Pinus sylvestris var. mongolica. From the perspective of soil nitrogen conservation, Populus spp. should be given priority among the two kinds of soil and water conservation species.