To study the seasonal variability of soil inorganic nitrogen (N) across borders at the woodland-farmland ecotone and potential mechanisms, contents of soil inorganic N were measured during the dry season (May 20 and June 30) and the rainy season (August 10 and September 20) of 2006 in the Songnen Plain of Northeast China. The borders between farmland and woodland were determined by a border-and-ecotone detection analysis (BEDA). The ecotone limits, often referred to as the depth-of-edge influence (DEI), are critical for determining the scale at which edge effect operates. The results showed that the soil inorganic N border between the woodland and farmland was located further toward the woodland interior during the rainy season (DEI=53.4±8.7 m, August 10) than during the dry season (DEI=35.0±12.6 m, May 20). The seasonal variability in the soil inorganic N border was found to be associated with seasonal changes of deposition flux of N (the correlation coefficients between them for the dry season and rainy season were 0.61 and 0.67, respectively), which resulted from foliation patterns of trees and crops. Accordingly, the leaf area index at woodland edges was lower than that in the woodland interior, so woodland edges captured large amounts of atmosphere nitrogen deposition. The average DEI was 44.1 m, which was in accordance with the values of other temperate forest boundaries in literatures; therefore, BEDA was an appropriate method to estimate the borders of ecotones. To study the seasonal variability of soil inorganic nitrogen (N) across the borders of the woodland-farmland ecotone and potential mechanisms, contents of soil inorganic N were measured during the dry season (May 20 and June 30) and the rainy season (August 10 and September 20) of 2006 in the Songnen Plain of Northeast China. The borders between farmland and woodland were determined by a border-and-ecotone detection analysis (BEDA). The ecotone limits, often referred to as the depth-of-edge influence ( DEI), are critical for determining the scale at which edge effect operates. The results showed that the soil inorganic N border between the woodland and farmland was located further toward the woodland interior during the rainy season (DEI = 53.4 ± 8.7 m, August 10 ) than during the dry season (DEI = 35.0 ± 12.6 m, May 20). The seasonal variability in the soil inorganic N border was found to be associated with seasonal changes of deposition flux of N (the correlation coefficients between them for th e dry season and rainy season were 0.61 and 0.67, respectively), which resulted from foliation patterns of trees and crops. Thus, the leaf area index at woodland edges was lower than that in the woodland interior, so woodland edges captured large amounts of atmosphere nitrogen deposition. The average DEI was 44.1 m, which was in accordance with the values ​​of other temperate forest boundaries in literatures; therefore, BEDA was an appropriate method to estimate the borders of ecotones.