Soil salinization and alkalization frequently co-occur in nature, but little is known about the mixed effects of salt-alkaline stresses on plants. An experiment with mixed salts （NaCI, Na2SO4, NaHCO3 and Na2CO3） and 30 salt-alkaline combinations （salinity 24-120 mmollL and pH 7.03-10.32） treating Medicago sativa seedlings was conducted. The results demonstrated that salinity and alkalinity significantly affected total biomass and biomass components of seedlings. There were interactive effects of salt composition and concentration on biomass （P 〈 0.001）. The interactions between salinity and alkalinity stresses led to changes in the root activity along the salinity gradient （P 〈 0.001）. The effects of alkalinity on seedling survival rate were more significant than those of salinity, and the seedlings demonstrated some physiological responses （leaf electrolyte leakage rate and proline content） in order to adapt to mixed salt-alkaline stresses. It was concluded that the mixed salt-alkaline stresses, which differ from either salt or alkali stress, emphasize the significant interaction between salt concentration （salinity） and salt component （alkalinity）. Further, the effects of the interaction between high alkalinity and salinity are more severe than those of either salt or alkali stress, and such a cooperative interaction results in more sensitive responses of ecological and physiological characteristics in plants.
Yong-Lin Peng Zhan-Wu Gao Ying Gao Guo-Fang Liu Lian-Xi Sheng De-Li Wang
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 ：k 8.7 m, August 10） than during the dry season （DEI = 35.0 =k 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.
