黄土高塬沟壑区不同坡位和植被下的土壤硝态氮特征研究

地形和植被会改变水分在土壤中的运移,进而影响土壤中硝态氮(NO_3~--N)的分布,并可能导致对水体污染的差异.在黄土高塬沟壑区黑河流域选取3个样点,采集刺槐林和草地在不同坡位(上、中和下坡位)的6 m深土样,分析了坡位和植被对NO_3~--N迁移的影响,并初步评估了其对地表水及地下水污染的潜在风险.不同坡位及植被条件下,土壤中硝态氮均没有出现累积,在表层土壤达到最大值后逐渐减小.2种植被下NO_3~--N达到稳定时的深度约为200 cm,稳定浓度均为下坡位上坡位中坡位,但在同一坡位的稳定浓度均有草地高于刺槐林的特点,说明坡位及植被覆盖类型均会影响NO_3~--N在土壤中的分布.整个流域地表水NO-3含量枯水期及汛期分别为(6.90±2.10)mg·L~(-1)和(5.84±2.86)mg·L~(-1),而坡地表层土壤(0~20 cm)中可移动态NO_3~-为(29.55±6.59)mg·L~(-1),明显大于地表水中的浓度,很有可能随径流流失造成地表水氮素污染.地下水枯水期和汛期的NO_3~-含量分别为(24.61±23.72)mg·L~(-1)和(15.70±10.78)mg·L~(-1),而坡地深层土壤(200 cm)中NO-3为(0.78±0.16)mg·L~(-1),由于浓度较低,对地下水造成污染的可能性较小.

Characteristics of nitrate nitrogen under different slope positions and vegetation in the tableland-gully region of the Loess Plateau

Topography and vegetation have great impacts on soil water movement, which further influences the redistribution of NO₃^--N in soil and results in water pollution. Therefore, it is important to analyze the effects of topography and vegetation on nitrogen migration. Soil samples within 0-6 m profile were collected from three sites in the tableland-gully region of Loess Plateau, for two types of vegetation (Robinia pseudoacacia and natural grassland without fertilization) and in three slope positions (upper, middle and lower slope). After determining the NO₃^--N concentration, the potential links between nitrogen, vegetation and slope position were discussed. Despite of slope position or vegetation conditions, no accumulation of NO₃^--N in soil profiles was observed. The NO₃^--N concentration is the highest in the surface layer (0~20 cm) and decreases gradually with depth. Under either vegetation, the average depth of the stabilized NO₃^--N concentration is 200 cm, and the stabilized NO₃^--N concentration is the highest in lower slope, intermediate in upper slope, and the lowest in middle slope. However, in the same slope position, the stabilized NO₃^--N concentration under natural grassland is higher than that under Robinia pseudoacacia. Obviously, the NO₃^--N concentration in soil profiles is influenced by the slope position and vegetation. The NO₃^- concentration of the surface soil layer (0~20 cm, (29.55±6.59) mg·L^-1) is higher than that in the surface water in dry and wet season ((6.90±2.10) mg·L^-1 and (5.84±2.86) mg·L^-1, respectively). It is very likely that the soil NO₃^--N can contribute to the nitrogen pollution of surface water. The NO₃^- concentration of the deep soil layer (>200 cm) is (0.78±0.16) mg·L^-1, which is lower than that in groundwater ((24.61±23.72) mg·L^-1 and (15.70±10.78) mg·L^-1 for dry and wet season, respectively). Therefore, the NO₃^--N in deep soil may contribute little to the pollution of groundwater.