在农村地区,地表水中的硝酸盐浓度通常来自农田中过量氮肥的浸出，而森林集水区往往有良好的水质。然而,道格拉斯冷杉人工林可能会引起氮循环失衡，这可能导致土壤中硝酸盐的过量产生。我们假设在种植道格拉斯冷杉的集水区中，土壤中硝酸盐的过量产生和硝酸盐淋溶到流水中的作用更大。我们在法国和卢森堡使用了不同土地覆盖的成对集水区(道格拉斯冷杉，云杉,落叶,草地和砍伐）在3-5年内进行了监测，以评估道格拉斯冷杉人工林对地表水化学成分的影响。还监测了土壤和地下水中的硝酸盐浓度。结果表明，排水道格拉斯冷杉集水区的溪流中的硝酸盐浓度比排水其他土地覆盖的溪流中的硝酸盐浓度高2至10倍，但与明挖集水区相似。道格拉斯冷杉下地下水（最高50mgL-1）和土壤中的硝酸盐浓度也高于所有其他土地覆盖下的硝酸盐浓度。土壤硝酸盐浓度与溪流硝酸盐浓度有关。这表明土壤过程，通过道格拉斯冷杉下硝酸盐的过量生产，正在驱动溪流水中的硝酸盐浓度，并且我们的假设是将相当大一部分的过量生产从土壤转移到溪流中。这项研究还表明，农村地区地表水和地下水中的硝酸盐浓度也可能来自道格拉斯冷杉森林的集水区。然而，通过稀释效应，道格拉斯冷杉的影响在下游降低：因此，在集水区尺度上混合树种可能是减轻道格拉斯冷杉对地表水硝酸盐浓度影响的解决方案。
In rural areas, nitrate concentrations in surface waters most often originate from the leaching of excess N fertilizer in agricultural lands, whereas forested catchments often have good water quality. However, Douglas-fir plantations may induce nitrogen cycle unbalances which may lead to an excess of nitrate production in the soil. We hypothesize that the excess of production of nitrate in the soil and nitrate leaching to streamwater is greater in catchments planted with Douglas fir. We used paired catchments in both France and Luxembourg with different land covers (Douglas-fir, Spruce, Deciduous, Grassland and clearcut) which were monitored over a 3-5 year period in order to assess the effect of Douglas-fir plantations on the chemical composition of surface water. Nitrate concentration in the soil and groundwater were also monitored. The results show that nitrate concentrations in streams draining Douglas-fir catchments were two to ten times higher than in streams draining other land covers, but were similar to the clearcut catchment. Nitrate concentrations under Douglas-fir in groundwater (up to 50 mg L-1) and in the soil were also higher than under all other land covers. Soil nitrate concentration was related to stream nitrate concentration. This suggests that soil processes, through excessive nitrate production under Douglas-fir, are driving the nitrate concentration in the stream water and our hypothesis of a transfer of a fairly large proportion of this excessive production from the soil to the stream is supported. This study also shows that nitrate concentrations in surface and ground waters in rural areas could also originate from Douglas fir forested catchments. The impact of Douglas-fir is nevertheless reduced downstream through a dilution effect: mixing tree species at the catchment scale could thus be a solution to mitigate the effect of Douglas-fir on nitrate concentration in surface waters.
