摘要:在宁夏六盘山香水河小流域，选择建立了4种典型森林的样地，测定了2011年生长季（5月24日-10月20日）大气降水（724.3 mm）、穿透水、干流、枯落物渗透水和主根系层（0-30 cm）土壤渗透水的无机氮（NH₄⁺-N、NO₃^--N）浓度及相应通量的变化。结果表明，在林外降水转化林下降水中，各样地林下降水携带的生长季NH₄⁺-N通量（kg/hm²）（华北落叶松人工林2.40、华山松次生林2.37、野李子灌丛2.29、桦木次生林2.09）均明显低于林外降水（3.04），NO₃^--N通量（kg/hm²）（华北落叶松人工林2.15、桦木次生林2.14、野李子灌丛2.09、华山松次生林1.92）更显著低于林外降水（4.27）。整体看来，阔叶林林冠的无机氮吸附（吸收）作用稍高于针叶林。在4种森林样地的枯落物层渗透水中，无机氮浓度变化在0.68-0.88 mg/L，稍高于林冠穿透水的无机氮浓度；4个样地的枯落物渗透水的无机氮通量（kg/hm²）（野李子灌丛4.10、桦木次生林3.24、华山松次生林3.22、华北落叶松人工林2.77）均低于林下降水。在华山松次生林和华北落叶松人工林的主根系层（0-30 cm）土壤渗透水中，无机氮浓度均高于枯落物渗漏水；因土壤淋出作用，华山松次生林和华北落叶松人工林的土壤渗透水的无机氮输出通量分别为16.34 kg/hm²和18.93 kg/hm²，均显著高于枯落物渗透水的输入通量。整体来看，林外降水在林地无机氮输入中占有重要地位，在研究年份生长季，林冠层和枯落物层的吸附（吸收）作用使降水输入的无机氮通量明显降低，但主根系层土壤淋出作用显著增大了土壤渗透水携带的无机氮输出通量，表现为土壤层氮素流失。



Abstract:In this study, 3-4 stand plots of 20 m×20 m to 30 m×30 m were selected from four typical forests located in a small watershed of Xiangshui River, the Liupan Mountain of Ningxia, China, to explore the spatial variability of inorganic nitrogen (NH₄⁺-N, NO₃^--N) fluxes and their relationships with precipitation. The concentration and flux of the inorganic nitrogen (NH₄⁺-N, NO₃^--N) in the infiltration water penetrating the soil of main root layers (0-30 cm) after precipitation, the water penetrating tree canopies (that is, canopy infiltration water), run-off water and the water infiltrating humus layers (that is, humus infiltration water) were measured during the growing season (24 of May to 20 of October) of 2011. The results showed that with the atmospheric precipitation of 724.3 mm in the open field, the concentration (kg/hm²) of inorganic nitrogen (NH₄⁺-N) in canopy infiltration water was 2.40 for the plantation of Larix principis-rupprechtii, 2.37 for the secondary forest of Pinus armandii, 2.29 for the bush of Prunus salicina, and 2.09 for the secondary forest of Betula platyphylla, respectively, and they all were lower than that in the rainfall of the open field, 3.04. The corresponding NO₃^--N fluxes (kg/hm²) were 2.15 for the plantation of Larix principis-rupprechtii, 2.14 for the secondary forest of Betula platyphylla, 2.09 for the bush of Prunus salicina, and 1.92 for the secondary forest of Pinus armandii, respectively, and also much lower than that in the rainfall of the open field, 4.27. Overall, the canopy adsorption effect of inorganic nitrogen was greater in the broad-leaved forests than in the coniferous forests. Among the four forest types, the humus infiltration water had an inorganic nitrogen concentration range of 0.68-0.88 mg/L, which was slightly higher than those in the canopy infiltration water. The inorganic nitrogen fluxes in the humus infiltration water were 4.10 for the bush of Prunus salicina, 3.24 for the secondary forest of Betula platyphylla, 3.22 for the secondary forest of Pinus armandii, and 2.77 for the Larix principis-rupprechtii plantation, respectively, and lower than those in the canopy infiltration water. In both the secondary forest of Pinus armandii and the plantation of Larix principis-rupprechtii, the concentrations of inorganic nitrogen in the soil infiltration water of the main root layer (0-30 cm) were higher than those in the humus infiltration water. Due to the soil leaching effect, the inorganic nitrogen flux carried by the soil infiltration water of the main root layer was 16.34 kg/hm² for the stand of Pinus armandii and 18.93 kg/hm² for the stand of Larix principis-rupprechtii, being obviously much higher than those in the humus infiltration water. In summary, precipitation plays an important role in the inorganic nitrogen fluxes input into the forest soils, and the absorption or up-taking effects of tree canopies and humus layers greatly reduce the amount of the inorganic nitrogen flux into soils through precipitation. However, the soil leaching effect in the main root layers can markedly increase the inorganic nitrogen flux of the soil infiltration water, indicating the loss of the inorganic nitrogens in the forest ecosystems.





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