Comparative nitrogen mineralization and its availability in certain woody plantations in Keerqin Sand Lands, China
Comparative nitrogen mineralization and its availability in certain woody plantations in Keerqin Sand Lands, China
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作者：
C. Fu
固沙林在减缓北方荒漠化进程中发挥了重要作用。土壤氮素矿化是贫桑迪生态系统的一个重要生态过程，可作为评价人工林生态系统功能的基本指标。本研究采用PVC管原位密闭培养和离子交换树脂袋法，研究了科尔沁东南部松树、白杨人工林及其邻近草原土壤氮素矿化过程及其有效性。结果表明，在生长季（2004年4月15日至10月15日），草地和赤松人工林的累积净矿化量分别为8.06 μg·g-1和9.06 μg·g-1种植园低于樟子松和赤松人工林;蒙古栎（18.36 μg·g-1）和小叶杨（17.88 μg·g-1）人工林（p0.05）。结果表明，草地硝化速率与净矿化速率的比值随草地类型的变化而变化。蒙古栎、密花杨和小叶杨人工林的土壤硝化速率分别为0.88，0.91，0.82，0.87，表明研究区土壤硝化作用较土壤净氨化作用强烈，NO3--N是研究区植物吸收的主要氮素形态。这可能与研究区土壤pH值较高（6.6）和人为干扰较强有关。草地净矿化量占土壤全氮含量的百分比，樟子松草地最高。蒙古松林、密花杨林和小叶杨林的氮素周转率分别为2.49%、4.42%、2.77%、4.26%，说明草地和密花杨林的氮素周转率很低，而樟子松林的氮素周转率很低。蒙古杨和小叶杨的生长量相对较高。从5月15日到10月1日，草原和小叶杨人工林树脂累积吸收的矿质氮最低，分别为283.50 μg·g-1干树脂和297.00 μg·g-1干树脂;蒙古松（440.10 μg·g-1干树脂）居中，密花松（835.65 μg·g-1干树脂）最高（p0.05）。研究了樟子松草地土壤相对硝化速率（以土壤硝态氮与矿质氮的比值表示）。蒙古松林、密花杨林和小叶杨林的平均土壤氮素含量分别为0.88、0.87、0.92和0.90，说明NO3--N是土壤矿质氮的主要来源。结果表明，原位密闭PVC管培养法比离子交换树脂法更能准确地测定土壤氮素矿化速率，而两种方法在研究土壤氮素有效性和植物氮素吸收方面更有说服力。综合上述结果，认为上述3种树种是研究区的选择性固沙树种，其适宜性排序为密花杨、樟子松和樟子松。蒙古杨和小叶杨。然而，为了全面评价研究区的生态系统功能，固沙林氮平衡的知识是必要的。
The sand-fixation plantations played an important role in cutting down desertification process in northern China. Soil nitrogen mineralization, which is a key ecological process in poor sandy ecosystems, can be used as a basic index to assess ecosystem function of tree plantations. In this study, soil nitrogen mineralization process and its availability were examined by the methods of in situ closed-top PVC tube incubation and ion exchange resin bags under plantations of pine and poplar, and their neighboring grasslands in the southeastern Keerqin region of China. Results indicated that the accumulative net mineralization in growing season (from April 15 to October 15, 2004) in the grassland (8.06 μg·g -1) and Pinus densiflora (9.06 μg·g -1) plantation was lower than that in both Pinus sylvestris var. mongolica (18.36 μg·g -1) and Populus simonii (17.88 μg·g -1) plantations (p0.05). The ratios of nitrification rate to net mineralization rate in grassland, P. sylvestris var. mongolica, P. densiflora, and Populus simonii plantations were 0.88, 0.91, 0.82, 0.87, respectively, which indicated that the soil nitrification rate in the study area is more intensive than soil net ammonification processes, and the NO-_3-N was the major N form absorbed by plants in the study area, which might due to the higher soil pH values (6.6) and strong human disturbances in the study area. The percentages of net mineralization amount to soil total N contents in grassland, P. sylvestris var. mongolica, P. densiflora, and Populus simonii plantations were 2.49%, 4.42%, 2.77%, 4.26%, respectively, implying that N turnover rates in grassland and P. densiflora plantation were very low, while those in P. sylvestris var. mongolica and Populus simonii were relatively higher. The accumulative mineral N absorbed by resin from May 15 to October 1 in both the grassland (283.50 μg·g -1 dry resin) and Populus simonii plantations (297.00 μg·g -1 dry resin) was the lowest, while that in P. sylvestris var. mongolica (440.10 μg·g -1 dry resin) was intermediate, and P. densiflora (835.65 μg·g -1 dry resin) was the highest (p0.05). Soil relative nitrification rate (which is expressed by the ratios of the soil NO-_3-N to mineral N) in the grassland, P. sylvestris var. mongolica, P. densiflora and Populus simonii plantations were 0.88, 0.87, 0.92, and 0.90, respectively, which also implied that soil NO-_3-N accounted for most of the soil mineral N. From this study, we conclude that the in situ closed-top PVC tube incubation method is more accurate than the ion exchange resin method in measuring seasonal soil N mineralization rate, while it is more convincing to use both methods in exploring both soil N availability and N absorption by plants. Based on the corresponding results, we concluded that the three tree species mentioned above were selective sand-fixation tree species in the study area, and the rank of suitability was P. densiflora, P. sylvestris var. mongolica and Populus simonii. However, to comprehensively assess the ecosystem functions in the study area, the knowledge of N balance in sand-fixation plantations is necessary.