|其他摘要||Nitrogen is the second limiting factor in desert ecosystem after water. Attributed to Human activities, such as the utilization of nitrogen fertilizer and nitrogen oxide emissions, So significant changes have taken place in the global N cycle that almost all the ecosystem N input increased. Agricultural nitrogen fertilizer, except those absorbed by crops, had quite a number of residues in the soil. Attributed to the irrigation or heavy rains, much nitrogen was leached and made the concentration of NO3--N in groundwater was increased. To investigate whether the leached Nitrogen could affect nitrogen utilization of desert plants after irrigation and drainage in desert oasis ecotone, the experiments were carried out in these plots including the desert Surrounded by farmland, the oasis desert ecotone and the desert away farmland. Three kinds of desert plants with different life forms, Haloxylon ammodendron, Tamarix ramosissima, Salsola brachiata, Erodium oxyrrhynchum, Nepeta micrantha and Schismus arabicus were choosed, to study concentration variations of NO3--N of groundwater in desert oasis ecotone, vertical distributions of TN and δ15N of farmland soil and desert soil of H. ammodendron and T. ramosissima, and δ15N values of different desert plants.
The experiment revealed that: (1) The farmland soil TN was accumulated in deep profiles (200~300cm), NO3--N concentrations of groundwater in farmland were significant higher than desert, and gradually decreased from the desert edge to the hinterland. (2) The soil δ15N value in desert was largest, ecotone second, followed by native desert and farmland was minimum, and they in whole profiles had a negative change with TN content. The closer the farmland, the smaller the varations of δ15N values in vertical profile, which may indicated farmland lateral seepage or groundwater had an impact on soil δ15N values. Besides, there were no significant differences of surface soil δ15N values of H. ammodendron and T. ramosissima in different plots, but the deeper soil δ15N values. The closer the farmland, the lower the deep soil δ15N values that implyed the deep soil δ15N values may be affected by NO3--N in groundwater, and agricultural irrigation had an indirect impact on deep soil δ15N values, it also suggested the impact gradually reduced from the edge to the hinterland. (3) The δ15N value of T. ramosissima was significant smaller than H. ammodendron, Salsola brachiata, Erodium oxyrrhynchum, Nepeta micrantha and Schismus arabicus (p<0.05), the δ15N values of T. ramosissima and H. ammodendron had a significant negative correlation with soil moisture and TN content (p<0.05), which may indicate T. ramosissima nitrogen utilization directly affected by agricultural irrigation and H. ammodendron indirectly affected, but Salsola brachiata, Erodium oxyrrhynchum, Nepeta micrantha, Schismus arabicus nitrogen utilization largely unaffected by agricultural irrigation.
In conclusion，in the desert oasis ecotone, the agricultural irrigation likely had an impact on the nitrogen utilization of desert plants with deep roots. So with increasing of nitrogen input and rising of NO3--N concentrations in groundwater caused by agricultural irrigation, the groundwater may become an important nitrogen source, which could change the oasis desert ecotone species composition and improve primary productivity in desert ecosystem. These find may improve our understanding of the impact of agricultural irrigation on the nitrogen and water utilization of desert plants. Additionally, the study may provide some theoretical support for the possible ecological evolution in the desert oasis ecotone.|