No₃^-Circulation In Vadose Zone And Its Response To Paleao-hydrology And Environment Of Badain

The nitrogen cycle has been a foucs of global change research since the elevated nitrate (NO3--N) has been found in the vadose zone in many arid regions in the world. It is possible that vadose zone of arid region is an important part of global nitrogen cycle and nitrogen reservoir. To study the cycle of the nitrate and its response to the past climate is quite significant for the global change and hydrological cycle. This work was carried out in the Badain Jaran Desert which is located at the marginal zone of monsoons, based on the stable isotopic technology and climate as well as hydrology information, we examined the distribution of the NO3-,δ15N and δ18O in the precipitation, lake, groundwater and the shallow vadose zone, and then explored the process and conversion of NO3-cycle in vadose zone and different water bodies, and finally reconstructed the recharge using the CMB method, which was compared with the records from the ice core and tree rings. The evolution of the NO3-in the vadose zone and its response to the climate during the past2000a was concluded through the various parameter. The results will make a better understanding of nitrogen cycle and water resource protection, and provide basic data for the research of the evolution of arid environment. The main conclusions are as follows:In Badain Jaran desert, the concentration of NO3-in precipitation ranges from1.12to3.18mg/L with an average value of1.83mg/L. The biggest value for the NO3-/Cl-is0.22, the lowest limit for δ15N-NO3-is-0.9‰and for δ18O-NO3-is+60‰. The concentration of NO3-in precipitation increases from the central region of the desert to the edge area, it has the same trend while the elevation goes up. The NO3-mainly origins from the natural process and the human pollution appear to be minimal. The content of NO3-is between31.29and47.93mg/L, the average is39.35mg/L. the mean value for the δ18O-NO3-and δ15N-NO3-is+8.48‰and-0.27‰, respectively, diatom and occurrence of nitrification have important influence on NO3-in desert lake. The NO3-in the lakes mainly originated from rainfall and NH4+of the sediments. The groundwater NO3-is from2.84to41.81mg/L with mean value of12.32mg/L. In natural condition, the groundwater has δ18O-NO3-values of+5.7%o andδ18O-NO3-of+1.6‰, the groundwater nitrogen is controlled by both nitrification and denitrification. The NO3-originates from mainly from atmospheric deposition and microbioal N fixation. The NO3-and stable isotope distribution in the shallow vadose zone varies with the coverage. The concentration of NO3-is2032mg/L in the sand dune where has the microbioal N. it is only11mg/L on the vegitation dune. The NO3-concentration of near-surface soil is generally very high owning to the mineralization which is caused by the dry-wet cycle of precipitation. The NO3-concentration rapidly decrease from near-surface to subsoil of lm in all shallow vadose, which could be a effect of the biological immobilization, assimilation of plants, as well as denitrification. The δ15N-NO3-value ranges from-3.35‰to+19.92%o with an average of+4.12‰, and the‰18O-NO3-value is between-9.47‰and+53.25%o, the mean value is+24.69‰. it is enriched in the naked and hale naked dune owning to the volatilize and inversion of N. The value is depleted in the vegetation dunes because of the microbial activity and nitrification.The NO3-concentration varies with the depth between8and20m, the biggest value is1174mg/L, and the smallest value is49mg/L. The transformation of NO3-is quite violent in the depth around lm, which has been affected by biological immobilization and assimilation of plants. However, the behavior of NO3-is similar to that of Cl-. The NO3-would be carried forward by rainfall event and thus the NO3-can be conserved. The enrichment of NO3-in the deep vadose zone is controlled by vegetation coverage, NO3-source, microbial activity, and the lake levels.The recharge history over the last1487years was reconstructed using the CMB methods. The estimated recharge is0.94mm yr-1. Periods of relatively dry years occurred during AD490-640、AD890-1190、AD1340-1440、AD1590-1710, while the relatively wet years occurred during AD700-850、AD1190-1320、AD1440-1580、AD1720-1880. The relationship between the NO3-concentration and past climate environment is very useful to study the past climate change, and the precipitation plays an significant role to the cycle of nitrogen, the evolution of NO3-can reflect the ancient regional hydrology. The concentration of NO3-is high in dry periods because of enhanced nitrification and leaching of nitrogen. The concentration of NO3-is low in wet periods because of denitrification and assimilation of plants.