[Nutrient Characteristics and Nitrogen Forms of Rhizosphere Soils Under Four Typical Plants in the Littoral Zone of TGR].

The Three Gorges Reservoir (TGR), which is the largest water conservancy project ever built in tne world, produced a drawdown area of about 348.93 km2 because of water level control. The biological geochemical cycle of the soil in the drawdown zone has been changed as the result of long-term winter flooding and summer drought and vegetation covering. The loss of soil nitrogen in the drawdown zone poses a threat to the water environmental in TGR. Pengxi river, is an important anabranch, which has the largest drawdown area has been selected in the present study. The four typical vegetation, contained Cynodon dactylon, Cyperus rotundus, Anthium sibiricum and Zea mays L. as the control, were studied to measure nutrient characteristics and nitrogen forms of rhizosphere and non-rhizosphere soils in three distribution areas with different soil types (paddy soil, purple soil and fluvo-aquic soils). The variables measured included organic matter (OM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), hydrolysis N, available P and available K, pH, ion-exchangeable N (IEE-N), weak acid extractable N (CF-N) , iron-manganese oxides N (IMOF-N), organic matter sulfide N (OSF-N), added up four N forms for total transferable N (TF-N) and TN minus TF-N for non-transferable N (NTF-N). The results showed: (1) pH of rhizosphere soil was generally lower than that of non-rhizosphere soil under different vegetation in different type soils because the possible organic acid and H+ released form plant roots and cation absorption differences, and the OM, TP, TN and hydrolysis N of rhizosphere soil were generally higher than those of non-rhizosphere soil, and that the enrichment ratio (ER) of all the four nutrient indicators showed Cyperus rotundus > Cynodon dactylon > Zea mays L. > Anthium sibiricum. Available P showed enrichment in the rhizosphere of three natural vegetations but lose under corn, and available K, TK showed different ER in different conditions. (2) IEF-N CF-N, IMOF-N, OSF-N and TF-N of rhizosphere soil were generally higher than those of non-rhizosphere soil, but the TF-N to TN ratio in rhizosphere of Cyperus rotundus and Cynodon dactylon were lower than those of non-rhizosphere soil, and in rhizosphere of Anthium sibiricum and Zea mays L. were higher, the rhizosphere effect of different vegetations on the N cycle was significant difference. (3) the correlation between nutrient characteristics and nitrogen forms was evaluated to explore the influence factor for the N forms changing. There was a significant correlation between soil OM and four N forms, TP was significantly correlated with CF-N, OSF-N, TF-N, and soil available P content was significantly negatively correlated with IMOF-N, OSF-N, TF-N and TN. Our research could provide that the drawdown zone covered with Cyperus rotundus and Cynodon dactylon was better than Anthium sibiricum and Zea mays L. to improve soil N holding and fixation. The vegetation recovery in the drawdown zone should consider the rhizosphere effect of different vegetations on N cycle.