Nitrogen Cycling And Its Influence On Nitrogen In Water Of Small Watersheds In The Three Gorges Area

Nitrogen is a ubiquitous biogenic element and widely distributed in the nature. The biochemical cycling of nitrogen has being severely challenged and significantly influences with intensive human activities. The extensive application of chemical fertilizers has greatly accelerated the nitrogen accumulation in the agro-ecological system that includes fields and water bodies. Approaches for improving nitrogen utilization ratio and reducing its losses, in order to ease the environmental pressure, have been explored all over the world. Investigating nitrogen cycle that includes nitrogen concentrations and migration in water at the watershed scale in the Three Gorges Area was the specific objective of this study.Different approaches, such as field survey, long-term monitoring, plot experiment and probabilistic risk assessment were linked to explore the characteristics of nitrogen cycle and reveal the role of nitrogen concentrations and migration in surface runoff, atmospheric deposition and leaching. We identified the major influencing factors, and put forward the target-oriented proposal and countermeasures for nitrogen loss. The main results were as follow:1) Farmer nitrogen cyclingAn effort was made to study the N cycle on Farm scale level, the results showed that the nitrogen discharge by breading households was living source of nitrogen, whereas the nitrogen load of growing households was the farm surplus nitrogen. By linking the nitrogen cycling and budget, farm surplus nitrogen and living discharging nitrogen were the most export items being potential sources of pollution to environment. Without proper control measurements, this could lead to serious nitrogen loss, and thus a threat to regional water environment.2) Nitrogen in water body1) By using the method of fixed-pointed monitoring in streams of two small watersheds, the contents of dissolved silicon (DSi), dissolved inorganic nitrogen (DIN) and dissolved phosphorus (DP) were analyzed. DSi, DIN and DP concentrations in small watersheds were higher than in Three Gorge Reservoir.2) NO3--N was the major form of nitrogen in base-flow, seasonal variations of NO3--N concentrations presented that winter>autumn>summer>spring, the values shown a gradually increasing trend.3) Rainfall was the major promoting factor for nitrogen export at the watershed scale, and 80% of nitrogen export occurred from April to September.4) Concentrations of Various nitrogen forms varied along with different treatments during the rainfall process. Increasing trend was observed initially, followed by decrease, finally the trend was stabilized. Nitrogen losses were different among treatments, but dissoluble nitrogen and sediment nitrogen losses were almost 40% of total nitrogen losses in runoff. NO3--N and NH4+-N were the dominated forms of dissoluble nitrogen, more than 80%. Nitrogen losses varied evidently among different land use types. The loss of nitrogen in Terrace and slope-contour hedgerow cultivation practices were only 50% of nitrogen loss from slope farmland, which indicted that the two cultivation practices have certain interception function for soil water and nitrogen losses. In addition, the study also indicated there were better effects for water and nutrients retention by planting citrus than crop or tea tree. Therefore, base on environmental and economical consideration, the combination of slope-contour hedgerow and plant citrus cultivation practice was the best land use type.5) Slope gradient was found to be an important factor, especially in new-ploughed fields. Water, soil and nitrogen loss increased with the increase in slope gradient.6) The contributing zone approach showed a significantly correlation between total nitrogen application and nitrogen concentrations in stream water, especially the nitrogen applied within the area of 200 m of upstream contributing zones. The results indicated that both sub watershed and contributing zone approaches are feasible methods to study the characteristics and influencing factors of nutrients in surface water. Furthermore, the present studies will provide a basis for the theory and method to study the influence of agricultural pollutants on stream water in some small watersheds.3) Nitrogen depositionIn this paper, nitrogen deposition was also considered, where wet deposition was found to be a main form of nitrogen deposition, accounts for more than 70% of the total. Generally nitrogen concentrations in rain water were 1.5～2.5 mg·L-1, and exceeded the threshold value for water eutrophication, especially in the dry season. The nitrogen deposition amount went beyond the critical value for feeble terrestrial ecosystem; thereby the effect on agricultural production cannot be ignored.4) Nitrogen leachingIn natural rainfall conditions, NO3--N leaching was studied in five different land use types. NO3--N leaching shown a declining trend with the soil depth, but the values maintained at a high level. In addition, fertilizer season was also the period with the largest NO3--N leaching, reasonably arrange fertilizer application rates and timing whether was a key issue for effective control NO3--N leaching. NO3--N leaching in different land use types presented that field>orchard>forest. Thereupon properly adjusting land use structures can effectively reduce the NO3--N leaching.5) Assessment of nitrogen loss riskThe probability of occurrence for nitrogen loss in Zhangjiachong watershed was evaluated both qualitatively and quantitatively by fault tree analysis. The results showed that the risk of nitrogen loss was mainly related to farm surplus nitrogen, living discharging nitrogen and livestock breed. Assessment results of the best management practices indicated that alternative practices (reduce fertilization, increase vegetation cover, install more riparian buffer along streams) can reduce the probability of occurrence for nitrogen loss.Conclusively, in the Three Gorges Area, higher nitrogen loss occurred coinciding with fertilizer application, greater precipitation, and higher stream flow. Some measures (e.g. Improve nitrogen use efficiency; understand seasonal variation of nitrogen loss and timely interception; find nitrogen sources and block these pathways; establish the long-term monitoring mechanism and increase rural diffuse pollution control) are important for nitrogen pollution prevention in the Three Gorges Area.