| Nitrogen (N) is a fundamental component of living organisms, and it has been strongly influenced by human activity. From the pre-industrial era to 1990, reactive N input to the global terrestrial system increased twofold. The massive N input has enabled humankind to greatly increase food production. However, excessive N can induce a series of economic and environmental problems such as the greenhouse effect, destruction of the ozone layer, acid rain, nitrate pollution in groundwater, eutrophication of lakes and offshore water, and biodiversity reduction locally, nationally and globally. The findings of manifold consequences of human alteration of the N cycle have led to a much improved understanding of the scope of the anthropogenic N problem and possible strategies for managing it.China is the third largest country in the world and has diverse climatic conditions ranging from tropical in the south to cold temperate in the north, and from humid in the east to arid in the northwest. Rapid economic development and expansion of the human population in the past three decades has resulted in a large increase in chemical fertilizer and fossil fuel consumption, and thus greatly altered the N cycle. However, the changes in the input and fate of reactive N are not well understood. This study compile a N budget for mainland China with spatial and temporal distribution by using measurements, household interview and more up-to-date activity data and flux parameters to analyze the fluxes of N inputs and output and their impacts on the environment. In the N budget model, N input include chemical fertilizer, N fixation, atmospheric deposition and net food/feed import, N output include ammonia volatilization, biomass burning, net food and feed export and N export to surface water. The difference between N input and output was assumed as denitrification and storage. The followings are the productions and conclusions of this dissertation:1.We conducted N budget calculations for a rice paddy dominated agricultural watershed(Jurong Reservoir Watershed, JRW) in eastern China for 2007-2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH3) volatilization and household interviews about N related agricultural activities. The results showed that total N input in JRW was 1272ton yr-1. Chemical fertilizer was by far the dominant N input, totaling 1001 ton yr-1, accounts for 78.7% of total N input, atmospheric N deposition was the second most N input in this watershed,39 kg N ha-1 yr-1. N fixation and seeds import accounts for 13.8%,0.6%, respectively.Although total N input to the watershed was up to 280 kg N ha-1 yr-1, riverine discharge was only 4.2 kg N ha-1 yr-1, accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha-1 yr-1 after reservoir storage and/or denitrification removal. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. The low riverine N output was because of the characteristics of paddy-dominated watersheds greatly influence the hydrologic flow path, increase the water residence time and the transport of N and associated elements. In addition, the low proportion of riverine N export in this watershed is also due to the fact that N is subject to other losses and export. Major N outputs included food/feed export, NH3 volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Most of the N inputs were on croplands through N fertilization and fixation, and this part of N inputs is susceptible to NH3 volatilization and biomass burning, as well as food export. On average,10% of the chemical N fertilizer was volatilized as NH3 in the watershed. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge.2. The Taihu Lake region in China is highly developed, but surface water pollution has become a serious environmental problem in recent years, with nitrogen (N) a major pollutant. A N-budget for Changshu, a representative county-level city in the Taihu Lake region, was established by using N-related human activities data from an intensive household survey conducted in 2007, measurement data on N fluxes and literature data on other necessary parameters. The total N input was 23927 kg N km-2 yr-1. Chemical fertilizer input was heavy and averaged 13553 km-2 yr-1, being the largest source of N input. Atmospheric N deposition contributed 15.5% to the total N input and food/feed import contributed another 22.3%. Average N input through biological N fixation was 1332 kg N km-2 yr-1, making it a minor contributor.Nearly half of the N input was denitrified or stored in soil, amounting to 12381 kg N km-2 yr-1. There was no N output through net food/feed export from the region. N transport to water was 7002 kg N km-2 yr-1, accounting for 29.3% of the total N input. NH3 volatilization from fertilizer and human and animal waste amounted to 3053 kg N km-2 yr-1, or 12.8% of the total N input. About 6.2% of the total N input was emitted to the atmosphere through burning of crop residue.N transport from human and animal waste, fertilizer and waste water, atmospheric N deposition directly on the water surface is an important source of N in water bodies. The largest sources of N load in the surface water were rural human and animal excreta and domestic sewage contributed 26.5%. Urban domestic water and human excreta, industrial waste, accounting for 25.7% of the total load. N leaching and runoff from farmland was the third most important source of N load, accounting for 17.9%. Runoff from other land uses also contributed 4.8%. Due to the wide cultivation of crabs and fish, aquaculture in the county also contributed 8.1% to the N load. The huge amount of N load to surface water would result in N concentrations of> 6.0 mg N L-1 even after denitrification removal in wetlands.3. The total N inputs in China mainland increased from 3081 kg km-2 in 1985 to 5426 kg km-2 in 2007. Chemical fertilizer N consumption dominated N input and accounted for 53.6% of the total N input in 2007. Atmospheric N deposition increased continuously, from 767 to 1300 kg N km-2 during 1985-2007. While the total amount of N2 fixation changed little from 1985 to 2007, its contribution to total N input decreased from 32.5 to 18.9%. Net N input through food/feed import increased steadily. Although there was net export of grains such as rice and maize in the past two decades, import of soybean with high N concentration increased greatly during 1995-2007.At a provincial scale, there was large spatial variability in total N inputs, ranging from 588 to 50582 kg N km-2 yr-1 for the Tibet Autonomous Region and the Shanghai Municipality, respectively. Total N inputs of different provinces in different years were significantly correlated with cropland areas, since chemical N fertilizer was the dominant source of N input. As a result, there was a large total N input in provinces in eastern and central China (e.g. Jiangsu, Sandong, Henan and Anhui) where land use is predominantly agriculture. Relatively large N inputs were also found in southern and southeastern provinces (e.g. Zhejing, Fujian, and Guangdong) where with high per capita Gross Domestic Product and 50% of the land area was forest, which has a higher N2 fixation rate. It is no wonder that the vast western area had very low N input as the major land use was desert. N input was relatively low in the most northeastern part of China due to the low chemical N fertilizer application rate and low crop index. Total N input steadily increased in all provinces during 1985-2007. The highest increase was in Ningxia and Tianjin, where total N input increased about tripled from 1985 to 2007. Tibet had the lowest increase in total N input, with a rate< 10%, due to N2 fixation being the dominant source of N input in Tibet, which changed little during the period. For most regions, the increase rate of total N input was 0-200 kg N km-2 yr-1, with greater increase rates in eastern and central China.The N that is stored and denitrified was estimated to amount to 1499kg N km-2 in 1985 to 3140 kg N km-2 in 2007, accounting for 48.7% and 57.9% of total N input, respectively. Both denitrification and storage are difficult to quantify by assuming a C/N ratio of 250 for vegetation and a C/N ratio of 10 for soils, the N storage in terrestrial ecosystems in China could be estimated at about 7.9 Tg N yr-1, and thus the total denitrification in Chinese terrestrial ecosystem would be over 20 Tg N yr-1 during the period, indicating that about 16% of the total N input was stored and about 42% was denitrified in Chinese terrestrial ecosystems in recent years.To more precisely account for the spatial and temporal variability in total N input and various N outputs, we conducted a correlation analysis between these N fluxes and land use types, human population density and per capita Gross Domestic Product (GDP). Total N input, N export to water bodies, denitrification and storage were highly correlated with population density, implying that most of the N is of anthropogenic origin. Total N input, N export to water bodies, denitrification and storage also had significant positive correlations with per capita GDP, indicating that economic development may enhance N load. Because of chemical fertilizer N which applied to cropland accounts for about half of the total N input, the percentage of cropland of total land area showed significant positive correlations with total N input and all N outputs. In contrast, the percentage of grassland and forestland of total land area were negatively correlated with total N input and all N output. The exception is biomass burning emission, which was positively correlated with the percentage of forestland of total land area since wood fuel was a major source of biomass burning.We summarized the amount of N export to water bodies for the three river basins (Yangtze River basin, Yellow River basin and Zhujiang River basin). The results showed that, the modeled riverine N exports agree reasonably well with the measured one for the Yangtze River basin and Zhujiang River basin. For the Yellow River basin, however, the estimated riverine N exports are much larger than the observed ones. This is likely due to the interception of the river water for irrigation and other purposes that may lead to low or zero flow in the lower part of the river in certain periods of the year.4. N cycling has been strongly influenced by human activity at different scales in China mainland. Chemical fertilizer was the biggest N input from a 45.5km2 scale watersheds to the whole China mainland and more than half of the N input was denitrificated or storaged in the system. At the small scale, although total N input is high, riverine N output can be <1.5% of the total N input, agricultural activities resulted in much more atmospheric N pollution. While in development area, food and feed import was much higher than the agricultural watershed, and large amount of total N transport to surface water body which resulted in water eutrophication in this area. There were large temporal variabilities in total N input and outputs at the national sacle, and total N input, N export to water bodies, denitrification and storage could be very well explained by human population density. Nitrogen input and major outputs were also positively related to per capita Gross Domestic Product and the percentage of total land area used as cropland. Large amout of N input resulted in soil acidification and the major river water pollution at national scale. |
PDF Full Text Request