Lake Okeechobee Watershed American Soil Nitrogen And Phosphorus Loss Of Spatial Distribution And Risk Assessment

In soils, N and P losses not only impact groundwater, but also cause non-pointsource pollution and eutrophication. In Lake Okeechobee, the losses of N and P insoils affect the water quality and the development of local tourism. Aiming at LakeOkeechobee in Florida of America, this paper analyzes the spatial distribution of soilP and N by geostatistics in combination with land uses and soil types within thewatershed, evaluating the risks of soil N and P losses within the lake, for whichconclusions are mainly reached as follows.1. In the research area, there is great variability of TN and NO3--N in surface andsubsurface soils and NH4+-N in surface soils, with moderate variability of NH4+-N insubsurface soils. Close spatial correlation exists among the TN in subsurface soils,NO3--N in surface soils and subsurface soils, whereas there is weak spatialcorrelation among TP in surface soils and NH4+-N in surface soils and subsurfacesoils. TP in surface soils and NH4+-N in surface soils and subsurface soils conform tolinear models. Exponential models are applicable for TN in subsurface soils andNO3--N in surface soils, while NO3--N in subsurface soils corresponds to sphericalmodels. In surface and subsurface soils, TN exhibits a trend of zonal distribution andNH4+-N is characterized by zonal and island-shaped distribution, while zonaldistribution and island-like distribution are found for NH4+-N in surface soils, andzonal distribution for NH4+-N in subsurface soils.2. In surface and subsurface soils of research area, the content of TN ismaximum and minimum in histosol and ultisol soils respectively. In surface andsubsurface soils, the content of NO3--N is maximum and minimum in histosol andmollisol soils respectively. In the subsurface, the histosol soils and ultisol respectivehave a maximum and minimum amount of soil NH4+-N. In subsurface soils, thecontent of NH4+-N is highest and lowest in histosol soils and inceptisol respectively.In surface soils, the TN is mostly from field crops and tree crop. In subsurface soils,the content of TN is maximum and minimum in field crops and ornamental plantsrespectively. In surface soils, field crops and cypress trees respectively have a maximum and minimum amount of NO3--N. In subsurface soils, the content ofNO3--N is respectively highest and lowest in field crops and coniferous forests. Themaximum and minimum content of NH4+-N are respectively presented fromunimproved pasture and shrubs on the subsurface. In the subsurface, the content ofNH4+-N is highest and lowest in field crops and ornamental plants respectively.3. In research area, the content of TN is higher in surface soils than that insubsurface soils in all cases and the content of NH4+-N is mostly higher in surfacesoils than that in subsurface soils. In many parts of subsurface, there is more NO3--Ncompared with surface soil. On the whole, the content of TN declines in sample areasranging from0-100cm in depth, with small changes in two sample areas ofunimproved pasture in depth ranging from70-100cm. The content of NO3--Nchanges in complicated manners vertically in soils. Except for unimproved pasture2,there are no remarkable changes in the content of NH4+-N in the vertical direction.4. In both the surface and subsurface soils of research area, the TP complieswith Gaussian model. The TP has strong spatial correlations in surface and subsurfacesoils. In surface soils, the TP is characterized by island-shaped distribution, while theTP shows a trend of multi-island distribution and there are several remarkable lowvalue regions. On the surface of research area, the content of TP is highest andlowest on inceptisol and mollisol soil, so do the content of TP on subsurface. There isa maximum and minimum amount of TP in surface soils of dairy farms and mixedshrubs, and in subsurface soils of improved pastures and mixed shrubs. In twosample areas, the content of TP is higher on the subsurface than that on the surface.There are spodosols and the lands are used for improved pastures in these sampleareas. As suggested from the cross sections of five sample areas, the content of TPtends to have dramatic decline in vertical direction except for improved pasture3,but there are no great changes in the content of total P in vertical directions inanother four sample areas.6. In the research area, the risk of N losses is highest and lowest in field cropsand cypress respectively in subsurface soils, field crops and coniferous forest, whilethe risk of NO3--N is highest in field crops in soils. In surface soils of research area, there are highest and lowest risks of N losses in histosol and ultisol respectively. Inthe subsurface soil, the highest and lowest risks of N losses are respectively indicatedfrom histosol and mollisol soils. Within this watershed, the risk of soil N lossesincreases from north to south. In the central part of the watershed, there are peakvalue areas and lands are used for planting crops where the soils are histosol. In thenorth of this watershed, there is low risk of losses and lands are utilized for plantingcypresses in which soils are histosol. In the south of this watershed, the risk of Nlosses is high and sugar cranes are planted, while the soils are histosol.8. In the research area, there are high risks of soil P losses in broad-leavedforests, residential areas, ornamental forests, dairies, improved pastures, shrubs,oranges and cane fields, with medium risks of P losses in improved pastures andmixed shrubs, and low risks of P losses in deciduous plants. High risks of P losses arediscovered in entisols, spodosols, inceptisols and alfisols. Besides, there are mediumrisks of P looses in mollisol soils and low risks of P losses in histosol soils. As regardsultisols, the risks of P losses are higher on the surface compared with subsurface. Inthis watershed, the soil P losses present a trend of island-shaped distribution and therisks of soil P losses are high in most areas. As a whole, the risk of P losses becomelower and lower from the south to north of this watershed. Around Lake Okeechobee,the risk of soil P losses is high and poses great threats to its water quality. In thenorth of the watershed, the risk of P losses is rather low and trees such as shrubs andarbors are planted, while there are entisols and ultisols. It is thus clear that the risksof P losses are comparatively lower in areas which are not so greatly impacted byhuman activities.