Phosphorus is one of a large number of elements that are necessary for plant growth and development. It is also the main limiting factor in crop yield [1
], and is of significance in global food security. In many parts of the world, in order to meet the growing populations’ demand for food, it is necessary to add mined rock phosphate or artificial phosphorus fertilizer to the cultivated land to maintain food production [2
]. A large amount of phosphorus accumulation in the soil enters into water bodies via soil erosion, causing the phosphorus content to become elevated and the water bodies to undergo eutrophication. When the total phosphorus content of water is more than 0.02 mg/L, it is likely to lead to algal blooms [3
], and therefore non-point source (NPS) pollution of phosphorus caused by soil erosion has generated wide public concern.
Phosphorus is lost from soils through several processes including leakage, surface runoff, and soil erosion. The loss of phosphorus is closely related to the physical structure of the soil, field capacity, rainfall, vegetation coverage, fertilizer application, and irrigation methods. Phosphorus load from surface runoff is three or four times that from the subsurface flow. Thus, the main part of the phosphorus output is suspended total phosphorus, while the percentage of dissolved total phosphorus and proportion of orthophosphate are very low [4
]. Migration of dissolved phosphorus through the soil occurs in two ways: one is a slow flow through normal (matrix) soil pores, and the other is rapid (preferential) flow involving macropores. The two types together contribute to the seepage erosion of soil phosphorus [5
The Sanjiang Plain is one of the largest grain production bases in China and has experienced rapid land cover change, making it a critical area in assessing the environmental impact of LUCC and proposing strategies for minimizing its impact on non-point source (NPS) pollution. Since the 1950s, the Sanjiang Plain has been subjected to massive agricultural development. A large number of marsh wetlands were drained and transformed into fertile land for cultivation, and deforestation and destruction of local grasslands have taken place. Consequently, significant changes have taken place in the mode of regional land use. The application of pesticides and chemical fertilizers, and changes in farming methods, have destroyed the long-standing soil nutrient equilibrium, and accelerated the migration rate of soil nutrients across the water/soil interface. These nutrients are mainly nitrogen and phosphorus that reach water bodies as pollutants, and have resulted in serious environmental pollution.
In recent years, there are many mechanism models that can be used to simulate and describe the migration and conversion of nutrients, such as the PSYCHIC model, the CREAMS model, the ANSWERS model, the AnnAGNPS model, the EPIC model, and the SWAT model [6
]. Among the above models, the SWAT model is the most widely used, and has been applied in North America, Europe, Africa, South Asia and other regions, as well as in China, and other countries. Several case studies analysing the impact of land cover changes on pollution via NPS nutrients have been conducted using SWAT model [13
]. Although the SWAT model is suitable for long-term continuous non-point phosphorus simulation in agricultural areas [16
], the SWAT model, which simplifies the model runs by dividing the hydrological response unit, belongs to a semi-distributed model. In addition, the SCS curve method, which is used in the hydrology module of SWAT, is an empirical model, and it has difficulties simulating the effects of best management practices (BMPs) on runoff because of its reliance on curve number methodology [17
]. Models such as the ANSWERS model, which can only simulate at the field-scale or small watershed scale, are not suitable for large-scale phosphorus cycle simulations. Although remote sensing technology cannot directly detect water quality, it can obtain the spatial information of NPS pollution model parameters. Spatial data can clearly meet the needs of digital simulation on NPS pollution. Thus, coupling multi-source remote sensing information, data assimilation, and a distributed NPS pollution model is the main trend in NPS pollution model development.
The Ecohydrological Assessment Tool (EcoHAT) [18
] is a type of ecological hydrology simulation system, which can be used to comprehensively simulate regional ecological and hydrological processes based on physical and chemical mechanisms. With EcoHAT, hydrological cycles, nutrient cycles and plant growth processes are integrated, while nutrient cycling and growth of vegetation are considered in relation to the hydrological cycle. EcoHAT, coupled with a remote sensing model to invert the land surface parameters, provides spatial data for ecological and hydrological process simulation. Driven by remote sensing data, EcoHAT makes full use of remote sensing technology in solving large-scale soil phosphorus, NPS pollution problems with time-series simulations. Meanwhile, EcoHAT integrates GIS spatial analysis tools, which can effectively analyse the simulation results [19
The NPSs of phosphorus pollution are important, such as leakage (loss of phosphorus), which may cause adverse effects in surface and ground water. The Sanjiang Plain is an important agricultural region with various types of land use and soil. Analysis of the amount of phosphorus load under different land use and soil types is helpful for protecting the water bodies, and for controlling NPS pollution in this agricultural area of the mid-high latitude region [26
]. However, there are few studies on non-point pollution by phosphorus in the Sanjiang Plain, and most of these focused on water quality of the river in marsh areas, or on the export mechanisms of phosphorus from specific land uses or specific soil types [27
]. The studies were done using experimental methods or empirical models, and the research requires further advances towards mechanism model development at the macro-scale. Questions remain about how to find a way to couple the processes in phosphorus cycling and a distributed hydrological model, with consideration for local hydrological features. Furthermore, studies of the non-point-source pollution by phosphorus in the Sanjiang Plain require solutions that can only be provided by access to the spatial data needed to drive the pertinent models, and to expand their scale.
To ensure good soil quality during agricultural development, the trend of changes in soil phosphorus due to land use change must be assessed. Understanding the characteristics of changes in soil phosphorus content is important for improving agricultural management practices and making land use practice more sustainable. The aims of this study are as follows: (1) to identify the temporal-spatial pattern of land use change occurring from 2000 to 2010 (11a) in the Sanjiang Plain; (2) to explore the transformation of soil phosphorus from six main soil types and five types of land use; and (3) to provide suggestions for future farmland management and establishment of sustainable land use systems while preserving soil phosphorus contents.