Cumulative effect of land use and cover changes in Naoli River basin in Sanjiang Plain on agricultural nonpoint source pollution load

Abstract

Unreasonable land use pattern is the one main driving factor of nonpoint source pollution (NPSP). Based on effect of land use change (reclaiming wetland into cultivated land) on NPSP, this paper analyzed the contents of main forms of nitrogen (N) and phosphorus (P) as well as key climate parameters in Naoli River Basin in Sanjiang Plain through field monitoring, sampling and indoor test. N and P are major non-point source pollutants. The spatial–temporal variation law of non-point pollution factors under different land use patterns and durations was analyzed. The cumulative effect of long-term land use and cover changes on non-point source pollution was simulated quantitatively by ArcSWAT model. The indoor test reported that (1) paddy field has the higher soil N and P contents, followed by dry land, wetland and forest land, successively; (2) in the whole river basin, mid-downstream shows the highest N and P nutrients in water, followed by midstream, downstream, mid-upstream and upstream, successively. N and P distributions in soil are similar to those in water. According to the quantitative simulation of NPSP in the Naoli River Basin, NPSP mainly concentrates on the two sides of the main stream. Compared to forest land, paddy field and dry land contribute more to NPSP. Cultivated land occupies 58 % of the whole basin area, which contributes at least 89.6 and 91.7 % of TP and TN. All of these areas were reclaimed from wetland, indicating that area and spatial position changes of wetland are the main causes of intensifying NPSP in the Naoli River Basin. The cumulative effect analysis revealed that since TN and TP in per unit of cultivated land area increase continuously, land use changes exert cumulative effect on NPSP. In other words, reclamation of wetland will accelerate increase of TP and TN loads.