Soil quality evaluation in typical coastal reclamation zones based on weighted Topsis method

Tidal flat is an important reserve of cultivated land resource in China. There is high spatial heterogeneity, properties and environment changes of soil in the transformation from wetland to agricultural soil. In order to evaluate the effect of soil quality under different reclamation durations and multiple land use in a typical coastal area in Cixi County, Zhejiang Province, 276 soil/sediment samples from farmland, vegetable land, orchard, forestland, aquafarm, wasteland and tidal flat reclaimed for 0 to 50 years were collected and analyzed. Considering the spatial heterogeneity, principal component analysis was used in combination with norm value, then minimum data set (MDS) with seven indexes was put forward. This included organic matter (OM), calcium oxide (CaO), sulfur (S), bromine (Br), boron (B), plumbum (Pb) and cadmium (Cd) under six reclamation duration and seven utility patterns. About 29.2% of the index accounted for 80.3% of the information. The Topsis method, which combined maximum difference normalization method and variable coefficient fixing weight method, was used to assess the soil quality. Compared with the traditional membership function method, the used method was a much more effective one. Besides, the distance to optimal vector directly was used to explain the obstruction factor, which showed the primary obstacle and was applicable in supervising soil environment and improving soil quality by the local government. The results suggested that the range of soil quality comprehensive indexes for the 276 soil/sediment samples was 44.6-74.1. The trend in the indexes increased sharply in first 20 years and then stabilized in the 20-30 years, and then it again increased sharply for 30-50 years of reclamation. In general, the overall soil quality in the study area was moderate to infertile. The class Ⅰ soil quality was only 7.1% of the samples. More than half of the soil samples belonged to classes Ⅳ or Ⅴ. Based on land use types, soil quality was ranked in the following order:vegetable field > orchard > farmland > forest > aquafarm > tidal flat > wasteland. Paddy field was better than dry land for the same reclamation period, which indicated that basic paddy farmland was much more environmental friendly in Cixi reclamation district. Irrigation-induced decalcification and desalination effectively improved the quality of reclaimed soil. Meanwhile, low OM and high CaO and Cd primarily limited soil quality. Next were salt content as Br, S and B, followed by heavy metal as Pb. Soil content of Cd increased quickly to critical point of pollution, which was easily enriched in paddy and caused diseases. Reasonable use of chemical fertilizers was a key to further promotion of soil quality in reclaimed tidal areas.