Construction and spatial variation of soil quality index in eroded farmland Mollisol of northeast China

Soil degradation is endangering agricultural and ecological sustainability in Mollisol, Northeast China. Among them, soil erosion can be the main cause of soil quality degradation. The spatial heterogeneity of erosion can affect the soil characteristics of farmland, leading to the spatial distribution of soil quality. However, only a few studies have been focused mainly on the spatial differentiation on soil quality degradation. This study aims to clarify the influence of spatial locations on the soil quality of eroded farmland Mollisol in Northeast China. Four hilly areas (Jiusan, Keshan, Hailun and Bin counties) under erosion environment were selected as the test objects along the latitudinal gradient from the cold to the middle temperate zone. Four representative long gentle slopes were chosen from the north to the south, and then divided into the slope top, upper, middle, lower and bottom position. The spatial heterogeneity of soil physicochemical properties was evaluated under soil erosion driven by different climatic and microgeomorphic features in agricultural soils. A systematic analysis was made to explore the effects of different spatial locations on the soil quality of eroded farmland, according to the physical, chemical and biological properties of soil. The soil quality index was constructed using the minimum data set (SQI-TDS), and then evaluated by principal component analysis and weighted synthesis. The results showed that the mean pH value of the tested soil was 5.48, indicating the weak acidity. The pH value in Bin County (5.29) was the lowest. There were the outstanding spatial variations of black soil properties in different regions, slope positions and soil depths in the four hilly regions. Total nitrogen and soil organic carbon shared an inverse relationship with the latitude, then gradually decreased with the decreasing slope position, and finally increased at the bottom of the slope. Moreover, the available potassium, saturated water conductivity and clay content were almost declined from the upper to lower position, and then increased continuously until the bottom position. Soil bulk density (BD), sand content, shear strength (SS) and penetration resistance (PR) increased with the decreasing latitude, while gradually increased from the upper to the lower position, and then decreased sharply at the bottom position. In the biological properties at different spatial locations, soil microbial biomass carbon (MBC) decreased gradually from the north to the south. There were the significant differences among the four hilly areas (P<0.05). The highest MBC content was found at the bottom position (238.39 mg/kg), and the lowest was at the lower position (189.69 mg/kg). In addition, the variation of enzyme activity was similar to that of MBC. The highest aggregate stability (MWD) was observed in Jiusan, the second in Keshan, and the lower in Hailun and Bin County. The formation of new aggregates was attributed to the deposition of soil fine particles at the bottom of the slope. As such, the MWD showed a "V" pattern of first decreasing and then increasing with the decrease of slope position. Combined with principal component and correlation analysis, the TN, clay, MWD, SOC, urease, PR, silt and AP were selected as the minimum data set (MDS) indexes. The soil quality index (SQI-TDS and SQI-MDS) was then calculated to evaluate soil quality. The total data set (TDS) was also fitted by linear regression. There was a significant positive linear correlation (R²=0.91, P< 0.05), indicating the reliability of SQI-MDS. Furthermore, the SQI-MDS decreased from the north to the south in the four hilly areas. At the same time, the lowest soil quality index was found in Bin County, which significantly decreased by 22.66%-26.52% than the rest areas. The SQI-MDS decreased in the descending order of bottom, top, upper, middle and lower position. The comprehensive soil quality was quantified to reveal the spatial differentiation of soil characteristics. These findings can also provide the important implications and scientific basis on the soil degradation, particularly for the restoration and sustainable development of eroded farmland in the Mollisol region